Summary Understanding of the pore structure of rocks and its pressure dependency has been a longstanding area of interest. More recently, pore structures and pore sizes have been mapped using induced polarization measurements (Revil et al. 2014, Niu et al., 2016). However, studies on the pressure dependency of Complex Resistivity (CR), also known as Induced Polarization (IP), and how they relate to pore deformation are almost nonexistent. In this study, we performed IP measurement on three sandstone samples with little to no clay content and observed the changes in real and quadrature conductivity with increasing confining pressure, while pore pressure was kept constant. The results from the IP measurements show that the real and quadrature conductivities both decrease with increasing pressure.
The mechanisms of ultrasonic attenuation in reservoir rock are known to be sensitive to multiple rock physical properties; this study focuses on ultrasonic experiments that measure the anisotropic attenuation in shales as function of hydrostatic confining pressure; four Eagleford and Bakken samples were measured using new experimental setup that allows measuring anisotropic acoustic properties of sample simultaneously with only one core plug. Our tests show that P-wave attenuation is sensitive to confining pressure, and attenuation anisotropy is stronger than velocity anisotropy, especially for more isotropic samples; the highly active change of attenuation with pressure supports the opinion that attenuation is a highly sensitive parameter to rock intrinsic properties. Moreover, attenuation as a function of pressure clearly suggests a two-phase attenuation mechanism exists in shale: high aspect ratio pores/microcracks closure and the related scattering attenuation on crack surfaces dominate attenuation behavior under low pressure, while at high pressure the main mechanism shifts to intrinsic attenuation caused by grain/crack friction and anelasiticity. The measured anisotropy data could be used for understanding the loss mechanisms responsible for seismic attenuation, and would benefit the development of theoretical attenuation rock physics models, as well as the interpretation of well logging and seismic surveys in shale reservoirs.
Presentation Date: Wednesday, October 19, 2016
Start Time: 10:20:00 AM
Presentation Type: ORAL
In tight sand reservoirs, a significant amount of reservoir fluids can be held in small, open, inter-granular, micro-cracks that connect larger pores. Determining the conditions required to either mobilize or immobilize the fluids in those micro-cracks can have significant economic implications, whether those fluids are oil or water. When a reservoir experiences a change in effective stress, for example from production (increased effective stress) or flooding (decreased effective stress), then knowing which pores are changing in volume with respect to a change in effective stress can provide insight into changes in the mobility of the micro-crack fluids and the overall contribution of those micro-cracks fluids to fluid transport. Mechanically, the change in porosity in clean, tight sand that accompanies a change in effective stress can be explained by changes in compliant, micro-crack porosity and aspect ratios. In this paper we take an experimental approach to explore whether or not elastic and electrical Self Consistent Approximation (SCA) modeling can be used together to predict changes in porosity and/or aspect ratios in a dual pore system of micro-cracks and nearly-spherical pores.
We analyze correlations between velocity, permeability, and porosity in three tight sand reservoirs. Flow zone indicator and effective specific surface area methods are introduced to analyze the relations between these properties. Correlations between velocity, permeability, and porosity of clean tight sands are shown to be well characterized after binning the data into zones with distinct effective specific surface or flow zones. Introduction Reservoir characterization, an important tool for analyzing production potential of a prospective zone, is challenging in unconventional reservoirs, due to limited understanding of the petrophysical characteristics and the governing factors of flow through these formations. In this paper, we analyze the petrophysical properties and correlations between velocity, permeability, and porosity behavior in tight gas sands.