Related Patent Applications

This PCT international patent application claims the benefit of pending Australian provisional patent applications identified as Serial Nos. 2008901528 filed April 2, 2008 and 2008905274 filed October 10, 2008.

Technical Field of the Invention

The present invention relates in general to hydrocarbon drilling operations, and more particularly to methods and apparatus for analysing gas desorbed in the drilling mud during the drilling operation.

Background of the Invention

Mud logging has been used for a long time in petroleum drilling to determine the approximate location of gas bearing strata during the drilling process. In particular, mud logging involves the process of examining the drill cuttings extracted from the drilling mud to identify gas, hydrocarbon and other constituents that exist at the particular location of the drill bit. To that end, a gas detector is usually set up at the surface to sample the outflow of the drill mud from the borehole. This location is frequently above a shale shaker, but may at other locations. The sampling equipment detects gases released from the drilling mud along with air that is drawn in by the sampling equipment. The system provides a qualitative analysis of the gases being released from the borehole. If the mud logging system monitors the progress of drilling operation and the drilling rnud flow rate, it is possible to calculate the approximate location in the borehole where the gas was released. This process involves a calculation of up-hole velocity with time and its correlation with the output of the mud logger,

The conventional mud logging systems lack quantitative estimates of gas release volume because of the nature of the sampling process. This may not be of great significance in the case of conventional petroleum reservoirs because the volume of gas released may not be related to the volume of reservoir drilled. In the case of a reservoir such as coal, where the gas is contained within the coal material itself, the gas volume release can be expected to be directly ^' related to the volume of coal drilled and directly related to the gas content of the coal on a volume per volume basis.

The usual method for obtaining gas constituents from coal seams is to core drill into the coal seam and pull the core as quickly as possible to the surface. The core is then removed from the core barrel and placed within a canister where the desorption of the gas from the core sample is monitored. There is invariably gas lost during the transit period from the depth of the coal scam to surface. This lost gas must be calculated from backwards extrapolation of the initial desorption rate of the core once it is placed in the canister, to the time at which it is considered the coal commenced desorption. As the gas released from the core slows down, it is customary to open the canister and sample the core, and crush the sample to expedite the desorption process, The gas released from the crushed sample is measured and used in the analysis of the total gas content of the core sample. This measurement is usually specified as a gas volume per unit weight of coal.

The limitations of this technique involve the requirement to conduct a coring process to obtain a core sample, as well as the inaccuracies in the estimation of the initial gas lost to the atmosphere during the analysis procedure. It can be seen that a need exists for a process in which the analysis of the gas constituents can be obtained during a conventional drilling of the strata, where the drill mud with the cuttings therein is not exposed to the atmosphere, but is contained until the gas analysis is completed. Yet another need exists for a gas analysing system that is dynamic, meaning that the gas is continually accumulated and analysed.

Summary of the Invention

The principles and concepts of the invention are especially applicable to the measurement of quantifiable total gas release from any borehole, but with particular reference to the measurement of gas release from strata such as coals or shales which contain the gas therein through the process of sorption.

According to an important feature of the invention, the exploration for gasses in a subterranean: strata is facilitated by conducting a drilling operation that captures any gasses desorbed from the formation as well as from the cuttings generated by the drilling operation. The drill fluid, cuttings and desorbed gasses are coupled from the downhole location to the surface equipment that processes the gasses to determine desired parameters thereof. The coupling of the desired gasses from the downhole location to the surface processing equipment is a closed system that prevents the desorbed gasses from being diluted or otherwise contaminated by air and other environmental gasses. The desired parameters resulting from the processed desorbed gasses are thus more accurate and provide a better assessment of the gaseous nature of the strata.

According to another feature of the invention, the drilling process is continuous, except for interruptions during additions to the drill string, whereby the analysis and processing of the desorbed gasses with the surface equipment is ongoing and thus provides a dynamic record of the gas content of the strata being drilled. The length of the borehole, the rate of movement of the drill liquid upwardly in the annulus and other factors are used to determine the depth of the strata from which the analysed gasses were released.

