A Review of Pore Structure Characterization of Unconventional Tight Reservoirs

The global growing energy demand driving the industry attention towards unconventional oil/gas resources due to limited conventional resources. Huge reserves of unconventional makes them promising and draw the industry attention, however oil/gas is stored in micro to nano scale pores with poor connectivity. It is very essential however difficult to quantify the flow characteristics in porous media in unconventional reservoirs due to complex pore network, irregular geometry of pore throat and non-homogeneous pore size distribution. Various experimental techniques to determine quantitative and qualitative characteristics of pore systems have been studied including scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), micro/nano computed tomography (XCT), and fluid invasion (mercury injection capillary pressure and gas adsorption/desorption). The comparison analysis of results has been carried out that exhibited the ability of these techniques to get the information about the pore size distribution and limitations for different pore sizes. Best and reliable technique for characterizing the pore structures in unconventional has been identified. SEM and FE-SEM are only able to provide the qualitative parameters for pore morphology, distribution and connectivity of pores. Three-dimensional image of pore structure and network could be studied through micro-CT scan images however, its high expense and huge processing time due to observation of small region at certain resolution make its use limited. Nitrogen adsorption is only able to study the micropores in tight sandstones but it destructive nature limits its usage. Pressure controlled mercury porosimeter technique is not able to determine the microporosity directly and determine the throat. It does not provide pore throat distribution. Application of high pressure may damage the pore structure. However, this mercury injection constant pressure rate can be applied for yielding both pore sizes and capillary pressure of pore throats. Both small throat and large pore body can be investigated through this technique, but this technique has limitation in maximum injection pressure. NMR is able to provide the qualitative and quantitative delineation of pore structures features such as pore throat distribution, sizes, and pore fluid saturation, total and effective porosity, and permeability directly if supplemented with other techniques. Detailed analyses of different analytical techniques resulted that none of the technique is able to fully characterize the pore structure of unconventional tight rocks. Combination of more than one technique is the best solution for complete description and accurate determination of pore structure characteristics.