Secondary Porosity and Permeability of Coal vs. Gas Composition and Pressure
- Matthew J. Mavor (Tesseract Corp.) | William D. Gunter (Alberta Research Council)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2006
- Document Type
- Journal Paper
- 114 - 125
- 2006. Society of Petroleum Engineers
- 5.4 Enhanced Recovery, 5.8.6 Naturally Fractured Reservoir, 1.6.9 Coring, Fishing, 5.2.2 Fluid Modeling, Equations of State, 5.5 Reservoir Simulation, 4.1.5 Processing Equipment, 5.6.4 Drillstem/Well Testing, 5.6.3 Pressure Transient Testing, 6.5.1 Air Emissions, 6.5.2 Water use, produced water discharge and disposal, 5.2.1 Phase Behavior and PVT Measurements, 5.8.3 Coal Seam Gas, 5.6.9 Production Forecasting, 5.4.2 Gas Injection Methods, 5.6.1 Open hole/cased hole log analysis, 6.5.7 Climate Change, 4.1.2 Separation and Treating
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We have been investigating the sequestration of atmospheric pollutants by injection into coal seams while at the same time enhancing hydrocarbon productivity by displacement of methane with pollutants. Our effort is one of several field-based research efforts into CO2 storage in coal seams that are currently operating in Canada (Mavor et al. 2004), China (Robinson et al. 2004), Japan (Nago and Komaki 2004), and Poland (Van Bergen et al. 2004). During the course of our field measurements, we have been using single-well injection, soak, and production tests to collect data required to understand and predict enhanced coalbed methane (ECBM) recovery potential and sequestration capacity. We found that changing the composition of the gas sorbed into the coal changes the porosity and permeability of the coal natural-fracture system owing to gas-content changes, which cause matrix swelling or shrinkage due to relative adsorption of different gases.
We collected sufficient information to develop a method for predicting the permeability and porosity of a coalbed as a function of the secondary porosity system (SPS) pressure and the gas content and composition of the primary porosity system (PPS). The method uses data from injection/falloff tests with water and/or a weaker adsorbing gas (WAG) than CH4 and a stronger adsorbing gas (SAG) than CH4. Estimates of effective permeability to gas and water obtained from these tests are used with an iterative computation procedure subject to constraints to solve for equivalent SPS porosity and absolute permeability at atmospheric pressure.
Once calibrated, the model can be used to predict a coalbed's permeability and porosity as a function of injection pressure and injected-fluid composition, which in turn are used to predict injection performance. The model is applicable to production forecasts to account for SPS permeability and porosity changes as reservoir pressure declines with changes in gas composition.
This paper describes the new model and discusses well-test procedures to obtain the data required for model calibration. Also included are coal property estimates resulting from Alberta Medicine River (Manville) coal core and test data and an example model calibration.
Commercial production of gas from coal seams is highly dependent upon natural fractures that control the absolute permeability magnitude and distribution throughout the reservoir. Natural-fracture absolute permeability and porosity vary as a function of location, pressure within the natural-fracture system, and the composition of gas within the coal matrix. Variations in fracture porosity cause variations in fluid saturations that in turn cause variations in the relative and effective permeability to gas and water.
Coal-gas reservoirs are dual-porosity reservoirs consisting of primary and secondary storage and mass transfer systems. The PPS contains the vast majority of the gas-in-place volume stored by sorption, while the SPS provides the conduit for mass transfer to wells (Mavor and Nelson 1997). The SPS generally consists of two or more natural-fracture sets created at different times. The more continuous, greater-permeability natural-fracture set is referred to as face cleats. There is often a more discontinuous, lower-permeability set referred to as butt cleats oriented at roughly right angles to face cleats. There also can be additional fracture sets present (Close 1993).
Coal permeability has been known for some time to be dependent upon net stress and changes in sorbed-gas content. Gray (1987) discussed both of these phenomena. The general belief was that as reservoirs are depleted, the increase in net stress decreases the aperture of natural fractures, which decreases the absolute permeability. However, Gray reported that permeability might increase as pressure and gas content decreased because an absolute permeability increase caused by gas desorption could overcome the decrease caused by net stress increase.
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