FIELD OF THE INVENTION

The present invention relates to processes and systems for the microbial conversion of a subsurface carbonaceous formation to methane. In particular, the invention relates to processes and systems for microbial conversion of subsurface carbonaceous formations incorporating an underground bioreactor and an aboveground bioreactor.

BACKGROUND OF THE INVENTION

Coal is estimated to comprise around 95% of Earth's fossil fuel resources, of which only around 10% is accessible by conventional mining technology. This leaves a substantial amount of energy locked away in stranded coal, which is more plentiful and more widely distributed than conventional oil and gas resources, including in many countries where development of stranded coal is the only option to alleviate energy poverty.

Whilst coal remains an important energy resource, its ongoing use is under pressure from environmental concerns around the role the emissions from coal combustion play in the ecosystem. The reduction of emissions through a transition to natural gas, represents one possible approach en route to renewable energy sources, and is being driven by unconventional natural gas developments such as coal seam gas (CSG). However, CSG development has a number of technical and social barriers to development due to substantial co-production of water, the use of hydraulic fracturing, and the scale required for the processes to be economic due to the low density of energy recovery. Any reduction in emissions from reduced coal use may be offset by impacts of unconventional gas development, particularly fugitive methane emissions.

Considering the important role of stranded coal as an energy source and the environmental challenges of unconventional gas development, it is desirable to develop new technologies that can extract energy from the coal with fewer environmental impacts.

Biogenic production of methane in coal seams is the result of the metabolic activities of anaerobic microbes which may have been entrained with the coal precursor material during burial and which are active during the early stages of coalification before thermogenic methane production commences under progressively elevated temperature and pressure. Biogenic coal seam gas formation is analogous to a number of natural processes where solid organic macromolecules are degraded via successive biological reactions to progressively smaller molecules, terminating in methane and carbon dioxide. Anaerobic microbes (anaerobes) are present in many low oxygen environments found in nature. Anaerobic biological processes are well understood and have seen the development of commercial biogas technologies where gas is generated from a range of biological waste products such as biomass, dairy effluent, animal manure, brewery effluent, and wastewater. These kinds of wastes streams have relatively easily digestible organic constituents such as proteins and carbohydrates.

Methods have been proposed to stimulate in situ biogenic methane production from subsurface formations through, for example: addition of non-native microbial consortia, or stimulating native microbes through the addition of nutrients. These processes seek to emulate the efficiencies of conventional (above ground) biogas systems in the underground environment. This is challenging because the main parameters that govern efficiency, such as available substrate area, pH, temperature, hydraulic retention time and nutrient compositions, cannot be controlled easily in the underground environment. Deviations from the preferred conditions can have a substantial reduction in process efficiency and therefore the amount of gas being produced.

One physical phenomenon affecting the biogenic stimulation of subsurface formations, is the inherent low permeability of sedimentary organic matter. Low permeability restricts microbial access to the volume of sedimentary organic matter that can be stimulated, and thus reduces the ability of microbes to induce change in the reservoir in the timeframes required to maintain a controlled process. Directional drilling approaches to improve surface area have been proposed to increase reach through a coal seam reservoir. However, these approaches still suffer from the drawback of limited process control.

Another physical phenomenon is the effect of generating new gas in a reservoir that naturally adsorbs methane and carbon dioxide into its structure. Given that few reservoirs are fully gas saturated, newly produced biogenic gas has the potential to be adsorbed into the coal matrix restricting the amounts of gas actually producible from the system. Moreover, the buoyancy of free gas in a saturated medium may limit production to areas where structural controls are favourable to collection up-dip of injection points.

SUMMARY OF THE INVENTION

In one aspect, the invention describes a method of generating methane from a carbonaceous formation, including: forming a fluid channel through the formation;

establishing an underground bioreactor in the channel; establishing an aboveground bioreactor connected to the channel;

passing nutrient fluid through the underground bioreactor into the aboveground bioreactor to provide a feedstock for the aboveground bioreactor; and

withdrawing methane from the aboveground bioreactor.

Preferably, the underground bioreactor is operated to promote hydrolytic and fermentative conversion of the carbonaceous formation for generation of oxygenated organic compounds in the fluid. In some embodiments, the fluid is recycled from the aboveground bioreactor into the underground bioreactor. Preferably, establishing the underground bioreactor includes providing a microbial consortium having hydrolytic, fermentative, acetogenic and/or acedogenic microbes. Preferably, establishing the aboveground bioreactor includes providing a microbial consortium having acetogenic, acedogenic and/or methanogenic microbes. Preferably, the belowground bioreactor includes a consortium serially adapted to convert the carbonaceous formation, into which it is injected, into oxygenated organic compounds. Preferably, the aboveground bioreactor includes a consortium serially adapted to convert the feedstock from the underground bioreactor to methane containing gas.

Preferably, formation of the fluid channel includes drilling one or more vertical wells into the formation and drilling one or more horizontal wells which intersect the vertical wells to form a sawtooth pattern. In some embodiments, a heating means is inserted into the fluid channel to modify temperature in the vicinity of the channel.

In another aspect the invention describes a system for generating methane from a carbonaceous formation, including: a fluid channel through the formation; an underground bioreactor in the channel; an aboveground bioreactor connected to the channel, injection means for passing nutrient fluid though the underground bioreactor into the aboveground bioreactor; and extraction means for withdrawing methane from the aboveground bioreactor.

Preferably, the underground bioreactor of the system contains a microbial consortium with hydrolytic, fermentative, acetogenic and/or acedogenic microbes. Preferably, the aboveground bioreactor of the system contains a microbial consortium with acetogenic, acedogenic and/or methanogenic microbes. In some embodiments, the system includes a fluid connection for recycling fluid from the aboveground reactor into the underground bioreactor.

In a third aspect the invention describes a method of generating methane from a carbonaceous formation, including: establishing a first bioreactor inside the formation, establishing a second bioreactor outside the formation, passing nutrient fluid into the first bioreactor, transferring modified nutrient fluid from the first bioreactor to the second bioreactor, and withdrawing methane from the second bioreactor.

In some embodiments of the third aspect, fluid is cycled between the first and second bioreactors. Preferably, establishment of the first bioreactor includes drilling one or more wells which pass through the formation, and a consortium of anaerobic microbes is provided in the wells. More preferably, one or more wells of the first bioreactor are configured in a sawtooth pattern of horizontal wells connected through one or more vertical wells to the second bioreactor.

In a fourth aspect the invention describes forming a subsurface network of channels for passing fluid through a geological formation, including: drilling an array of one or more vertical wells, and drilling an array of two or more horizontal wells, wherein each horizontal well has at least one or more branches in the formation, and wherein at least two horizontal wells intersect the same vertical well.

Preferably, the branches of the array of horizontal wells are laid out in a sawtooth pattern.

In a fifth aspect the invention describes a subsurface network of channels for a bioreactor system, including: an array of one or more horizontal wells which form an underground bioreactor, and an array of two or more vertical wells which connect the underground bioreactor to an aboveground bioreactor, wherein each horizontal well has two or more branches which form a sawtooth pattern, and wherein each branch of a horizontal well intersects a vertical well.

