skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Measurement of stress-dependent permeability in coal and its influence on coalbed methane production

Abstract

It is known that coal permeability measured in the laboratory is highly sensitive to the net confining pressure applied on the sample. Since many coalbed methane modeling studies assume that the permeability of a coal seam does not change with reservoir pressure depletion, skepticism about the accuracy of future predictions is understandable. This paper presents laboratory-measured permeability reduction during reservoir pressure depletion using a whole-core sample of coal in uniaxial strain at reservoir conditions. These laboratory results have been used in a numerical simulation study to determine the influence of pressure-dependent permeability on coalbed methane production. The productivity of a coalbed methane well is substantially reduced because of the decline in coal permeability during reservoir pressure depletion. More significantly, most of the reduction in gas and water production rates because of pressure-dependent permeability occurs during well start-up. Thereafter, the well productivity loss due to stress dependent permeability is fairly gradual over a very long period of time. The characteristic gas and water production-rate profiles obtained from a numerical simulator are almost the same regardless of whether the coal seam permeability is assumed to be constant or stress-dependent.

Authors:
 [1];  [2]
  1. (Amoco Production Co., Denver, CO (US))
  2. (Amoco Production Co., Tulsa, OK (US))
Publication Date:
OSTI Identifier:
7103499
Alternate Identifier(s):
OSTI ID: 7103499
Resource Type:
Journal Article
Resource Relation:
Journal Name: In Situ; (United States); Journal Volume: 16:3
Country of Publication:
United States
Language:
English
Subject:
03 NATURAL GAS; COAL; PERMEABILITY; METHANE; PRODUCTION; FLUIDIZED BEDS; INHIBITION; LABORATORIES; NATURAL GAS WELLS; PRESSURE DEPENDENCE; SIMULATION; SIMULATORS; STRESSES; ALKANES; ANALOG SYSTEMS; CARBONACEOUS MATERIALS; ENERGY SOURCES; FOSSIL FUELS; FUELS; FUNCTIONAL MODELS; HYDROCARBONS; MATERIALS; ORGANIC COMPOUNDS; WELLS 030300* -- Natural Gas-- Drilling, Production, & Processing

