Micro-occurrence of formation water in tight sandstone gas reservoirs of low hydrocarbon generating intensity: Case study of northern Tianhuan Depression in the Ordos Basin, NW China

Abstract

A series of experimental techniques which contain casting thin slice observation, field emission scanning electron microscopy, high-pressure mercury injection, constant-velocity mercury injection, and NMR(nuclear magnetic resonance) were used to study the throat microstructure and microscopic occurrence of formation water in the Upper Paleozoic tight sandstone gas reservoir in northern Tianhuan Depression, Ordos Basin. The results show that intragranular dissolution pores, intercrystalline pores, and residual intergranular pores represent 33 %, 31 %, and 16 % of the pore types in the reservoir of the He8-Shan1 members in the northern Tianhuan Depression, respectively. Pore radii range from 80 μm to 300 μm, with an average of 154.18 μm, while throat radii range from 0.01 μm to 1.60 μm, with an average of 0.55 μm. Micron pores and nano throats control the seepage ability of the reservoirs, and the throat radius serves as the controlling factor. Formation water has four microscopic occurrences: bound water, capillary water, free water, and adsorbed water. At low charging pressure, a mixture of gas and water was found in the intergranular pores and dissolution pores controlled by macro throats, with a high gas content and existent free water; a small amount of membrane adsorption water was observed at high charging pressure. However, in the intergranular pores and dissolution pores controlled by medium-small throats, the mixture of gas and water at low charging pressure presents a low gas content and a large amount of capillary water, while at high charging pressure, the mixture of gas and water or pure gas presents a high gas content and a small amount of capillary water. Pure water controlled by tiny throats, on the other hand, was found at low charging pressure in the intergranular pores, whereas gas and water mixture was found at high charging pressure, but the gas content is low and bound water appears. In the intergranular micropores, pure water, which is bound water, is the primary composition at both low and high charging pressure. The throat radius cut-offs of the four kinds of formation water are 0.10 μm, 0.26 μm, and 0.28 μm, the permeability cutoffs are 0.21 × 10−3 μm2, 0.51 × 10−3 μm2, and 0.55 × 10−3 μm2, and the porosity cutoffs are 5.86 %, 7.99 %, and 8.18 %, respectively. The starting pressure gradient and throats less than 0.10 μm are controlling factors on the micro-occurrence and residual water saturation of formation water. As the amount of gas driving water increases in the natural gas accumulation process, the percentage of formation water controlled by large throats gradually decreases. Free water makes up 50 % of the research area, capillary water makes up 18 %, bound water compensates 30 %, adsorbed water is 2 %, and residual water saturation is about 32 %.

Keywords

• Tight sandstone
• Formation water
• Micro-throats
• Gas-water relationship