Unconventional/complex reservoirs

Fracture-Matrix Modeling Technique Unlocks CO2 Enhanced Shale Gas Recovery

The paper presents a model for shale gas production in which CO2 is injected by huff ’n’ puff into a hydraulic fracture surrounded by a shale matrix.

Gas recovery (left) and total gas pressure (right) vs. time
Fig. 1—Gas recovery (left) and total gas pressure (right) vs. time for cases with different mechanisms. Cyclic CO2 injection/CH4 production cycle of 200 days each.

With technology available at the time of writing, only 3–10% of gas from tight shale is recovered economically through natural depletion, demonstrating a significant potential for enhanced shale gas recovery (ESGR). Experimental studies have demonstrated that shale kerogen/organic matter has a higher adsorption affinity for carbon dioxide (CO2) than methane (CH4). CO2 is preferentially adsorbed over CH4 with a ratio of as much as 5:1. The complete paper examines CO2 ESGR in compressible shale during huff ’n’ puff injection to understand better the parameters controlling its feasibility and effectiveness. The authors present a mathematical model in the complete paper in which the CO2/CH4 substitution mechanism is implemented in an injection/production setting representative of field implementation.

Introduction

Modeling of CO2 injection and the interplay between CO2 and CH4 sorption has been extremely challenging for scientists and engineers. The presence of CO2 with methane during the CO2 ESGR process makes gas-desorption behavior and measurement more difficult. Few researchers have evaluated the efficiency of CO2 ESGR in compressible shale.

To improve the understanding of this technique, the authors present a numerical modeling approach using a 1D+1D fracture-matrix model in order to study the feasibility of CO2 injection in shale formations.

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