Secondary porosity and permeability of coal vs. gas composition and pressure
- Tesseract Corp., San Francisco, CA (United States)
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. 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 CH{sub 4} and a stronger adsorbing gas (SAG) than CH{sub 4}. 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.
- OSTI ID:
- 20752186
- Journal Information:
- SPE (Society of Petroleum Engineers) Reservoir Evaluation and Engineering, Vol. 9, Issue 2; ISSN 1094-6470
- Country of Publication:
- United States
- Language:
- English
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