Although the occurrence mode and enrichment law of coalbed methane are different from those of conventional natural gas, the exploration methods also have their own characteristics. However, like conventional oil and gas exploration, the exploration of coalbed methane has stages. First, based on basin evaluation, regional exploration, pre-exploration and evaluation drilling of coalbed methane are carried out on the basis of coal exploration, and a series of coalbed methane resources and reserves from single well trial production to well group test is gradually established. According to the Specification for CBM Resources/Reserves (DZ/T 02 16—2002), this paper introduces the calculation method of CBM reserves.
3.4. 1 coalbed methane resources
Coalbed methane resources: refers to the coalbed methane rich body with underground coal seam as reservoir and economic significance. Its quantitative expression is divided into resources and reserves.
Coalbed methane resources: refers to the amount of coalbed methane that exists in coal seams, can be mined at present or may be mined in the future, and has practical and potential economic significance according to certain geological and engineering foundations.
3.4.2 Geological reserves of coalbed methane
Geological reserves of coalbed methane: refers to the total amount of coalbed methane found in coalbed methane reservoirs with clear calculation boundaries in the original state.
Original recoverable reserves (hereinafter referred to as recoverable reserves): the recoverable part of geological reserves refers to the amount of coalbed methane that can be finally recovered in a known coalbed methane reservoir with clear calculation boundaries by using existing technologies under the current economic conditions and government regulations.
Economically recoverable reserves: the economic part of the original recoverable reserves refers to the part of coalbed methane reserves that can be extracted from known coalbed methane reservoirs with clear calculation boundaries by using existing technologies under the current economic conditions and government regulations, and that is considered economically beneficial by economic evaluation. Economically recoverable reserves are the sum of accumulated production and remaining economically recoverable reserves.
Remaining economically recoverable reserves: refers to the amount of coalbed methane that can be extracted from known coalbed methane reservoirs with clear calculation boundaries by using existing technologies under the current economic conditions and government regulations, which is considered to have economic benefits by economic evaluation.
3.4.3 Classification and classification of coalbed methane resources/reserves
3.4.3. 1 classification and grading principle
The classification of coalbed methane reserves is based on the principle that economic benefits can be obtained from production and sales under specific policy, law, time and environmental conditions. Through technical and economic evaluation in different exploration stages, it can be divided into three categories according to economic feasibility: economy, sub-economy and internal economy. Based on the basic principle of geological understanding level of coalbed methane resources, according to the different exploration and development projects and geological understanding level, coalbed methane resources are divided into two levels: to be discovered and discovered. The discovered coalbed methane resources, also known as coalbed methane geological reserves, are divided into three levels according to geological reliability: prediction, control and verification. Recoverable reserves can be determined according to geological reserves.
3.4.3.2 classification
Economy: Under the conditions of market economy at that time, the production and sales of coalbed methane were technically feasible, economically reasonable and geologically reliable, and the whole business activities could meet the requirements of return on investment.
Sub-economy: Under the condition of market economy at that time, the production and sales of coalbed methane have no economic benefits and are uneconomical for the time being, but it can be transformed into economy under the conditions of economic environment changes or government support policies.
Intrinsic economy: Under the condition of market economy at that time, it is still impossible to judge whether the production and sales of coalbed methane are economical, including the part that cannot judge the economic attribute at present.
3.4.3.3 classification
Prediction: preliminarily understand the distribution law of coalbed methane resources and obtain the reservoir parameters under the typical structural environment of coalbed methane reservoir. Because there is no drainage test, there are only some coal-bearing and gas-bearing parameter well projects, and most reservoir parameter conditions are inferred. The reliability of coalbed methane resources is very low, and the confidence coefficient of reserves is 0. 1 ~ 0.2.
