US20080135236A1

US20080135236A1 - Method and Apparatus for Characterizing Gas Production

Info

Publication number: US20080135236A1
Application number: US11/697,786
Authority: US
Inventors: Martin Schoell
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-04-10
Filing date: 2007-04-09
Publication date: 2008-06-12

Abstract

A method of characterizing the productivity of open hole gas well involves characterizing the chemical and/or isotopic composition of the gas contributing strata from a series of discrete samples by degassing the mud during drilling This data is then used to de-convolute the combined or mixed gas during well production after drilling is complete to determine the relative contributions of the now characterized strata.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to the U.S. Provisional Patent Application for a “Method and Apparatus for Characterizing Gas Production”, filed on May 16, 2006, and having application Ser. No. 60/747,327, which is incorporated herein by reference. The present application also claims priority to the U.S. Provisional Patent Application for a “Method of Characterizing Open Hole Gas Production”, filed on Apr. 10, 2006, and having application Ser. No. 60/791,169, which is also incorporated herein by reference.

BACKGROUND OF INVENTION

The present invention relates to improvements in the characterization and production of natural gas deposits, and in particular to deposits accessed by open hole drilling methods.
The exploration for and production of fossil fuel deposits of oil and natural gas is a capital intensive industry. Production of oil and gas is accomplished by drilling into the ground to perforate discrete geological formations or zone for which logging analyses, that is the chemical and/or physical analysis of either the product or the earth removed, indicates the presence and nature of the hydrocarbon reservoirs. Success depends on making skilled estimates of the potential productivity of known deposits in completed wells, as well as projections on the optimum locations to drill additional wells to efficiently tap such deposits. Other techniques of fossil fuel production chemically or physically modify the geological formation to enhance the extraction potential from existing wells.
Natural gas is extracted from the subsurface rock formations via the lining or sides of the bore hole. This is accomplished with two principal types of bore hole completions or finishes. One type is characterized as cased hole completion in which steel casing is lowered into the borehole and the steel casing is cemented to the rock formations by filling the space between the steel pipe and the rock formations with cement. Generally as natural gas wells produce mostly from discrete strata in the well bore, the well casing need only be open in discrete locations in these strata and production is from discrete perforations
In contrast, the other principle bore type is open hole completion. However, its practice is limited currently to special geologic conditions and formations. These are typically the so-called tight gas sands and coal formations where the gas-containing zones are more difficult to identify and fracking of large intervals is necessary to stimulate the gas flow from the formation. The “open hole production” well bores are also cased but all perforations are opened and contribute to the gas stream that is commingled in the well bore. During gas production from this type of well, it is not known how much gas is flowing from each specific perforations and frack compartments. It should be appreciated that production allocation in open hole production with flow from multiple gas-bearing strata is very difficult as the individual gas sands are not well known, the zones of gas reservoirs are often not discrete, and frack zones typically extend over large sections of the borehole.
The present practice in open hole production is to periodically generate a so-called production log by lowering a flow meter(s) to record the total mass flow of gas at different depths and infer the productivity of the specific strata in the geological deposit being exploited. This practice is very expensive as it requires the physical lowering of an instrument, which in turn requires an interruption of the production.
The use of gas isotopes for production allocation have been described in cased hole production by Schoell, M., P. D. Jenden, M. A. Beeunas, and D. D. Coleman, 1993, Isotope Analysis of Gases in Gas Field and Gas Storage Operations: SPE Paper No. 26171, p. 337-344, which is incorporated herein by reference. Briefly, stable isotopes vary in natural gas reservoirs from reservoir to reservoir. Therefore, each of the reservoirs has characteristic “isotopic fingerprints”. Isotope analyses are concentration measurements and therefore follow a mass balance. This is the fundamental principle of the use of isotopes in gases for a quantification of the contributions of different gases in mixtures. In cased hole production, contributions from each reservoir can be determined in a commingled gas stream from two discrete zones if the composition of each zone is known.
However, production allocation from the aforementioned production log in open hole production is not only expensive, but does not produce the accurate information that is generally available from cased hole production wells. The problem is especially complex because the zones of gas reservoirs in tight gas production are often not discrete. Thus, any measurements made after drilling of the well will involve mingled gas streams from different strata.
It is therefore a first object of the present invention to provide an improved and inexpensive method of production logging open hole natural gas wells.
It is a further objective to accomplish such a method without stopping or reducing the wells productive capacity.

