Simulating Carbon Dioxide Sequestration/ECBM Production in Coal Seams: Effects of Permeability Anisotropies and the Diffusion-Time Constant

Abstract

Coalbed methane now accounts for a significant fraction of domestic natural-gas production. Injection of carbon dioxide (CO 2 ) into coal seams is a promising technology for reducing anthropogenic greenhouse-gas emissions and increasing ultimate production of coalbed methane. Reservoir simulations are an inexpensive method for designing field projects and predicting optimal tradeoffs between maximum sequestration and maximum methane production. Optimum project design and operation are expected to depend on the anisotropy of the permeability along the face-cleat and butt-cleat directions, the spacing between cleats, and the sorption isotherms for methane and CO 2 .In this work, a dual-porosity coalbed-methane simulator is used to model primary and secondary production of methane from coal for a variety of coal properties and operational parameters. It is assumed that the face and butt cleats are perpendicular to each other, with horizontal wells parallel to one type of cleat and perpendicular to the other. The well pattern consists of four horizontal production wells that form a rectangle, with four shorter horizontal wells centered within the rectangle. In the limiting case of no permeability anisotropy, the central wells form a "plus" sign within the square of production wells. All wells are operated as producers of methane and water until a specified reservoir pressure is reached, after which the central wells are operated as injectors for CO 2 . Production of methane continues until the CO 2 concentration in the produced gas is too high.The simulation results predict the optimum lengths of the injection wells along the face-and butt-cleat directions and show how these optimum lengths depend on the permeabilities in the two directions. If the cleat spacing is sufficiently small, and diffusion of the gas through the pores to the cleats is sufficiently rapid, instantaneous sorption may be assumed. Otherwise, the field performance depends on the diffusion-time constant that characterizes the rate of transfer between the cleats and the coal matrix. The pressures at which the injection wells are operated also affect the amounts of CO 2 sequestered through the pressures and volumes of the sorption isotherms.

Competitive Sorption of Methane/Ethane Mixtures on Shale: Measurements and Modeling

“…It is, in general, a challenge to model sorption in natural materials, such as shale, because of their heterogeneous nature and hierarchical pore structures (manifested, typically, by multimodal pore size distributions), which makes the representation of competitive sorption phenomena in these systems quite difficult. Past modeling efforts, for example, have attempted to describe multicomponent gas sorption in shales from pure component isotherm data via the use of the extended Langmuir model (ELM), and also the ideal adsorbed solution (IAS) theory, the main motivation behind such efforts being the simplicity and low/moderate computational cost of these commonly utilized methods. − However, it has been demonstrated that neither of these models is capable of accurately describing the multicomponent sorption behavior that is relevant to coalbed methane recovery processes. , Here, we test, instead, the accuracy of the multicomponent potential theory of adsorption (MPTA) approach for the calculation of gas sorption on shale. The choice of the MPTA method, which originated from the potential theory concept suggested by Polanyi, is because it directly models excess adsorption data, which is a key advantage, while ELM/IAS require that one converts the excess adsorption data into absolute adsorption, and that entails significant, and completely untested (for the CH 4 /C 2 H 6 pair in shale) assumptions about the density of the adsorbate layer.…”

Field-Project Designs for Carbon Dioxide Sequestration and Enhanced Coalbed Methane Production

“…Many factors affect sequestration in coal seams and enhanced production of coalbed methane. − Among these are sorption isotherms 26 and any anisotropy of the coal permeability . Cleat permeability and seam thickness are very important properties of coal because they may determine the profitability of a coal seam sequestration project. , However, Eastern coal seams often are very thin; for them the thickness of the seam has little effect on the flow patterns because gravitational effects are small, and thickness effects need not be studied in any detail.…”

Development of Empirical and Artificial Neural Network Model for the Prediction of Sorption Time to Assess the Potential of CO2 Sequestration in Coal

“…Several studies have indicated that sorption time can significantly affect the sequestration potential of a coal seam. ,– Inaccurate estimation of the sorption time may lead to an unreliable assessment of the CO 2 injection behavior and gas production. Despite this, sorption time is often disregarded in the case of CBM project’s feasibility studies, as coals are assumed to show equilibrium desorption behavior in which release of sorbed gas is instantaneous .…”