CO₂ Sequestration in, and Residual Gas Recovery from, Depleted Shale Gas Reservoirs

The primary objective of this simulation study is to investigate the technical feasibility and to evaluate the operational practices and the system properties and conditions that can maximize concurrently the potential for (a) significant CO₂ sequestration in, and (b) substantial residual gas recovery from, depleted, ultra-low permeability shale gas reservoirs. Additionally, this study provides guidance on reservoir development and operational practices related to CO₂ sequestration in ultra-low permeability reservoirs. We use high-resolution gridding and full physics models to analyze fluid and heat flow and transport using a reservoir "stencil" — i.e., the minimum repeatable unit in a system of two parallel, multi-fractured horizontal wells. After the end of the primary gas production, supercritical CO₂ injection and residual natural gas production commence at two offset hydraulic fractures. The investigation accounts for all currently known flow and thermophysical processes and evaluates the effects of key properties and conditions (e.g., matrix permeability, initial aqueous saturation, water salinity, etc.) as well as operational factors that affect the dual goals of maximal CO₂ sequestration and residual gas production.

The conclusions from this study are that, under certain conditions: (a) the dual objectives of CO₂ sequestration in, and residual gas recovery from, depleted shale gas reservoirs are technically feasible, (b) the volumes of both the sequestered CO₂ and of the recovered residual gas can be substantial, (c) significant mixing of the produced residual gas with the injected CO₂ can be significantly delayed. These results can support the commercial viability of this approach. The rates of CO₂ sequestration and residual gas recovery increase (i) with a decreasing initial aqueous phase saturation S_A and (ii) with an increasing reservoir permeability that should not reduce gas recovery to impractically short times, and (iii) is adversely affected, but to a negligible degree, even by high levels of salinity. The potential benefits of the alternative STOP-PRO method are unclear, and can only be evaluated on a case-by-case basis.