In according with one embodiment of the invention, a seal is installed at the top of the wellbore .casing to seal the drill string thereto. A port is situated below the seal so that the drilling fluid or mud (with the cuttings therein) returned from the bottom of the borehole is forced out through the port. Normally, the borehole being drilled would be drilled by open hole techniques rather than by coring. The seal would normally be of a rotary type to permit drilling by rotation of the drill string. The drill fluids carried out of the port contain both drilling fluid, cuttings and gas release ^' d from the formation and from desorption of the cuttings. If the rriud pressure in the borehole exceeds the formation pressure then no formation fluids will enter the borehole. As a consequence, the only gas that would be liberated would be from the strata being drilled, and would be either from the direct release of gas contained within pore space, or from gas absorbed into the strata and released by desorption.

According to one embodiment of the invention, the fluids passing from the port below the rotary seal are directed into a separator that separates the gas from the liquids and solids. By holding the liquids and cuttings within the separator for an adequate period of time, most of the gases that can be desorbed will be released. This process is particularly enhanced by the fact that the cuttings are ground finely by the drilling process. The detention time of the cuttings within the separator is a function of the volume of the separator and the inflow rate of the drilling fluid into the separator.

A rudimentary form of the separator includes a vessel or containment floating in a. drilling mud pit. The vessel is open on the bottom side and is partially submerged in the mud pit. The gas in the drilling fluid and that desorbed from the cuttings rise to surface of the mud pit and would normally escape. However, according to one technique, the gasses emitted from the cuttings are collected by the vessel. The volume of the gas released is measured by a flow meter. Typically the vessel would have the form of an upturned water tank which floats at a constant level above the surface of the mud pit. This level is maintained by floats. The vessel has an outlet port for discharge of the gas released from the well or cuttings, so that the discharged gases are passed through a flow meter. The flow meter monitors the gas produced with respect to time. At the outlet to the flow meter there may be a gas analysis system to sample the gas and provide the, constituent components of the gas. The gas thus released may be measured with respect to volume and type. The volumetric measurement can be accomplished by a variety of gas flow meters. These can include pressure differential and positive displacement measuring devices. According to a feature of the invention, the gas type can be sampled without dilution with air and analysed by a variety of devices such as infra red sensors or gas chromatographs. The information from these flow and gas type sensors is gathered by a data acquisition system.

According to another embodiment of the invention, a gas impermeable blanket or cover is placed over a mud pit containing the drilling fluid, cuttings and desorbed gasses therein. The desorbed gasses are collected under the cover and coupled to the gas processing equipment for analysing the same to determine whether the subterranean strata has sufficient gas to pursue production of the same. The drill fluid, cuttings and the desorbed gasses are conveyed from the downhole location to the gas analysing equipment in a closed system to prevent air and other contaminants from contaminating the desorbed gasses.

According to yet another embodiment of the invention, the drilling fluid, cuttings and the desorbed gasses are coupled from the dowπhoJe location to processing equipment which separates the drill liquid and cuttings from the desorbed gases. A separator is adapted for separating the gasses form the liquids and solids, whereby the liquids and solids are released from the separator and the gasses are coupled to gas processing equipment for analysing the same. The separator is adapted for separating the liquids and solids from the gasses while maintaining the system closed so that air does not dilute or contaminate the desorbed gasses. A second downline separator can be employed to further separate the desorbed gasses from any residual liquid droplets.

Jn order to precisely correlate the gas sample being analysed to a drilling depth, it is necessary to monitor the drilling process so that the depth and progress of drilling is monitored, as well as the drilling mud inflow or outflow rate. The location of the mud sample containing cuttings and gas bubbles can therefore be quite accurately determined. Such information is gathered by the data acquisition system.

The process of determining gas content of the formation being drilled from the apparatus is one where the gas volume released is measured and related to the position in the borehole from whence it is being cut by analysis of drilling records. These involve knowing the position and penetration rate of the drill bit during drilling and of a record of mud flow bringing chips to surface. This information is used to derive a model of chips being cut and rising to surface in the pumped fluid stream in the annulus. When pumping is not occurring consideration is given to chips settling in the annulus and the presence of rising bubbles in the drilling fluid. While this model can be kept simple or become complex the basic information derived from it is to relate gas release to a specific strata group being drilled. The process can be simplified by drilling a segment of say one drill pipe length and flushing all chips to surface and analysing the same before stopping pumping. This assures that all the information from the zone drilled is obtained before drilling recommences. The volume of gas released can be related to the volume of strata being drilled through a knowledge of the drill bit size and cutting diameter. This chip volume information should be refined where possible by obtaining a geophysical calliper log of the hole after it is drilled. Thus the basic information on the gas content of the strata is obtained as information on gas content per unit volume drilled. A geophysical log of the hole which includes a. density log may be used to convert this information to the more customary unit of gas content per unit weight of the strata from whence it was released.

