Digital rock-based investigation of conductivity mechanism in low-resistivity gas hydrate reservoirs: Insights from the Muli area's gas hydrates

Abstract

Gas hydrate reservoirs in the Muli area of China are characterized by complex lithology, high hardness, developed micro-fractures, and low porosity-permeability, in which an unusually low-resistivity phenomenon is observed. This phenomenon has led to considerable uncertainty in determining the gas hydrate layers and measuring the gas hydrate saturation. Therefore, it is necessary to understand the underlying factors appropriately to optimize exploration and production. In the present study, digital rock technology was combined with field-scale well-logging data to analyze the conductivity mechanism and identify the causes of the observed low resistivity. The obtained results and sensitivity analysis indicated that the rock porosity was the main controlling factor, and then the formation water salinity, pore structure, micro-fractures, clay components, heavy minerals, and gas hydrate pore-habits were the other significant parameters in the order of high to low significance with the corresponding significance values of 3.64, 2.51, 1.97, 1.72, 1.36, 1.04, and 0.56, respectively. Moreover, wells DK-8, DK11-14, and DK12-13 were taken as examples to analyze the formation mechanism of low-resistivity gas hydrate, and the results showed that the high reservoir porosity, developed micro-pores, high irreducible water saturation and good pore connectivity are the main reasons for the low-resistivity gas hydrate in DK-8; and micro-fractures and the banded distribution of clay are responsible for the low-resistivity gas hydrate in wells DK11-14 and DK12-13, respectively. These findings provide fundamental information for accurate interpretation of the key well-logging data in the study area as well as in similar reservoirs and help improve the exploitation of gas hydrate reservoirs both in the permafrost regions and marine environments.