酸压裂缝体形态与流动能力的控制因素——以鄂尔多斯盆地大牛地气田下奥陶统马家沟组马五5亚段储层为例

Controlling factors of morphology and flow capacity of acid-etched fracture system:A case study of the reservoir in the Ma5^5 sub-member of Ordovician Majiagou Formation in the Daniudi Gasfield of the Ordos Basin

立体改造技术是实现非均质碳酸盐岩油气藏立体开发的关键技术之一,而控制酸压裂缝体的形态和流动能力则是其核心。为了揭示控制酸压裂缝体形态和流动能力的主要因素,选取鄂尔多斯盆地大牛地气田下奥陶统马家沟组马五5亚段露头岩样,针对“滑溜水+高浓度酸液”酸压工艺,采用岩石力学真三轴耦合系统开展室内物理模拟实验;采用CT扫描技术来表征酸压裂缝体形态及获取裂缝宽度;在此基础上,研究了酸压裂缝体形态控制因素;然后,通过对裂缝系统的渗透率进行测试,分析了酸压裂缝体流动能力的控制因素。研究结果表明:①大牛地气田马五5亚段储层平均水平应力差异系数为0.25,酸压后能够形成复杂裂缝体;②酸液对裂缝条数的影响不明显,其对水力裂缝的溶蚀、疏通作用增加了裂缝宽度,酸蚀缝宽是酸液在水力裂缝主缝与分支缝、靠近注液端的水力裂缝与远端水力裂缝中竞争流动、反应的结果;③该气田马五5亚段储层进行酸压改造时的合理注酸强度为12.0 m^3/m,注酸排量应大于等于6.0 m^3/min;④进行立体酸压设计时,需要针对特定储层改造对象,确定注酸量的合理范围及最小合理注酸排量,进而优化注酸规模,以实现对储层的充分改造。结论认为,所取得的研究成果可以为致密碳酸盐岩储层立体酸压的优化设计提供指导。

Three-dimensional stimulation technology is one of the key technologies in the three-dimensional development of heterogeneous carbonate oil and gas reservoirs,and its core lies in controlling the morphology and flow capacity of acid-etched fracture system.In order to reveal the main factors controlling the morphology and flow capacity of acid-etched fracture,we selected outcrop samples from the Ma5^5 sub-member of Majiagou Formation,Lower Ordovician in the Daniudi Gasfield of the Ordos Basin,to conduct laboratory physical simulation experiments on the acid fracturing technology of"slick water+high-concentration acid"by using the true triaxial coupling system of rock mechanics.Then,CT scanning technology was used to characterize the morphology of acid-etched fracture system and calculate fracture width.On this basis,the main factors controlling the morphology of acid-etched fracture system were studied.Finally,the factors controlling the flow capacity of acid-etched fracture system were studied by testing the permeability of the fracture system.And the following research results are obtained.First,the average horizontal principle stress difference coefficient of the reservoir in the Ma5^5 sub-member of Majiagou Formation in the Daniudi Gasfield is 0.25,so a complex fracture system can be generated after acid fracturing.Second,the influence of acid on the number of fractures is not obvious,but its dissolution and communication action on hydraulic fractures increases the fracture width.The width of acid-etched fractures is the competitive flow and reaction result of acid between the main hydraulic fractures and the branches and between the hydraulic fractures near the acid injection entrance and the ones far away.Third,as for the reservoir in the Ma55 sub-member of Majiagou Formation in the Daniudi Gasfield,the reasonable acid injection volume per reservoir thickness of acid fracturing is 12.0 m^3/m,and the acid injection rate shall be no less than 6.0 m^3/min.Fourth,when designing three-dimensional acid fracturing,it is necessary to determine the reasonable range of acid injection volume and the minimum reasonable acid injection rate for a specific reservoir to be stimulated,and then optimize the acid injection scale so as to fully stimulate the reservoir.In conclusion,the research results can provide useful guidance for the optimal design of three-dimensional acid fracturing in tight carbonate reservoirs.