鄂尔多斯盆地宁县-合水地区长6、长7、长8储层裂缝差异性及开发意义

Differences of natural fracture characteristics and their development significance in Chang 6,Chang 7 and Chang 8 reservoir,Ningxian-Heshui area,Ordos Basin

根据露头、岩心及测井等资料,对比研究了宁县—合水地区长6、长7、长8储层裂缝发育特征的差异性,并讨论其开发意义。宁县—合水地区长6、长7、长8储层均发育构造裂缝和成岩裂缝两种成因类型,但各储层的裂缝产状、裂缝规模(纵向高度及平面长度)及裂缝发育程度存在较大差别。长6储层主要发育近东西向和近南北向两组裂缝,长7储层北东东—南西西向裂缝最为发育,其次为近东西向和近南北向,长8储层北东东—南西西向裂缝最为发育,其次为近东西向裂缝和北西西—南东东向;各储层均以发育高角度构造裂缝为主,但长6储层还发育一定数量中低角度泥岩滑脱裂缝,比例相对较大;在裂缝规模和裂缝发育程度上,长7储层裂缝最为发育,规模相对较小,长8储层裂缝发育程度最差,但具有较大的裂缝规模,长6储层裂缝发育程度和规模介于以上两者之间。由于长6、长7、长8储层裂缝发育特征的差异性,在现今地应力场作用下不同层位、不同产状裂缝开启能力会存在差异,因此在进行注水开发时,要根据各储层裂缝的发育情况,准确计算不同层位裂缝开启压力值,进而确定合理的注水参数,避免注水压力过大造成裂缝大规模开启。此外,通过计算长6、长7、长8储层脆性指数可知,长7储层脆性相对较大,且天然裂缝最为发育,适合采取体积压裂进行储层改造。

Based on the outcrops,cores and logging data,fractures developing characteristics of Chang 6,Chang 7 and Chang 8 reservoir in Ningxian-Heshui area were compared and their development significances were discussed.There are two kinds of fractures in Chang 6,Chang 7 and Chang 8 reservoir in Ningxian-Heshui area,i.e.tectonic fractures and diagenetic fractures,but the occurrence,scale and development degree of fractures in each reservoirs are of great difference.Chang 6 reservoir develops nearly E-W and S-N tectonic fractures.In Chang 7 reservoir the fractures in the direction of NEE-SWW are developed the best,followed by the E-W and S-N.In Chang 8 reservoir,the NEE-SWW fractures are dominant,followed by the nearly E-W and NWW-SEE.The tectonic fractures in each reservoir are all mainly high-angle,while Chang 6 reservoir also develops low-angle mudstone slippage fractures with relatively high percentage.On the development degree and the scale of fractures,Chang 7 reservoir is of the best development degree with relatively small scale and Chang 8 reservoir is of the worst degree with a big scale.The situation in Chang 6 reservoir is between the two.Because of the different fracture characteristics above,fractures of different occurrence in each reservoir have different opening pressure values under the action of present ground stress.Therefore,reasonable water injection parameters should be determined in order to avoid large-scale open fractures which were caused by high water injection pressure being formed during water flooding development.Moreover,the calculation results of brittleness index of each reservoir showed that Chang 7 reservoir has relatively high values.With the high rock brittleness index and well-developed fractures,Chang 7 reservoir is suitable for volumetric fracturing to modify the reservoir.