STIMULATED OIL PRODUCTION USING REACTIVE FLUIDS

Technical field [0001] This invention relates to methods for stimulating oil production from well by

pumping reactive stimulation fluids from the well into the formation. The

methods are particularly relevant to cold heavy oil production.

Background art

[0002] Cold heavy oil production with sand (CHOPS) is one of the many methods

currently employed to extract viscous heavy oil from deposits in Canada.

Not all fields or local reservoirs are amenable to this technique, but due to

its low cost it is often the method of choice whenever it can be applied.

[0003] CHOPS is a well documented technique and is a standard method of

producing heavy oil in Northern Alberta and Saskatchewan. Further details

of this technique can be found in

http://www.energy.gov.ab.ca/docs/oilsands/pdfs/RPT_Chops_ chptr3.pdf .

It comprises the deliberate initiation of sand influx during the completion

procedure, maintenance of sand influx during the productive life of the

well, separation of the sand from the oil, and finally the disposal of the

sand. No sand exclusion devices (screens, liners, gravel packs, etc.) are

used in the wellbores, and no filters, cyclones or high pressure separators

are used at the surface. The sand is produced along with oil water and

gas, and separated from the oil by settling before being cleaned and sent

to a facility for upgrading to a synthetic crude.

[0004] One stimulation treatment routinely performed on oil and gas wells in low-

permeability reservoirs is hydraulic fracturing. Specially engineered fluids

are pumped at high pressure and rate into the reservoir interval to be

treated, causing a vertical fracture to open. The wings of the fracture

extend away from the wellbore in opposing directions according to the

natural stresses within the formation. Proppant, such as grains of sand of

a particular size, can be mixed with the treatment fluid keep the fracture

open when the treatment is complete. Hydraulic fracturing creates high-

conductivity communication with a large area of formation and bypasses

any damage that may exist in the near-wellbore area.

[0005] Fracture acidizing (sometimes called 'acid frac') is a variation on the

hydraulic fracturing well-stimulation operation in which acid, usually

hydrochloric [HCI], is injected into a carbonate formation at a pressure

above the formation-fracturing pressure. Flowing acid tends to etch the

fracture faces in a non-uniform pattern, forming conductive channels that

remain open without a propping agent after the fracture closes. The

length of the etched fracture limits the effectiveness of an acid-fracture

treatment. The fracture length depends on acid leakoff and acid spending.

If acid fluid-loss characteristics are poor, excessive leakoff will terminate

fracture extension. Similarly, if the acid spends too rapidly, the etched

portion of the fracture will be too short. The major problem in fracture

acidizing is the development of wormholes in the fracture face; these

wormholes increase the reactive surface area and cause excessive leakoff

and rapid spending of the acid. To some extent, this problem can be

overcome by using inert fluid-loss additives to bridge wormholes or by

using viscosified acids. Fracture acidizing is also called acid fracturing or

acid-fracture treatment.

[0006] Reactive chemical systems have been considered for stimulating the

diatomite formations in California (note that these are not produced by

CHOPS).

[0007] It is an object of the invention to provide a technique for improving oil

recovery that can be used in heavy oil formations without some or all of

the problems of the previous techniques.

Disclosure of the invention

[0008] This invention provides methods of stimulating production from a formation

surrounding a well, comprising:

- pumping a fluid from the well into the formation so as to create a

hydraulic fracture, wherein the fluid contains one or more reactive species

that destabilise cohesive forces in the formation matrix; and

- allowing the fluid to leak of into the formation and react with the formation

matrix so as to produce a destabilised zone in the formation around the

location of the fracture such that formation fluids and sand particles can be

produced from the formation through the zone and into the well.

[0009] The methods preferably comprise selecting the fluid so as to produce an

at least partially unconsolidated formation matrix in the destabilised zone.

[0010] It is particularly preferred to produce fluids from the formation so as to

cause worm-holing in the destabilised zone.

[0011] The fluid can contain additives in liquid form, solid or granular form. It is

also preferred that the fluid acts as a diluent for heavy oil and can also

modify formation fluid rheology.

[0012] One embodiment of the invention comprises alternately pumping the fluid

containing the reactive species and a diverting fluid into the formation.

Another comprises alternately pumping into the formation the fluid

containing the reactive species and a fluid that acts to provide local

consolidation in the matrix.

[0013] A method according to the invention can also comprise periodically

injecting CO2 into the formation and shutting in the well to allow the CO2 to

dissipate and dissolve followed by production from the formation.

[0014] Chemical, isotopic or radioactive tracers can be provided in the fluid.

[0015] Methods according to the invention have particular uses in wells producing

heavy oil from the formation.

Brief description of the drawings

[0016] Figures 1-3 show a top view of a borehole at various stages of a

procedure according to a first embodiment of the invention; and

Figures 4-6 show a top view of a borehole at various stages of a

procedure according to a second embodiment of the invention.

