Dual Porosity Simulation of Gravity Drainage Mechanism Induced by Geological Acid Gas Storage in Naturally Fractured Reservoirs

Abstract

Acid gases, containing CO₂ and H₂S, are by-products of gas sweetening. Geological sequestration of these gases in naturally fractured reservoirs (NFRs) is a practical method to reduce greenhouse gas emission. An industrially accepted approach to simulate fluid flow in NFRs is the dual-porosity method; however, this method needs multiple parameters' specifications. The main goal of this study is to develop a dual-porosity model with improved parameters that can be used for simulation of both hydrocarbon gas gravity drainage and acid gas injection in the gas-invaded zone of NFRs. To do so, a single-porosity model, as the reference model, is constructed for a single matrix block (SMB) with which the equivalent dual-porosity model's (DP) parameters are determined and matched. Then, DP is improved by a dual-porosity vertical discrete (VD) model to consider gravity drainage. This was later enhanced by non-neighborhood connections (NNCs) to account for re-infiltration in stacked matrices, yielding comparable results to the reference CPU-intensive single-porosity simulation. A thorough sensitivity analysis is performed on acid gas injection in VD model. The results show that the most effective parameter is porosity. The permeability and NNC transmissibility only change the rate of acid gas storage and more acid gas is trapped as H₂S content increases. Also, the heterogeneous distribution of porosity only influences the rate of storage when the mean porosity is constant, while permeability heterogeneity does not affect acid gas storage. The recovery factor is considerably increased to nearly 100% when the acid gas replaces hydrocarbon gas in fractured surrounding. About 7000 kmole of acid gas is stored in SMB over 4.5 years. Similar results are obtained for stacked matrices, and trapped gas is about 22,000 kmole, after 9 years.