Cement should more accurately be regarded as a poroelastic material rather than elastic material. Early researchers have measured the strengths and elastic properties of oil well cement, but Biot effective stress coefficient (α) remains an important unknown despite its significance to poroelastic fluid flow and effective stress. Here, we present measurements of Biot effective stress coefficient for Class G cement, which is calculated by the gradient of confining pressure to pore pressure under changing total stress conditions with volumetric strain held constant. High-pressure and high-temperature GCTS triaxial equipment was used. This system includes four pore pressure flow lines, two for flow inlet and outlet, and two others for static measurement lines or bleed lines. This enables the precise measurement of pore pressure. Cement samples with three different curing periods (3, 10, and 28 days) were measured under different effective stress. Results show that Biot effective stress coefficient of cement ranges between 0.6 and 0.8. This work contributes to poroelastic modeling of cement and serves as groundwork for future experiments that are meant to directly measure the effective stress state in the cemented annulus of a well at reservoir conditions.
Oil well cement is usually regarded as a continuous, single-phase, linear-elastic material for basic wellbore integrity analysis (Thiercelin et al, 1998; Saint-Marc et al, 2008; Liu et al., 2017; Liu et al, 2018; Meng et al. 2019). However, inter-granular porosity is generated during cement curing; therefore, cement should more accurately be regarded as a poroelastic material rather than an elastic material. In poroelastic theory, it is the effective stress that governs the deformation of the matrix. To calculate the effective stress of cement, we need to determine α first.
Compared with other geomechanical parameters (i.e., modulus, Poisson's ratio, cohesion, internal friction angle, fatigue life), α is technically more difficult to measure because it requires precise control of the pore pressure. Either the triaxial compression equipment's limits, or technical challenges in the experiment may cause this kind of test to fail. For example, the pore pressure gauge may be installed far away from the end of the sample. The pressure loss in the pipeline between gauge and sample end could be significant if there is air/fluid/contamination blockage. The porous disc can be blocked by sample grains after several regular triaxial compression failure tests, or confining oil if there is leakage of the membrane. This blockage can prevent the propagation of pore pressure inside the samples.