Abstract
Due to the special micro-pore structure and the seepage law of tight reservoirs, the research on the development of tight oil is quite different from conventional reservoirs. For the tight oil reservoirs recovered with the gas injection, the gas breakthrough is an eternal theme as a result of the preferable mobility of the gas and the strong heterogeneity of the reservoirs. It is extremely important to evaluate the sweep efficiency. Based on the stream-tube method and the non-Darcy theory, this paper establishes a rapid evaluation technique of sweep efficiency considering the mechanism of the gas flooding and the seepage characteristics of the tight oil reservoir.
Firstly, the relative permeability under different miscible condition are determined through the revised Coats model. Besides, the Todd-Longstaff model is adopted to describe the varying viscosities of oil and gas. Secondly, the stream-tube model of the inverted nine-spot well pattern with fracture is established. Next, the seepage equations of oil and gas in the stream-tube is constructed considering the threshold pressure and the variation of the viscosity and relative permeability. Then, the sweep efficiency is obtained by solving these equations. Furthermore, an application example for evaluating the sweep efficiency is presented and sensitivity analyses are conducted to study the effect of the viscosity, pressure difference, fracture permeability and well spacing taking the case of a real tight reservoir.
Through the analyses, it can be concluded that the factors have remarkable impacts on the sweep efficiency. The threshold pressure increases the resistance and reduces the flow rate, leading to a lower sweep of the injected gas. Even worse, the excessive threshold pressure results in that the effective displacement cannot be established. The fracture greatly shortened the breakthrough time and result in early channeling of the injected gas. The sweep efficiency is improved through the increase of the pressure difference and decrease of the well distance. Consequently, in order to improve the sweep efficiency of the tight reservoirs, a reasonable displacement pressure difference and a well pattern adapted to the reservoir are needed. This paper presents a rapid and effective technology to evaluate the sweep efficiency of the tight reservoirs recovered with gas injection, which provides an important basis for improving the sweep efficiency and fine development of the tight reservoir.