Brief Description of the Drawing

Further features and advantages will become apparent from the following and more particular description of the preferred and other embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters generally refer to the same parts, elements or components throughout the views, and to which:

Fig. 1 illustrates one embodiment of a well drilling system for analysing gas released from a borehole; Fig. 2 illustrates another embodiment of a gas analysing system; and

Fig. 3 illustrates yet another embodiment of a gas analysing system adapted for use in a drilling operation.

Detailed Description of the Invention Fig. 1 illustrates a downhole drilling operation of the type well adapted for analysing gasses obtained from a substrata coal seam. Over long periods of time, the coal seam absorbs or generates gasses which are contained in the pores of the coal material. It is to be understood that the principles and concepts of the invention can be employed in many other drilling situations and applications, including oil and gas shales, and other geological formations. The gas recovery and processing system shown in Fig. 1 illustrates a wellhead providing a closed system for recovering the drill liquid, cuttings and any desorbed gas from the downhole formation. The drill liquid, cuttings and desorbed gas are coupled from the wellhead in a closed separator system in which the gas is separated from the drill fluid and cuttings. The desorbed gas is then coupled to the gas processing equipment to deteππine predefined parameters, such as the extent of gas and /or the gas constituents in the formation being drilled.

According to the embodiment of the invention illustrated in Fig. 1, a borehole (1) is drilled ■ by drill bit (2) that is attached to the end of a drill string (3). A conventional drill mud is forced under pressure by a mud pump down the drill string (3). The drill bit (2) is rotated by either the drill string (3) or by a mud powered downhole motor (not shown). If a downhole motor is not used it is normal practise to place a pressure relief/non return valve(not shown) behind the bit (2) to prevent fluid flow in the drill pipe (3) unless pumping takes place. The mud carries cuttings from the formation to the surface via the wellbore aiinulus (5). The borehole (1) is shown being drilled through a coal seam (4). The drilling operation produces coal cuttings that contain gas absorbed therein. The cuttings suspended in the drill mud rise up the annulus (5) between the drill string (3) and the borehole (1) and into the casing (6). Attached to the casing (6) is a diverter (7) with ports (8) and (9). Above the diverter (7) is an optional blowout preventer (10). A seal (I I) is located above the blowout preventer (10). The seal (11) would normally be a rotary device through which the drill string (3) passes. The seal (11) prevents the escape of gasses desorbed from the drill mud during the upward travel from the bottom of the borehole (1) to the surface equipment. As used herein, the terms drill mud and drill fluid are interchangeable. The port (8) on the diverter (7) is shown connected to a valve (12) and pipe (13) which would normally be a kill line for well control. The other port (9) of the diverter (7) is shown connected to a valve (14) which is connected via a conduit (15) to a choke (16), shown as an annular adjustable pressure relief valve. A conduit (17) is connected to the outlet of the choke (16) and discharges the drilling fluid at outlet (18) and into a mud pit (19). The surface of the mud in the mud pit (19) is shown as numeral (20).

Situated on the mud pit (19), and above the outlet (IS) of the conduit (17), is collector vessel (21) for collecting gasses desorbed from the cuttings of the coal seam (4). Importantly, the gasses collected are those that were desorbed during upward travel of the drilling fluid in the borehole annulus (5), as well as during the residence time in the mud pit (19). In other words, no gasses are lost, and the gasses collected are not diluted by atmospheric air. The vessel (21) is supported by floats (22) and (23) on the surface (20) of the mud pit (19). The annular skirt of the vessel (21) is submerged under the surface (20) of the mud pit (19) so that the gasses accumulated within the vessel (21) are contained. The vessel (21) is tethered loosely by ropes (33) and (34) to respective anchor points so that the bottom opening of the vessel (21) is generally over the drill mud outlet (18). The mud pit (19) and the vessel (21) thus form a separator for separating the gas from the drill liquid and cuttings.