Controlled: The geological characteristics, reservoir distribution and gas-bearing property of coalbed methane reservoirs are basically ascertained, and the mining technical conditions are basically controlled. Through single well testing and reservoir numerical simulation, the single well productivity of coalbed methane surface drilling under typical geological background is understood. However, due to the limited number of parameter wells and trial production wells, it is not enough to fully understand the gas occurrence conditions and gas production potential within the calculation range of the whole gas reservoir, so the reliability of coalbed methane resources is not high, and the confidence coefficient of reserves is about 0.5.
Proved: geological characteristics, reservoir distribution law, gas-bearing property and mining technical conditions of coalbed methane reservoir (including reservoir physical properties, pressure system and gas flow capacity, etc.). ) has been identified; Through the implementation of small well pattern and/or single well coalbed methane test or development well pattern, the coalbed methane resources and recoverability within the exploration scope are determined. The reliability of coalbed methane resources is very high, and the confidence coefficient of reserves is 0.7 ~ 0.9.
The remaining proven economic recoverable reserves can be divided into two categories according to the development status: ① Developed refers to the estimated amount of coalbed methane produced by existing wells in the proven area; (2) The term "to be developed" refers to the amount of coalbed methane that can be expected to be produced from an existing well in an undrilled area or a proven area to another reservoir.
Classification and grading system of coalbed methane resources and reserves in 3.4.3.4
According to the classification standard of coalbed methane resources and its corresponding relationship with exploration control projects, the classification system of coalbed methane resources and reserves is established (Table 3.5).
Table 3.5 Classification and Classification System of CBM Resources/Reserves
3.4.4 Calculation of CBM resources and reserves
3.4.4. 1 Initial conditions of reserves and calculation unit
(1) reserve starting conditions
The calculation of coalbed methane reserves is based on the lower limit of single well production, that is, proven reserves can only be calculated in areas where the gas production of coalbed methane wells reaches the lower limit of production. According to the domestic average conditions, the lower limit of single well average production is shown in Table 3.6. The exploration degree and understanding degree of reserves at all levels given in Table 3.7 are the basic requirements for reserves calculation.
Table 3.6 Lower Limit Standard of Single Well Production with Reserves
Table 3.7 Requirements for exploration degree and understanding degree of coalbed methane reserves at all levels
(2) Reserves calculation unit
The calculation unit of reserves is generally coalbed methane reservoir, that is, gas-bearing coal reservoir controlled by various geological factors. When the geological boundary of coalbed methane reservoir is not clear, the boundary shall be calculated according to the calculation of coalbed methane reservoir. Computational units are generally called blocks in plane, and blocks with large areas can be subdivided into well areas (or well areas), and the same block should basically have the same or similar structural conditions and gas storage conditions. Generally, a single coal seam is taken as the calculation unit in the longitudinal direction, and the coal seam groups with relatively concentrated coal seams can be combined with the calculation unit, and the shallow coal reservoirs in the weathered zone of coal seams do not calculate the reserves. See Code for Geological Exploration of Coal Resources for indicators of weathered zone.
(3) Reserves calculation boundary
The boundary of reserve calculation unit should be determined by various geological boundaries of coalbed methane reservoir, such as faults, stratigraphic changes (thinning, pinch-out, denudation, metamorphism, etc. ), the lower limit of gas content, the lower limit of net thickness of coal seam (0.5~0.8 m), etc. (Coal seam group can be adjusted according to the actual situation); If the geological boundary is not found out, it is mainly delineated by coalbed methane wells that have reached the lower limit of production. For various reasons, it can also be delineated by the boundaries of mineral rights areas, natural geographical boundaries or artificial reserves. The lower limit value of coal seam gas content (see Table 3.8) can also be adjusted according to specific conditions, such as different coal seam thickness.
Table 3.8 Lower Limit Standard of Coal Seam Gas Content
Calculation method of reserves in 3.4.4.2
(1) Calculation of geological reserves
A. Simulation method
The analogy method mainly uses the comparison with developed coalbed methane fields (or similar reservoirs) to calculate reserves. When calculating, it is necessary to draw a typical curve of the correlation between production characteristics and reserves in developed areas, obtain comparable reserves parameters in the calculation area, and then calculate reserves with other methods. The analogy method can be used to predict the calculation of geological reserves.
