Exploration of Pore Structure and Permeability Evolution of Carbonate under Drilling Fluid Invasion Based on Adaptive Quantitative Multiscale Pore Structure Characterization

The carbonate reservoir of the Dengying Formation in the Penglai Gas Field, Sichuan, China, has a 6000 m buried depth and over 160 °C central temperature. This reservoir is characterized by a complex multiscale pore structure and significant heterogeneity. Furthermore, drilling fluid filtration often occurs during drilling, leading to the solid phase invasion of the drilling fluid into the rock pores. This invasion complicates the characterization of the complex multiscale pore structure and affects permeability evaluation. A series of related experiments investigated the pore structures and the permeability evaluation of drilling fluid damage. The core samples were tested using a nuclear magnetic resonance (NMR) instrument and a gas permeability tester before and after high-temperature immersion in a drilling fluid. An adaptive quantitative multiscale pore structure characterization method was introduced to investigate carbonates’ pore structure and permeability evolution under drilling fluid invasion. The results indicate the following: (1) Solid phase invasion of drilling fluid caused a significant reduction in macropores and micropores, while the changes in mesopores are slight. (2) Original pores may be transformed into smaller ones or even form unsaturated pores by the invading solid phase, resulting in additional porosity reduction. (3) The permeability estimation model, which accounts for the influence of multiscale pore structure, demonstrated an accuracy of over 91% in permeability calculations. It is designed to predict permeability under corresponding conditions, such as drilling fluid damage or acidizing, by obtaining changes in multiscale pore structure parameters. (4) In the undamaged condition, the permeability contributions from pores of various scales exhibited orders of magnitude differences. The permeability of most samples was predominantly attributed to macropores, which contributed 58.1%. Drilling fluid damage decreased the contribution from macropores to permeability (reduced to 51.2%), while the contribution from mesopores increased from 38.9% to 46.8%.