Abstract
Biot’s coefficient can be expressed by a few different equations, and consequently, these equations may represent different approaches to measure this coefficient. We developed a combined method to measure the Biot’s coefficient by three different approaches based on three different equations using a single laboratory setup. The methodology is to measure the bulk modulus of a rock sample by conducting a drained hydrostatic test, and then to adjust the confining pressure and pore pressure to derive their correlation upon a constant volumetric strain condition, and then to conduct a series of drained hydrostatic compression and hydrostatic depletion tests to obtain the relationship between pore volume and rock bulk volume changes. The last step is to unify the pore pressure and confining pressure or keep a constant difference between confining and pore pressure in case the pore fluid and confining fluid are different to simulate an unjacketed hydrostatic test to acquire the grain bulk modulus of the rock. This method requires accurate and direct strain measurement and a fully saturated rock sample. This method allows us to acquire the Biot’s coefficient using a single rock applying different loading conditions based on three different equations. We demonstrated the measurement methods for both permeable rocks (Berea sandstone and Indiana limestone) and relatively impermeable rocks (Westerly granite and Asmari limestone). Different pore fluids (mineral oil, de-ionized water and nitrogen gas) were used for different rocks in order to achieve better measurements, and thus there are minor differences of the operation when dealing with different types of rocks.