SUMMARY OF INVENTION

In the present invention, the above objects are achieved by extracting gas samples from the drilling mud at the surface of a well. These mud gas samples are characteristic of the material being removed by the drill bit as a function of depth. Each mudgas sample is analyzed for the chemical and/or isotopic concentration of hydrocarbon gas constituents that are associated with a particular geological strata. Thereafter the relative contributions from the particular strata are calculated using chemical and/or isotopic analysis obtained during the drilling process. The objective of the isotope analysis of mud-gases during drilling is to provide reference data for isotope analysis of commingled production. Numerical analysis of chemical and/or isotope composition and other logging data permits the determination of the major contributing strata.
In the present invention, the above objects are also achieved collecting gas samples after completion of the well during production logging using a specially equipped down-hole logging device for gas sample collection. These gas samples are characteristic of the gas that is flowing through the downhole logging device as a function of depth. Each gas sample is retrieved and is analyzed for the chemical and/or isotopic concentration of hydrocarbon gas constituents that are associated with a particular geological strata. Thereafter the relative contributions from the particular strata are calculated using chemical and/or isotopic analysis obtained during the drilling process. The objective of the isotope analysis of gases during production logging is to provide reference data for isotope analysis of commingled production. Numerical analysis of chemical and/or isotope composition and other logging data permits the determination of the major contributing strata.
In the present invention, the above objects are also achieved by collecting gas samples after completion of the well during fracking of the rock formations by collecting gas samples during the backflow of gas after the fracking procedure. Each gas sample is retrieved and is analyzed for the chemical and preferably the isotopic concentration of hydrocarbon gas constituents that are associated with a particular geological strata. Thereafter the relative contributions from the particular strata are calculated using chemical and/or isotopic analysis obtained during the fracking process. The objective of the isotope analysis of gases from the backflow of fracking procedures is to provide reference data for isotope analysis of commingled production. Numerical analysis of chemical and/or isotope composition and other logging data permits the determination of the major contributing strata.
Further aspects of the invention thus involve monitoring through time the performance of the contributing strata. Additional aspects of the invention involve monitoring a plurality of a wells to provide a spatial representation of reservoir changes over time. Accordingly, chemical and gas isotope analyses described in the detailed description that follows can replace current and expensive production allocation practices that are not accurate and do not fully map changes in a reservoir. Further, visualization of lateral variations can help in the planning of infield drilling patterns
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic illustration of mud-gas collection for chemical and isotopic analysis during the drilling of test and/or production wells.

FIG. 2 is schematic illustration of a production logging device that is equipped for collection of gas samples from discrete depth of the bore hole for chemical and isotopic analysis

FIG. 3 is a cross sectional elevation of a first embodiment of the production logging device shown in FIG. 2

FIG. 4 is a cross sectional elevation of another embodiment of the production logging device shown in FIG. 2.

FIG. 5A-D are schematic illustrations of collection of gas samples of the backflow after the fracking of the open hole at various depth. There are multiple fracks for each hole and the gas of each backflow needs to be collected for compositional and isotope analysis.

FIG. 6A is a schematic graph of the hypothetical variation of isotopic composition of methane (C1), ethane (C2) and propane (C3) as a function of drilling depth as determined from mud samples.

FIG. 6B is a schematic double carbon isotope plot of methane and ethane illustrating the expected hypothetical results from mixed gas production from strata A and B.

FIG. 6C is schematic diagram illustrating the hypothetical time variation of the relative contributions related to changes in the plot in FIG. 2B

FIG. 7 is a hypothetical illustration of the spatial distribution of methane carbon 13 isotope values across a reservoir in a gas field.

FIGS. 8A and 8B schematically depict the change in spatial distribution of the ¹³C isotope signatures in the produced gas from an initial state (FIG. 8A) with that at a latter time (FIG. 8B).

