Impact of Testing Temperature on Stiffness and Yield Properties of North Sea Chalk

Successful management of chalk reservoirs for various subsurface applications often depends on reliable estimates of rock mechanical properties. Yet, rock stiffness and strength data collected at in situ temperature are typically limited. Accordingly, a mechanical testing program was designed to investigate how testing temperature impacts stiffness and yield properties of high porosity North Sea chalk. Tests were conducted at ambient (≈ 25 ℃) and in situ temperatures (75 or 85 ℃) and as uniaxial strain and hydrostatic compaction. Core plugs were either oil or water saturated, and the static moduli and yield strength were derived from stress–strain curves. Irrespective of saturating fluid, no clear temperature effect was observed on the static moduli or yield strength, yet a water weakening effect was seen. The impact of testing temperature on stress–strain behaviour and static moduli are thus of similar magnitude to effects from initial fractures and natural sample variations identified from CT-scans. In contrast, the dynamic moduli derived from elastic wave velocities and density indicated a minor stiffness decrease at increased testing temperature. The observed decrease in dynamic moduli for increased testing temperature aligns with the temperature-induced stiffness decrease of the constituting calcite minerals. Accordingly, the largest temperature-induced stiffness decrease is for the uniaxial compressional modulus and the lowest for the shear modulus. The observed effect of increased temperature on elastic moduli of the studied chalk may be due to two opposing effects on the solid frame; stiffening from thermal expansion that opposes the thermal softening of the constituting calcite minerals.