Swelling of Coal in Response to CO2 Sequestration for ECBM and Its Effect on Fracture Permeability

Abstract

Summary The "swelling" of coal by a penetrant refers to an increase in the volume occupied by the coal as a result of the viscoelastic relaxation of its highly crosslinked macromolecular structure. Projects relating to CO2 sequestration in coal seams suffer a serious setback in terms of injectivity loss resulting from the swelling of coal. Volumetric swelling associated with CO2 sorption on coal has a significant influence on the fracture porosity and permeability of the coal. Two coal samples differing in rank were used for volumetric strain measurements. With CO2-, the high-rank Selar Cornish coal showed a maximum volumetric strain of 1.48% corresponding to an average pore pressure of 13 MPa. A matrix swelling coefficient (Cm) of 1.77× 10-4 MPa-1 was calculated for this Selar Cornish coal. The low-rank Warndt Luisenthal coal exhibited higher strain of 1.6%, and a matrix swelling coefficient (Cm) of 8.98×10-5 MPa-1 was calculated. The rank dependence of swelling holds true in this set of experiments. Repeat volumetric strain measurement on the same Warndt Luisenthal coal core shows higher volumetric strain values for all pressure steps. A volumetric strain of 1.9% corresponding to a mean pore pressure of 14 MPa was measured. This confirms the process of sequential swelling. A unique feature of this work is that real-time permeability measurements were done under unconstrained conditions. Permeabilities were measured, reducing the pore pressure from 16 to 1 MPa at constant flow rate. Although measured permeability increased with increasing pore pressure under unconstrained swelling, in-situ permeability will actually decrease because of fracture closure in a constrained coal. To validate the permeability swelling relationship, both permeability measurements under unconstrained conditions and volumetric strain measurements were used. Introduction Maturation of coalbed methane (CBM) production operations in some basins, the emergence of injection schemes for enhanced coalbed methane (ECBM), and carbon sequestration of greenhouse gases has led to renewed focus on the behavior of coalbed reservoir properties under these conditions. Cleat permeability of coal is the most important parameter for coalbed methane production. Being normal to the bedding plane and orthogonal to each other, the face and butt cleats in coal seams are usually subvertically oriented. Thus, changes in the cleat permeability are primarily controlled by the prevailing effective horizontal stresses that act across the cleats, rather than the effective vertical stress, defined as the difference between the overburden stress and pore pressure (Harpalani and Chen 1997). Coal swelling accompanying CO2 sorption would decrease the permeability of the coal as the volume increase is compensated within the fracture porosity.

Role of adsorption and swelling on the dynamics of gas injection in coal

“…It was further observed, that if an adsorbing gas is used, the matrix shrinks when significant desorption starts: this phenomenon counteracts the narrowing of the cleats resulting in some cases in a net permeability increase [Harpalani and Chen, 1997]. More recently, the effect of CO 2 injection on the permeability of coal samples was investigated with a high-pressure core flooding setup [Mazumder et al, 2006;Mazumder and Wolf, 2008]. The experiments were carried out at 45°C by imposing a constant effective stress on the sample.…”

“…The experiments were carried out at 45°C by imposing a constant effective stress on the sample. Injection of CO 2 resulted in volumetric expansion (swelling) of the sample and the observed increase in permeability with CO 2 pressure was attributed to the fact that the coal sample was allowed to expand freely inside the pressure cell [Mazumder et al, 2006]. Moreover, differential swelling was obtained by measuring the axial changes of the core dimension during the injection of CO 2 in a coal core, which has been presaturated with CH 4 , and its effects on the core permeability were estimated [Mazumder and Wolf, 2008].…”

High pressure gas flow, storage, and displacement in fractured rock—Experimental setup development and application

“…Syringe pumps are commonly used for pressurising the experimental gases. 33,34,16 In this work, however, adopting a gas booster was preferred due to its lower cost compared to the syringe pump. One limitation of the gas booster is that it has a limited capacity and providing the required volumes of pressurised gas might be time consuming, especially for the experiments with high gas demand.…”

“…The apparatus and approaches reported in the literature include (1) a True Triaxial Stress Coal Parameter (TTSCP) facility to measure the coal permeability to CO 2 using a quasi-steady flow method; 14,15 (2) a highpressure core flooding setup comprising a pressure cell, a syringe pump, and a micro gas chromatographer; [16][17][18][19] and (3) a high pressure triaxial apparatus capable of measuring the flow rate and deformation of the core samples. [20][21][22][23] Among the mentioned methods, the triaxial flooding approach has received more attention compared to other methods due to its capability to replicate the in situ stress conditions at different depths.…”

Discrete Fracture Modeling of 3D Heterogeneous Enhanced Coalbed Methane Recovery with Prismatic Meshing

“…In addition, COMET2 and SIMED II are specialized CBM simulators which are widely used in modeling primary and secondary CBM recovery processes. The later developed COMET3 [31] enables the triple-porosity dual-permeability option to model the cleat and large-scale fracture system. Nevertheless, all these simulators are in the context of a highly-conceptualized equivalent-continuum approach, which assumes fractures are evenly distributed and interconnected and can be represented by structured blocks.…”