In this paper the coning of water or gas in vertical and horizontal wells is examined. The existing analytical solutions and correlations do not predict accurately the water breakthrough for horizontal wells; also solutions or correlations for predicting the gas breakthrough time do not exist. A three-dimensional, three-phase, black-oil, commercial simulator is used to examine the effects of various rack and fluid properties, well configurations, reservoir anisotropy, etc. on the breakthrough time and to curve correlations which can be used to predict the time at which gas or water cones into a vertical or horizontal well. A correction is applied to the simulator breakthrough times to incorporate the effects of numerical dispersion. The results obtained from the newly derived correlations are compared with existing analytical and numerical solutions for a number of cases; these comparisons indicate that the correlations developed in this work can be used to predict the water or gas breakthrough time in vertical and horizontal wells quite accurately.
Water and/or gas coning is a serious problem in many reservoirs with wells producing from an oil zone underlying a gas cap, overlying an aquifer or both, Coning occurs in a well on production, when the water or gas zone moves up towards the wellbore in the form of a cone.
Eventually, the water or gas breaks through into the well and water from the aquifer and/or gas from the gas cap is produced along with oil. The water or gas production increases progressively after breakthrough time and may reduce significantly the crude oil production.
The main factors affecting the water and/or gas coning tendency are the density difference between oil and gas or oil and water, the viscosity of water or gas, formation permeability, pressure drawdown, flow rate, etc. More specifically, the tendency of a fluid to cone is directly proportional to the density difference between the fluid and crude oil, but inversely proportional to the fluid viscosity and reservoir permeability.
The water and/or gas coning can be reduced by (i) decreasing the well production rate; (ii) improving the productivity of the well; (iii) using horizontal instead of vertical wells to produce the formation; (iv) selectively partially penetrating the well at the top of the reservoir in the case of water coning, at the bottom of the reservoir in the case of gas coning, and close to the center of the pay zone in the case of simultaneous water and gas coning; (v) recompleting the well at a different elevation to increase the distance between the gas-oil or water-oil contact and the perforated interval; and (vi) infill drilling.
Most of the research efforts in the area of water and gas coning have concentrated on estimating the critical oil rate and the post breakthrough well behavior. In the subsequent parts of this section some of these studies will be discussed for both vertical and horizontal wells.
One of the first papers published on the coning phenomena was by Muskat and Wyckoff1.