Analytical and numerical study on radius of investigation in composite reservoirs

Radius of investigation (ROI) is of great significance in well testing and reserve estimation. The studies on ROI over the past decades were mainly focusing on homogeneous reservoirs. In heterogeneous formations, such as composite reservoirs and fractured reservoirs, the problem remains unsolved. Few ROI evaluation methods for composite reservoirs have been validated and commonly accepted. This paper conducts extensive analytical and numerical analysis on the pressure propagation of composite reservoirs, analyzing the limitations and misunderstandings of previous methods on the evaluation of ROI. The pressure solution of a two-zone composite reservoir is presented in the Laplace domain and is used to analyze the impact of heterogeneity parameters on pressure transient propagation and the evaluation of radius of investigation. A new ROI formula for composite reservoirs is proposed, which is in great agreement with the analytical solution. The study reveals that the inner zone of a composite reservoir cannot be treated as homogeneous when the pressure front approaches the interface of the two zones with high heterogeneity. A large mobility ratio will slow pressure propagation in composite reservoirs, while a low storativity ratio will increase the value of ROI. Reservoir simulation and real case studies are used to demonstrate the findings of the analytical study. The new study sheds insights into the pressure propagation and eases the calculation of ROI in composite reservoirs.