Additional embodiments and features are set forth in part in the following detailed description and accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the specification, or may be learned by the practice of the technology. The features and advantages of the technology may be realised and attained by means of the instrumentalities, combinations, and methods described in the specification.

BRIEF DESCRIPTON OF THE DRAWINGS

Exemplary embodiments of the present invention are illustrated by way of example in the accompanying drawings in which like reference numbers indicate the same or similar elements and in which:

FIG. 1A illustrates an overview of a process and system for the two-stage conversion of a subsurface carbonaceous formation. The overview includes: a fluid channel located in the formation with a bioreactor located therein, an aboveground bioreactor in fluid communication with the underground bioreactor, and a return circuit for process fluid from the aboveground bioreactor back to the formation. Auxiliary tanks are also illustrated.

FIG. IB is an expanded view of the underground bioreactor depicted in FIG. 1A.

FIG. 2 schematically illustrates an overview of the biological pathways for conversion of the macromolecules contained in a subsurface carbonaceous formation to breakdown products, and then on to short chain fatty acids (SCFAs), acetates, methane gas and other products.

FIG. 3 schematically illustrates biological pathways for conversion of macromolecules to oxygenated organic compounds such as SCFAs, acetates, C0 ₂ and H ₂

FIG. 4 schematically illustrates biological pathways for conversion of acetate, C0 ₂ and hydrogen to methane.

FIG. 5A illustrates a heating means inserted in the fluid channel terminating at the vertical well open-hole.

FIG. 5B illustrates a heating means inserted in the fluid channel through an opening and extending through the fluid channel in the subsurface formation exiting the channel at a second opening of the vertical intercept well. FIG. 6 illustrates a plan view of a representation of an array of horizontal and vertical wells interconnected to form continuous channels.

FIG. 7 illustrates a plan view of a representation of an alternative array of horizontal and vertical wells interconnected to form continuous channels.

FIG. 8 illustrates a three dimensional view of a representation of an array of horizontal and verticals wells interconnected to form continuous channels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the exemplary embodiments illustrated in the drawings, and specific language will be used herein to describe the same. It will nevertheless be appreciated that the invention may be implemented in various forms, and that this description is given by way of example only and that no limitation of the scope of the invention is thereby intended. Alterations and further modifications of the inventive features, process steps, and materials illustrated herein, and additional applications of the principles of the inventions as illustrated herein, which would occur to one of ordinary skill in the relevant art and having possession of this disclosure, are to be considered within the scope of the invention.

The drawings are provided for illustrative purposes only and are not necessarily drafted to scale. As such, variations may be had as to dimensions and proportions illustrated without departing from the scope of the present invention. Further, these drawings are illustrative of specific aspects and are not inclusive of all potential variations which fall within the scope of the invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practise or testing of the present invention, preferred methods and materials are described.

A fluid channel 101 formed by the intersection 103 of a horizontal well 102 with a vertical well 104 is depicted in FIG. 1A. This fluid channel, the path of which is indicated by arrows 101, extends from the surface into the subsurface where it further extends horizontally in the underground, or subsurface, carbonaceous formation 105 before returning to the surface.

As used herein the term "subsurface" refers to the stratum or strata below the earth' s surface and the term "underground" refers to below the surface of the ground. Whilst for many intents and purposes the two terms may be used interchangeably, the term "subsurface" has been used more particularly in reference to geological formations and the term "underground" has been used more particularly in a locational sense with respect to the surface of the ground. Similarly, as used herein the term "aboveground" refers to on or above the surface of the ground.

The fluid channel 101 may allow injected material to flow extended distances through the carbonaceous formation 105 in a controllable and predictable manner. Fluid injection through an opening 106 in a designated wellhead 107 is conducted at a rate that allows relatively constant pressure to be maintained along the length of the horizontal well 102 and may be done under active injection (pumping) or passive injection (gravity feeding). The injected liquids flow through the horizontal well 102 and are recovered through the vertical well 104 via artificial lift.

As used herein, the term "subsurface carbonaceous formation" refers to any sub-surface organic- carbon-containing sediment including, but not limited to: coal, shale, carbonaceous mudstone, tar sands, and oil retaining formations. Preferably the carbonaceous formation is a coal formation. As used herein the term "coal" refers to any of the series of carbonaceous substances ranging from the lowest rank including lignite and brown coal substances to the highest rank including anthracites. Coal rank is generally relatable to geological maturity and may be quantitatively observable, for example, through oxygen and moisture content (high in low rank coal decreasing with increasing rank).

Referring again to FIG. 1A, the vertical intercept well 104, with an opening 109 is drilled and cased to the top of the formation 112 with the case terminating in a casing shoe 11 1. Generally, the well 104, when drilled into the carbonaceous formation 105 itself, is an uncased open-hole completion 113, which is under-reamed 114. The open-hole completion 1 13 may be under- reamed at a range of diameters larger than the casing, for example, from 6" to 18". Optionally, the open-hole completion 1 13 may be jet drilled to provide an even larger cavity. The open-hole completion 113 provides a target area for the directionally drilled horizontal well 102, as well as a significant open area for the bore.

The horizontal well 102 may be constructed by one or more known techniques for well formation achieved by drilling from the surface and deviating the well bore in the subsurface to traverse the carbonaceous formation horizontally. The horizontal well 102 is cased through to the top of the formation 1 12 terminating in casing shoe 116. Typically, the horizontal open-hole section, extending from casing shoe 116 through to the intersection point with the vertical well at 103, is drilled through the subsurface carbonaceous formation 105 using a steerable bottom-hole assembly. Optionally, the open hole section may be opened up to a larger diameter through the use of standard j etting tools to provide an increased bore diameter and wellbore rugosity which, for example, provides a larger initial substrate surface area upon which an anaerobic biofilm may be established.

Typically, the horizontal well 102 is constructed in a similar manner to a vertical bore, with the only difference being the deviated path 115. A low angle of entry (for example, 30° from the horizontal) may be used, for shallow deposits. The radius bend of the horizontal well may be short (5° - 10 three feet), medium (6° - 307100 foot) or long (up to 67100 foot). The radius bend used may depend, for example, upon formation type and depth. The well 102 may enter the subsurface carbonaceous formation sub-horizontally with the target horizontal drilling horizon further along the formation, for example, at a 50m - 100m drilling horizon from the entry point into the formation. The well 102 may follow the subsurface carbonaceous formation contour, for example, down-dipping of the formation.

The open-hole section of the horizontal well 102 may be drilled to a range of different lengths, before interception 103 with the vertical bore 104. The length may be determined by formation type and economics. Low pressure water bores may be suitable for the formation of the fluid channel. The open-hole section of the fluid channel, extending from the horizontal well casing shoe 1 16, through the vertical well casing shoe 111, provides a region in the subsurface carbonaceous formation 105 in which an anaerobic microbial consortium can be established. Fluid that flows through the channel will be in direct contact with the carbonaceous formation in the open-hole section of the channel.

