Numerical simulation study on differences in acid dissolution mechanisms between fractured dolomite and limestone reservoirs

The fractures often alter the acid dissolution patterns and affect the acid treatment effectiveness during acid treatments in fractured carbonate reservoirs. Based on the two-scale continuum model， scholars have conducted numerous studies on the acid dissolution mechanisms in porous and fractured carbonate reservoirs using numerical simulation methods. However， relatively few studies focused on the differences in how fractures influence the acid dissolution mechanisms in fractured dolomite and limestone reservoirs. The two-scale continuum model was extended， and an acid dissolution model suitable for simulating acid treatment in carbonate reservoirs with varying calcite and dolomite contents was developed. Then， the related research was conducted in conjunction with a pseudo-fracture model. Acid dissolution simulation results for dolomite and limestone reservoirs were obtained by changing the positions， lengths， and dip angles of fractures. The acid treatment effectiveness was evaluated using the acid dissolution patterns and breakthrough volumes， clarifying the influence of fractures on the acid dissolution mechanisms in dolomite and limestone reservoirs. This provided a theoretical foundation for the design and optimization of acid treatments in fractured dolomite and limestone reservoirs. The results indicate that the optimal acid pump rates for limestone reservoirs are higher than those for dolomite reservoirs. When the pump rates are lower than the optimal ones for dolomite reservoirs， the breakthrough volumes of the two reservoirs are similar. However， when the pump rates exceed the optimal ones for dolomite reservoirs， the breakthrough volumes in dolomite reservoirs remain consistently higher than in limestone reservoirs. The primary factor influencing breakthrough volume is fracture length， while the secondary factor is fracture dip angle. As fracture length increases and dip angle decreases， the breakthrough volumes decrease in dolomite and limestone reservoirs， whereas fracture position has little effect on breakthrough volume. Short fractures have a greater impact on dolomite reservoirs than on limestone reservoirs， but as fracture length increases， their influence on both reservoir types becomes more similar. In dolomite and limestone reservoirs， fractures at different positions， with various lengths and dip angles， attract acid flow， and wormholes consistently connect the fractures. Compared to acid-etched wormholes， the amounts of acid dissolution on the fracture walls are minimal. The influence of fracture dip angles on acid dissolution patterns is mainly reflected in the directions of the main wormholes. Fractures facilitate rapid acid breakthrough； few branches develop on the fracture walls when the fracture dip angles are small. However， fractures hinder rapid acid breakthrough， and numerous branches develop along the fracture walls as the fracture dip angle increases to 75° in dolomite reservoirs and 60° in limestone reservoirs.