An outlet (24) of the collector vessel (21) discharges gasses via conduit (25) into a gas flow meter (26). The gas flow meter (26) is connected to a data acquisition device (27) which records cumulative gas release volume or gas release rate versus time, and monitors the temperature of the gas as well. The flow meter (26) exhausts the gasses via conduit (28). The top end of the conduit (28) is protected from rain by a cover (29), The gas exhausted by the conduit (28) is shown being sampled by a sample conduit (30) which is located directly within the gas stream being exhausted by the conduit (28). A gas analyser (31) analyses the gas sampled by the sample conduit (30). The gas analyser (31) is connected to the data acquisition device (27). Another data acquisition system (32) is illustrated for gathering information concerning the drilling process, including the monitoring of the drill bit position and drilling mud inflow and outflow rates. In its simplest form, the data acquisition system (32) is used to count the number of drill pipes between the top and bottom of the drill string (3). Data acquisition devices (27) and (32) record data in real time so that their outputs can be synchronised. In other words, by correlating the data output from the data acquisition devices (27) and (32), the sample of gas being analysed can be correlated to a particular downhole location. The concentration and type of gas at the various depths of the coal seam (4) can thus be accurately determined and recorded.

In operation of the embodiment illustrated in Fig. 1, the drill mud is pumped under pressure down the drill string (3) to the bit (2) which drills the borehole (1 ) downwardly. In forming the borehole (1), the bit (2) cuts through the strata and as a result form chips or cuttings that are carried with the drill mud up the annulus (5) between lhe drill siring (3) and lhe borehole (1). The cuttings are small parts of the formation being drilled. Thus, as the borehole (1) is drilled through the coal seam (4), with gasses absorbed therein over time, the cuttings will also include gasses-absorbed therein. As the cuttings are carried upwardly with the drilling mud in the annulus (5), the gasses are also desorbed from the cuttings into the drill mud. The drilling mud with the cuttings are carried in a closed system to the mud pit (19). In this manner, all of the gasses that are desorbed from the cuttings into the drilling mud, as well as the gasses yet in the cuttings, are all coupled to the outlet (18) and deposited into the mud pit (19). Moreover, the gasses carried in the closed system are not diluted or contaminated with air or other environmental gasses. An advantage of the gas analysis system of the invention is that continuous and real time evaluations can be made of the gas content of the strata being drilled during a drilling operation, although there is a delay between the time the cuttings are generated at the downhole drilling site, and when the cutting chips are deposited in the mud pit (19). Those skilled in the art will find it elementary to determine the time in transit of the cuttings so that the gas content at each location in the coal seam (4) can be determined.

Another embodiment of the invention is illustrated in Fig.2. Here, the wellhead is of a different configuration, as compared to that shown in Fig. 1, but nevertheless provides a closed system for recovering the drill fluid, cuttings and any desorbed gas from the downhole location to the surface. From the wellhead, the drill fluid, cuttings and desorbed gas are coupled to a separator system in which the drill fluid and cuttings are separated from the desorbed gas. The desorbed gas is then coupled to apparatus that processes the same to determine specific parameters thereof, such as volume and/or constituent components. The details of the wellhead and the gas separator system are described below, it being understood that a drill string can employed as shown in Fig. 1. The top of the borehole easing (6) includes a flange (41). Attached to the flange (41) of the casing (6) is a valve (42) to enable the borehole (1) to be closed. The top of the valve (42) is connected to a flange (43) which is connected to the base (44) of a blow out preventer (46). The body of the blow out preventer (46) is attached to the base (44). The body of the blow out preventer (46) includes an annular seal member (45) that closes around the drill rod (50) when actuated by fluid via a conduit (47). At the top of the blow out preventer (46) is a rotary seal (48) with a bearing set (49) to enable the drill stem (50) to rotate freely. This rotary seal assembly (48) and (49) is adapted to enable the drill stem (50) to be rotated, advanced and withdrawn from the borehole (1) without leakage under conditions of low pressure in the borehole annulus (5). There are pipe connections below the rotary seal (48) and (49), and in this case, below the blow out preventer annular seal element

