Storage CO2 in shale formation is considered as a promising option to reduce CO2 emissions and enhance shale gas recovery. Many simple analytical and semi-analytical techniques have been proposed to support screening analysis and performance assessment for potential carbon sequestration sites. However, these analytical techniques have ignored the effect of fracture occurrences, which are important to the carbon sequestration in shale formation.

In this paper, numerical simulation technology is applied to model different complex hydraulic fracture occurrences and evaluate the CO2 seepage rule during carbon sequestration. First, multi-component Langmuir isotherm is applied to simulate the adsorption desorption phenomenon of CO2. Combining Langmuir isotherm with the reservoir parameters of Barnett shale formation, shale reservoir simulation model is constructed to simulate the CO2 seepage law during the CO2 sequestration. Then, based on the shale reservoir simulation model, local grid refinement technology is used to characterize the five typical fracture occurrences caused by hydraulic fracturing. Subsequently, the numerical simulation cases with five different fracture occurrences are run to evaluate the performance during CO2 storage process. Finally, some critical important parameters, including engineering and geologic parameters, are evaluated through the sensitivity study. In order to identify and illustrate the performance which progressively occur over time, the log-log plot of CO2 injection rate versus time is adopted.

The simulation results indicate that the flow regimes of CO2 injection can be classified into eight flow regions. Among these flow regions, the inner boundary dominated flow regime is a unique one which can occur only when realizing strongly stimulated reservoir volume. Further, the sensitivity study indicates that the fracture topologies and the primary fracture conductivity are the two key factors dominating the early-time flow behavior during the carbon sequestration. And the final-time flow behavior is dominated by the shale reservoir parameters.

This work systematically analyzes the effect of fracture topologies on carbon sequestration and enlarges our knowledge of CO2 storage in shale gas systems.

1. Introduction

Shale gas has become an increasingly important source of natural gas (CH4) in the United States over the last decade. Due to its unconventional characteristics, injecting carbon dioxide (CO2) to enhance shale gas recovery (CO2-EGR) is a potentially feasible method to increase gas-yield while realizing CO2 sequestration (CS). Therefore, it is more and more important to quantitatively demonstrate the influence of various reservoir and completion parameters on performance of multiply-hydraulic fractured horizontal wells (MFHW) in the process of CO2 injection.

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