奥陶系深层碳酸盐岩酸压机理与深度酸压工艺优化

Acid-fracturing mechanism and deep acid-fracturing technique optimization of Ordovician deep carbonate rocks

奥陶系深层碳酸盐岩已成为长庆天然气勘探的重要领域,但与常规白云岩储层相比,奥陶系深层碳酸盐岩储层埋藏深、储层致密、灰质含量高,酸岩反应速度快,常规酸压改造酸蚀裂缝缝长短,单井产量较低,需开展酸压机理研究和深度酸压工艺优化,提高酸压改造效果。针对储层地质特征和酸压改造需求,通过开展酸岩反应动力学实验、酸蚀程度评价、酸压数值模拟,明确了不同白云石含量碳酸盐岩储层酸岩反应机理的差异性:随着白云石含量的降低,灰质含量的增加,反应速度加快,岩石表面酸蚀程度高,但酸蚀裂缝长度降低,应强化不同白云石含量碳酸盐岩储层改造的差异化设计。以“先造压裂缝、后酸蚀”的改造思路,优化形成了“前置液造缝、多体系酸液交替注入、缝内转向”为核心的深度酸压技术,通过酸液和压裂液共同作用,形成水力裂缝、酸蚀裂缝与基质溶孔连通的裂缝网络,现场试验取得较好增产效果。

Ordovician deep carbonate rocks have become an important field of natural gas exploration in Changqing.Howev⁃er,compared with the conventional dolomite reservoirs,the Ordovician carbonate reservoirs are deeply buried and tight with high limestone content and fast acid-rock reaction.Conventional acid-fracturing is confronted with short acid-etched fractures and low single-well production.Therefore,it is necessary to study acid-fracturing mechanism and optimize the deep acid-fracturing technique to improve the acid-fracturing effect.In accordance with the reservoir geological character⁃istics and the demand for acid-fracturing,the kinetic experiments of acid-rock reaction,the evaluation of acid etching,and the numerical simulation of acid fracturing were carried out to clarify the differences in the acid-rock reaction mechanism of carbonate reservoirs with different dolomite contents:With the decrease in dolomite content and increase in limestone content,the acid-rock reaction is accelerated with severer acid etching on the rock surface,while the length of acid-etched fractures witnesses a decline.Hence,the differential design of carbonate reservoirs with different dolomite contents should be strengthened.The deep acid-fracturing technology with the core of“hydraulic fracture generation by prepad fluid,alter⁃native acid injection by multi-system,and internal fracture turning”was optimized based on the idea of“forming hydraulic fractures first and then conducting acid etching”.Through the joint action of acid solution and fracturing fluid,a fracture network connecting hydraulic fractures,acid-etched fractures,and matrix dissolved pores was formed.In the field test,our technology has played a role in raising production.