A criterion for evaluating the efficiency of water injection in oil sand reservoirs

Steam Assisted Gravity Drainage (SAGD) has been implemented at a large scale in the development of ultra-heavy oil reservoirs in Karamay city, the Xinjiang province of China. Before circulating steam, byproduct water from previous SAGD projects is injected into a SAGD wellpair in order to create a dilative and highly permeable zone in the inter-well domain. A criterion is desired to evaluate how efficient this injection/stimulation has been performed. The criterion was based on pore pressure distribution in the domain encompassing the dual wells, which was the outcome from finite element analysis using the Drucker-Prager model to describe oil sand. The analysis used constitutive parameters calibrated by laboratory results and the real-time injection data input as loading conditions. Based on the simulated pressure profile, communication parameter (CP) was proposed to represent the efficiency of water injection, with a magnitude varying from 0 to 1. It was found that CP gradually and continuously increased with the injection process, demonstrating that the efficiency increases with time and pressure. Moreover, by monitoring the time point at which the injection rate dramatically increased in the field, a group of threshold CPs were established based on the previously completed stimulation projects on a variety of oil sand reservoirs. The criterion was then fulfilled by comparing the calculated CP to the threshold CP of a specific reservoir type. By applying the criterion in the stimulation projects of two wellpairs in the field, it was observed that immediately after the CP exceeds the threshold value of the corresponding reservoir type, the pressure of the dual wells was quickly responsive to each other and the injection rates sharply increased. The preheating periods of the two wellpairs decreased from conventionally 180-300 days to 60 and 90 days, implying the success of reservoir stimulation. The proposed criterion can not only be used to quantitatively describe how efficient a water injection process has achieved, but also to determine whether the stimulation of an oil sand reservoir has been successfully accomplished. When the criterion is satisfied, the field engineers can confidently move on to the steam circulation stage.