DETAILED DESCRIPTION

In this disclosure gas composition means the chemical and isotopic identity and concentration, both absolute and relative, of hydrocarbons in the well bore or other representative sampling of the strata or bore hole depth profile. Gas composition analysis also includes means that identify and quantifying non-hydrocarbon gas that may be characteristic of a strata such as hydrogen sulfide and the like.
Owing to the deficiencies of the prior art, it is now appreciated that production allocation and production monitoring in open hole gas production requires a combination of techniques for the quantification of production and planning of the most efficient, productive and economical means of gas exploitation.
The previously mentioned obstacle in the art can be overcome in part by first analyzing the mud gas composition as the well is drilled. Mud gas isotope analyses is generally described in Muehlenbachs, K., Szatkowski, B. J. and Miller, R. R. (2000) Carbon Isotopic Ratios in Natural Gas: A Detailed Depth Profile in the Grand Prairie Region of Alberta. Extended Abstract. GAC-MAC Annual meeting, GeoCanada 2000, Calgary, Alberta, May 31, 2000 and Ellis, L., Brown, A., Schoell, M. and Uchityl, S. (2003); Mud-gas Isotope Analyses (MGIL) assists in oil and gas drilling operations. Oil and Gas Journal, May 26, 2003. PennWell, both of which are incorporated herein by references.
FIG. 1 schematically illustrates the application of this technique during exploration and initial drilling of a gas well. The drill bit of the well penetrates different gas containing formations A, B and C. The drilling mud carries the cuttings with the gases to the surface where the gases can be collected from the drilling mud with a collection device. The gas compositions from the different strata A, B and C are recorded in the mud-log as so called gas shows, which are gas concentrations that are higher than the normal “background”. With this technique, by properly extracting the mud samples, it is possible to determine the gas composition of virtually every zone in the bore hole.
FIG. 2 is schematic illustration of another aspect of the invention showing a an apparatus 100 for collecting of gas samples from discrete depth of the bore hole for chemical and isotopic analysis. Such collection can be used an alternative to mud sampling in characterizing the well after drilling or after drilling an subsequent frack operations, as well as when the well is in production. The apparatus 100 in FIG. 2 comprises at least one of, but preferably both, a gas sampling device 110 and flow meter 120 that is isolated from upper and lower portions of the well bore by upper and lower packing materials 101 and 102. Device 100 is held by device support 106 so that it can be lowered and raised from the well bore.
Thus, sampling of gas occurs in the discrete zone 105 between the gas flow isolating upper 101 and lower packing material 102, to sample primarily the perforations in the well casing between the upper and lower packing. The device 100 has open side walls 100 a. The upper and lower packing block gas flow into the region from other portion of the well such that the gas capture or sampling device 110 and flow meter 120 provide a separate measure from isolated perforations. It should be appreciated that the flow meter 120 and sampling device 110 need not be physically isolated between the packing, as a tube or conduit 115 can be used to draw flowing gas that is between the packing to a region outside the packing for such measurement, including even at the well surface. Such a tube or conduit 115 is desired such that the flow of gas is not limited by the discrete volume between the upper and lower packing.
The upper and lower packer define there between an isolated chamber for receiving gas either directly from the adjacent formation or holes in the well casing 50. In the device shown in FIGS. 2 and 3, both the gas flow meter 120 and the collection device 110 are disposed within the chamber between the upper and lower packers.
The gas collection devices 100 shown in FIGS. 3 and 4 comprises a series of preferably removable gas sample tubes 310 arranged in series between an upper 320 and lower manifold 330. Flow meter 120 has an inlet 120 a for receiving gas. The lower manifold 330 is in fluid communication with the output of the gas flow meter 120 via connection or pipe 125 and is connected to each of the gas sample tubes via a valve 331. The upper manifold is connected to the opposite side of each of the gas sample tubes via another series of valves 321 and is fluid communication at the other end with a conduit 315 through the upper packing into the well bore. Gas is sampled in each tube by opening each tubes upper and lower valve then closing both of these valves after a sufficient quantity of gas has flushed way any residual gas in the sample tube so that a representative sample is captured. Further, there is an outlet 315 on the upper manifold.
The output of the flow meter 120, which is communicated to operators that control the valves for each sample tube allows the determination that a sufficient quantity of gas has passed through the sample tube. Such information can be sent by telemetry means known in the art of well logging. Further, as conventional telemetry, power and controls for operating the sampling device 100 are well known to one of ordinary skill in the art, they are omitted from FIG. 2-4.
