AN ANALYTICAL MODEL TO DETERMINE A RESERVOIR CAPACITY FOR WASTEWATER DISPOSAL WITH CONSIDERATION OF GEOMECHANICS AND EXTRACTIONS

Abstract

In many scenarios, we need to determine the capacity of a reservoir for water injection or disposal. The capacity is typically controlled by the container size, total compressibility, and the maximum pressure above which the injection may jeopardize the formation integrity related to formation parting pressure. It can be potentially correlated to and sometimes even cause fault reactivation. Even though current studies only have correlations rather than causality to prove that SWD causes fault reactivation, it has been established that other factors like permeability, reservoir fluid, and mineralogy can also be potential factors causing seismic activities. Given that full-scale numerical modeling and simulation needs extensive data that isn’t usually available, we propose an analytical model to determine the maximum injection volume based on pressure superposition. This model provides a practical solution to calculate the reservoir capacity that can help fill knowledge gaps by removing human bias. In the proposed model, a reservoir container has homogeneous properties with boundaries that can be infinite acting, open, or closed. Multiple wells, including injection (disposal) wells and extraction wells, are arbitrarily placed in the reservoir. The model assumes that individual extraction rates are constant with time but may differ from well to well. A geomechanical model considering thermoelastic and poroelastic stresses determines the maximum injection pressure. An equation system can be established for injection wells to a given time based on the pressure superposition in the reservoir. The solution of the equation system is the rates for the injection wells. The analytical solution is validated by numerical simulation with 16 equally spaced wells in a 100 km by 100 km reservoir with closed boundaries, and the reservoir only contains water. We get the amount of water volume and pressure profiles at different times through numerical modelling. We use the determined rates from the equation system and known extraction rates to compute the pressure along the fault to see when the activation pressure is reached. With known rates and time, the reservoir's total capacity can be determined. The proposed analytical model is robust, easy to implement, and doesn’t need extensive subsurface characterization. Because of its high performance, the model can be used in uncertainty studies, such as Monte Carlo simulation, to determine the range of capacity with probability. This will make the model very attractive in practice. This geomechanical approach is a valuable tool that can provide optimal water injection pressure while minimizing the risk of formation integrity.