(45) to a choke (56). These pipe connections are shown as a pipe nipple (51), a valve (52), a pipe nipple (53), a pipe bend (54), and a pipe nipple (55) connected to the choke (56). The choke (56) is a fluid actuated unit, although it can be activated by other means. A hydraulic hose (58) supplies hydraulic fluid to actuate the choke (56). The outlet (57) of the choke (56) is the discharge from the borehole (1) under controlled flow conditions when the blow out preventer seal member (45) is closed around the drill stem (50). The base (44) of the blow out preventer

(46) has an alternate flow path through a pipe nipple (59) to a pipe tee (60). Fluid can be pumped into the borehole annulus (5) below the blow out preventer seal (45) via a conduit (63), a valve

(62), a pipe nipple (61), to the tee (60) for well control purposes.

Under the normal operating conditions of the system, the valves (52) and (62) are closed and valve (65) is open. This allows the drill fluid from the annulus (5) of the borehole (1) to pass through conduits (66), (61) and (68) to a port (69) on a primary gas separator. The gas separator is constructed with a body (70), a top gas outlet (74), a bottom outlet (73) for draining liquid and cuttings from, the separator. The drill liquid and cuttings are input to the separator (70) via the inlet port (69). The inlet port (69) is attached tangentially to the body (70) of the separator so as to induce a swirl to the incoming flow and to enhance separation, of liquids and drill cuttings from the gas. The separator contains a float valve system adapted to maintain a substantially constant liquid level within the separator (70). The float system includes a main float (73) connected to a movable valve member and weight (71) by a connecting rod (72). The bottom of the valve member (71) is adapted to seat with the annular opening in the separator outlet (73). Also shown is a bottom projection from the valve member (71) in the form of a spike (98). This spike (98) assists in dislodging blockages caused by cuttings in the bottom outlet (99) of the separator (70).

In operation of the separator system, should a blockage occur within the separator outlet (73) when the float (73) is in a down position, the rising liquid level in the separator (70) will cause the float (73) to also rise. The upward movement of the float (73) will lift the valve member (71) and the spike (98), whereby the upward movement of the spike (98) will free the blockage in the outlet (99). Similarly, if a blockage occurs in the outlet (99) when the float (73) is in a raised position and the valve member (71) is open, the separator may be manually or automatically drained by a valve (not shown) and the float assembly (73), (72), (71) and the spike (98) will drop, thereby dislodging cuttings lodged in the separator outlet (73). The float valve assembly in the separator body (70) is adapted to rηaintain the volume of the drill liquid at a constant level within the separator (70). The separator (70) is adapted to provide adequate volume to maintain a residence time of several minutes so that gas can substantially desorb and be separated from cuttings. It is believed that within about two to ten minutes of residence time, up to about 80%-90% of the gases in the drill liquid and cuttings will be released to the top of the separator. Of. course, the residence time depends on many factors. It is noted that most of the gas is desorbed from the cuttings during the transit from the downhole location to the separator system. It is also possible to periodically sample the cuttings being released from the separator so as to determine any residual gas they may hold.

The outlet (74) from the top of the separator (70) is coupled to a second separator of the hydrocylcone type. The second or downline separator includes an inlet (75), a body (76), a float (77), a lower liquid outlet (78), and an upper gas outlet (79). The downline separator (76) is adapted to strip any remaining water droplets from the gas stream exhaust from the upstream separator (70).

The outlet (79) of the second separator (76) couples gas to a flow metering system. The flow metering system incudes a high flow gas flow meter and a low flovv gas flow meter and a data acquisition system. The high flow gas flow meter includes a flow restriction (83) in the flow path. The pressure of the gas is monitored by a pressure transducer (81), and the differential pressure across the flow restriction is monitored by a differential pressure transducer (82). Data from these transducers (8.1) and (82) is gathered via electronic cable (95) which delivers the information to the data acquisition unit (97). The gas flow continues to a low flow gas flow meter (86) which is protected from high gas flow rates by a low pressure relief valve (87). The low pressure relief valve (87) causes high pressure gas flows to bypass the low flow gas flow meter (86). The low flow gas meter (86) is thus protected from high flow rates because the pressure relief valve (87) which opens under high gas flow rates. The low flow meter (86) can therefore be a common domestic gas flow meter. Electronic data signals from the low flow gas meter (86) are transmitted to the data acquisition system (97) via cable (96). Also connected to the data acquisition system (97) is information from the drill rig concerning pump flow rate, drill bit position, etc.