Alternatively, the opening and closing of the valve surrounding each gas tube can be under the operation of an automatic controller, such as a microprocessor that closes the valves in response to the measurement of either a specific amount of gas or after a predetermined flow rate is maintained for a predetermined time.
In contrast, in the device shown in FIG. 4 both the gas flow meter and the collection device are disposed above the upper packer with a gas conduit 410 penetrating the upper packer to collect the gas entering the cavity between the upper and lower packer. In this embodiment gas initially flows into the inlet 405 from central chamber 105.
While it is preferable to run the gas flow meter and the sampling device in parallel it is also optional that the gas flow meter is placed down stream from the sampling device with respect to the flow of gas out of the central chamber 105.
The packer can be inflatable as described in U.S. Pat. No. 6,578,638 (to Guillory, et al. and issued Jun. 17, 2003), which is incorporated herein by reference. Alternatively, the wellbore isolation apparatus described in U.S. Pat. No. 7,086,481 (to Hosie, et al. issued Aug. 8, 2006) provides another alternative configuration for the packers, which is incorporated herein by reference. In addition, another alternative formation fluid sampling and hydraulic testing tool and packer assembly is disclosed in U.S. Pat. No. 7,066,281 (to Grotendorst issued Jun. 27, 2006), which is incorporated herein by reference. Grotendorst discloses a drilling apparatus that includes a formation fluid sampling and hydraulic testing tool for a drilling apparatus that includes a drilling string comprised of a drilling pipe and a drilling bit is provided. The tool is mounted on a string that comprises from top to bottom a pressure reduction valve, an expandable and collapsible packer bladder assembly, the sampling tool, and a lower expandable and collapsible packer bladder assembly, disposed between the drilling pipe and the drilling bit.
In either of the embodiments shown in FIG. 2-4 it is preferable that the gas sampling tubes are arranged in an annular organization with respect to the core of the well to most efficiently utilize the limited space in the well. Likewise, in such an embodiment both the upper and lower manifolds are also annularly shaped.
FIG. 5A-D are schematic illustrations of collection of gas samples of the backflow after the fracking of the open hole at various depth. The backflow means either the gas that enters the region between the packing, or flows from this region via an isolated tube or conduit. As there are multiple fracks for each hole and the gas of each backflow needs to be collected for compositional and isotope analysis. In FIG. 5A the apparatus shown in FIG. 2 is lowered to sample the gas flowing through the casing perforations at frack region 1. In FIG. 5B the apparatus shown in FIG. 2 is raised to sample the gas flowing through the casing perforations at frack region 2. In FIG. 5C the apparatus shown in FIG. 2 is raised further to sample the gas flowing through the casing perforations at frack region 3. In FIG. 5D the apparatus shown in FIG. 2 is raised even further to sample the gas flowing through the casing perforations at frack region 4. Accordingly, the movement and repeated sample by the apparatus shown in FIG. 2, provides an alternative means to mud-gas analysis to generate a useful compositional production log.
Further, it has been found most useful in addition to characterizing the gas composition of the strata to also determine the isotopic composition of each hydrocarbon component, as is shown in FIG. 6A. It is this isotope data that provides the original input data from the formations that can contribute to the gas stream in the open hole well bore. The isotopic data can be obtained from either the mud gas analysis shown in FIG. 1, or via the sampling method and apparatus of FIG. 2.
Individual hydrocarbon gas components, i.e. methane, ethane, propane, isobutene, n-butane and the like are characterized by their stable carbon and hydrogen isotopic composition, which are respectively (¹³C/¹²C) and (²H/¹H). The heavy isotopes are rare, accounting for circa 1% of the total carbon and circa 150 ppm of the total hydrogen, respectively. The minor variations characteristic of different fossil fuel deposits are accurately measured by well known methods of mass spectrometry after first burning the isolated hydrocarbon fractions totally to CO₂and H₂O, and then measuring the isotopic mass ratio of CO2 and hydrogen, respectively. Conventionally, these ratios are quantified and represented as a δ:
$δ^{13} C (%_{c}) = [\frac{{(^{13} C /^{12} C)}_{?}_{}}{{(^{13} C /^{12} C)}_{PDS}} - 1] \times 1000$ $and$ $δ D (%_{?}) = [\frac{{(D / H)}_{?}}{{(D / H)}_{SMOW}} - 1] \times 1000$ $? indicates text missing or illegible when filed$
FIG. 6A is an example of a down hole recording of mud-gas composition and isotope analyses during drilling of a well. The trace (C1) depicts methane isotopes, trace (C2) ethane isotopes and the trace (C3) propane isotopes (data are from Muehlenbachs et al. 2000). The isotopic signatures of the gases that are encountered during drilling change and indicate different gases that can contribute to the gas stream when production starts. A, B and C are inferred strata that contribute to the commingled production.
