Effects of pore fluids on the hardening of shear modulus in tight dolomite: experimental observations and theoretical modelling

Abstract

SUMMERYUnderstanding the influence of pore fluids on acoustic characteristics of rocks containing pores of low aspect ratio, that is, cracks is crucial for both laboratory and field observations of seismic exploration. Unlike the petrophysical aspects of conventional rock species, those of tight dolomites are relatively more complex and poorly understood. To address this problem, we investigated the ultrasonic velocity of dry and saturated tight dolomite in the effective pressure range of 2–60 MPa and analysed the variation in shear modulus upon fluid saturation. Our results showed that while undergoing the change in form from the dry state to being saturated with water, the rock may weaken or harden, and the shear modulus may correspondingly either decrease or increase. Shear modulus hardening was more apparent in dolomites, with microcracks mostly dispersed between and within particles of a high proportion of soft pores with a wide distribution range of aspect ratio. This study used the Walsh–Hudson model to satisfactorily simulate the hardening of shear modulus in fluid-saturated tight dolomites. The suppression of squirt flow that is responsible for the increased shear modulus of fluid-saturated tight dolomites tested at sufficiently high frequencies. As the change in shear modulus affects the prediction of longitudinal and shear wave velocities, using Gassmann's theory to fluid substitution in unconventional reservoirs such as tight dolomite may yield inaccurate results. Therefore, it may be necessary to consider variation in shear modulus while selecting the appropriate simulation model.