Influence Factors and an Evaluation Method Study About the Breakthrough Pressure of Carbonate Caprocks

Abstract

Many oil and gas fields around the world are sealed by carbonate caprocks. A carbonate caprock is an indispensable factor for the accumulation, distribution and exploration of carbonate reservoirs. The breakthrough pressure (BP) is a significant indicator that indicates the sealing capacity of a caprock. Carbonate rocks are different from shale (fine clastic rock) and are characterized by strong heterogeneity, the ease of development of cracks and many factors that affect the sealing ability. Therefore, we conducted a series of experiments, which included the determination of the BP, clay content, mineral composition and microstructural characteristics (i.e., porosity and permeability, specific surface area and pore size distribution) on carbonate samples from the Yingshan and Penglaiba Formations in the Kalpin area, in the Tarim Basin of China to study factors that affect carbonate caprocks. The BP determination were completed for water-saturated samples with a step-by-step approach.

According to an analysis of the experimental data, a non-linear decreasing relationship exists between the BP, porosity, and permeability. The BP deceases with the macropore volume and increases with the micropore and mesopore volume. The effect of microstructures on BP of carbonate rocks is far below than in shale. The presence of clay in the intercrystalline pores and clay minerals of carbonate rocks, especially illite/smectite (I/S) mixed layers that expand with water in the samples, may reduce the porosity and permeability and increase the BP of carbonate rocks. Because linear correlations exist between the BP and the reciprocal of porosity, the reciprocal of permeability, and the clay content, a principal component regression procedure (PCRP) was implemented with the statistical analysis system software to eliminate the correlations among three dependent variables and obtain a fitting formula for the BP. The formula indicates that these dependent variables are positively correlated with the BP, consistent with our analysis. The curve of the calculated BP with depth nicely matches with the interpretation of gas logging results. This study provides a research idea and a mathematical calculation model of the BP of carbonate caprocks to further valuate and make predictions about carbonate caprocks.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018