Application of mercury injection and rate-controlled mercury penetration in quantitative characterization of microscopic pore structure of tight reservoirs: A case study of the Chang7 reservoir in Huachi-Heshui area, the Ordos Basin

Huachi-Heshui area in the Ordos Basin is a typical enrichment region for tight reservoirs. It has poor physical properties and complicated characteristics of microscopic pore throat structure, which has a great effect on oil and gas accumulation and exploitation.Mercury injection and rate-controlled mercury penetration were used to quantitatively characterize nanometer pore throats and to study fluid mobility characteristics in tight sandstone reservoirs in the seventh member of the Yanchang Formation (Chang7). The reservoir in the study area has high displacement pressure, low average throat radius, large pore/throat volume ratio, big pore throat and poor permeability. The distribution range of pore radius of rock samples with different properties is consistent, while the distribution of throat geometry among them is obviously different. Mercury injection saturation increases with the increase of pore numbers. The curve of S_Hg-ΔS_Hg/ΔP_c can preferably reflect mercury injection velocity and pore throat structure. Nanometer throat pore sare well developed in tight reservoirs, which make a great contribution to reservoir capacity and permeability. In the course of injection, fluid first enters the main controlling area of the pore, then the common control area of pore and throat, and finally the main control area of the throat.Rate-controlled mercury penetration is useful for the reservoirs with good physical properties, but it can not reflect the characteristics of nanometer pore throats in studying the pore throat structure of tight reservoirs.