The use of a horizontal well 102, intersecting with a vertical intercept well 103, to form the fluid channel 101, may provide a greater degree of control of process conditions within the vicinity of the wellbore. For example, this approach may facilitate high microbial conversion of a smaller reservoir area, as opposed to lower conversion of a larger reservoir area.

The fluid channel 101, extending from opening 106 to opening 109 is an integrally connected continuous fluid channel, providing a low resistance fluid path between separate injection points 106 and production points 109. Direct flow connection between wells may allow for greater process control, for example, control of flow rate and fluid composition along the fluid channel, which may result in improved control over the physical, chemical and biological exchange of the circulating fluid and the carbonaceous formation. The presence of a continual flowing stream of constant composition through the fluid channel may allow for diffusive effects to maintain a relatively constant and controlled composition away from the borehole. The overall efficiency of the biological process may be enhanced through higher available substrate surface area and maintenance of optimal environmental conditions over a large area relative to vertical wells. An underground bioreactor, comprising an anaerobic microbial consortium, for conversion of the subsurface carbonaceous formation 105, may be established within the open-hole section of the fluid channel. The open-hole section extends from the horizontal well casing shoe 1 16 through the vertical well casing shoe 111, as depicted by the double headed arrow 117, which indicates the bioreactor. A first anaerobic microbial consortium, for injection into the fluid channel, may be cultured batch-wise in a surface tank or tanks 118. Alternatively, the anaerobic microbial consortium may be prepared off-site at a bio-culturing facility and transported to site and transferred to the surface tank 118.

Optionally, a sample of the subsurface carbonaceous formation may be extracted and used in tank 118 as a growth medium for the anaerobic consortium. Using samples of the carbonaceous formation may assist in selecting, identifying and/or adapting anaerobic microbes with particular activity for that carbonaceous formation. Alternatively, a sample of the extracted subsurface carbonaceous formation may be taken off site to a laboratory and used there for the culturing and adaptation of an anaerobic consortium.

The underground bioreactor may be operated under a range of environmental conditions, for example - pH and temperature, to promote different types of microbial activity.

In some embodiments, the first anaerobic consortium has been serially adapted to the environmental conditions prevailing in the subsurface environment into which the microbes are to be injected. In some preferred embodiments, the first anaerobic consortium has been serially adapted to the temperature, salinity and pH of the subsurface environment.

In other embodiments, the subsurface environment is adapted to increase the rate of production of oxygenated organic molecules by the first anaerobic consortium. In some preferred other embodiments the subsurface temperature and pH are adapted to increase the rate of production of oxygenated organic molecules by the first anaerobic consortium.

In some further embodiments, both the first anaerobic consortium may serially adapted to the environmental conditions prevailing in the subsurface environment into which the microbes are to be injected, and, the subsurface environment is adapted to increase the rate of production of oxygenated organic molecules by the first anaerobic consortium. Preferably the first anaerobic consortium has been adapted to biodegrade the coal in situ to produce oxygenated organic molecules.

One method of serial adaptation of microbes involves serial batch culturing of microbial populations growing in flasks or tubes. For example, the microbial populations such as bacteria, archaea and protozoa populations, which may form part of an anaerobic consortium. At the end of a defined period of time, an aliquot from the current population may be introduced into a new flask containing fresh media that permits further growth. These transfers may occur at any time during the growth phase of the bacterial population. Typically, most transfers occur once the population has exhausted the resources in the media and entered stationary phase.

As an example, the conditions in the fresh media may be modified to gradually approach (over a series of transfers), the environmental conditions identified in a subsurface formation. This could occur, for example, through gradual variation (e.g decrease) of pH and temperature.

Alternatively, for example, the conditions in the fresh media may be modified to gradually approach (over a series of transfers), the chemical composition of a fluid feedstock derived from a subsurface bioreactor. This could occur for example, by gradually feeding, over a series of adaptations, model oxygenated organic compounds of increasing concentration and diversity, to a population. Alternatively, a sample from a carbonaceous could be biologically converted to provide a feedstock.

Alternatively, an adaptation could be accomplished by gradually introducing to a consortium, residing in a continuous flow system such as in a test UASB reactor system, a feedstock that emulates a feedstock that would be derived from a subsurface bioreactor. The feedstock can be introduced as a co-feed at very low concentrations with its concentration gradually increasing over time until the microbes have adapted to the composition of the new feedstock. This could also be conducted at larger scale than a test system, if desired.

Formation water may be withdrawn from the fluid channel in the subsurface and transferred to a surface water storage tank 125. This water may then be used as the liquid medium for batch growth of microbial consortia and subsequently for reinjection to the formation. The first anaerobic consortium may be adapted to growth in this formation water. Generally, water for use in the process, may also be stored in a separate tank or tanks 125 and used as required.

A filtration system may be employed to concentrate the microbes above normal batch levels before injection into the fluid channel. The first anaerobic consortium in fluid form may be transferred 1 19 from tank 1 18 into mixing tank 120 and subsequently transferred 121 through to well opening 106 into the fluid channel 101 to form the underground bioreactor 1 17. Most preferably, the transfer is done under anaerobic (anoxic) conditions. The first consortium may be transferred to the subsurface by active means (pumping) or passive means (gravity). The first consortium may establish itself in the formation, using the fluid medium in the open-hole channel and/or the open-hole channel surface as a growth medium upon which to establish a bioactive surface.

To promote the establishment of an underground bioreactor, nutrients may be added to the subsurface carbonaceous formation, along with the anaerobic microbial consortium. Water quality may also be amended. Nutrients and water quality amenders or modifiers may generally be used to optimise environmental conditions for growth of the first consortium within the subsurface formation. An amendment tank 122 may be used to add nutrients and modify water quality before transfer 123 to the subsurface. For example: nutrients, as well as water quality amenders may be combined in the amendment tank to form a nutrient amendment mixture. This nutrient amendment mixture may then be transferred 123 from amendment tank 122 into mixing tank 120, and subsequently transferred 121 to the subsurface carbonaceous formation. The nutrient amendment mixture may be transferred to the mixing tank and combined, for example, with an anaerobic consortium, which has previously been transferred to mixing tank 120, or the nutrient amendment mixture may be transferred without further addition to mixing tank 120 and subsequently to the subsurface formation. Additional water may be supplied 124 from a water tank 125.

The length of time required for the microbial consortium to establish itself within the subsurface carbonaceous formation may depend on factors such as: cellular division rates of microbes of the consortium, environmental conditions, and substrate biological availability as determined by the physical and chemical properties carbonaceous formation. It is expected that as the rate of cellular division increases the rate of production of metabolism by-products will increase.

Samples of fluid and/or solids taken from the underground bioreactor can be analysed to determine the progress of the establishment of the first microbial consortium within the underground bioreactor, and to quantify the rate of product formation. Rate of production can be determined, for example, through change in concentration of the products within fluid contained in the fluid channel. Analysis techniques may include: chemical oxygen demand (COD), dissolved oxygen (DO), oxidation reduction potential (ORP), gas chromatography (GC), high performance liquid chromatography (HPLC), H, hardness, alkalinity and the like. Such analysis methods and techniques are known to the art.

