Measurements of elastic and electrical properties of an unconventional organic shale under differential loading

Post date: Jul 27, 2015 7:43:57 AM

W. F. Woodruff1, A. Revil2, M. Prasad3 and C. Torres-Verdín4

1Colorado School of Mines, Department of Geophysics, Golden, Colorado, USA. E-mail: wfwoodruff@gmail.com; mprasad@mines.edu.

2Colorado School of Mines, Department of Geophysics, Golden, Colorado, USA and Université Savoie Mont Blanc, ISTerre, CNRS, UMR CNRS, Le Bourget du Lac, France. E-mail: arevil@mines.edu

3Colorado School of Mines, Department of Geophysics, and Department of Petroleum Engineering, Golden, Colorado, USA.

4University of Texas at Austin, Department of Petroleum and Geosystems Engineering, Austin, Texas, USA. E-mail: cverdin@austin.utexas.edu.

We have developed an experimental approach to simultaneously measure the stress dependence of ultrasonic wave velocities at 1 MHz, and therefore the components of the undrained elastic stiffness tensor, as well as the components of the complex conductivity tensor in the frequency range from 100 mHz to 10 Hz. We performed the experiments on a cylindrical core sample from the Haynesville Formation (porosity of approximately 0.08, bound water excluded, and clay content, mostly illite, approximately 30–40 wt%). We performed experiments under controlled confining and pore fluid pressures, achieving differential pressure states representative of autochthonous reservoir conditions. Directional measurements were made using independent acquisition arrays (piezoelectric crystals and nonpolarizing electrodes) distributed azimuthally on the core sample external surface, the bedding plane being along the axis of the cylindrical core sample. Ultrasonic waveforms were recorded on a high-resolution oscilloscope, and complex impedance spectra were recorded with a four-electrode acquisition system using an impedance meter with precision of 0.1 mrad. Experiments were repeated under drained and undrained conditions, over loading and unloading sequences for fully water-saturated conditions. Measurements show strong stress dependence on ultrasonic and complex conductivity measurements, which can be ascribed to the opening and closing of cracks within the samples. The data were used to estimate the anisotropic electrical and elastic effective pressure coefficients of the core sample, resulting in effective stress coefficients smaller than one for both elastic and electrical properties. These effective stress coefficients were also smaller for the electrical and acoustic properties once the cracks have closed. The anisotropy ratio for the components of the complex conductivity tensor was on the order of 30, whereas it was only 2 for the compressional components of the stiffness tensor (C11/C33).