The gas flow passes out of both flow meters, via non-return valve (88) to an outlet (93). However, the gas stream is sampled at a tee (90) and conveyed to a gas analyser (91) to be analysed as to the content and constituents of the gas, and then exhaust at an outlet (92). The gas analyser (94) is connected to the data acquisition system (97) by a cable (94).

In operation of the gas analysing system of Fig. 2, it can be seen that again the gas desorbed from the cuttings of the downhole boring location are carried to the analyser in a closed circuit and are not exposed to external gasses such as atmospheric air or other gasses. In normal drilling operations when the blow out preventer (46) is not operated, and the seal (45) provides a seal around the drill stem (50), the drilling fluid with the cuttings are carried up the annulus (5) of the borehole (1), up the casing (6) and into the open valve (42). From the valve (42) the drilling fluid and cuttings are carried under some pressure out of the blow out preventer port 59 through the tee 60 and valve (65) to the first separator (70). The first separator (70) separates the gas from the liquids and solids. The solids, including the drilling liquid and cuttings are discharged from the bottom outlet (99). A float system controls the volume of the drilling mud in the separator (70), as well as maintains the bottom outlet (99) clear of accumulated cuttings. The gas and any residual water droplets are carried out of the top of the first separator (70) and to the second separator (76) where the remaining water or other liquid is removed, From the second separator, the gas is processed to determine the flow rate thereof, as well as the constituents of the gas. The data collected concerning the makeup of the gas is combined with the data concerning where in the borehole (1 ) the drill mud being processed was combined with the formation cuttings, to thereby arrive at a determination as to where in the formation the gas sample originated. reason for this is that any liquids in the formation are maintained in the formation and do not run into the borehole to be combined with the drilling mud. This could alter the composition of the drilling mud to the extent that an accurate analysis of the gas would be hampered. This is accomplished by either maintaining the density of the drill mud or by adjusting the pressure of the drill mud forced downhole by the mud pump so that the pressure in the borehole is always greater than the formation pressure. Sensors attached to the well head can monitor the various pressures to adjust the pressure by which the mud pump operates or adjust a choke to maintain well bore pressure. It should also be appreciated that when the cuttings are smaller in size, the gas desorbed therefrom is expedited. This reduces the residence time in which the gasses are desorbed from the cuttings, thus allowing the same to be analysed more quickly. Those skilled in the art will understand how to conduct the drilling operation to obtain smaller cuttings, such as slowing down the rotary motion of the drill bit, using drill bits with teeth that make smaller chips, and other techniques.

While the analysis of the gas desorbed by the cuttings is considered continuous, it is noted that certain discontinuities may exist when a drill stem is added to the drill string. In order to minimize any change in the drill mud caused by atmosphere air, or otherwise, it is preferred that a , pressure relief valve, similar to a check valve, be installed at. the bottom of the drill stem, above the drill bit. With such a valve, when the mud pump is interrupted to install another drill stem to the drill string, the reduced pressure within the drill string will allow the valve to close and maintain the downhole parameters al Lhe status quo. In addition, the drill mud at the bottom of the borehole will not tend to rise in the drill string. When the drill stem has been added Io the string and the mud pump commences operation, the pressure of the drilling mud within the drill string will open the valve so that normal drilling can be continued.

The foregoing describes the various embodiments in connection with the drilling in a coal seam. However, this is not a limitation of the invention as the principles and concepts of the invention can be employed with equal effectiveness with other types of formations, such as oil shale formations, gas shale formations and other formations where the presence of gas is suspected. In addition, while the different configurations of wellheads are disclosed in the various embodiments, it should be understood that many other different configurations can be employed with equal effectiveness, as long as the wellhead systems provide a closed system to prevent air from contaminating the desorbed gasses from the borehole formation. The various embodiments describe the use of a mud pit, however, a tank or other container-type reservoir can be used with equal effectiveness.

While the preferred and other embodiments of the invention have been disclosed with reference to specific drilling apparatus, separators and gas processing equipment, it is to be understood that many changes in detail may be made as a matter of engineering choices without departing from the spirit and scope of the invention, as defined by the appended claims.