US 20080135236 A1
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.
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
3. A process for characterizing production from an open bore gas well according to
4. A process for characterizing production from an open bore gas well according to
5. A process for characterizing production from an open bore gas well according to
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
8. A device according to
9. A device according to
10. A device according to
11. A device according to
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
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
a) fracking the geological deposit at the fixed location prior to positioning the sampling device.
15. A method according to
16. A method according to
17. A method according to
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.
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.
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.
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.
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
The gas collection devices 100 shown in
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
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
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
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
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 CO2 and H2O, and then measuring the isotopic mass ratio of CO2 and hydrogen, respectively. Conventionally, these ratios are quantified and represented as a δ:
As is shown in
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
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.
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.
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
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.