Porosity and permeability characteristics of compact Carboniferous limestone in the eastern Qaidam basin

The pore system of carbonate rock reservoirs is complex. The study of the pore structure characteristics and flow mechanism has a direct impact on the exploitation of oil and gas resources. This work analyzed five tightly consolidated Carboniferous limestone samples taken from the borehole ZK5-2 in the eastern Qaidam basin. The mineral and chemical compositions of these samples were analysed using X ray diffraction (XRD) and X ray fluorescence (XRF) techniques; and the porosity and pore structure characteristics were further studied respectively using helium double chamber method and aperture distribution tests (using the mercury intrusion, nitrogen adsorption and carbon dioxide adsorption methods). Four kinds of gases, i.e. methane, nitrogen, carbon dioxide and helium, were used in the seepage experiments to measure the apparent permeability of each sample. The results show that there is no direct connection between the apparent permeability and porosity of these limestone samples. The measured values of the apparent permeability, which are jointly governed by the Darcy, slip and diffusion flows, increase in the order of CH4, N2, CO2 and He. The apparent permeability values measured using He are significantly higher than other gases. The contribution of the slip flow to the apparent permeability is controlled by both the molecular mean free path and the pore size. When the molecular mean free paths are close to the pore size of the rock, there will be a significant slippage in these pores and the slip flow will make significant contribution to the apparent permeability. Gases with large molecular mean free path flow mainly by diffusion in dense rock. The contribution of diffusion flow to apparent permeability increases with the proportion of rock pores that are smaller than the molecular mean free path.