As is shown in FIG. 6B, by plotting the δ¹³C of one carbon compound (i.e. methane) versus another carbon compound (i.e. ethane) a mixing line can be calculated by isotope mass balance that provides a sensitive “fingerprint” characteristic for gas mixtures from gases of individual strata pierced as the well is being drilled. Likewise, evaluation of δD versus δ¹³C for different carbon compounds may also be useful.
In the first step of the process one determines the chemical and/or isotopic composition of gas along the whole borehole through mud-gas isotope analyses as is schematically illustrated in FIG. 6B.
With this capability to characterize the drill hole composition and assign a unique or fingerprint composition to each discrete strata of interest, the following steps are then deployed for successful production allocation and production monitoring in open hole gas production.
In the next step, during production, one then analyzes the isotopic and chemical composition of the commingled production gas. Identification and characterization of a particular strata with a unique isotopic composition may be accomplished solely by reference to the chemical and preferably the isotopic analysis of the deposit, but more preferably includes characterizing the geophysical nature of the layer and its relationship with adjacent geological structure along with other information generally tracked in a well evaluation log, such as mud-gas composition, density log, porosity and frack position and the like as a function of well depth. Such supporting characterization of the geological nature of the layers or strata may include all available down hole logging data generated during and after drilling, including but not limited to petrography, mineral composition analysis such as by gamma ray analysis, x-ray fluorescence, crystallographic and microscopic analysis as well as the differential mass flow of the discrete or commingled gas streams.
Such analysis requires mathematical deconvolution of complex mixtures using the techniques described in the following paragraph and results in the identification and quantification of the major contributing strata in the well bore that fit the isotopic composition of the commingled production. FIG. 6B is a simplified application of this method showing a double carbon isotope plot of methane and ethane for different reservoirs A and B. A mixing line (the series of crosses) for A and B can be calculated using simple mass balance. Isotopes of a mixture of A and B will follow the mixing line. The contribution of each reservoir to the commingled production can be deduced from the position of the commingled production along the mixing line.
It should be appreciated that for production allocation from multi component mixtures of gases un-mixing algorithms such as Alternating Least Square fits (ALS), Polytopic Vector Analysis (PVT), Multi-Variate Resolution (MVR) need to be applied, using as input data the characteristic gas isotope composition and other formation evaluation data assigned as representative of each strata. Such algorithms are now well known to analytical chemists and can be applied for production allocation in open hole production.
FIG. 6C is a depiction of the concept of production monitoring of a well using stable isotope analyses of commingled production. The initial production has an isotopic composition that is closer to that of A than to B and using mixing algorithms from FIG. 6B one can calculate contributions from A and B. During the production of the well gas isotopes are monitored over time and indicate a change after time T1 such that the isotopic composition of the commingled production becomes more similar to that of B. Together with volume production data this would provide engineers valuable information on the performance of the well.
FIG. 7 is hypothetical plot of the spatial distribution of methane carbon (δ¹³C1) isotope values across a reservoir in a gas field. The lateral variations of the methane carbon isotopes are mostly indicating geologic boundaries such as faults (upper left corner of map) where there is a rapid spatial change in isotope concentration, i.e. the large gradient of zones having a δ¹³C of −42 to −41 over less than about 1 relative unit of longitude. These boundaries are in general baffles to gas flow and result in a compartmentalization of gas fields. Such compartments require special drilling patterns for complete and efficient exploitation.
It should be appreciated that another aspect of the invention is to monitor with time the spatial and/or depth variation contributing components to the commingled production gas, and thus identify any changes in the nature of the extract and connection of the geologic deposits and their compartmentalization.
It is anticipated that monitoring the spatial distribution of the concentration and isotope values of gas components across a reservoir in a gas field over time will enable the petroleum engineer to readily interpret the contribution of production gas from different strata. A hypothetical example of such a comparison is shown in FIG. 8A, representing an initial or early state, and FIG. 8B, representing a latter state. It should be appreciated that the spread, that is the enlargement, of the region with a δ¹³C1 isotope value of 41.5, at the expense of the regions having a δ¹³C1 isotope value of 40.75 to 41.25, indicates that the strata contributing above the fault barrier at the upper left corner is no longer compartmentalized as in the earlier state at FIG. 8B.
While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.