In addition to establishing a bioactive surface on the channel walls, the microbes of the anaerobic consortium infiltrate the pores of the formation, migrating from the walls of the channel in response to substrate availability and nutritional amendment (from diffusion), to establish a bioactive zone The underground bioreactor comprises all of: the bioactive surface, fluid in the open-hole channel, and the bioactive zone.

Referring to FIG. IB, an expanded view of the underground bioreactor is depicted. Arrows indicate the direction of the fluid stream 101 1 through the bioreactor (see FIG. 1A, 117).

Bioactive films 145 are formed on the surface of the fluid channel 1021. The respective bioactive zones 146, 147, and 148 progressively extend from the bioactive surfaces 145 into the carbonaceous formation with a microbial density gradient represented directionally by the arrow 149 generally away from the surface 145 into the formation.

Microbial density is expected to be higher at or near the channel surface with the density gradient (cell count) per cubic centimetre decreasing with further penetration into the formation from the channel surface. Additional vertical bores may be drilled adjacent to the horizontal channel in the carbonaceous formation and nutrients deployed into the vertical wells in order to encourage microbial migration from the horizontal well out into the formation. The degree of microbial penetration into the carbonaceous formation may depend, for example, on the degree of porosity, pore diameter and connectivity of pores.

Microbial activity may open the porosity of the formation in the vicinity of the channel 1021 thus increasing penetration into the formation over time. Cleat structures and fractures in the carbonaceous formation which are in fluid communication with the channel 1021 may provide avenues for further microbial penetration of the carbonaceous formation as the microbes migrate down these passages. Arrows 150 indicate the direction of transfer of nutrients and water quality modifiers from the fluid stream into the carbonaceous formation. Arrows 151 indicate the direction of transfer of products from biochemical microbial processes occurring on and in the carbonaceous formation into the fluid stream 1011 to be carried through the bioreactor system 117 to the surface.

The radius of influence from the wellbore channel 1021 increases progressively due to diffusive transport, microbial population growth and migration, and temperature equilibration. High conversion of the substrate may promote increased permeability which may enhance production rates due to increased substrate surface area and improved mass transport to the extremities of the bioreactor zone. Periodic stoping may occur as the cavity grows, providing further substrate surface are

Referring back to FIG. 1A, after the first microbial consortium has been established in the subsurface carbonaceous formation in the form of a bioreactor, the products (or effluent) of the cellular metabolism of this first consortium, in the form of a modified fluid composition, may be withdrawn 126 through an opening 109 from the production well 110. The modified fluid composition may be considered the effluent of the underground ground bioreactor and commensurately the feedstock for an aboveground bioreactor.

Preferably the modified fluid composition is comprised of oxygenated organic compounds. Preferably, the first anaerobic microbial consortium is adapted to promote hydrolytic and fermentative (acedogenic/) conversion of the carbonaceous formation, to generate oxygenated organic compounds. More preferably, the consortium is adapted to generate low molecular weight oxygenated organic compounds. Preferably the consortium is comprised of hydrolytic, fermentative, acedogenic and anaerobic microbes.

The terms "anaerobic consortium" and "anaerobic consortia" refer to a group or groups, an ensemble or ensembles, or a population of anaerobic microbes. Microbes of the consortia may be obligate, facultative and have a greater or lesser sensitivity to oxygen. A facultative anaerobe is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation or anaerobic respiration if oxygen is absent. An obligate aerobe, by contrast, cannot make ATP in the absence of oxygen, and obligate anaerobes die in the presence of oxygen.

Anaerobic consortia are found throughout nature including in locations such as: vertebrate and invertebrate guts - such as termite hind-gut microbes (e.g. ATCC ^® 55801 ^™ ); peat bogs; rice paddies; wastewater treatment plants; benthic zones; soil; coal seams; shale; oil reservoirs; marshland; and numerous other low oxygen environments.

The use of anaerobic consortia for biological processes such as wastewater treatment or in situ bioremediation is known to the art. Microbiological techniques to adapt anaerobic consortia to a range of carbonaceous feedstocks (e.g. different wastewaters streams, coals, etc) and environmental conditions (e.g. temperature and pH) are also known to the art.

As used herein, the term "low molecular weight" refers to molecules that have a molecular weight of less than 1000, preferably less than 500 and more preferably less than 150 atomic mass units (Daltons (Da)).

As used herein, the term "oxygenated organic compound" refers to an organic compound that contains at least one oxygen atom in its structure. Examples of oxygenated organic compounds include: alkanoic acids (including low molecular weight organic acids and short chain fatty acids (SCFAs)), alkanoates, alcohols, esters, ketones, aldehydes and ethers. Short-chain fatty acids (SCFAs), including volatile fatty acids (VFAs), are a sub-group of fatty acids with saturated or unsaturated aliphatic tails of less than six carbons. SCFAs may be further substituted with other functional groups, such as hydroxyl, keto- or carboxylic acid groups. Examples of unsubstituted SCFAs include: formic acid (methanoic acid), acetic acid (ethanoic acid), propionic acid (propanoic acid), butyric acid (butanoic acid), succinic acid (butanedioc acid), valeric acid (pentanoic acid) and caproic acid (hexanoic acid). Substituted SCFAs include: isobutyric acid (2-methylpropanoic acid), isovaleric acid (3-methylbutanoic acid), leucic acid (2-hydroxy-4- methyl-pentanoic acid), isocaproic acid (4-methyl-pentanoic acid), pyruvic acid (2-oxopropanoic acid), lactic acid (2-hydroxypropanoic acid), fumaric acid (fr ra-butenedioic acid), and malic acid (hydroxybutanedioic acid).

Broadly, as used herein, the term "fermentation", and related terms such as "fermentative", refers to the bulk growth of microorganisms on a growth medium. The grown medium may be a carbonaceous substrate such as coal. Fermentation may be a staged process comprised of, for example, primary and secondary fermentation stages.

The first anaerobic consortium may comprise anaerobic organotrophs - organisms that use organic compounds as electron donors. These consortia can catabolise organic macromolecules, such as those contained in subsurface carbonaceous formations.

As used herein, "catabolism", and related terms such as "catabolise" refer to the metabolic pathways that break down molecules into smaller units (breakdown products) to release energy. Through catabolism, large macromolecules are broken down into smaller units or breakdown products.

The term "organic macromolecule", as used herein, refers to a polymeric or non-polymeric molecule with a high molecular mass. Organic macromolecules contained in subsurface formations often have a biopolymeric origin and have been transformed by temperature and pressure, and other prevailing conditions of the subsurface (water chemistry, regional minerology), over geological timeframes. For example, the structure of coal may in part ultimately be relatable back to organic biopolymers such as lignin. The organic macromolecules contained in subsurface carbonaceous formations have varied structural features, including: aromaticity, aliphatic substituents, heteroatom content and degree (for example, oxygen, nitrogen, and sulphur), functional groups, covalent and non-covalent cross-links, physical associations, and other surface structural features that determine chemical and physical properties and reactivities. Properties such as the degree of aromaticity, heteroatom content, etc., may vary with rank, for example, low rank coal may be have a higher oxygen content, whilst bituminous coal may have a higher degree of polycyclic aromaticity and less oxygen. An "organic macromolecule" includes but is not limited to: fulvic acids, humic acids, humates, humins, bitumen and kerogen.

