Characterization of Pore Structure in Tight Oil Reservoir Rock

AbstractThe concern of global energy shortage forces people to divert their attention from conventional oil and gas resources to unconventional ones such as tight oil, shale gas, basin-centered gas, coal bed methane and gas hydrate. Tight oil, in consideration of technical feasibility and economical affordability, is regarded as one of the most promising unconventional resources. And it is necessary to make the characterization of realistic pore structure for tight oil, which is the foundation for continuous production and enhanced recovery. In this paper a novel analysis procedure to characterize the pore structure of the tight oil sample in three scales, millimeter, micrometer and nanometer, is proposed and performed on two core samples of tight oil reservoir from Chuanzhong Oilfield. Macro-CT is used to scan a whole core and to obtain millimeter scale images which represent the porosity distribution and the connectivity on the reservoir scale. Micro-CT and Nano-CT are used for identifying micropores and nanopores respectively, and the quantity and the connectivity of pores and throats are obtained by reconstruction for the pore scale network. Nevertheless, paradoxical issues of Micro-CT and Nano-CT scanning are the lack characterizations for pore size, pore type and pore origin. These issues can be resolved with the aid of SEM which generates a 2D image of the surface of the core sample covering the range of scales from nanometer to millimeter. In the millimeter scale level, the distribution frequency of porosity under 0.5% is about 40% which forms the main porosity area for both two core samples. In the micrometer and nanometer scale levels, pore scale networks show little connectivity for the matrix pore. Analyzing SEM images, it indicates that intergranular pore is identified with the diameter from 1 micrometer to 6 micrometer in the micrometer scale level and intragranular pore is identified with the diameter from 300 nanometer to 800 nanometer in the nanometer scale level. The microfracture system, which contributes to improve the effective permeability for tight oil, can also be well observed in SEM images with average length from 10 micrometer to 100 micrometer and averaged width from 1 micrometer to 3 micrometer.