Experimental study on the pore structure characteristics of tight sandstone reservoirs in Upper Triassic Ordos Basin China

Abstract

Tight sandstone reservoirs typically show a wide pore size distribution, which ranges from several nanometers to several hundred micrometers, requiring a combination of several techniques to properly characterize the pore structure characteristics. In this article, scanning electron microscopy, nitrogen gas adsorption, pressure-controlled porosimetry, and rate-controlled porosimetry were applied to investigate the pore systems of five tight samples of Yanchang Formation in Upper Triassic Ordos Basin China. Pore throat types and shapes were qualitatively identified and classified by scanning electron microscopy and nitrogen gas adsorption, and pore size distribution was calculated by combination of nitrogen gas adsorption, pressure-controlled porosimetry, and rate-controlled porosimetry, which is proposed as a new method to obtain the overall pore structure characteristics of tight sandstone reservoirs; then analyzing microscopic pore structure controls of permeability stress sensitivity of tight sandstone reservoirs. Results indicate that three typical pore types exist in tight sandstone reservoirs, which are interparticle pores, grain dissolution pores and micro cracks, and pore shapes contain sheet and bent sheet, cylindrical, and bottle neck shapes. Pore size distribution develops from nano-micrometer scale completely in tight sandstone reservoir, nitrogen adsorption experiments can accurately characterize nanometer scale pore size distribution mainly ranging 2–50 nm; nano-micrometer scale pore throats distribution characteristics are quantitatively analyzed by pressure-controlled porosimetry and rate-controlled porosimetry, pore throats radius ranges 10 nm–40 µm using the former, pores radius ranges mainly 80–300 µm, throats radius ranges 100 nm–5 µm and pore throat ratio difference is bigger by the latter, and the pore throat structures determine the typical reservoir characteristics in Ordos basin. The emphasis of study on mechanism of permeability stress sensitivity currently is microscopic pore throat structures instead of macroscopic permeability, indicating that the strongest is micro cracks and neck pores, intergranular cylindrical pores and intragranular dissolution pores is the weakest. This is significant to the development of tight sandstone reservoirs.