ABSTRACT:

To monitor a geomechanical response of injecting CO2 into relatively shallow coal seams, tiltmeters were set as an array to cover the ground surface area surrounding the injection well, and to measure the ground deformation during a well fracturing stimulation and a short-term CO2 injection test. In this paper, an attempt to establish a quantitative relationship between the in-situ coal swelling and the corresponding ground deformation was made by means of numerical simulation study.

1. INTRODUCTION

It is nowadays internationally well recognized that anthropogenic emission of CO2 which is the main green house gas causes global warming. CO2 geological storages are developed as the timely and feasible technological main force to mitigate the atmospheric carbon dioxide concentration. The CO2-enhanced coalbed methane (ECBM) recovery, in which CO2 is sequestrated into unmineable coal seams for a geologically significant time, is an economically advantageous developing technology as it enhances the value-added natural gas production and offsets the CO2 sequestration cost [1]. Although the pressure-depletion scenarios such as oil and gas reservoir compaction and the land subsidence induced by oil and natural gas production have been abundantly studied [2], geomechanical aspects during CO2 geological injection, however, still remains as a fresh subject that hasn't been sufficiently studied. However, understanding the mechanisms of the swelling in CO2 adsorbed coal and its induced ground surface deformation is very important for better evaluating and real-time monitoring the environmental safety of CO2 sequestration into coal seams. In this paper, an attempt to establish the relationship between underground coal swelling and corresponding ground surface deformation has been made by numerical simulations using FLAC, FLAC3D and GEM. Both the hydraulic fracturing and the CO2 injection into coal induced ground deformations were modelled. Geomechanical simulations using Itasca's FLAC/FLAC3D were combined with reservoir flow simulation on the transport and sorption of gases within the coal seam using CMG's GEM. Thermal expansion is used as an analogy of coal swelling. Through the thermomechanical, hydromechanical and geomechanical simulations of stresses/deformations through their linear elastic superposition, the coal swelling under in-situ confinements and its induced ground heaves were studied. After the short-term CO2 injection history was well matched, the determined model is expected to be capable of preliminarily predicting the ground surface heave and estimating the maximum ground displacement that possibly occurs during a long-term CO2 injection process. Reversely, based on plenty of measured data from the ground surface deformation in in-situ tests and the coal swelling in experimental samples, we put a numerical simulation strategy into practice to study the in-situ coal swelling itself and its related geomechanical behavior during CO2 injection.

2. GROUND SURFACE DEFORMATION MEASURED BY TILTMETERS

In Alberta, there was a CO2-ECBM/sequestration multiwell pilot test on low-rank coal and relatively shallow reservoir. As a monitoring means, surface tiltmeters were deployed around the drilled injection well as shown in Figure 1. A tiltmeter is an instrument designed to measure very small ground surface changes from the horizontal level. The continuous data were downloaded monthly.

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