The effects of water content on transversely isotropic properties of organic rich gas shale

Abstract

Organic-rich gas shales are often composed of thin laminated structures and, thus, may be considered as transversely isotropic material. The shale's anisotropic properties are affected by many factors, including confining pressure, water content, total organic content (TOC), etc. In this work, ultrasonic velocity measurements were performed at various confining pressures and water content conditions to study the effects of confining pressure and water content on anisotropic properties at Eagle Ford shale. The test results show that Eagle Ford shale has weak elastic anisotropy. In addition, the results confirm that transversely isotropic media is an appropriate model to characterize Eagle Ford shale.Both the compressional (P) and shear (S) wave velocities increase as the confining pressure increases, especially at low confining pressure range. The horizontal elastic modulus, vertical elastic modulus and shear modulus also increase with the increase in confining pressure. Varying the confining pressure show minimal to no effect on the horizontal and vertical Poisson's ratio. On the other hand, the increase in the confining pressure decreases both P- and S-wave anisotropy. The study shows that P-wave anisotropy is more sensitive to confining pressure than S-wave anisotropy. The imbibed water significantly decreases P- and S- wave velocities. For a given sample at the same confining pressure, the decrease of P-wave/S-wave velocity is proportional to the unit increase of water content, which is defined as water content impact factor. Increasing water content also significantly reduces horizontal Young's modulus, vertical Young's modulus and shear modulus. Water makes the shale softer. The study shows that increase in the water content decreases the horizontal Young's modulus, the vertical modulus as well as the shear modulus by 8%, 11% and 15%, respectively. Both horizontal and vertical Poisson's ratio increase with increasing water content. Rock samples tested confirm that the increase in water content increases the P- and S-wave anisotropies by approx. 2.1% and 2.6%, respectively. The presence of water narrows the difference of VP and VP(900) at small angle of the near vertical range and, therefore, decreases the near vertical P-wave anisotropy.