Evidence for paleo-condensate water in tight sandstone reservoirs in the eastern Ordos Basin, China: hydrochemistry, stable isotope (δ2H, δ18O, 87Sr/86Sr) and production data

Abstract

Understanding the origin and gas–water relationships of formation water in rocks can help promote the economic and effective development of tight sandstone gas reservoirs. Previous studies suggest that water produced in low-permeability tight reservoir sandstones is a mixture of injected fluid and formation water, and that tight sandstones have high water saturation. After the water saturation reaches 50%, no significant gas permeability exists in the formation, and the subsequent produced water largely determines the relative gas permeability. These assumptions form the basis for the current understanding of fluid sources and production behavior in tight reservoir sandstones. However, the mechanisms which have low-permeability and high water saturation yet produce low/no water cut require further investigation. In this study, we first proved the source of important produced water in tight sandstone gas wells in the Ordos Basin, which is paleo-condensate water subjected to ancient overpressure and high temperature and, subsequently, condensed from the gas phase. The characteristics of paleo-condensate water are high total dissolved solids (TDS), high Sr content, 87Sr/86Sr, Br depletion, and a hypersaline or brine CaCl2 water type. The residual brine formed after strong evaporation because of high temperatures, retains a heavier or more enriched 18O oxygen isotope composition. Sodium/calcium ion exchange occurs primarily to form albitization and slight dolomitization in tight sandstone reservoirs, whereas a considerable degree of mixing occurs with evaporated brine. Based on the single-well production of tight sandstone gas wells, it was concluded that the well production proves gas/water were stored together. For example, as water production increased, the daily gas production gradually increased. Moreover, the water/gas ratio was low, and no sustained increase in the ratio was observed in the water-producing gas wells. Studies have shown that the ability of natural gas to evaporate and carry water is 1136.7 g/m3 in a formation environment of 27.57 MPa and 100 °C (Bennion et al., 1996). During the Late Early Cretaceous period, pressure coefficients of 1.14–1.66 and formation temperatures of 120–200 °C within the Upper Paleozoic strata of the study area led to the formation of paleo-condensate water. The presence of paleo-condensate water not only elucidates the reason for low water production and/or no water production in tight sandstone gas wells in the study area but also emphasizes the need to consider the properties and composition of formation water when evaluating natural gas resources with high water saturation in tight sandstones.