LAUSR

Abstract Coalbed reservoirs are considered as suitable media for injection operations, especially for geological sequestration of carbon dioxide (CO2). This type of reservoirs might be stimulated to enhance the efficiency of injection as the permeability of these formations is usually low (0.1–10 mD). The hydraulic fracturing technology is one of the effective methods to attain a greater injectivity. To design an efficient hydraulic fracture, it is vital to systematically investigate important changes (e.g., pressure and stresses) occurring over the injection process on the fracturing performance. An increase in the reservoir pressure is an inevitable occuerrence during the fracturing operation. In situ stresses will also vary due to the fluid injection. As a consequence, the permeability of coalbed will be altered during the reservoir life. In this research study, a novel model for the hydraulic fracture design is introduced to consider the changes in the reservoir permeability during the injection process. A Direct Boundary Element Method (DBEM), which is a fundamental concept in the Unified Fracture Design (UFD), is employed to determine the fracture injectivity. In this study, the pressure change is estimated based on the injectivity. The variations of in situ stresses are determined using a model, which uses the Boundary Element Method (BEM) and theory of inclusions. Permeability is then updated based on the new values of stresses and the design dimensions of the fracture for the new time steps. Therefore, the model presents a suitable design of hydraulic fracture by considering the reservoir pressure changes during the injection. The proposed method is implemented on Latrobe Valley brown coal. The results suggest that the reservoir life is an important parameter to optimize the hydraulic fracturing design. Fractures with the same width and different penetrations yield very different efficiencies during the injection. The highest amount of cumulative injection can be achieved after 10 years using a fracture, which seems to be not optimum at the initiation stage of the injection process. This study proposes that it is necessary to take into account the stress changes in the design of a hydraulic fracture system in a coalbed to attain the effective injection operation regardless of high penetrations.