Experiment Research on the Effect of CO2 on Acid-Rock Reactions in Fractured Reservoirs

To enhance acid fracturing performance in fractured reservoirs in the Tahe Oilfield,research has been conducted to highlight the impact of CO2 on acid rock reactions in such reservoirs.By using independently developed acid rock reaction instruments and a fracture network model,tests were conducted to study the impact of CO2 on acid rock reactions with different acids,viscosity and concentration.Test results showed that flow resistance of acid may decrease sharply as CO2 generated during acid rock reaction become supercritical,then dissolve in acid massively as back pressure increases.Compared with gelled acid and Type I surface cross-linked acid,Type II surface cross-linked acid with concentration of 20% has less filtering loss,higher viscosity and susceptibility to CO2 and lower acid rock reaction speed.This kind of acid may facilitate acid fracturing to achieve deep penatration in fractured reservoirs.A pilot test was conducted for Type II surface cross-linked acid in the acid refracturing of the Well YJY.Compared with the primary acid fracturing with gelling acid,both fluid efficiency and fracture length increased dramatically.With cumulative oil production of 1.2×104 t after refracturing,productivity enhancement performance has been observed.So research results showed supercritical CO2 could effectively reduce flow resistance of acid,enhance acid penetration and promote the performance of acid-fracturing of fractured reservoirs in the Tahe Oilfield.