基于“扩缝串洞”理念的超深层小缝洞群碳酸盐岩储层改造数值模拟

Numerical simulation of stimulation in ultra-deep small-scale fractured-vuggy carbonate reservoirs based on the concept of"fracture expanding and vug connecting"

随着四川盆地和塔里木盆地碳酸盐岩油气藏勘探开发逐渐迈向超深层领域,受地质条件影响,储层由大型缝洞向以厘米—毫米级小缝洞群转变,以往针对大型缝洞储层改造的酸压技术系列难以适应小缝洞群储层。为充分释放超深层小缝洞群碳酸盐岩储层的天然气产能潜力,基于小缝洞群碳酸盐岩储层特征,构建了多尺度离散缝洞方法,建立了考虑岩石变形、裂缝延伸、酸液流动反应、温度变化的热流固化多场耦合数值模型,最后提出了以多级转向扩面积、精准控黏串缝洞、定点生酸扩缝长为核心的“扩缝串洞”缝控改造理念,并对小缝洞群碳酸盐岩储层“扩缝串洞”缝控改造方式进行了优化分析。研究结果表明:①多级交替注入形成的黏性指进现象可以促进缝控改造面积的增加和裂缝非均匀刻蚀,从而提高改造后的产能,且缝控改造效果最佳;②酸液注入黏度比增加,缝控改造无因次产能指数呈现先增大后减小的规律,当黏度比超过30,缝控改造无因次产能指数开始迅速减小,缝控改造中酸液交替黏度比需控制在25~35区间;③酸液用量越大,缝控改造面积及改造后无因次产能指数越大,但对于一定地层参数和单井控制面积的缝控改造存在极限酸液用量,超过该酸液用量后缝控面积趋于极限值,而无因次产能指数仍随酸液用量的增加而稳定增大。结论认为,提高缝控改造面积和激活动用小缝洞群是小缝洞群碳酸盐岩储层提产的关键,该认识为超深层小缝洞群碳酸盐岩气藏的高效开发提供了理论指导。

As the exploration and development of carbonate oil and gas reservoirs in the Sichuan Basin and the Tarim Basin steps into ultra�deep field,the ultra-deep geological conditions lead to the reservoirs transforming from large-scale fractured-vuggy type to small-scale(centimeter to millimeter scale)fractured-vuggy type.However,the previous acid fracturing technologies for the stimulation of large-scale fractured-vuggy reservoirs are not suitable for small-scale fractured-vuggy reservoirs.In order to sufficiently give full play to the natural gas productivity of ultra-deep small-scale fractured-vuggy carbonate reservoirs,this paper establishes a multi-scale discrete fracture-vug method based on the characteristics of small-scale fractured-vuggy carbonate reservoirs.Then,a thermal-hydraulic-mechanical-chemical field coupling numerical model is established,which takes into account rock deformation,fracture propagation,acid flow and reaction,and temperature change.Finally,the"fracture expanding and vug connecting"concept with multi-stage diversion for expanding area,accurate viscosity controlling for connecting fractures and vugs,and fixed-point acid etching for increasing fracture length as the cores is put forward,and the fracture controlled stimulation mode of"fracture expanding and vug connecting"for small-scale fractured-vuggy carbonate reservoirs is optimized and analyzed.And the following research results are obtained.First,viscous fingering caused by multi-stage alternative injection can promote the increase of fracture controlled stimulation area and the non-uniform etching of fracture,thus increasing the dimensionless productivity index and achieving the best fracture controlled stimulation effect.Second,as the acid injection viscosity ratio increases,the dimensionless productivity index of fracture controlled stimulation increases first and then decreases.When the viscosity ratio exceeds 30,the dimensionless productivity index begins to decrease rapidly.Therefore,the alternation viscosity ratio of acid in fracture controlled stimulation shall be controlled between 25 and 35.Third,the larger the acid consumption,the greater the fracture controlled stimulation area and dimensionless productivity index.For certain reservoir parameters and single well controlled area,however,there is an acid consumption limit for fracture controlled stimulation.When acid consumption exceeds this limit,the fracture controlled stimulation area approaches to a limit value,but steady increase is still observed in dimensionless productivity index.In conclusion,the key to stimulate small-scale fractured-vuggy carbonate reservoirs is to increase the fracture controlled stimulation area and activate small fractures and vugs.The research results provide a theoretical guidance for the efficient development of ultra-deep small-scale fractured-vuggy carbonate gas reservoirs.