Referring to FIG. 2, the biological process for breakdown of these organic macromolecules can be broadly illustrated by the following stages: breakdown 201 of the large macromolecular structures contained in carbonaceous formation through the fermentative and hydrolytic processes of hydrolytic-type microbes, for example, through the secretion of exoenzymes; further fermentative-type conversion 202 of these breakdown 201 products, by microbes such as acedogens, to low molecular weight oxygenated compounds, such as short chain fatty acids (SCFA), including volatile fatty acids (VFAs), and other oxidised hydrocarbons such as alcohols, ketones, and aldehydes, conversion 204, by microbes such as acetogens, of VFAs to acetic acid (CH ₃ COOH) hydrogen and CO ₂ ; and, conversion of acetic acid, and other byproducts such as hydrogen and carbon dioxide, by methanogenic microorganisms to methane. Methanogenesis may be hydrogenotrophic 206a, or acetoclastic 206b. As used herein, the term acetogens refers to microbes that produce acetic acid, hydrogen and carbon dioxide from fermentation products. Acetogens fall into two main groups: hydrogen producing acetogens and homo-acetogens. Acetate may also be produced through direct fermentation 203 and by hydrogen producing acetogens 205a and 205b. Acetogens form syntrophic relationships with methanogens.

The use, herein, of the term "acedogen" includes reference to acid forming microbes that can produce oxygenated organic compounds, such as short chain fatty acids, from fermentative breakdown products.

As used herein, the terms "exoenzyme" and "extracellular enzyme", refers to enzymes that are secreted by a cell and function outside of that cell. Many of these enzymes are involved in the breakdown of larger macromolecules to hydrolytic and fermentative breakdown products. These breakdown products can enter into cells and be utilized for various cellular functions. Examples of exoenzymes include: hydrolytic exoenzymes, oxidoreductases, decarboxylases and the like.

As used herein, the term endocytosis refers to the process whereby a cell absorbs molecules by engulfing them. An example of endocytosis is phagocytosis, a common feeding mechanism of protozoan (protist) cells.

In the context of anaerobic microbial processes, the term "hydrolysis" generally refers to the biological conversion, by hydrolytic microbes, of macromolecules to breakdown products. This may occur by phagocytosis where a macromolecule, or even a small coal particle is ingested by a microbe, such protozoan cell, and digested with breakdown products typically as by-products of the process. These may be excreted and taken up by other cells

The term "acedogenesis" generally refers to the conversion of breakdown products, by fermentative type microbes (acedogens), to oxygenated organic compounds such as short chain fatty acids (SCFAs) and alcohols, for example: methanol, ethanol, propanol and butanol. The term "acetogenesis" refers either to: (i) the conversion of low molecular weight oxygenated organic compounds by acetogens to acetates (acetic acid), or (ii) the conversion of hydrogen and carbon dioxide to acetic acid. The term methanogenesis refers to the conversion of acetates, by acetoclastic methanogens, to methane, and also refers to the conversion of carbon dioxide and hydrogen, by hydrogenotrophic methanogens, to methane. The term methanogen, as used herein, includes reference to one or more of acetoclastic and hydrogenotrophic microbes.

Returning to FIG. 1A, the modified fluid composition, or feedstock to the aboveground bioreactor, may then be filtered 127, through means such as gravity (settling), flocculation and/or size exclusion, to reduce particulate matter load. Filtered solids may periodically be transferred 128 to a separate storage vessel 129. Filtered solids may include useful fertiliser substances such as humins and humic acids - depending on the process fluid pH. Humins are organic compounds that are largely insoluble in water irrespective of the pH. Humic acids are solubilised by increasing alkalinity.

The process depicted in FIG. 1A may optionally be a continuous or continuous batch process. A build-up of breakdown products and metabolites within the underground bioreactor system may lead to inhibitory effects. Such inhibitory effects may be overcome by use of the continuous flow system. Some of the hydrolytic and fermentative microbes of the anaerobic consortium of the underground bioreactor are sessile in nature, and therefore adhere to surfaces in the

carbonaceous formation. When water is recycled to the system, and commensurately water withdrawn from the system, inhibitory products may be removed from the underground bioreactor, allowing for a positive effect on biochemical equilibria, such as promotion of the production of secondary fermentation products including oxygenated organic molecules. Semi- continuous and continuous circulation through the underground bioreactor may assist in maintaining a constant composition of nutritional amendment, and the recovery of oxygenated organic compounds to surface.

The process in the subsurface, including the underground bioreactor, is conducted at a pressure approximately that of the prevailing hydrostatic pressure of the carbonaceous formation to reduce formation losses. Pressure is controlled by the rate of injection in the injection well 107 and the rate of withdrawal from the production well 1 10, with the two being maintained approximately equal for the purposes of standard operation. This pressure control may be determined specifically for each underground bioreactor 117 and may be dependent upon the formation permeability and borehole characteristics.

The pressure in the borehole channel 117 may be periodically lowered by increasing the rate of pumping from the production well 110 or reducing injection rates in the injection well 107. This may have the effect of drawing product back from the outer extremities of the horizontal bioreactor for recovery to surface. The timing of this may be a function of the preferred hydraulic retention time and variation of the process conditions from optimal ranges due to the build-up of products such as SCFAs .

The use of a continuous fluid channel in the subsurface also provides a practical mechanism for flushing of boreholes. In this respect, the direction of fluid flow through the channel may be reversed. For example, fluid may be inj ected through an opening 109 in wellhead 110 and withdrawn from the opening 106 in wellhead 107. This reversal of flow 130 may be used to "back-flush" the system which may assist, for example, in dislodging blockages that may have occurred in one or more parts of the fluid circuit including the continuous channel in the subsurface. A high velocity flush may reduce the build-up of waste biomass, fines or other undesirable materials. Biocides or other chemical treatments may also be used in the flushing process.

Returning to FIG. 1A, after filtration 127, the modified fluid composition, which has been transferred 126 from the subsurface, may then be transferred 131 to a subsurface product holding tank 132, where it may be preconditioned by modification of environmental factors such as: temperature, water quality and nutrients, after which it may be transferred 133 to an

aboveground bioreactor 134 which comprises a second anaerobic microbial consortium.

