Gas Coning in Vertical and Horizontal wells, a Numerical Approach

Abstract

Abstract In the presence of gas cap, production from a well is limited by the maximum, critical oil flow rate. If oil production rate is above this critical value, gas breakthrough occurs. After the breakthrough, the gas phase may dominate the total production rate to the extent that further operation of the well becomes uneconomical and the well must be shut in. In the oil industry this phenomenon is also referred to as "gas coning."" Until recently, several techniques have been used by the industry to prevent gas breakthrough due to coning. These methods include: keeping production rates below the critical value, perforating as far from the initial gas-oil contact (GOC) as possible, or creating a gas-blocking zone around the well by injecting crosslinking polymers or gels. Unfortunately, all these conventional methods do not solve the gas breakthrough problem. It is usually uneconomical to keep production rate of a well below the critical rate. Perforating far above the GOC reduces the length of the perforation interval and, thus, increases pressure drawdown around the well. The increased pressure drawdown may enhance gas coning. Despite the fact that the coning has been studied extensively since the fifties, it is still difficult to answer to the following two questions:How to perforate a well subjected to the coning?What is the optimum oil flow rate of the well? In this paper a numerical approach is used to study the gas coning using a 3-D radial model, and gas dipping using a 3-D irregular cartesian model, where well A, from Hassi-R'mel field in Algeria, is chosen for a case study. In the first part, a sensitivity study was performed to analyze the most relevant parameters that affect the coning. In the second part, a new technique is proposed to interpret the variation of the different parameters that affect gas coning. It consists in plotting the free gas oil ratio (GOR-RsBHP) against the cumulative oil production on a log-log scale. This model reveals straight lines that occur after gas break through. In the last part of this study, regression analysis was performed to develop empirical gas coning correlations to predict critical oil rate, gas breakthrough time and gas oil ratio (GOR) after breakthrough for both vertical and horizontal wells. Introduction When a reservoir is produced by primary methods, production economics can be influenced by controlling the number and location of wells and the flow rate of each well. However, flow rates are usually restricted when facing coning problems. Production from oil reservoirs with bottom water or gas cap, or from a gas reservoir with bottom water is always associated with coning. Gas or water coning is a serious problem in many oil field applications. It can reduce oil production significantly. Therefore, it is important to minimize or at least delay coning. In reservoirs with a gas cap, vertical wells are normally perforated as low as possible to minimize or delay the gas coning. This assumes that there is no bottom water. Similarly, in a reservoir with bottom water, vertical wells are normally completed in the top section of the pay zone to minimize or delay water coning, if there is no gas cap. If an oil reservoir has both, gas cap as well as bottom water table, then the vertical well is normally perforated either near the center of the oil zone or below the center, toward the water zone. This is because coning tendencies are inversely proportional to the density difference and the viscosity of the fluids. The density difference between gas and oil is normally larger than the density difference between water and oil. Hence, gas has less tendency to cone than water. However, gas viscosity is much lower than the water viscosity, herefore, for the same pressure drawdown in a given reservoir, the gas flow rate will be higher than the water flow rate. Thus, density and viscosity differences between water and gas tend to balance each other. Therefore to minimize gas as well as water coning, a preferred perforated interval is at the center of the oil pay zone. From the practical standpoint, however, many wells are perforated closer to water-oil contact than to the gas-oil contact.