Optimisation and Design for WAG-CO2 Combined with Soft Micro Gel SMG in Complicated Carbonate Reservoirs Containing with High Permeability Streaks

摘要

Carbonate reservoirs in the Middle East are highly heterogeneous with great reservoir physical property difference in longitudinal direction, water breakthrough easily caused by high permeability streaks when the reservoirs were put into commingled production. This paper concerns a highly layered gas cap carbonate reservoir with an Upper zone and a Lower zone, which has been developed for nearly 60 years with recovery factor by 16% and water cut by 12% and remained about 52% recoverable reserve in tight zones. Nearly 40% undeveloped reserves of the oilfields lied in low permeability reservoirs, which has low well productivity with vertical well and achieved low sweep efficiency by waterflooding due to high permeability streaks. Horizontal well technology has been or will be implemented for these low permeability reservoirs to enhance well productivity in succession and WAG-CO2 combined with Soft Micro Gel (SMG) (140°C temperature resistance, 300,000 mg/L salinity tolerance) can plug off large pore throat and high permeability streaks. The paper completed preliminary screening of CO2 WAG process for the reservoirs in B oilfield based on reservoir parameters, empirical principal, laboratory physical simulation experiment and numerical simulation. The paper optimized horizontal well pattern for the reservoirs simultaneously for horizontal waterflooding and the CO2 WAG process. Multi-parameter sensitivity analysis has been completed to establish the empirical model for the relationship of well spacing and recovery factor by CO2 WAG process. Simulation models also have been built up with relevant data of the reservoirs to provide the optimum ratio of CO2 to water, CO2 injection timing and CO2 injection rate. The paper also compared WF and CO2 WAG combined with Soft Micro Gel (SMG) through modelling result. Soft Micro Gel (SMG) can assist CO2 WAG to achieve better results and enhance vertical sweep efficiency. CO2 WAG combined with Soft Micro Gel (SMG) can enhance recovery factor by 12% to 18% for the carbonate reservoirs containing with high permeability streaks and is the most effective EOR method whether or not previous water flooding. The study in the paper provides important EOR reference for other similar reservoirs in Middle East to achieve high ultimate RF after WF.