ABSTRACT: Shale formations containing above a critical amount of clay present significant rock-fluid interactions that affect drilling, completion, stimulation, production and economic viability of the operations. Correlating the impacts of generated osmotic potential and physico-chemical effects to formation mechanical properties and strength will provide better efficiency in the shale field developments and production optimization. In this paper, we aim to minimize the uncertainties surrounding the structural changes observed with the osmotic pressure by conducting precisely controlled coupled measurements of membrane efficiency, permeability, dynamic and static Young’s moduli, and shear strength under triaxial stress state with elevated pore pressures up to 4000 psi. The results indicate that osmotic membrane efficiency increases from 16.5% to 30.3% due to shale structural changes associated with the increase in effective stress and changing salinity of the pore fluid.
1. INTRODUCTION
Shales are typically considered to behave as a semipermeable or leaky membrane. Therefore, correlating the impacts of generated osmotic potential and physico-chemical effects to formation mechanical properties and strength will provide better efficiency of the drilling and completion operations in shale formations whether they are sealing the reservoir formations or are organic-rich source rock that are produced from. The type of fluid shale formations are exposed make changes in their mechanical properties and strength characteristics. For example, static Young’s modulus increases when Pierre II outcrop core samples containing up to 65% smectite are exposed to high salinity brine solution of 257,448 ppm NaCl at 22°C. Alternatively, water imbibition from low salinity injection adversely alters the structure of the same formation containing swelling clays. While there has been many research studies conducted on the effect of exposure to various fluids, the effect of this exposure on geomechanical properties of shales have not been sufficiently investigated under in situ reservoir stress, elevated pore pressure and temperature conditions.
