SAGD Operation in Interbedded Sands with Application of Horizontal Multistage Fracturing: Geomechanics and Fracturing Aspects

摘要

Steam assisted gravity drainage (SAGD) is a widely used thermal recovery method. During steam injection in interbedded sands, the portion of the deposits above the vertical permeability barriers is likely not recoverable with current practices. This paper discusses some of the geomechanics and fracturing aspects of using multistage fracturing in SAGD operations in interbedded sands such as IHS, where the vertical permeability barriers impede the gravity drainage process. The first part of this work (to be presented in a separate manuscript) covered the reservoir engineering aspects of this method, where it was assumed that it is possible to successfully create vertical fractures in the oil sands and through shale layers. In this paper, the geomechanical challenges in implementing the method and after the start of operation are discussed, starting with a review of relevant hydraulic fracturing field trials in the oil sands. Some interface crossing criteria are reviewed and the extended Renshaw and Pollard criterion is used to study fracture propagation through sand-shale interfaces. These results are then compared to results of a numerical simulation using a coupled reservoir-geomechanics-fracturing simulation software. It is shown that tensile hydraulic fractures can be created in the oil sands, and fractures can cross sand-shale interfaces and be filled with desired concentration of proppant. It is shown that small but high conductivity fractures that are required for the success of the proposed method, can be generated by using currently available proppants and fluid systems. After start of the steam injection, the conductivity of the fracture might be reduced by various mechanisms such as proppant embedment. It is shown that the reduction due to embedment would not be significant at high range of proppant concentrations and also due to the low stress regime in the Alberta oil sands. Furthermore, the design considerations for various liner systems that can combine the fracturing process and the SAGD phase are discussed and a few new designs are proposed. Finally, a step by step roadmap for field implementation of this method is presented. The novelty and main contribution of this paper is the analysis and design considerations of the application of multistage fracturing in SAGD process in the field. Our results indicate that the field execution is feasible, and a field trial of the technique is justified.