超深层裂缝性碳酸盐岩力学特性及其主控机制

Mechanical property and main control mechanism of ultra-deep fractured carbonate rocks

超深层海相碳酸盐岩储层中发育的天然裂缝有可能显著增加钻完井过程的井壁失稳和钻井液漏失风险,并且关于天然裂缝发育程度及裂缝微观特征对超深层碳酸盐岩力学性质的影响机制尚需系统深入研究。为了给超深层碳酸盐岩地层防塌防漏和酸压工艺技术改进提供实验依据,以四川盆地川西坳陷中三叠统雷口坡组碳酸盐岩储层为例,开展了三轴力学实验,结合CT扫描重构、裂缝充填物分析、裂缝面扫描成像和摩擦系数测试等成果,从裂缝产状、充填物和摩擦学特征等方面研究其力学特性。研究结果表明:①川西坳陷雷口坡组超深层碳酸盐岩储层天然裂缝多被高纯度方解石充填,岩石具有泊松比低、力学强度低且离散性强的特点;②雷口坡组岩样普遍发生高角度天然裂缝剪切破坏,破裂面倾角介于46°~80°,天然裂缝轮廓平整,裂缝面微凸体欠发育且坡度小,方解石充填物硬度低且胶结程度弱;③裂缝产状、充填物和摩擦学特征导致岩石抗剪切破坏能力弱、抗压强度整体偏低,裂缝面摩擦系数小是导致其抗剪切破坏能力弱的关键原因;④水基钻井液对方解石充填层的良好润湿性导致岩石强度进一步降低,且裂缝主控特征更为显著。结论认为,该研究成果可为超深层裂缝性碳酸盐岩储层井壁失稳控制和酸压设计优化提供更具针对性的基础指导。

Natural fractures developed in ultra-deep marine carbonate reservoirs can aggravate the risks of wellbore instability and circulation loss in the process of drilling and completion.What's more,the influence mechanisms of development degree and microscopic characteristics of natural fractures on the mechanical property of ultra-deep carbonate rocks need further studying.In order to provide experimental basis for improving the anti-collapse and anti-leak and acid fracturing technologies of ultra-deep marine carbonate reservoir,this paper carries out triaxial mechanical experiments by taking the carbonate reservoir of Middle Triassic Leikoupo Formation in the West Sichuan Depression of the Sichuan Basin as an example.Then,based on the results of CT scanning reconstruction,fracture filling analysis,fracture plane scanning image and friction coefficient test,its mechanical properties are studied from the aspects of fracture occurrence,fillings and friction characteristics.And the following research results are obtained.First,natural fractures in the Leikoupo Formation ultra-deep carbonate reservoir of the West Sichuan Depression are mostly filled with high-purity calcite,and the rocks have the characteristics of low Poisson's ratio,low mechanical strength and strong discretion.Second,the rock samples of Leikoupo Formation generally suffer shear failure from high-angle natural fractures,and the dip angle of failure surfaces is in the range of 46°-80°.The contour of natural fractures is flat,the micro asperities on the fracture surfaces are underdeveloped with low slope,and calcite fillings are of low hardness and weak cementation.Third,fracture occurrence,fillings and friction characteristics weaken the shear failure resistance of rocks and reduces the compression strength overall,and the low friction coefficient of fracture surface is the key cause of weak shear failure resistance.Fourth,the good wettability of water-based drilling fluid to calcite filling layers leads to the further reduction of rock strength,and fractures'control characteristics are more obvious.In conclusion,the research results provide basic guidance more specifically for wellbore instability control and acid fracturing design optimization of ultra-deep fractured carbonate reservoirs.