B. volumetric method
Volume method is the basic method to calculate the geological reserves of coalbed methane, which is suitable for the calculation of geological reserves of coalbed methane at all levels. Its accuracy depends on the understanding of geological and reservoir conditions of gas reservoirs, as well as the accuracy and quantity of related parameters.
The calculation formula of volumetric method is
Gi= 0.0 1 AhDCad
or
Coal-formed gas geology
In which: CAD =100cdaf (100-mad-ad); Gi is the geological reserve of coalbed methane,108m3; ; A is the gas-bearing area of coal seam, km2；; H is the net thickness of coal seam, m; D is the dry basis mass density of coal (unit weight of coal), t/m3; Cad is the basic gas content of coal air drying, m3/t; Ddaf is the dry ash-free mass density of coal, t/m3; Cdaf is the dry ash-free gas content of coal, m3/t; Mad is the raw coal-based moisture in coal,%; Ad is ash in coal,%.
(2) Calculation of recoverable reserves
A. Numerical simulation method
Numerical simulation is an important method to calculate the recoverable reserves of coalbed methane. This method uses special software in computer (called numerical simulator) to fit the obtained reservoir parameters with the early production data (or trial production data), and finally obtains the expected production curve and recoverable reserves of gas wells.
Selection of data simulator: The selected numerical simulator must be able to simulate the unique dual pore characteristics of coal reservoir, three flow modes of gas-water two-phase fluid (desorption, diffusion and seepage) and their interaction process, as well as the mechanical properties and mechanical properties of coal and rock.
Reservoir description: it is the study of spatial distribution and plane distribution characteristics of reservoir parameters, and it is the basis of quantitative evaluation of coalbed methane reservoirs. The description should include four parameters: basic geology, reservoir physical properties, reservoir fluid and production performance. Through the description of these parameters, the reservoir geological model is established to predict the productivity.
Historical fitting and productivity prediction: using the reservoir geological and engineering parameters calculated by reservoir simulation tools, the calculated gas-water production and pressure values are historically matched with the actual production and measured pressure values of gas wells. When the simulated gas-water production performance matches the actual production performance of gas wells, a gas reservoir model can be established, a gas production curve can be obtained, the future gas production can be predicted, and the final cumulative total production of coalbed methane, namely the recoverable reserves of coalbed methane, can be obtained.
According to the degree of data mastery and calculation accuracy, the calculation results of reservoir simulation method can be used as control recoverable reserves and proven recoverable reserves.
B. Production decline method
Production decline method is to predict reserves by studying the gas production law of coalbed methane wells and analyzing the production characteristics and historical data of gas wells. Usually, the slope of the production decline curve is used to calculate the future production after the coalbed methane well experiences the peak gas production and begins to stabilize or decline. Production decline method is actually an extrapolation method of production characteristics of coalbed methane wells, and the following conditions must be met when using production decline method:
1) It is reasonable to believe that the selected production curve has typical representative significance of gas production potential of gas reservoirs;
2) Gas production area of gas well can be clearly defined;
3) The gas production decline curve has a stable slope value for at least half a year after the gas production peak on the production-time curve;
4) The influence of production changes caused by non-geological reasons such as market shrinkage, workover or surface water treatment on the determination of slope value of decline curve must be effectively eliminated.
Production decline method can be used to calculate proven recoverable reserves, especially in the production and development stage of gas wells. Production decline method can improve the calculation accuracy of reserves together with volume method and reservoir simulation method.
C. Calculation method of oil production
Recoverable reserves can also be calculated by calculating gas reservoir recovery ratio, and the calculation formula is as follows
Coal-formed gas geology
Where: Gr is the recoverable reserves of coalbed methane,108m3; ; Gi is the geological reserve of coalbed methane,108m3; ; Rf is the recovery factor,%.
