In oil and gas production, the reservoir is a porous medium that stores oil and gas. Many macroscopic properties of the reservoir (such as porosity, permeability and capillary pressure) depend on its micro structures, as well as on the physical properties of solids that compose it and fluids in the spaces of its pores. Therefore, in order to enhance the oil recovery remarkably, the theoretical study and technical development thereof should not stay on the macro level, but the research should be directed into the interiors of the porous medium on the micro level.
The oil and gas reservoir micro pore structure refers to the geometrical shapes, sizes, distributions and the interconnecting relationships of the pores and throats of reservoir rocks, generally on the micrometer (micron) scale (less than 1 millimeter) or smaller. The oil and gas reservoir micro pore structure decides the reservoir features, preserving mechanism and production process of the oil and gas resources, in particular, as for a tight sandstone reservoir which mainly comprises nanoscale micro pore structures, the micro pore structure thereof is an important factor that decides the pore permeability features thereof. Therefore, how to accurately characterize a reservoir micro pore structure has become an important topic in the process of oil-gas exploration.
Currently, the conventionally employed characterization method for a reservoir micro pore structure is CT scanning technology. The CT scanning technology is a technique that performs an all-around and large-scale scanning and imaging of a rock sample using X-ray and then reconstructs the micro pore 3D structural features using values of the scanned images. The CT scanning technology is classified into nanoscale CT and micron scale CT, wherein, the micron scale CT can only characterize pore structures over micron scale, of which the maximum resolution ratio is 0.7 micron; the nanoscale CT has a resolution ratio higher than that of the micron scale CT, but the maximum resolution ratio thereof during measuring process is also only 50 nm.
In oil and gas development, it is found that there are large amount of micro pore structures which are smaller than 50 nm in the unconventional reservoirs such as tight sandstone and shale etc. Since the maximum resolution ratio of the CT scanning technology is 50 nm, therefore it is hard to satisfy the requirements for the characterization of such micro pore structures by using conventional CT scanning technologies, and this has to some extent restricted the process of oil and gas exploration and development of the unconventional reservoir such as tight sandstone and shale etc. Therefore, there is an urgent need for a characterization method for a reservoir micro pore structure for characterizing a reservoir micro pore structure smaller than 50 nm.