Citation Formats

Puri, R., and Seidle, J.P. Measurement of stress-dependent permeability in coal and its influence on coalbed methane production. United States: N. p., 1992. Web.
Puri, R., & Seidle, J.P. Measurement of stress-dependent permeability in coal and its influence on coalbed methane production. United States.
Puri, R., and Seidle, J.P. Wed . "Measurement of stress-dependent permeability in coal and its influence on coalbed methane production". United States. doi:.
@article{osti_7103499,
title = {Measurement of stress-dependent permeability in coal and its influence on coalbed methane production},
author = {Puri, R. and Seidle, J.P.},
abstractNote = {It is known that coal permeability measured in the laboratory is highly sensitive to the net confining pressure applied on the sample. Since many coalbed methane modeling studies assume that the permeability of a coal seam does not change with reservoir pressure depletion, skepticism about the accuracy of future predictions is understandable. This paper presents laboratory-measured permeability reduction during reservoir pressure depletion using a whole-core sample of coal in uniaxial strain at reservoir conditions. These laboratory results have been used in a numerical simulation study to determine the influence of pressure-dependent permeability on coalbed methane production. The productivity of a coalbed methane well is substantially reduced because of the decline in coal permeability during reservoir pressure depletion. More significantly, most of the reduction in gas and water production rates because of pressure-dependent permeability occurs during well start-up. Thereafter, the well productivity loss due to stress dependent permeability is fairly gradual over a very long period of time. The characteristic gas and water production-rate profiles obtained from a numerical simulator are almost the same regardless of whether the coal seam permeability is assumed to be constant or stress-dependent.},
doi = {},
journal = {In Situ; (United States)},
number = ,
volume = 16:3,
place = {United States},
year = {Wed Jan 01 00:00:00 EST 1992},
month = {Wed Jan 01 00:00:00 EST 1992}
}
  • For deep coal seams, significant reservoir pressure drawdown is required to promote gas desorption because of the Langmuir-type isotherm that typifies coals. Hence, a large permeability decline may occur because of pressure drawdown and the resulting increase in effective stress, depending on coal properties and the stress field during production. However, the permeability decline can potentially be offset by the permeability enhancement caused by the matrix shrinkage associated with methane desorption. The predictability of varying permeability is critical for coalbed gas exploration and production-well management. We have investigated quantitatively the effects of reservoir pressure and sorption-induced volumetric strain on coal-seammore » permeability with constraints from the adsorption isotherm and associated volumetric strain measured on a Cretaceous Mesaverde Group coal (Piceance basin) and derived a stress-dependent permeability model. Our results suggest that the favorable coal properties that can result in less permeability reduction during earlier production and an earlier strong permeability rebound (increase in permeability caused by coal shrinkage) with methane desorption include (1) large bulk or Young's modulus; (2) large adsorption or Langmuir volume; (3) high Langmuir pressure; (4) high initial permeability and dense cleat spacing; and (5) low initial reservoir pressure and high in-situ gas content. Permeability variation with gas production is further dependent on the orientation of the coal seam, the reservoir stress field, and the cleat structure. Well completion with injection of N2 and displacement of CH{sub 4} only results in short-term enhancement of permeability and does not promote the overall gas production for the coal studied.« less
  • Fabric and composition of a series of Upper Permian high-volatile to low-volatile bituminous coals of the Sydney basin have a marked effect on stress sensitivity of permeability, and thus on the reservoir characteristics of the coal. The coals vary in composition from end members of predominantly bright-banded coal comprised mainly of the microlithotype vitrite and the maceral vitrinite, to dull coal composed of significant amounts of ash, inertinite group macerals, and the microlithotype inertite. The brighter coals are more extensively fractured with one or, more commonly, two or three regularly spaced fracture sets (cleats) spaced at 5-20 {mu}m. Fusinite andmore » semifusinite, common macerals in the dull coals, are characterized by phyteral porosity (mainly cell lumens) and fabric-selective intergranular porosity. The permeability of tested samples varies significantly with composition and effective stress. The fabric of the samples is the most important factor in determining permeability and stress sensitivity of permeability. Coals with the highest permeability are those with at least one well-developed, throughgoing fracture set; these samples generally include abundant vitrite bands. The lowest permeability samples are nonbanded, with an attrital fabric and significant authigenic mineralization. At 0.8 MPa effective stress, the permeability of the sample suite ranges from 10 to over 2 x 10{sup 6} {mu}d, a difference of five orders of magnitude. Even at high effective stresses (i.e., 12 MPa), the difference in permeability between samples varies as much as three orders of magnitude. The variation in permeability with effective stress (stress sensitivity) is marked; a twofold increase in effective stress causes a sixfold decrease in permeability. At effective stresses of over 9 MPa, the permeability of all samples is less than 10 {mu}d.« less
  • Methane resources are frequently associated with deeply buried coal seams which are also saturated with water; therefore, knowledge of their hydrologic properties is essential. As the formation pressure is lowered during dewatering, permeability may decline by one to two orders of magnitude. Theoretical relationships have been developed which fit laboratory data well for porosity and permeability as a function of effective stress. It was discovered that for practical purposes permeability is a function only of effective stress and the ratio of initial fracture porosity to matrix compressibility (fracture closure pressure). An approximate analytical solution for well testing has been obtainedmore » using the model developed. A new method for pump test analysis is then proposed.« less
  • In this work, the Palmer-Mansoori model for coal shrinkage and permeability increases during primary methane production was rewritten to also account for coal swelling caused by CO{sub 2} sorption. The generalized model was added to a compositional, dual porosity coalbed-methane reservoir simulator for primary (CBM) and ECBM production. A standard five-spot of vertical wells and representative coal properties for Appalachian coals was used. Simulations and sensitivity analyses were performed with the modified simulator for nine different parameters, including coal seam and operational parameters and economic criteria. The coal properties and operating parameters that were varied included Young's modulus, Poisson's ratio,more » cleat porosity, and injection pressure. The economic variables included CH{sub 4}, price, Col Cost, CO{sub 2} credit, water disposal cost, and interest rate. Net-present value (NPV) analyses of the simulation results included profits resulting from CH{sub 4}, production and potential incentives for sequestered CO{sub 2}, This work shows that for some coal seams, the combination of compressibility, cleat porosity, and shrinkage/swelling of the coal may have a significant impact on project economics.« less