Water Injection Optimization Using Geomechanics and PWRI Fracture Modeling, South of Sultanate of Oman

Abstract

In the quest to increase production in parallel with reservoir pressure maintenance and to reach the ultimate recovery factor, PDO has been developing the fluvial stacked sandstones using water injection concept under matrix condition. The reservoir is good quality sandstone formation with average porosity (14-24 %) and permeability (20-500mD). The reservoir has weak aquifer support and develops with peripheral water injection. The target VRR is 1 was maintained fairly well in the area in the past few years, until Aug 2020 when few water injectors faced injectivity issues mostly related to the formation itself rather than water quality, so there is a need to compensate for the low cumulative voidage replacement ratio (VRR). Previous study done in year 2015 recommended matrix injection in peripheral mode with possibility to go for controlled fracture injection mode with the need to understand the timeline for fracture propagation to reach the producer wells. Geomechanical modeling results were integrated with well and reservoir management (WRM) activities and surveillance technologies to optimize the injection strategy for improved waterflood performance. Formation fracture pressure data were used to provide guidance on maximum allowable injection pressure in injectors to manage the risk of induced fracture growth. A Produced Water Re-Injection (PWRI-FRAC) fracture modeling and analysis was performed to determine the potential fracture dimensions to provide input to development decisions of injection rate and water quality. Field data utilized to generate different scenarios of PWRI-FRAC fracture model containment and uncontainment in which the contained fracture scenario indicates that injection higher rates >400 m3/day and TSS=140 ppm, would result in fracture half-length exceeding 1/3 of PI distance towards the end of field life. For small thickness units, models show low risk of fracture growth for rates 100 and 200 m3/Day and TSS=18 and 80 ppm. The rate of fracture growth is primarily influenced by water quality and injection rate. Mapped well spacing for critical wells are around 800-1000m away with orientation towards NE-SW direction, fracture growth modeling results give indication that water injectors are less likely to interact or result in short circuiting with producers if injection with a rate is less than 400 m3/Day and less than 140 ppm of TSS and this is also in line with SH max direction (NE-SW) as seen from drilling induced fractures interpretation from borehole images. Fracture injection is feasible as long as the injection rate and water quality are operated under recommended limits (rates <400 m3/Day and TSS< 140 PPM) and this will significantly impact the field waterflood performance. Results provided inputs to reservoir simulations and injection rate envelope. The study benefits the field to minimize risk of injector producer short-circuiting for improved waterflood management.