Prediction method of rock physics S-wave velocity in tight sandstone reservoir: a case study of Yanchang Formation in Ordos Basin

The reservoir space of tight sandstone reservoirs in Trias sic Yanchang Formation of Ordos Basin is dominated by polygonal pores such as triangular and square ones, with a few cracks. Traditional prediction methods of rock physics S-wave velocity in tight sandstone reservoirs are based on the theoretical assumption of elliptical pores, which cannot achieve an ideal effect of describing polygonal pores. Assuming that the pores in the tight sandstone reservoirs of Yanchang Formation are equivalent to triangular and square pores and the combination of these pores and fractures, this paper establishes a prediction method of S-wave velocity in tight sandstone reservoir that considers the coexistence of polygonal pores and fractures. The method combines the rock physics model of regular polygonal pores with that of differential equivalent medium and introduces the volume ratios of convex triangle pores and concave square pores and iv2) to the total porosity, so as to describe the impact of complex types of pores in rocks on P-wave and S-wave velocity. These two parameters (i. e., UL and iv2) are calculated under the constraint of P-wave velocity, based on which S-wave velocity in tight sandstone reservoir can be predicted. Through numerical forward simulation, comparison of core test data, and well logging interpretation of S-wave velocity, the validity of the calculation method for S-wave velocity in tight sandstone reservoir proposed herein has been verified. © 2022 Science Press. All rights reserved.