A visualized investigation on the mechanisms of anti-water coning process using nitrogen injection in horizontal wells

Abstract

Horizontal wells have been widely used in oil reservoirs with bottom aquifer. As bottom water coning occurs, water cut of the production well rises sharply and the output declines rapidly. In this paper, an experimental study on the mechanism of anti-water coning and oil production increment by injecting nitrogen in horizontal wells was carried out. A theory of bubbling behaviors induced by gas permeating through a porous medium is skillfully applied to analyze the mechanisms of anti-water coning by injecting nitrogen. A series of visual experiments and a one-dimensional displacement experiment are conducted to intuitively study bubble characteristics and blocking mechanisms in porous media, as well as the microscopic mechanisms of the improvement of well performance by injecting nitrogen. The results show that there are three forms of nitrogen in the water channeling path: tiny bubble, elongated bubble, and gas channeling, which are related to water saturation and gas injection volume in the water channel. Elongated bubble, the main mechanism of anti-water coning by injecting nitrogen, can block high-permeable water channeling path to a certain extent. As the viscosity of crude oil increases, the negative effect of anti-water coning and the increment of bottom water sweeping range show up. It is suitable for non-homogeneous reservoirs, which is conducive to the exploitation of oil on the top layer in low permeability oil region. It can also largely increase the sweeping area of bottom water in low permeability reservoirs. But for high permeability formation, water suppression effect is poor. The earlier the injection of nitrogen gas, the better the effect of inhibiting water encroachment which can even reduce the water cut to zero. However, the effect of oil production increment will be not that good. This paper provides reference for the study in anti-water coning and increasing oil production by injecting nitrogen through the horizontal wells.