Influence factors and an evaluation method about breakthrough pressure of carbonate rocks: An experimental study on the Ordovician of carbonate rock from the Kalpin area, Tarim Basin, China

Abstract

There are many oil and gas fields around the world that are sealed by tight carbonate caprocks. Compared with gypsum rocks and shales, tight carbonate caprocks are characterized by brittleness and ease of structural fracturing and dissolving in acidic groundwater. The breakthrough pressure (BP) is a significant indicator that indicates the sealing capacity of the caprocks. Therefore, we conducted a series of experiments, including effective porosity, permeability, casting thin-sections, BP, mineral composition, microstructural characteristics and clay minerals contents on carbonate samples from Ordovician system in the Kalpin area, Tarim Basin, China to research factors that affect the BP of carbonate caprocks. According to the experimental data analysis, the BP deceases with macropore proportions and increases with proportions of micropore and mesopore volume. Due to the large macropore proportion and low clay content (<10%), expansion of clay minerals with water hardly reduces effective porosity and permeability. Pore throat and cement influence effective porosity and permeability, and thus influence the BP. Generally, the BP decreases with the increase of effective porosity and permeability, and linear correlations exist between the reciprocal of effective porosity (A), −0.48 power of permeability (B) and the BP. Due to the interdependency between A and B, a principal component regression procedure (PCRP) is implemented with the statistical analysis system (SAS) software to eliminate the interdependencies and obtain a fitting formula of BP. The calculated BP with depth by this method matches gas logging results. This study provides a research idea and a mathematical calculation model of the BP of carbonate caprocks to further evaluate and predict carbonate caprocks behaviour.