Preferably, the second consortium is comprised of a microbial consortium adapted to generate biogas (methane and carbon dioxide), including microbes such as methanogens, acetogens, and acedogens. The second consortium may be batch grown in a tank 135 before being transferred 136 to the bioreactor 134. A second consortium seed may be prepared in a laboratory before being cultured in tank 135 for subsequent use in bioreactor 134. Alternatively, the second consortium may be cultured at a dedicated off-site facility, transported to site, and transferred to culturing tank 135 or transferred directly to the aboveground bioreactor 134. Preferably the second consortium is adapted to use the effluent of the first underground consortium, which is the modified fluid composition withdrawn from the subsurface, as a feedstock. Product gas generated in the aboveground bioreactor 134, which contains methane, is withdrawn or extracted from the bioreactor and transferred 137 to a dedicated storage tank 138 before it may transferred 139 for sale into a gas pipeline 140 or used for power generation.

In some embodiments, the gas may be subjected to an acid gas removal process to remove C ⁽ ¾ and increase the calorific value, and to remove other acid gases, such as sulphur containing acid gases, if present. Bioreactor process water from the aboveground bioreactor 134 may be transferred 141 to a storage tank 142 where it optionally may be amended to adjust water quality and nutrient content.

Optionally, some or all of the bioreactor process water may be filtered and treated to meet environmental standard requirements and discharged. The bioreactor process water contained in tank 142 may be transferred 143 to a mixing tank 120 before it is returned to the subsurface through well opening 106. Additional make-up water from water storage tank 125 may be transferred 124 to the mixing tank 120. A water amendment composition comprising one or more of nutrients and water quality amenders or modifiers, may optionally be transferred 124 to the mixing tank from tank 125 to amend the chemical composition of the process fluid.

By using a dual reactor system comprising: (a) an underground bioreactor 1 17 comprising a first microbial consortium and (b) an aboveground bioreactor 134 comprising a second microbial consortium, greater control over the overall biochemical conversion of macromolecules contained in subsurface carbonaceous formations may be obtained. This control may be accomplished optionally through selective modification or amendment of environmental factors for each of the underground bioreactor and for the aboveground bioreactor or by adaptation of the consortia to different environmental conditions. The above ground portion of the dual reactor system, as enclosed within the hashed line 144, comprises a return circuit for process fluid to be returned to the subsurface.

As used herein the term "environmental factors" refers to any factor, abiotic or biotic, that influences a living organism's environmental conditions. Examples of environmental factors relevant to anaerobic microbial growth include: temperature, water quality, light and nutrients. As such, modifications, adaptations or amendments to environmental conditions include changes to environmental factors such as: nutrient type and concentration, water quality, light, and temperature

Through identifying environmental factors, which influence the microbial activity of for example: hydrolytic microbes, acedogens, acetogens and methanogens, process improvements may be made. In particular, the environmental factors for the first consortia may be different to the environmental factors for the second consortium.

For example, environmental factors influencing the underground bioreactor environmental conditions may be amended to promote hydrolysis, acedogenesis and acetogenesis in order to increase the rate and quantity of production of oxygenated organic compounds. Optionally, microbes may be adapted to the prevailing environmental conditions, such as found in a target coal seam, in order to improve microbial activity under those prevailing conditions.

Accordingly, either the environmental conditions in the underground bioreactor may be adapted to optimise one or more of the biological processes indicated in FIG. 3, or the microbes may be adapted to optimise one or more of these processes, or both may be undertaken. As depicted therein, macromolecules 301 contained in the subsurface carbonaceous formation are converted 201 by hydrolytic and fermentative microbes to breakdown products 303. These breakdown products (303) may be taken up by microbes such as acedogens and are subsequently converted via a range of pathways such as: acedogenesis 202, direct fermentative acetogenesis 203, homoacetogenesis 204, to low molecular weight oxygenated organic molecules including SCFAs, alcohols (305) and acetate 306. The biochemical conversion processes may also result 205a, 307 in other products such as hydrogen and carbon dioxide 308. For example, a byproduct of hydrogen-producing acetogens is hydrogen gas. The relative proportion of hydrogen, carbon dioxide and methane formed depends, at least in part, on environmental conditions such as pH and temperature.

Similarly, environmental factors influencing the aboveground bioreactor 134 environmental conditions may be amended to promote methanogenesis. Accordingly, environmental conditions for the aboveground bioreactor may be adapted to optimise the biochemical processes illustrated in FIG.4. As depicted in FIG. 4, anaerobic acetogens convert 204, 205a and 205b oxygenated organic molecules 305 into acetate 306. Carbon dioxide and hydrogen 308 may also be produced as byproducts of the biochemical conversion by the consortium of oxygenated organic molecules 305. Acetoclastic methanogens convert 206b acetate 306 into methane and carbon dioxide 401. Hydrogenotrophic methanogens convert 206a hydrogen and carbon dioxide 308 into methane 401. Furthermore, microbes may be adapted to bicoonvert a specific feedstock, such as a feedstock derived from the breakdown of coal in the subsurface.

An environmental factor affecting the underground bioreactor is temperature. The temperature of the subsurface formation is a function of depth and the geothermal gradient. The underground bioreactor may be designed to incorporate a longitudinal temperature control system, providing an additional degree of control over the process. Referring to FIG. 5A, a heating means 501a may be installed along the length of the fluid channel 1021 from opening 1061, optionally, extending partially along, or completely along the horizontal well terminating 502 at the horizontal well's intersection with the open hole section of the vertical intercept well 1031. The heating means may be used to adjust the temperature of the fluid in the channel and the carbonaceous formation in the vicinity of the fluid channel. The heating means may be an electrical element, such as a heating trace, or a fluid heat exchanger. Optionally, the electrical heating element may be enclosed by a flexible pipe to assist in minimising damage to the cable. Referring to FIG. 5B, optionally, the heating element 501b may also extend from opening 1061 all the way through the fluid channel 1021 and exit the channel at opening 1091. The heating means may take the form of a rigid narrow diameter pipe (nominally between 1" and 3") which may be run inside of drill pipe after drilling of the horizontal borehole. The narrow diameter pipe may be polyethylene, plastic or steel pipe. For example, the end of the narrow diameter pipe can be fitted with a barbed deployment tool which opens up at a desired point, usually the base of the vertical borehole that has been intercepted. This secures the narrow diameter pipe to the desired point in the horizontal borehole allowing the outer drill pipe to be retracted, leaving the narrow diameter pipe in place. In some embodiments a fishing tool may be deployed in the vertical well to allow the pipe to be pulled up the inside of the vertical well to surface to allow direct flow to be established through the narrow diameter pipe. The narrow diameter pipe may be used to circulate fluids through the system at a desired temperature independent of the flow of microbe and nutrient mixture. Alternatively the steel pipe may include a wire resistance heater for conductive heat transfer.

Temperature is also an environmental factor affecting the aboveground reactor The

aboveground bioreactor may be adapted to provide a stable temperature environment to the consortium therein. A temperature in the range of 30°C - 40°C is preferred for mesophilic methanogens. A temperature change of 2°C - 3°C may cause an accumulation of SCFAs and a decreasing methane generation rate. Mesophilic methanogenesis may be suppressed by low temperatures. The temperature of the aboveground bioreactor (133) may be maintained at a relatively constant temperature through heat exchange (heating and/or cooling). Preferably, the temperature of the aboveground bioreactor is maintained in the range of 20 - 50°C, and more preferably in the range of 30°C - 40°C.

