Application of the combination of high-pressure mercury injection and nuclear magnetic resonance to the classification and evaluation of tight sandstone reservoirs: A case study of the Linxing Block in the Ordos Basin

Abstract

Tight sandstone gas reservoirs have poorer porosity–permeability relationships, so conventional reservoir classification schemes can hardly satisfy the classification and evaluation demand of this type of reservoirs. To solve this problem, this paper took the Permian tight sandstone gas reservoir in the Linxing Block along the eastern margin of the Ordos Basin as an example to describe the micro-structures of the tight sandstone reservoirs by means of high-pressure mercury injection, nuclear magnetic resonance (NMR), scanning electron microscope (SEM) and so on. Then, the control effect of micro-structure parameters on the macrophysical properties was studied. Finally, classification and evaluation of tight sandstone reservoirs were carried out on this basis. And the following research results were obtained. First, NMR can identify the distribution of pores of different sizes, and high-pressure mercury injection can reflect the pore–throat configuration and percolation capacity of a reservoir. Second, both methods are better coincident in the description results. With an increase of the right peak of T2 spectra, the mercury intrusion curve presents a concave shape and the pore throat radius increases while the pore type gradually changes from intragranular dissolution pores and intercrystalline pores to intergranular pores and intergranular dissolution pores and the reservoir quality gets better. Third, micro-pore structure controls reservoir physical properties and fluid mobility. And the porosity of large pores is best correlated with the effective porosity, so it can be used to evaluate the reservoir capacity of tight sandstone. Fourth, the throat radius R15 obtained by high pressure mercury injection is in the best correlation with porosity and permeability, so it can be used to evaluate the percolation capacity of tight sandstone. Fifth, by combining the porosity of large pores with the R15, the tight sandstone reservoirs in the Linxing Block are classified into 4 categories, and the classification results are in a good agreement with the on-site well test data. It is concluded that the combination of high-pressure mercury injection and NMR can effectively identify the key parameters which reflect the reservoir capacity and percolation capacity of tight sandstone, and improve the reliability and integrity of reservoir classification. And by selecting the key parameters that reflect reservoir capacity and percolation capacity, it can provide guidance for the classification and evaluation of tight sandstone reservoirs.