Complex gas-water contacts in tight sandstone gas reservoirs: Distribution pattern and dominant factors controlling their formation and distribution

In recent years, extensive exploration and exploitation activities in tight sandstone gas reservoirs have highlighted the common phenomenon of water production, indicating complex gas-water contacts. Exploring gas layers while avoiding water layers has become critical to the efficient exploration and exploitation of tight sandstone gas reservoirs. This study presents comprehensive geological analyses of gas-water contacts in simple gentle tectonic zones (tight sandstone gas reservoirs in the Sulige and Daniudi areas in the Ordos Basin), a transition zone of simple gentle to complex uplift (Hangjin Banner in the Ordos Basin), and complex uplift zones (tight-gas reservoirs in the western Sichuan Basin). Combined with the core-scale and pore-scale physical simulations of gas-water contact in tight sandstone, we clarify the types and characteristics of gas-water contacts in tight-gas sandstone reservoirs, reveal the dominant factors controlling the formation and distribution of intricate gas-water contacts based on the sand bodies, cores, and pores, and establish corresponding gas-water distribution patterns. Key findings are as follows. In terms of sand body, there are primarily six types of gas-water contacts within, including (1) the simple type of gas layer without water layer; (2) the normal type with gas layer underlain by water layer; (3) the inverted type with gas layer overlaid by water layer; (4) the hybrid type with gas and water in the same layer; (5) the isolated type with water layer within a gas layer; and (6) the simple type of water layer without gas. The distribution range, style, and boundary of gas-water contacts are governed by hydrocarbon-generating intensity, reservoir heterogeneity, and a combination of source rock-reservoir pressure differences and tectonic activity, respectively. At core-scale, permeability coupled with charging dynamics of the tight sandstone governs the critical conditions for the formation and distribution of gas-water contacts. At pore-scale, the coupling of pore throat size and coordination number with charging pressure dictates the fluid occurrence and seepage characteristics, determining the critical conditions for the formation and distribution of gas-water contacts. Owing to the collective effects of dominant factors from sand body, core-scale, and pore scale and their differences, tight-gas reservoirs with different source rock-reservoir assemblages exhibit different gas-water distribution patterns.