The impact of clay fraction on the strength and stress ratio (K0) in Gulf of Mexico mudrocks and quartz silt mixtures: implications for borehole stability and fracture gradient

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, ϕ′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 ϕ′cs at two stress levels: 1 and 10 MPa. Values of K0 and ϕ′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 ϕ′cs, with increases in clay content at both stress levels. Stress dependence is observed through increases in K0 and decreases in ϕ′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