Dual pore shape modeling using experimental elastic and electrical data and pore-scale imaging results

Abstract

The objective of this research is to use elastic and electrical models to characterize which pore shape is most affected by a change in effective stress in a dual porosity system. The influence of pore geometry (pore aspect ratio) on elastic and electrical properties in a dual porosity tight sand reservoir is explored. The dual porosity system that was considered is a reservoir that contains both small, disk-shaped micro-crack pores and larger, nearly-spherical pores. During the study, both modeling and experimental approaches were applied. For modeling methods, elastic and electrical self-consistent approximation methods are introduced, their relationship with elastic and electrical Hashin-Shtrikman bounds was studied, and the ability of the models to fit experimental electrical and elastic data as a function of effective stress was explored. For experimental methods, an experimental setup was built to make simultaneous elastic and electrical measurements, and this system was used to make measurements as a function of effective stress on clean, elastically-isotropic tight sand reservoir samples from the Mesaverde Group, Utah. Additionally, a 3D micro-CT image was processed on one sample to acquire the quantitative, detailed pore characteristic information, including pore volume, surface area and aspect ratio. The results from micro-CT imaging confirmed that tight sand reservoirs in this study consist of a dual pore characteristics system - (1) large volume fraction pores with low pore count and (2) disk-shape pores with high pore count. Due to the limitations in image resolution, the micro-CT image could only provide quantified information about large pores. The pore shape information determined from the micro-CT image processing results was used as input to the elastic and electrical self-consistent approximation models to try to model changes in electrical and elastic data as a function of effective stress, explained by changes in porosity of each of the dual pore shapes. This modeling demonstrated that in the tight sand dual porosity system, the reduction of porosity with effective stress increase is mainly contributed to a reduction of micro-crack shaped pore porosity. While a number of publications focus on either elastic or electrical modeling of pore shape, until recently, few focus on joint modeling of both elastic and electrical joint measurements, which is the focus of this research. Generally speaking, the results in this research may be further used to improve characterization of tight sand reservoirs, thereby improve the identification of gas in place, reserves calculation, production potential estimates and other problems during production and recovery process. To be more specific, being able to model which pore shape (micro-crack or nearly spherical) is responsible for a change in porosity due to change in effective stress has potential to be applied to dual porosity reservoir simulation and using seismic and/or resistivity data in field to monitor the effects of change in porosity when a reservoir experiences a change in effective stress, such as during either production or flooding.