Study on Boundary of Control Models and Mechanisms of Acid Rock Reaction in Carbonate Reservoirs with Different Dolomite Mass Fraction

Abstract

AbstractThe understanding of acid rock reaction kinetics is the basis for the proposal of carbonate rock acidizing and acid fracturing. So far, the study of acid rock reaction kinetics in carbonate reservoirs is mostly focused on limestone reservoirs. The difference in acid rock reaction characteristics between limestone and dolomite reservoirs is obvious. In order to clarify the boundaries of control models and mechanisms of acid rock reaction in dolomite reservoirs, to guide the optimization of acidizing and acid fracturing proposal for carbonate reservoirs with different dolomite contents.The experimental study of acid rock reaction kinetics was completed with carbonate cores with different dolomite mass fraction, and the difference of acid corrosion mechanism between limestone and dolomite was analyzed by scanning electron microscope.The dolomite acid rock reaction kinetics equation under the control of different factors was established, and it was clarified that the temperature 90°C and the rotation speed 500r/min are the boundary of the surface reaction and mass transfer control mode. The study found that under the same experimental conditions, the acid rock reaction rate of dolomite is much lower than that of limestone; as the dolomite mass fraction increases, the acid rock reaction rate decreases rapidly, when the dolomite content exceeds 75%, the reaction rate of acid rock is reduced to the lowest and tends to be stable. Limestone is dominated by surface dissolution, with good dissolution effect, fast dissolution rate; dolomite is dominated by point dissolution, with holes on the surface, poor dissolution effect and low dissolution rate. Compared with limestone, the particle size of dolomite crystal is much larger, and the specific surface area is small, which is also the reaction rate of acid rock in dolomite is slower than limestone, this is also a main reason why the reaction rate of dolomite acid rocks is slower than that of limestone. So the acidizing or acid fracturing methodology of the dolomite reservoir is different from that of the limestone reservoir. The acid rock reaction rate and hydrochloric acid concentration should be appropriately increased within the allowable range to improve the uneven etching of the fracture wall, thereby increasing increase the conductivity of acid fracture.