Investigation of gas and water coning behavior for the enhancement of oil production

Abstract

This study presents the extensive simulation to control the concurrent behavior of gas and water coning in oil reservoir with existence of a bottom aquifer. From simulation results, coning phenomena were observed even with the critical oil rate obtained analytically. It is because the critical rate is calculated using a steady state expression. In order to examine the coning behavior, firstly, we have run for various oil layer thicknesses. The result in case of thin layer shows early breakthrough of gas and water cones and the increase in water-oil ratio from the beginning of production. Meanwhile, for the thick case of 200 ft, there is no water breakthrough observed even though water cone has been already formed because it is stable. Since gas and water cones move mainly in a vertical direction, cone development is affected by a vertical permeability. As a result of runs for vertical permeabilities, the breakthrough time is getting delayed as the vertical permeability is smaller. In the case of a high vertical permeability, the shape of the water cone is developed in a concave form at the beginning. After two years of production, however, this cone shape becomes almost flat since the water-oil contact is elevated uniformly throughout the whole reservoir. In the analysis of coning behavior for different aquifer sizes, it is found that the aquifer size does not affect both cone shape and water-cut. But with a strong bottom aquifer the behavior of gas coning is greatly decreased since the pressure is maintained by the active aquifer. The extent of well penetration into the oil layer has a considerable effect on coning phenomena. As the completion interval is decreased, the breakthrough time is delayed. However, a large pressure drop occurs in the shortest interval so that it worsens the well productivity. The most practical method to control coning is the oil production rate. Production of gas and water can be minimized by keeping oil rates as low as possible. However, a low rate is directly linked to well's economics, and therefore, the optimizing process for the production rate is essential.