Coalbed methane recovery (Rf) can be calculated by the following methods:
1) analogy method: it is obtained by analogy with geological parameters and engineering parameters of developed gas fields or adjacent gas fields, and can only be used to predict recoverable reserves.
2) Reservoir simulation method: directly calculated on the reservoir simulation productivity curve, which can be used to calculate the controlled recoverable reserves and proven recoverable reserves.
Coal-formed gas geology
Where: GPL is the cumulative gas production of gas well,108m3; ; Giw refers to the geological reserves within the well control range, 108m3.
3) Isothermal adsorption curve method: the calculation of waste pressure on isothermal adsorption curve can only be used to predict recoverable reserves, and can also be used as a reference for controlling recoverable reserves.
Coal-formed gas geology
Where: Cgi is the coalbed methane content under the original reservoir condition, m3/t; Cga is the content of coalbed methane under abandonment pressure, m3/t. ..
4) Production decline method: directly calculated on the production decline curve with stable decline slope, which can be used for the calculation of proven recoverable reserves.
Coal-formed gas geology
Where: GPL is the cumulative gas production of gas well,108m3; ; Giw refers to the geological reserves within the well control range, 108m3.
3.4.5 Selection and value of calculation parameters of coalbed methane resources and reserves
3.4.5. 1 volumetric method parameter determination
(1) Coal seam gas-bearing area (hereinafter referred to as gas-bearing area)
Gas-bearing area refers to the coal seam distribution area where the coalbed methane production of a single well reaches the lower limit of production. It is necessary to make full use of geological, drilling, logging, seismic and coal sample test data, comprehensively analyze the geological laws and geometric shapes of coal seam distribution, and delineate them on the coal seam roof and floor structural map compiled by drilling control and seismic interpretation. The well (hole) control degree of the reservoir shall meet the well spacing requirements specified in Table 3. 13 and Table 3.7. The principles for delimiting the boundaries of gas-bearing areas are as follows:
The boundary of coalbed methane reservoir determined by drilling and earthquake, that is, the geological boundary such as fault, pinch out and dissolution; The lower limit of net thickness of coal seam can not reach the lower limit of production; Gas content lower limit boundary and gas weathering zone boundary.
When the boundary of coalbed methane reservoir is not found out or the coalbed methane well is too far from the boundary, it is mainly delineated by extrapolation of coalbed methane well. The extrapolated distance of the proven area boundary does not exceed 0.5 ~ 1.0 times of the well spacing specified in Table 3. 13, which can be divided into the following situations (assuming that the distance specified in Table 3. 13 is 1 well spacing): ① When only 1 well reaches the lower limit of gas production, (2) When several adjacent wells reach the lower limit of gas production, if the distance between two adjacent wells exceeds 3 well spacing, the well spacing of 1/2 can be extrapolated around these two wells respectively; (3) When several adjacent wells reach the lower limit of gas production, if the distance between two adjacent wells exceeds two well spacings but is less than three well spacings, all the inter-well areas are counted as proven areas, and at the same time, 1 well spacing can be extrapolated from these two wells as the boundary of proven areas; (4) When several adjacent wells reach the lower limit of gas production and the well spacing does not exceed two well spacings, extrapolate the proved area boundary with 1 well spacing as the center.
For various reasons, it can also be defined by the boundaries of mineral rights areas, natural geographical boundaries or artificial reserve calculation lines. The boundary of the proven area shall not exceed 0.5 ~ 1.0 times of the well spacing specified in Table 3. 13.
