Effect of temperature on the compressive strength of eagle ford oil shale rock: An experimental study

Although many shale oil wells have been drilled recently in the Eagle Ford shale oil field, wellbore stability problems in the shale oil reservoirs have not been fully understood. During drilling, fracturing, and production operations, the wellbore temperature is not constant but varies during these operations. This temperature fluctuation results in thermal cycling which tends to alter rock physical properties such as compressive strength, Young's modulus (E), and Poisson's ratio (v), and consequently, result in wellbore stability problems. Wellbore stability problems are one of the most costly problems that can occur during drilling operations, especially in unconventional reservoirs. These problems include but are not limited to tight holes, pipe sticking, fishing, sidetracking, and well abandonment. Since the majority of wells are drilled horizontally in shale oil reservoirs, the likelihood of wellbore stability problems is dramatically increased. Thus, to have more efficient and effective drilling operations through these formations, a better understanding of their properties as well as the effects of drilling conditions on rock properties such as compressive strength, Young's modulus (E), and Poisson's ratio (v) are required. Not many experiments have been performed to investigate the effect of temperature on sedimentary rocks' properties such as compressive strength. Some experiments have been conducted to assess the effect of temperature on shale rock samples, since shale oil rock samples are quite different from common shale rock samples, the results cannot be equally applied to shale oil and shale gas formations. For this study, the mineralogy of shale oil core samples from the Eagle Ford field was determined. To simulate actual well conditions a High Pressure High Temperature (HPHT) setup was built and used which let us apply different axial and radial confining stresses, equivalent formation pore pressure, and drilling fluid wellbore pressure. The experiments were conducted under elevated temperatures to better mimic real drilling operations. Saturated shale oil core samples from the Eagle Ford field were tested under various temperatures including reservoir temperature. We also performed Unconfined Compressive Strength (UCS) tests to investigate the effect of temperature on the compressive strength of the core samples. The experimental setup was modified to accommodate five Linearly Variable Displacement Transducers (LVDTs) to measure Young's Modulus (E) and Poisson's ratio (v). Various experiments were run to quantify the effect of temperature on the rock compressive strength, E, and v. Experiments have shown a distinct change in the mechanical properties of the rock. These effects will be discussed in details.