Mudrock strength parameters are required to improve the prediction of fracture gradient during drilling in poorly consolidated formations. Understanding the influence that mineral composition and consolidation stress have on the mechanical properties of mudrocks aids safe well design. Small changes in the mudrock clay content are shown to have significant effects on two important engineering parameters: the lateral stress ratio at rest, K₀, and the critical state effective friction angle, '_cs. A laboratory-testing programme using quartz silt and a Gulf of Mexico clay mixture investigates the effects of mudrock clay content on K₀ and '_cs at two stress levels: 1 and 10 MPa. Values of K₀ and '_cs were obtained by subjecting composition-controlled clay–silt specimens to a drained uniaxial vertical strain (K₀ consolidation) phase, followed by an undrained compression triaxial shear phase. Triaxial tests illustrate consistent and systematic increases in K₀ and decreases in '_cs, with increases in clay content at both stress levels. Stress dependence is observed through increases in K₀ and decreases in '_cs with increases in stress level. Stress-dependent behaviour is shown to be more pronounced in mudrocks of high clay content. A predictive model is presented for K₀ in normally consolidated mudrocks, as a function of clay content and effective stress. The model is used to calculate the fracture gradient (least horizontal stress) profile at the International Ocean Drilling Program (IODP) Expedition 308 Site 1324. The estimated fracture gradient illustrates the effects of clay content and consolidation stress on safe mud weight design.

Thematic collection: This article is part of the Geopressure collection available at: https://www.lyellcollection.org/cc/geopressure

需要泥岩强度参数来改进在弱固结地层中钻井期间裂缝梯度的预测。了解矿物成分和固结应力对泥岩力学性能的影响有助于安全井设计。泥岩粘土含量的微小变化对两个重要的工程参数有显着影响：静止侧向应力比K ₀和临界状态有效摩擦角，' _cs。使用石英粉砂和墨西哥湾粘土混合物的实验室测试程序研究了泥岩粘土含量对K ₀和' _cs在两个应力水平：1 和 10 MPa。K ₀的值和' _cs是通过使成分控制的粘土-粉砂试样经受排水单轴垂直应变（K ₀固结）阶段，然后是不排水压缩三轴剪切阶段获得的。三轴测试表明 K 0 一致且系统地增加，而K ₀减少' _cs，在两个应力水平下粘土含量都增加了。通过增加K ₀和减少' _cs随着压力水平的增加。在高粘土含量的泥岩中，应力依赖性行为被证明更为明显。给出了正常固结泥岩中K ₀的预测模型，作为粘土含量和有效应力的函数。该模型用于计算国际海洋钻探计划 (IODP) 远征 308 站点 1324 的裂缝梯度（最小水平应力）剖面。估计的裂缝梯度说明了粘土含量和固结应力对安全泥浆重量设计的影响。

专题合集：本文是 Geopressure 合集的一部分，可在以下网址获得：https://www.lyellcollection.org/cc/geopressure