基于能量演化的超深层高温页岩脆性评价方法

深层/超深层页岩气是未来勘探开发重点攻关对象。目前川东南某页岩气井的最大钻探垂深已达6 600 m,地层温度高达180 ℃。为揭示高温页岩力学性质及脆性特征,以该井垂深为5 942.34～ 5 951.75 m的龙马溪-五峰组真实岩心开展180 ℃实时高温三轴压缩实验,提出了基于矿物组分权重的能量演化脆性评价方法,并以此评价其脆性。该方法同时考虑岩石矿物组分和破坏过程中的能量演化,能更准确地定量评价高温岩石脆性。结果表明:页岩抗压强度随温度/围压的增加而增加,弹性模量、泊松比受温度/围压的影响较小;破坏模式随围压的增加由脆性张剪性贯穿多裂纹破坏向脆塑性单裂纹剪切破坏转变;计算得到该超深页岩层段脆性指数高,表明其具备形成复杂缝网的物质基础。

An Evaluation Method of Rock Brittleness for the Ultra-Deep and High-Temperature Shale Based on Energy Evolution

Deep / ultra-deep shale gas is a crucial target for future exploration and development. Currently, the maximum drilling vertical depth of some shale gas wells in southern Sichuan reaches 6 600 meters, and the formation temperature reaches 180 ℃. Triaxial compression experiments with a real-time maximum temperature of 180 ℃ are carried out using actual shale cores from the Longmaxi-Wufeng formation with a depth from 5 942.34 to 5 951.75 meters below the surface, which is found in a region of southern Sichuan. A brittleness evaluation method based on the weight of mineral components and energy evolution is employed to reveal the mechanical properties and brittle characteristics of ultra-deep and high-temperature shale. The formation of brittleness is assessed based on the process. This approach allows for a more precise quantitative assessment of rock brittleness because it considers both the composition of the rock's minerals and the development of energy during the whole failure process in a high-temperature environment. It is shown that the compressive strength of shales within the experimental setting temperature and confining pressure increases with temperature and/or confining pressure. The elastic modulus and Poisson's ratio are less affected by temperature and confining pressure. With the increase of confining pressure, the failure mode of shale changes from brittle tensile-shear multi-crack failure to brittle-plastic single-crack shear failure. The calculated brittleness index of the ultra-deep shale interval is high, indicating that it has the physical basis for forming complex fracture networks.