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Understanding of the pore structure of rocks and its pressure dependency has been a long-standing 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 non-existent.
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. However, the quadrature conductivity further shows a systematic shift in the peak frequency towards lower frequencies. This shift towards lower frequencies implies that surface area is more sensitive to pressure change than bulk pore volume.
Presentation Date: Tuesday, October 16, 2018
Start Time: 1:50:00 PM
Location: 202A (Anaheim Convention Center)
Presentation Type: Oral
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
Characterizing shale mechanical properties requires five independent stiffness coefficients: C33, C44, C66, C11, and C13. In vertical wells, C33 and C44 are determined from the vertically propagating P- and S-waves. Because the commonly implemented models, such as the ANNIE model and the modified ANNIE (M-ANNIE 1) model use the Stoneley-wave velocity to interpret C66 and two other assumptions to predict C11 and C13, they are not applicable in cased-hole conditions when the Stoneley-wave cannot be measured.
In this paper, three new models without the Stoneley-wave as an input are introduced: the velocity regression model (V-reg), the further modified ANNIE (M-ANNIE 2), and the integration model combining both. The bases of the V-reg model are the observed near-linear relationships among measured 0°, 45°, and 90° P- and S-wave velocities. M-ANNIE 2, which is based on M-ANNIE 1, uses the linear relationship between the Thomsen compressional-and- shear-wave anisotropy parameters to replace the Stoneley-wave constraint to predict C66.
By applying the new models to the ultrasonic core data of multiple organic shales, their predictive power for the stiffness coefficients, elastic moduli, and closure stress are evaluated. Generally, the new models provide predictions that are as good as the M-ANNIE 1 and better predictions than the ANNIE and isotropic models. They provide good estimates for C66, with a small bias of approximately 1%. They also reduce the underestimation bias of the ANNIE and isotropic models. Finally, the log examples show that the new models yield predictions consistent with the Stoneley-wave models.
An accurate prediction of Young’s modulus and Poisson’s ratio is crucial for predicting fracture deformation, minimum horizontal stress (σhmin), and rock brittleness. Hence, it is important for selecting where to drill and perforate or designing a fracturing pumping strategy (Khan et al., 2012; Gokaraju et al., 2015). Elastic moduli computed from measured or predicted sonic-wave velocities are referred to as the small-strain elastic properties or “dynamic moduli,” in contrast to those measured in a rock mechanics laboratory with triaxial tests, which are referred to as the large-strain deformational properties or “static moduli.” Both the short-term brittle deformation during drilling/ fracturing and the long-term ductile deformation during production are related to the static moduli. So, all dynamic moduli must be transformed to static moduli based on the empirical relationships calibrated to laboratory data for their future use in drilling/completion designs.
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.