Pore throat characterization of bioturbated heterogeneous sandstone, Bhuj Formation, Kachchh India: An integrated analysis using NMR and HPMI studies

Abstract

Nuclear magnetic resonance (NMR) relaxometry and high-pressure mercury intrusion (HPMI) are substantially utilized techniques to characterize the inherent heterogeneity of reservoirs. Bioturbation is one such type of microscale heterogeneity that provides challenges to optimal recovery planning and volumetric estimation of hydrocarbon reserves. A comprehensive approach is determined by acknowledging the applicability and limitation of each method. Due to the wide variation in petrophysical properties resulting from the effect of bioturbation, the sample size ranges from mm to cm; hence, a proper methodology needs to be formulated. The present study extends the available method of generating pseudo Pc curves from NMR studies to analyze the effect of multiscale bioturbation heterogeneity. The samples were chosen from two different classes on a bioturbation scale referred to as the bioturbation index (BI) ranging from 0 to 6, 0 being 0% bioturbation and 6 meaning 100% bioturbation. The results indicate that lower BI samples had unimodal behavior, while the higher BI sample had bimodal characteristics in pore network distribution. Increased anisotropy was evident in the higher BI samples, and the HPMI study revealed a contrasting nature of pore throat connectivity with a lower threshold and displacement pressure and an increased median throat diameter in the bioturbated part compared with the nonbioturbated. The bioturbated volume essentially had better and less tortuous pore throat connectivity. The effects of bioturbation were identifiable in a core-scale study using NMR. The bimodal characteristic behavior with two distinct pore throat classes with the larger throat class as dominant claims the bioturbation-assisted pore throat connectivity established in them. The purpose of the paper is to investigate pore throat distribution in bioturbated or heterogeneous reservoirs by integrating NMR and HPMI methods. This study proposes a method that can correlate NMR responses from logging into pore throat distribution in bioturbated zones of the reservoir.