Development of horizontal well technology offers a new approach toreducing gas/water coning effect during oil production. However, further improvement of the oil production rate of horizontal wellsis still limited by the encroachment of the water cone when bottom water exists. This paper provides reservoir engineers with a general solution of the gas/water coning problem for horizontal wells. The objectives of this investigation include:
to determine the location of the stable water/gas cone,
to estimate the critical oil production rate, and
to find out the optimum placement of the horizontal wellbore so that the maximum critical oil rate can be obtained.
Using conformal mapping method, the location of the stable water/gas cone is determined as a function of oil rate and wellbore location. The critical oil rate is estimated by examining the profile development of the stable cone. It is found that the critical rate is directly proportional to the effective permeability, thickness of the oil reservoir, and density contrast between oil and the coning fluid, and inversely proportional to the oil viscosity. The critical oil rate also depends upon the wellbore location in the oil reservoir and takes its maximum value when the horizontal wellbore is placed at about 70 % of reservoir thickness from the unwanted fluid.
Oil production through a horizontal well causes the water (or gas) -oil interface to deform into a crest (or dip) shape. As the production rate is increased, the height (or depth) of the water-crest (or gas-dip) also increases until at a certain production rate the water crest (or gas-dip) becomes unstable and water (or gas) flows into the well. This phenomena is referred to two-phase interface coning. The maximum production rate without water (or free gas) is called the critical rate.
The encroachment of the water (or gas) cone is a limitation for improving oil production rate in horizontal wells, especially for some cases where the thickness of the oil reservoir is small. Maximizing the critical rate is a very practical problem. It is also of vital importance to know the location of the water crest (or gas dip) for evaluating the oil recovery from horizontal wells. Since horizontal wells have been drilled commercially in the oil industry only in recent years, very few studies have been reported about their water/gas-oil coning behavior.
Giger presented an analytical 2-D model of water cresting before breakthrough for horizontal wells. Since he used the free surface boundary condition and assumed that the free surface is at a large distance, the oil height in the model may be difficult to choose. Although the mathematical solution was modified by the author, he still suggested that these solutions should not be used for small values of dimensionless drainage radius. In fact, an ideal solution similar to Giger's solution was also given by Efros in the early 1960s. But again, this solution is not advisable to use in practice. Chaperon studied the behavior of cresting towardhorizontal wells in an anisotropic formation assuming constant interface elevation at a finite distance. Her approach is identical to that used by Muskat. Since she neglected the flow restriction due to the immobile water in the crest, her theory might give an over optimistic evaluation of critical rate. Kuchuk et al. analyzed the pressure transient behavior of horizontal wells with and without a gas cap or aquifer. They considered the irregular two-phase boundary as a constant pressure boundary, and assumed that the gravity effect is negligible and the viscosity of fluid is constant throughout the medium. The existence of the gas or water crest does not affect the pressure response according to their solution. Ozkan and Raghavan investigated the time-dependent performance of horizontal wells subject to bottom water drive.