As mentioned, a decrease in temperature may supress methanogenesis, which may be desirable if the suppression of gas production in the subsurface is preferred. Accordingly, in some embodiments, it is preferable for the below ground bioreactor to be operated at a lower temperature than the above ground bioreactor. In some embodiments the below ground reactor is at 20°C - 30°C.

Water quality is an environmental factor that generally includes reference to the chemical, physical and biological characteristics of water. Indicators of water quality include but are not limited to: turbidity and clarity; colour; salinity; suspended solids; dissolved solids; pH;

hardness; alkalinity; dissolved oxygen; oxidation reduction potential (ORP); dissolved organic and inorganic compounds including: acids, bases, organic salts and mineral salts; odours; taints; colour; and, floating matter. To adjust, amend or modify water quality refers to chemical and/or physical means that alter one or more water quality indicators.

Acids and/or bases may be used to modify water quality through adjusting the water pH. As used herein, the term "pH" refers to a measure of the acidity or basicity of an aqueous solution. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. pH is a significant environmental factor for anaerobic digestion processes. For example, a more preferred pH for growth of methanogens may at higher pH, in the

approximately neutral to basic range (6.5 - 8.5). Methanogens may be more sensitive to pH variations than, for example, acedogens. Many microbes thrive only in a relatively narrow pH range. Accordingly, it may be desirable to keep the pH of, for example, a bioreactor, within a narrow range. A pH buffer solution is an aqueous solution consisting of at least a mixture of a weak acid and its conjugate base, or vice versa. A buffer solution's pH typically changes very little when a small amount of strong acid or base is added to it and thus it may used to prevent changes in the pH of a solution.

Acids may be mineral or organic acids. Examples of mineral acids include: HC1, H ₂ SO ₄ , HNO ₃ , and H ₃ PO ₄ . Examples of organic acids include: acetic acid, citric acid, formic acid, pyruvic acid, succinic acid, gluconic acid, fumaric acid, lactic acid, oxalic acid, humic acids and fulvic acids.

Alkalinity is an environmental factor that may have an effect on bioreactor performance. The term "alkalinity" as used herein, is a measure of the acid-neutralizing capacity of a fluid. It is an aggregate measure of the sum of all titratable bases in the sample. Alkalinity in most natural waters is due to the presence of carbonate (CO3 ^2" ), bicarbonate (HCO3 ^" ), and hydroxyl (OH ^" ) anions. However, borates, phosphates, silicates, and other bases also contribute to alkalinity if present. Alkalinity is usually reported as equivalents of calcium carbonate (CaCC^) The alkalinity of a fluid may be modified by the addition of alkaline substances such as inorganic bases, for example: KOH, citrate, NaOH, CaC0 ₃ , Ca(OH) ₂ , NaHC0 ₃ , Na ₂ C0 ₃ , KH ₂ P0 ₄ , and Na ₂ P0 ₄ . Metallic cations such as calcium, magnesium and sodium are important in many biological processes as are counterions (anions such as carbonate and sulphate). In addition, alkaline or basic substances may be used to bring the pH of a bioreactor up if production of SCFAs and acetic acid overtakes methanogenesis and the pH of the system begins to drop. This is significant, for example, in maintaining control over environmental conditions in the aboveground bioreactor efficiency, as some methanogens have known sensitivities to high concentrations of SCFAs. In some instances, basic solutions may be used in the subsurface to modify the porosity of the carbonaceous formation and to make available substrates, such as humic acids and humins to, for example, to the first consortium. Low rank coals such as lignites and brown coals are known to contain humic acids. Low rank coals may have a gel-type structure due to particle surface and molecular surface functionalization. Therefore, low rank coal may be peptized with alkali treatment which releases, for example, humic acids.

Related to alkalinity is the water quality indicator "hardness", which refers to the sum of polyvalent cation concentrations dissolved in the water.

A range of buffer systems may be used depending on the pH requirements of the microbes. Many biological systems lie in the pH range of 5.0 - 10.0. A good buffer system should be water soluble. Examples of buffer systems include: bicarbonate-carbonate, citrate buffer and phosphate buffer. Sodium citrate buffer solutions may be made and adjusted to the desired pH by mixing citric acid and trisodium citrate. Other buffers are made by mixing the buffer component and its conjugate acid or base by using Henderson-Hasselbalch calculations. For example, phosphate buffers are made by mixing monobasic and dibasic sodium phosphate solutions in a specific ratio Sodium bicarbonate buffer systems are made by mixing solutions of sodium carbonate and sodium bicarbonate. The bicarbonate-carbonate buffer system is commonly in the pH range of 9.2 - 10.8, the citrate buffer in the range of 3.0 - 6.2, and the phosphate buffer system in the range of 5.8 - 8.0. A phosphate buffer, for pH 6.0 at 25°C is prepared as follows:

Solution A: 0.2M sodium phosphate, dibasic dehydrate (Na ₂ HP0 ₄ .2H ₂ 0, FW = 178.05).

Solution B; 0.2M sodium phosphate, monobasic monohydrate (NaH ₂ P0 ₄ .H ₂ 0 FW = 138.01).

Combine 61.5 litres of solution A with 438.5 litres of solution B to provide a final pH of 6.0 at 25°C.

As used herein, the term nutrient refers to those components in foods that an organism utilizes to survive and grow. The term "nutrients" encompasses both "macronutrients" and

"micronutrients". Typically, nutrients needed in small amounts are micronutrients and those that are needed in larger quantities are macronutrients. Use of the term macronutrients also includes reference to macro-minerals. Examples of micronutrients include: selenium (Se), Iron (Fe), Calcium (Ca), Cobalt (Co), Magnesium (Mg), Molybdenum (Mo), and Nickel (Ni), Copper (Cu), Tungsten (Wo); Zinc (Zn), and Boron (B). Use of the term micronutrients also includes reference to vitamins and other trace compounds and elements. Vitamins are organic compounds required as nutrients in tiny amounts by an organism. Examples of vitamins include vitamin B ₁₂ , Pantothenic acid, riboflavin, biotin, folic acid, thiamine, nicotinic acid, pyridoxamine-HCl and the like. Examples of macrominerals include: calcium (Ca), chloride (CI), magnesium (Mg), nitrogen (N), phosphorus (P) and potassium (K). Nutrients may include: organic compounds, such as: carbohydrates, for example, complex and simple sugars (e.g. mono, di-, tri-, oligo-, and polysaccharides), molasses; proteins; fats; citric acid; humic acids; pyruvic acid). The term nutrient can also refer to solutions containing ions that will promote and/or modify bacterial growth.

Another type of micronutrient is a cofactor. A cofactor is a non-protein chemical moiety that is required for a protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations. Cofactors may be used, for example, to promote or assist the activity of exoenzymes involved in hydrolysis of organic macromolecules.

An exemplary nutrient composition for culturing anaerobic microbes is provided in Table 1, below.

