Production Data Analysis of CBM Wells
- Christopher R. Clarkson (ConocoPhillips Canada) | Colin L. Jordan (Apache) | Roger R. Gierhart (BP Amoco PLC) | John P. Seidle (MHA Petroleum Consultants)
- Document ID
- Society of Petroleum Engineers
- Rocky Mountain Oil & Gas Technology Symposium, 16-18 April, Denver, Colorado, U.S.A.
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 5.5.8 History Matching, 5.7 Reserves Evaluation, 4.1.2 Separation and Treating, 5.8.2 Shale Gas, 4.1.4 Gas Processing, 4.6 Natural Gas, 5.1 Reservoir Characterisation, 5.4.2 Gas Injection Methods, 5.8.3 Coal Seam Gas, 5.6.3 Pressure Transient Testing, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 5.6.9 Production Forecasting, 5.6.4 Drillstem/Well Testing, 5.3.2 Multiphase Flow, 5.5 Reservoir Simulation
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Recent advances in production data analysis (PDA) techniques have greatly assisted engineers in extracting meaningful reservoir and stimulation information from well production and flowing pressure data. Application of these techniques to coalbed methane (CBM) reservoirs requires that the unique coal storage and transport properties be accounted for. In recent work, the authors and others have demonstrated how new techniques such as the flowing material balance (FMB) and production type-curves may be adapted to account for CBM storage mechanisms (i.e. adsorption), but to date the focus has been on relatively simple CBM reservoir behavior. Although adaptations of PDA to include more complex CBM reservoir characteristics were introduced, the focus of the current work is advancement of modern PDA techniques to incorporate reservoir behavior such as single-phase flow of water in undersaturated reservoirs, two-phase (gas+water) flow in saturated reservoirs, effective permeability changes during depletion, and changing gas composition due to relative adsorption. Specifically, the FMB technique is modified in this work to include several complex CBM reservoir characteristics. FMB can be a powerful diagnostic technique when relative and absolute permeability changes are apparent during depletion. Several synthetic and field examples are given to demonstrate how FMB, type-curve analysis and analytical simulation can be used in parallel to provide a particularly useful data analysis tool.
The adapted PDA techniques used in this work make use of the pseudotime and pseudopressure concepts, modified to account for CBM reservoir behavior, to linearize the (constant-rate) diffusivity equation for CBM. Material balance pseudotime was used to account for variable operating conditions. These techniques were used successfully to extract quantitative reservoir information from single- and two-phase CBM simulated and field production and pressure data. The techniques for two-phase CBM require further evaluation, however.
Several key assumptions were used in deriving the PDA techniques including (but not limited to) instantaneous desorption (small sorption times) and single-layer behavior. Although the former is not considered a serious limitation, as most commercial reservoirs analyzed to date by the authors have exhibited single-porosity behavior during production, the latter may be quite important for some producing fields. PDA of multi-layered CBM reservoirs will be discussed at length in a future paper.
Over the past several decades, significant advances have been made in production data analysis (PDA) of conventional oil and gas reservoirs1-10 (select references given only). Although numerical reservoir simulation certainly qualifies as a PDA technique, we limit our discussion mainly to FMB, and production type-curves, used, in some instances, in parallel with analytical simulation. These modern methods have greatly enhanced the engineers' ability to obtain quantitative information about reservoir properties and stimulation/damage from data that is routinely gathered during the producing life of a well, such as production data, and in some instances, flowing pressure information. The information that may be obtained from detailed production data analysis includes oil or gas-in-place (GIP), permeability-thickness product (kh) and skin (s), and can be used to supplement information obtained from other sources such as pressure transient analysis (PTA), material balance and reservoir simulation.
Complex reservoir behavior encountered in CBM plays has precluded analysis using modern PDA techniques, such as FMB, until recently. The analyst could be faced with several reservoir complexities during primary depletion of a CBM reservoir, depending upon the CBM play of interest, such as:
- Gas storage by adsorption and free-gas compression in the pore space.
- Relative permeability effects in two-phase (gas+water) coals.
- Stress- and/or desorption-dependent fracture pore volume and absolute permeability.
- Multi-layer characteristics caused by contrasting permeability from coal seam to seam, that have limited or no vertical communication.
- Dual porosity behavior, when sorption times are large.
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