Mechanical instability induced by water weakening in laboratory fluid injection tests

Abstract

To assess water‐weakening effects in reservoir rocks, previous experimental studies have focused on changes in the failure envelopes derived from mechanical tests conducted on rocks fully saturated either with water or with inert fluids. So far, little attention has been paid to the mechanical behavior during fluid injection under conditions similar to enhanced oil recovery operations. We studied the effect of fluid injection on the mechanical behavior of the weakly consolidated Sherwood sandstone in laboratory experiments. Our specimens were instrumented with 16 ultrasonic P wave transducers for both passive and active acoustic monitoring during loading and fluid injection to record the acoustic signature of fluid migration in the pore space and the development of damage. Calibration triaxial tests were conducted on three samples saturated with air, water, or oil. In a second series of experiments, water and inert oil were injected into samples critically loaded up to 80% or 70% of the dry or oil‐saturated compressive strength, respectively, to assess the impact of fluid migration on mechanical strength and elastic properties. The fluids were injected with a low back pressure to minimize effective stress variations during injection. Our observations show that creep takes place with a much higher strain rate for water injection compared to oil injection. The most remarkable difference is that water injection in both dry and oil‐saturated samples triggers mechanical instability (macroscopic failure) within half an hour whereas oil injection does not after several hours. The analysis of X‐ray computed tomography images of postmortem samples revealed that the mechanical instability was probably linked to loss of cohesion in the water‐invaded region.

Ultrasonic monitoring of spontaneous imbibition experiments: Precursory moisture diffusion effects ahead of water front

“…In a recent study, David et al . [] recorded the P waveforms transmitted between two pairs of ultrasonic transducers (~0.5 MHz) located at two heights along vertical rock specimens subjected to spontaneous water imbibition experiments. Simultaneously, X‐ray CT images were acquired with a Siemens SOMATOM Definition AS 64‐slice.…”

“…Two pairs of P wave ultrasonic transducers (two source/receiver pairs) are attached to vertical rock samples (length ~80 mm) at two different heights (~25 and 50 mm). A specific stand is designed to allow water to be in contact with the bottom end of the rock specimen, allowing a spontaneous water imbibition process to occur [ David et al ., ]. In addition, this arrangement allows for a time‐lapse imaging of the water distribution within the rock specimen using, for instance, X‐ray CT monitoring.…”

“…The first results of this kind of experiments on four rock samples were published by David et al . [, ]. Here we present unpublished results which allow us to extend our data set to 14 different rock samples that experienced similar spontaneous imbibition tests with imaging and ultrasonic monitoring (Table ).…”

“…Note that the results for SMX, MAJ, and SH‐hor have been published in David et al . [, ]; the others are unpublished results. This sample set covers a porosity range between 14% and 39% and a permeability range between 26 mD and 3400 mD.…”

New method for measuring compressibility and poroelasticity coefficients in porous and permeable rocks

“…This result proves the repeatability of the method for this rock sample, even using a liquid as viscous as the glycerine. Although strong elastic weakening from rock‐fluid interaction have been observed in some porous and ill‐cemented sandstones [e.g., Pimienta et al , ; David et al , ], this result further highlights the absence of any elastic weakening for this sandstone sample.…”