(2) Effective (net) thickness of coal seam (referred to as effective thickness or net thickness)
Effective thickness of coal seam refers to the thickness of coal seam after deducting gangue layer, also known as net thickness. The proven effective thickness should be determined according to the following principles: ① The trial production of coalbed methane wells should confirm that the reserve threshold has been reached, and the untested coal seams should be continuous and similar to those of adjacent wells that have reached the reserve threshold; ② The well (hole) control degree shall meet the requirements of Table 3. 13 well spacing, and the area balance method is generally adopted to take the value; ③ The effective thickness should be determined mainly by drilling coring or logging, and the well position and thickness should be corrected when the well deviation is too large; ④ The lower limit of the effective thickness of a single well is 0.5~0.8 m (which can be adjusted according to the gas content), and the tripping thickness of the gangue layer is 0.05 ~ 0.1m. ..
(3) the mass density of coal
Coal density is divided into pure coal density and apparent coal density, which correspond to different gas content standards in reserve calculation. See GB 2 12—9 1 industrial analysis method of coal for the determination method.
(4) Gas content
Coalbed methane reserves can be approximately calculated by dry ash-free basis or air-dried basis, and the conversion relationship can be calculated by the following formula:
Coal-formed gas geology
Where: Cad is the air-dried gas content of coal, m3/t; Cdaf is the dry ash-free gas content of coal, m3/t; Mad is the raw coal-based moisture in coal,%; Ad is ash in coal,%.
However, in order to ensure the accuracy of the calculation results, it is best to use the gas content of raw coal to calculate the coalbed methane reserves. The gas content of raw coal base needs to be corrected for the balance moisture and average ash on the basis of air drying gas content, and the correction formula is:
Coal-formed gas geology
Where: Cc is the original coal-based gas content of coal, m3/t; Cad is the basic gas content of coal air drying, m3/t; Aav is the average ash content of coal,%; Meq is the equilibrium moisture of coal,%; β is the slope of the correlation curve between air-dried base gas content and (ash+moisture).
For the determination of various benchmark coalbed methane content and equilibrium moisture, refer to the USBM coalbed methane content determination and ASTM equilibrium moisture determination methods of the US Bureau of Mines.
The determination principle of coalbed methane content is as follows:
1) When calculating the proven geological reserves, the gas content measured by the on-site coal core direct desorption method (USBM method of the US Bureau of Mines) should be adopted, and the gas content measured by the Coal Core Analysis Method in coal exploration (MT/T 77-94) can also be referenced, but necessary corrections should be made. Sampling interval: within the coal seam thickness 10 m, every 0.5 ~ 1.0m 1 sample; The thickness of coal seam is greater than 10 m, and more than 10 samples are evenly distributed (every 2 m or more can be distributed 1 sample). The well (hole) control degree reaches 1.5 ~ 2.0 times of the well spacing specified in Table 3. 13. Generally, the area balance method is used to get the value, and the contour line circled by the calibration well is larger than that of the adjacent coalbed methane well, and the gas content above it does not participate in the balance.
2) When calculating the unproven geological reserves, the gas content determined by on-site coal core direct desorption method and coal core analysis method (MT/T 77-94 coalbed methane determination method) can be used. The gas content obtained by analogy with neighboring areas with similar geological conditions and coal quality can be used to predict the calculation of geological reserves. When necessary, the gas content can also be estimated according to the coal quality and buried depth, and the estimated gas content can be used to predict the calculation of geological reserves.
3) After comprehensive analysis of relevant geological environment and structural conditions such as coal seam, roof and floor, adjacent layers and mined-out areas, when calculating and estimating the resources, the relative gas emission of the mine can be taken as the reference value for calculating the gas content. Although the isothermal adsorption curve used for gas outburst prevention can also provide the coalbed methane capacity value, it must be corrected in terms of moisture and temperature when quoting, and the corrected curve can be used to estimate the resource amount.
4) Refer to GB/T 136 10-92 Gas Composition Analysis Method for the determination of coalbed methane composition. Coalbed methane reserves should be calculated according to the different classification of gas components. In general, non-hydrocarbon gas components with a concentration exceeding 10% should be excluded from the measured values of coalbed methane content involved in reserve calculation.