Claims

1. A process for characterizing production from an open bore gas well, the process comprising steps of:

a) drilling an open bore well,

b) extracting gas samples characteristic of the well depth,

c) characterizing the gas isotope composition of the extracted gas samples,

d) defining the gas isotope compositions characteristic of two or more strata penetrated by the well,

e) extracting commingled production gas samples from the producing open bore well,

f) characterizing the gas isotope composition of the extracted production gas samples,

g) determining with mathematical un-mixing algorithms the contribution to the production gas sample from the gas isotope compositions characteristic of two or more strata.

2. A process for characterizing production from an open bore gas well according to claim 1 wherein said step of extracting gas sample comprises the step by extracting gas samples from the drilling mud at the surface of a well.

3. A process for characterizing production from an open bore gas well according to claim 1 wherein said step of extracting gas sample occurs while the well is being drilled.

4. A process for characterizing production from an open bore gas well according to claim 1 further comprising the step of lowering a sampling device in the bore hole after the well is drilled.

5. A process for characterizing production from an open bore gas well according to claim 4 further wherein the gas is sample from isolated fracks by sealing the portion of the bore hole above and below the frack.

6. A device for collecting of gas samples from discrete depth of the bore hole for chemical and isotopic analysis, the device comprising:

a) an upper packing layer,

b) an lower packing layer disposed below and coupled to said upper packing materials so as to form a cavity for receiving gas there between, wherein both said upper and lower packing layer are substantially impermeable to gases,

c) a gas sampling device in fluid communication with the cavity between said upper and lower packing layer.

7. A device according to claim 6 that further comprises a gas flow meter in fluid communication with the cavity between said upper and lower packing layer.

8. A device according to claim 7 wherein said flow meter and said gas sampling device are disposed between the packing.

9. A device according to claim 7 wherein said flow meter and said gas sampling device are disposed above said upper packing and further comprising a conduit through said upper packing to provide fluid communication with the cavity between said upper and lower packing layer.

10. A device according to claim 7 wherein the gas sampling devices comprises a plurality of isolatable sample tubes connected at the top by a common upper gas manifold and the bottom by a common lower gas manifold.

11. A device according to claim 10 wherein a plurality of the isolatable sample tubes are each isolated from the common upper gas manifold by a valves and from the common lower gas manifold by a second valve.

12. A method of gas sampling in a gas well, the method comprising the steps of:

a) providing the sampling device having a gas sampling inlet disposed between an upper and lower packer member,

b) lowering the sampling device in the well bore,

c) positioning the sampling device to capture gas samples at a fixed location,

d) activating the upper and lower packer member to isolate the gas sampling inlet from the region of the gas well above and below the fixed location,

e) acquiring gas samples,

f) removing the gas samples.

13. A method according to claim 12 that further comprises the steps of:

a) characterizing the gas isotope composition of the extracted gas samples,

b) defining the gas isotope compositions characteristic of two or more strata penetrated by the well,

c) extracting commingled production gas samples from the producing open bore well,

d) characterizing the gas isotope composition of the extracted production gas samples,

e) determining with mathematical un-mixing algorithms the contribution to the production gas sample from the gas isotope compositions characteristic of two or more strata.

14. A method according to claim 12 that further comprises the step of:

a) fracking the geological deposit at the fixed location prior to positioning the sampling device.

15. A method according to claim 12 that further comprises multiple step of fracking different fixed location or geological deposits surrounding the well bore and sampling the gas at two or more frack locations.

16. A method according to claim 12 wherein the well has an open bore.

17. A method according to claim 12 wherein the well has a casing.