Mode(s) for carrying out the invention

[0017] It has been extensively documented that many heavy oil formations

produce more oil when sand is also produced. This observation has led to

the extensive deployment of the CHOPS method in the heavy oil fields of

Alberta and Saskatchewan. A goal of this invention is to stimulate a

CHOPS-like process by a process of pumping a reactive chemical system

in a similar fashion to an acid frac. The objective, however, is not to

create an etched-face fracture as in acid frac, but to create a lens of

"destabilized" rock extending a distance from the well bore. The reactive

chemicals can destabilize the rock matrix, facilitating production of sand

and oil in the leakoff zone surrounding the fractured region. High

permeability channels can develop in the lens as the oil is produced,

essentially opening more communication possibilities from the formation

up to the wellbore.

[0018] The invention is similar to acid fracturing in that a hydraulic fracture is

created using a reactive liquid (the process of acid fracturing is broadly

described above). However, in the case of this invention, an open, channel

or fracture after the treatment is completed is not intended. In fact it does

not particularly matter if the fracture is completely healed after the

treatment is completed and the formation closes. Rather, this invention

creates a lens of destabilized rock matrix surrounding the "ghost" of the

hydraulic fracture. This rock will have a higher propensity to failure and

worm holing than the native rock in the formation. Production such as

CHOPS can therefore be stimulated in this region.

There are a number of chemical systems that can be used as the reactive

liquid, depending on the rock type and other operational parameters.

Examples include:

• Strong bases such as sodium hydroxide (NaOH). These have

previously been used to dissolve silicates, and can be used in this

invention to destabilize the cementation between particles.

• Delayed systems such as magnesium oxide (MgO), solid NaOH

pellets, or alkaline glasses can be left in the fracture and allowed to

react after pumping has finished.

• Simple mineral acids can be used to destabilize rock when the

cementatious materials are of a carbonate nature and are prone to acid

dissolution.

• Hydrofluoric acid and mud acid can be used to destabilize sandstones,

clays and other silicate and aluminosilicate cementatious materials.

• Hydrofluoric acid precursors such as ammonium bifluoride can be

pumped with acid precursors such as esters, polylactic acid, sodium

bisulfate, etc.

• Various types of organic chelating agents (EDTA's, etc.).

• If the cementatious materials are clays, then some simple brines

(NaCI ₁ ) fresh water, or simple surfactants may destabilize the rock.

[0020] The fluid system are designed to have the correct rheology and leakoff

characteristics in order for it to be pumpable, and for it to place the

reactive materials sufficiently far from the wellbore. The basic techniques

for this are essentially the same as are used in other fracturing operations.

[0021] By adjusting the leakoff characteristics of the fluid, the total volume

pumped, and the chemical nature of the reactive additive (liquid, solid,

etc.) the width and length of the region affected can be controlled. Tuning

this method allows subsequent worm holing to be directed and optimized

for a given formation. For example, by including solid state reactive

materials the destabilized rock would tend to be located primarily in a

narrow region occupied by the ghost (re-healed) fracture. Figure 1 shows

an axial view of a borehole 10 which has been pressurized with a fluid 12a

to create a fracture 14 in the formation 16 . The fluid 12a contains solid

state or encapsulated reactive chemicals 18 (see above). After pumping

stops and the fluid 12a leaks off into the formation 16, the fracture.14

closes on the solid state materials 18 which react to destabilize the

formation 16 (see Figure 2). The localization of the reactive chemicals in

the fracture means that when production starts from the formation 16, rock

failure and worm-holing 20a are found close to the 'ghost' 22 of the

fracture and closely aligned with it (see Figure 3).

[0022] By including a liquid reactive chemical, the wider, leaked off region can be

made susceptible to failure as is illustrated in Figures 4-6. Initially, the

fluid is pumped to create a fracture in essentially the same manner as is

described above in relation to Figure 1 (see Figure 4). However, when

pumping stops, the reactive fluid 12b leaks off into the region of formation

24 surrounding the fracture 14 (see Figure 5). This in turn leads to a wider

region of rock failure and worm-holing 20b around the 'ghost' 22 of the

fracture 14 that is less constrained and aligned (see Figure 6).

[0023] Also, as a means of forcing localized failure in the affected region, the

fluid, whether aqueous, hydrocarbon, or solvent based, can be chosen to

interact differently with the fluids in the formation. For example, the carrier

fluid can be chosen to be a solvent of the heavy oil in the formation. By

diluting the heavy oil with a solvent, the fluid in the destabilized region, or

adjacent to the destabilized region, has a lower viscosity, and a higher

likelihood of been induced to flow more readily than the virgin oil. If an

aqueous fluid is chosen with a very high salt concentration (compared with

the water cut in the virgin formation), then there could be a localized high

fluid pressure due to osmotic forces.

[0024] Diversion and viscous fingering techniques can be used to further direct

the channels of rock failure.

[0025] One method of obtaining feedback from the process, and thereby

increasing control, is to include conservative tracers in the fluid, in the

solid additives or in both. That way, long term analysis of the produced

fluid can help determine the worm holing profile, or help identify how the

formation is failing.

[0026] Other changes within the scope of the invention will be apparent. For

example, periodic pressurization with CO2, shut in, followed by production

("huff 'n puff') can be used with the technique of the invention.