Grading evaluation of pore throats in a tight sandstone reservoir: a case study of the Gao 3 section reservoir in the Qijia area of the Songliao Basin

Abstract

The micropore throat of the Gao 3 section tight sandstone reservoir was studied using sampling and experimental (high-pressure mercury injection and scanning electron microscopy) methods. Based on the relative permeability, the reservoir can be divided into three categories: conventional, unconventional and low permeability. To determine the displacement pressure, porosity and other relevant parameters of various reservoirs, the mercury injection curve of all samples was projected to the same coordinate system. According to the position of the inflection point on the mercury injection curve of all levels of reservoirs, the pore throat was divided into three types: micropores, transition pores and macropores. Based on the theory that the throat is characterised by the mercury withdrawal curve, the throat in the present study is divided into three categories: the large throat, the medium throat and the small throat. Combined with the ordinate position at the inflection point, the critical point of micropore-throat classification of all kinds of reservoirs was determined. By analysing the histogram of pore-throat distribution of all kinds of reservoirs, the peak value of reservoir pore-throat distribution was determined. From the definition of expectation and variance in mathematics, the concept of grading parameters was introduced to determine the size and concentration degree of micropore-throat distribution at all levels of reservoirs, and a correlation analysis with permeability was performed to determine the control law of permeability.