Application of Optimized Differential Acid Fracturing and Acidizing for Mixed Injection-Production Well Patterns in Low-Permeability Carbonate Reservoir

Reservoir M in Oilfield B is a carbonate reservoir characterized by medium to high porosity and low permeability, with 40% of its porosity exhibiting poor connectivity. Having been in production for nearly 12 years, this reservoir employs a reverse 9-spot injection-production well pattern with vertical wells at the structural top and primarily horizontal wells in a linear injection-production pattern at the edges. The current development faces major challenges: (1) difficulty in establishing an effective drive system, low production and injection capacity in vertical wells, and severe pressure build-up in horizontal injection wells; (2) poor water injection efficiency and significant remaining oil accumulation under the existing well pattern. This paper proposes multi-scale acid fracturing and acidizing technology for different well types as a feasible option for further developing tight carbonate reservoirs. Firstly, a multi-scale acid fracturing design is implemented for production and injection wells dominated by vertical wells to create fractures of varying sizes suitable for the five-spot injection-production well pattern, thereby enhancing productivity and injectivity, shortening the distance between producers and injectors, and establishing a better displacement system to ensure optimal water drive efficiency. For horizontal well production areas, a variable-density flow-limiting screen deep acidizing process is proposed, which improves staged acidizing injection volumes by varying hole density and diameter to optimize profiles, thereby reducing seepage resistance around the wellbore and improving connectivity. Sector models for both vertical well and horizontal well areas are established to analyze the impact of fracture scale, conductivity, and effective time. Based on laboratory and field experimental results, key parameters for this technology and process scheme are optimized and recommended. Dynamic and well test interpretation results after pilots are used to establish a numerical dynamic model for the full reservoir to evaluate and predict the effects before and after implementing multi-scale acid fracturing. M-351 successfully achieved 8 times increase in PI, nearly 240 MB of incremental oil, a half-fracture length of 61.3 m, conductivity of 18,800 mD·m, and a total acid consumption of 200 m3. The results indicate that after acid fracturing, daily oil production increased by 2 to 5 times with rapid reservoir pressure recovery, and the estimated recovery factor can be improved by 3.6%.