Recovery efficiency of tight oil reservoirs with different injection fluids: An experimental investigation of oil-water distribution feature

Abstract After depletion development stage, the well-utilized approach to enhance recovery of tight oil reservoirs is fluid-injection, involving nitrogen, water, and CO2. Although a great deal of research efforts has been devoted to relevant scope, it is still challenging to identify the appropriate fluid type which can achieve favorable recovery efficiency. Also, the oil-water distribution feature in tight oil reservoirs from the microscopic view is rarely investigated, especially for that after fluid-injection. Moreover, inherent TPG (threshold pressure gradient) will make a great difference on the assignment of well-spacing, which have not well investigated. Given the current situation, this work expects to bridge up the knowledge gap. First, the original oil-water distribution feature and that after fluid-injection are revealed through laboratory experiments, in which different kinds of fluid are measured. Then, the relationship between TPG and fluid type is quantified based on another set of experiments. After that, considering the pressure distribution characteristics in whole tight oil reservoirs, the optimization plate for well-placing under different fluid types is concluded. Results show that (1) Gas-flooding shares advantages compared with water-flooding, displacement efficiency for gas-flooding is over 40% and that for water-flooding is 36% on average; (2) the main pore types initially occupied by oil consists of pore-throat, cluster, corner and intergranular, which account for 9.51%, 26.42%, 15.51%, and 13.82% respectively. And the major contributor to oil recovery comes from cluster pore-type; (3) gas-flooding approach is recommended as for the low-permeability case, and the water-flooding approach becomes favorable as for the high-permeability case.