Na ₂ Se0 ₃ .5H ₂ 0 0.001 g/L

Resazurin 0.001 g/L

Na ₂ S.9H ₂ 0 0.3 - 0.5 g/L

L-Cysteme-HC1.H ₂ 0 0.3 - 0.5 g/L

Yeast Extract 1.0 g/L

Tryptic Soy Broth 1.0 g/L

Vitamin Solution 10 mL

Trisodium Citrate 0.1 - 0.5 g/L

Methanol 0.4 g/L

Method for preparation of nutrient composition

Dissolve ingredients except bicarbonate, vitamins, methanol cysteine and sulphide and sparge medium for 30 - 45 mins with a mixture of N ₂ :C0 ₂ (4: 1). Alternatively, dissolve ingredients except bicarbonate, vitamins, methanol cysteine and sulphide and bring to then cool to room temperature under a mixture of NiiCCh (4: 1). Dispense the medium under the same gas atmosphere into culture vessels and autoclave. Add sulphide from a sterile anoxic stock solution prepared under nitrogen and carbonate from a sterile stock solution prepared under 80% 2 and 20% C0 ₂ .

Table 2: Exem lar Vitamin Solution

Vitamins are prepared under N2 gas atmosphere and sterilised by filtration.

Nutrients, water quality, temperature, and other environmental factors that affect bioconversion of samples collected from a subsurface carbonaceous formation (such as a specific bed within a basin) may be identified in the laboratory through experimentation. Environmental factors, such as water quality modifiers and nutrients, affecting growth of the first consortium in the underground bioreactor and product production therefrom, may be added to the process water before it is returned to the underground bioreactor. The process water may be modified by the addition of macro- and micronutrients to stimulate microbial growth, and the water quality, such as pH and hardness, may be adjusted through the addition of acids, bases and salts.

The environmental conditions of the aboveground bioreactor may be modified to optimise the methanogenic activities of the anaerobic microbes contained therein. For example, fluid returning from the subsurface which may have a relatively low pH, may be adjusted to raise the pH of the fluid, for example to at or above pH = 7, if required.

The fluid channels may be connected in subsurface arrays as illustrated in FIG. 6 by the diagrammatic representation of arrays: 600a, 600b and 600c. As shown in further detail in the diagrammatic representation 600a, the connected fluid channels may be comprised of an array of vertical well (1062a - 1062d) and branched (630a - 630c) horizontal wells (1092a - 1092c). In a representative sense, the triangular shape, or chevron, formed by the points 1062a, 1092a, and 1062b, may be considered a single tooth of a sawtooth pattern. Alternatively, the chevron formed by points 1092a, 1062b and 1092b, may be considered a single tooth of a sawtooth pattern. A chevron, or tooth, may therefore be formed from the intersection of a branched horizontal well with two separate vertical wells, or two branches of separate horizontal wells with a single vertical well.

The horizontal wells branch to intersect one or more horizontal wells. For example in FIG. 6, horizontal well 1092a, branching at point 630a, is shown to intersect with vertical wells 1062a and 1062b. Fluid, such as: fluid containing microbes for conversion of the subsurface; nutrients to support microbes in the subsurface; effluent from an above-ground bioreactor (such as illustrated in FIG. 1 (144)); or amendment solutions such as pH modifiers (e.g. acids, bases, buffer systems), may be added to the subsurface through an opening such as 1062a or 1092a. The fluid passes along the reactor system (620a - 620f) where it may be withdrawn from an opening (1062d) in the subsurface. The number of vertical and horizontal wells in an array, and the number of arrays, may be commensurate with the required levels of gas production and consistent with world-scale above ground anaerobic bioconversion technologies, such industrial and municipal UASB technology.

Sawtooth arrays, such as illustrated by diagrammatic representations 600a - 600c, may be injected with fluid, and have fluid withdrawn from them, in parallel or series, or in combinations of parallel and series. For example, fluid may be simultaneously injected into wells 1062a, 1062f and 1062g and simultaneously withdrawn from wells 1062d, 1062e and 1062h. Alternatively, fluid may be, for example, injected into 1062a, withdrawn from 1062d, reinjected into 1062e, withdrawn from 1062f, reinjected into 1062g, withdrawn from 1062h, and so on.

Referring now to FIG. 7, a diagrammatic representation (700) of an alternative array of horizontal and verticals wells is shown. As depicted in FIG. 7, vertical wells (1063a - 1063d) are intersected by multiple horizontal well laterals (e.g. laterals 740a - 740d) intersecting vertical wells 1063a and 1063b to create a diamond shaped or double sawtooth patterns. Fluid may be injected into vertical well 1063a where is pass down either lateral channel 740a or 740b, around branch points 710a and 710b, along respective laterals 740c and 740d to recombine at the intersection (1063b) of the lateral wells with a vertical well. The fluid then continues to branch and recombine as it passes along the fluid channels in the subsurface. The fluid may be removed from the wellhead of a horizontal or vertical well aperture, for example, vertical well 1063d. Microbes that are injected into the channels in the subsurface in order to generate a subsurface bioreactor may be encourage to move out into the formation by injecting fluids such as nutrients and vitamins, into locations that are peripheral to the fluid channels created by the array of horizontal and vertical wells, such as through injecting into peripheral vertical wells 750a - 750d.

Turning now to FIG. 8, a three dimensional diagrammatic representation of three borehole arrays 800a - 800c is illustrated. Referring in particular to the diagrammatic representation 800c, horizontal and vertical wells are shown intersecting (e.g. intersection points 1032a - 1032c) in the subsurface formation (1051). The arrows 820a - 820c indicate that fluid may be injected into or withdrawn from any of the vertical well heads 1094a - 1094c. Under some operating conditions, one or more well heads may be temporarily or permanently sealed. For example, horizontal wellheads 1094a and 1094b, and vertical wellhead 1064b may be sealed with only inlet (1064a) and outlet (1064c) remaining connected to the fluid circuit. Under a typical operating scenario: fluid containing microbes is injected into the formation at well head 1064a; the fluid travels down the vertical well (820a), along one of the horizontal well laterals (830a); around the lateral branch point (810a); back down the second lateral (830b) extending from the branch point (810a); along a further lateral (830c) connected (1032a) at a vertical well around a second branch point (810b), along a further lateral (830d) where it may be then withdrawn (820c) at a well head (1064c). The number of horizontal and vertical wells, forming an array or arrays, as represented diagrammatically in FIGs. 6 - 8, should in no way be considered limiting and commercial application of the technology would comprise considerable larger arrays of horizontal and vertical wells than those pictured.

In some instances, arrays may be comprised of 20 or more horizontal wells, and a commensurate number of vertical wells. In other instances arrays may be comprised of 100 or more horizontal wells with a commensurate number of vertical wells. In yet other instances, arrays may be comprised of 1000 or more horizontal wells and commensurate vertical wells. The number of horizontal and vertical wells employed in an array may depend, in part, on: the scale of above ground technology used to convert the oxygenated organic compounds withdrawn from the subsurface to a gas product comprised of methane, carbon dioxide and hydrogen; the volume of the carbonaceous resource to be converted; and/or on the gas production requirements.