Determination of parameters of 3.4.5.2 numerical simulation method and production decline method
The parameters of numerical simulation method, gas-water properties, coal quality and composition, reservoir physical properties, isothermal adsorption characteristics, temperature, pressure and gas-water production are implemented with reference to GB 2 12-9 1, GB/T 13 10-92 and related standards.
Parameter selection of reserves calculation in 3.4.5.3
1) The parameters in reserves calculation can be obtained from various data and methods, and their accuracy and representativeness should be compared in detail for comprehensive selection, and the basis for determining the parameters should be discussed in the reserves report.
2) When calculating the average parameters of geological units, the thickness of coal seam should be based on the actual structural development law in principle, and the isoline area balance method or well point control area balance method should be adopted. However, in the detailed survey area and detailed survey area of coal exploration, the arithmetic average method can be directly used to calculate, and other parameters should generally be calculated by the balance method of well point control area of coalbed methane parameter test well.
3) The names, symbols, units and significant figures of all parameters are shown in Table 3. 13, and the rounding method is adopted for calculation.
4) The coalbed methane reserves should be expressed as dry unit of volume in standard state (temperature 20℃, pressure 0. 10 1 MPa).
3.4.6 Evaluation of coalbed methane reserves
3.4.6. 1 comprehensive geological evaluation
(1) reserve scale
According to the size of reserves, the geological reserves of coalbed methane fields are divided into four categories (Table 3.9).
(2) Rich reserves
According to the reserve abundance of coalbed methane fields, the geological reserve abundance of coalbed methane fields is divided into four categories (Table 3. 10).
Table 3.9 Classification of Reserves Scale
Table 3. 10 Classification of Reserves Abundance
(3) production capacity
According to the stable daily production of gas wells, the gas reservoir productivity can be divided into four categories (Table 3. 1 1).
(4) Buried depth
According to the buried depth, gas reservoirs can be divided into three categories (Table 3. 12).
Table 3. 1 1 productivity classification of coalbed methane wells
Table 3. 12 Classification of buried depth of coalbed methane reservoir
Economic evaluation of 3.4.6.2
1) Use NPV analysis method to predict the cost and benefit of all levels of reserves submitted in each stage of coalbed methane exploration and development in the future, analyze and demonstrate its financial feasibility and economic rationality, and optimize exploration and development projects to obtain the best economic and social benefits.
2) The economic evaluation of reserves should run through the whole process of coalbed methane exploration and development, and the corresponding economic evaluation should be carried out for all levels of reserves.
3) All declared proven reserves must be evaluated economically.
4) The estimation of investment, cost and expenses in economic evaluation should be based on the actual situation of coalbed methane fields and fully consider the statistical data of similar developed or adjacent coalbed methane fields in the current year.
5) The productivity prediction of coalbed methane wells in new gas fields must be based on the development concept design compiled by the development department, and the average stable daily output of a single well should be demonstrated according to the reservoir numerical simulation.
Table 3. 13 Control requirements of basic wells (holes) for calculation of proven geological reserves of coalbed methane
Further reading is recommended.
1. Song Yan, Zhang Xinmin and others, 2005. Formation mechanism of coalbed methane and theoretical basis of economic development. Beijing: Science Press, 1 ~ 9.
2. Zhao Qingbo et al. 1999. Geology and exploration technology of coalbed methane. Beijing: Geological Publishing House, 45 ~ 53.
3. Zhang Xinmin and others, 2002. Geology and resource evaluation of coalbed methane in China. Beijing: Science Press, 5 1 ~ 6 1.
4. Ministry of Land and Resources of the People's Republic of China. 2003. People's Republic of China (PRC) Geology and Mineral Industry Standard (DZ/T 02 16—2002). Specification for coalbed methane resources/reserves. Beijing: Geological Publishing House.