Experimental investigation of the pore structure characteristics of the Garau gas shale formation in the Lurestan Basin, Iran

Abstract

Gas storage and flow characteristics of shale gas reservoirs are greatly affected by their complex pore structure. These structures contain a wide range of pore size distribution with significant contribution of nanometer scaled pores. In this work an integrated experimental approach including geochemical and mineralogical analysis, Mercury Injection Capillary Pressure (MICP) and helium expansion porosimetry along with nano-scale Field Emission-Scanning Electron Microscopy (FE-SEM) imaging techniques was used to characterize pore structure of Garau formation, a shale gas play recently explored in the western Iran.MICP was used in this study as an acceptable laboratory method to identify pore throat size down to three nanometers. FE-SEM image analysis was also applied for better understanding of distribution, connectivity and nature of sub-micron pore systems. Experimental measurements were made on 23 plugs collected from two depth intervals from two wells in the Garau formation. X-Ray Diffraction mineralogy (XRD) analysis represents that shale samples are calcite rich with low clay contents. The kerogen analysis shows kerogen type II with Total Organic Carbon (TOC) values ranging from 0.41 to 3.18%. The total porosity obtained from helium expansion test, varies between 0.1 and 3.6% under ambient test conditions and between 0.1 and 1.2% under overburden stress test conditions suggesting high stress dependency of the porosity in the studied samples. Based on the MICP test results, the pore throat sizes in the Garau shale samples belong to the mesopores and macropores realm. In addition, two types of pore size distributions are observed in the samples according to their mineralogical compositions. The majority of pores in the first group with high calcite contents are in macro-scale (>50 nm) with the dominant pore radius of around 50 μm, while the dominating pore radii in the second group with illite traces are found to be in the meso-scale (5–50 nm). Furthermore, comparing the MICP and helium porosimetry test results shows that the total porosity of the Garau shale increases with the increase of the meso-scale pore volume. Observations from FE-SEM images were also in good agreement with MICP test results. FE-SEM analysis revealed that pores are located both in the organic matter and inorganic matrix whether in isolated or interconnected types. These observations showed that a portion of kerogen body is occupied by nano-scaled pores with different shapes and sizes; as a result, samples with higher TOC contents and maturity offers generally higher total porosity values.