Experimental investigation of the pore structure characteristics for the high-salt formation in Qianjiang depression, China

Abstract

Special attention has recently been paid to the high-salt formation in Qianjiang depression as its abundant oil resources with ultra-low permeability and porosity. However, the complex pore structure characteristic of the high-salt formation has not been well understood so far because of its pore size ranging from nanometer to micrometer scale. This paper aims to investigate the complex pore structure characteristic of the target high-salt formation based on an integrated experimental approach involving X-ray diffraction (XRD), scanning electron microscope (SEM), mercury injection capillary pressure (MICP), casting lamella observation and nuclear magnetic resonance (NMR) techniques. XRD analysis results indicate that the target formation mainly includes clastic mineral, carbonate mineral and salt mineral, and the content of them are 32.16%, 36.59%, and 24.65%, respectively. The results of SEM and casting lamella observation indicate that storage space of target formation can be divided into intercrystal pores, solution pores, and fractures. Solution pores have great impact on oil production, which includes dolomite dissolved pore and glauberite dissolved pore. Based on the MICP test results, the samples can be divided into pore type and fracture-pore type. The average pore radius of pore type samples and fracture-pore type samples are 0.0144 μm and 0.40 μm, respectively. Pore size distribution (PSD) results from MICP indicate that single peaks appeared in PSD curves of pore type samples and multi-peaks appeared in PSD curves of fracture-pore type samples. Permeability contribution peaks of pore type samples range from 0.01 to 0.03 μm, and permeability contribution peaks of fracture-pore type samples range from 1 to 10 μm. Comparing the permeability contribution of pore type samples and fracture-pore type samples reveals that mesopores and macropores make the vast majority of permeability contribution of the two kind type samples. A method to get permeability contribution of pore throats from PSD results has been introduced in this paper based on T2 NMR test. Comparing the PSD results from MICP and T2 NMR test results shows that T2 NMR test can identify smaller pore throat.