碳酸盐岩酸压裂缝内净压力变化规律研究

Study on Changing Law of Net Pressure in Acid-Fracturing-induced Fracture of Carbonate Rock

酸化压裂是开发碳酸盐岩储层最常用的增产技术,而酸压裂缝内净压力是反映裂缝扩展特征、推断裂缝几何形态的重要参数。以水力压裂裂缝延伸机理为基础,基于酸岩反应理论,考虑酸岩反应导致的缝宽变化、CO_(2)气体膨胀、酸液滤失及岩石力学性质变化,结合PKN模型,建立酸压裂缝内的净压力计算模型并进行验证。研究分析酸压裂缝内净压力的影响因素及其规律,结果表明:距缝口距离越远,水力裂缝与酸压裂缝的缝宽均越小,酸液的溶蚀作用会使裂缝宽度增大。随着液体的滤失,离缝口越远液体越少,水力裂缝与酸压裂缝净压力逐渐减小,越靠近裂缝尖端净压力衰减越快。随着作用时间延长,酸压裂缝缝宽逐渐增大,前期随着大量CO_(2)生成,净压力迅速增大,随着酸液消耗与滤失增加,之后逐渐减小并趋于稳定。随着排量的增大,滤失增大,酸压裂缝缝宽及净压力均逐渐增大且增量越来越小。在黏度较小的情况下,滤失较大,净压力逐渐减小;在黏度较大的情况下,CO_(2)膨胀作用大于滤失作用,净压力逐渐增大。随着酸液浓度的增大,酸岩反应速率增大,反应生成CO_(2)增多,净压力逐渐增大。

Acid fracturing is the most commonly used stimulation technology to develop carbonate reservoirs,and the net pressure in acid-fracturing-induced fractures is an important parameter that reflects the fracture propagation characteristics and infers the fracture geometry.Based on the fracture extension mechanism of hy-draulic fracturing and the theory of acid-rock reaction,the calculation model of net pressure in the acid-fracturing-induced fractures is established and verified with the PKN model by considering the change in fracture width,CO_(2) gas expansion,acid filtration,and change in rock mechanical properties caused by the acid-rock reaction.Research and analysis of the factors affecting the net pressure in the acid-fracturing-induced fractures and rules show that the farther away from the mouth of the fracture,the smaller the width of both hydraulic fractures and acid-fracturing-induced fractures are,and the dissolution of the acid will make the width of the fracture increase.With liquid filtration,the farther away from the mouth of the fracture,the less liquid,the net pressure of hydraulic fractures and acid-fracturing-induced fractures gradually decreases,and the closer to the fracture tip,the faster the net pressure decreases.The width of the acid-fracturing-induced fractures gradually increases as the action time prolongs,and the net pressure increases rapidly as a large amount of CO_(2) is generated in the early stage,and then gradually decreases and tends to stabilize with the increase of acid consumption and filtration.With the increase of the discharge volume,the filtration increases,and the width of the acid-fracturing-induced fractures and the net pressure both increase gradually and the increment is getting smaller and smaller.In the case of smaller viscosity,the filtration is greater,and the net pressure gradually decreases.In the case of larger viscosity,the CO_(2) expansion effect is greater than the filtration effect,and the net pressure gradually increases.With the increase in acid concentration,the acid-rock reaction rate increases,the CO_(2) generated by the reaction increases,and the net pressure gradually increases.