Predicting Gas Well Performance Coning Water in Bottom-Water-Drive Reservoirs

Abstract

Kabir, C.S., Schlumberger Technical Services Member SPE-AIME Abstract Depletion strategy of a gas reservoir underlain by an active water drive requires careful thought and planning. Producing rate and completion interval are the apparent key Producing rate and completion interval are the apparent key considerations in a depletion scheme because of the potential water coning that may affect the ultimate recovery. As this study indicates, both the considerations are relatively unimportant from the stand point of theoretical ultimate recovery. The primary purposes of this investigation are to develop an analytical solution for the water coning problem and to provide guidelines on the depletion strategy in thin reservoirs (less than 60 ft). A systematic study of the variables indicates that permeability and pay thickness are the most important variables governing coning phenomenon. Other variables such as penetration ratio, horizontal to vertical permeability ratio, well spacing, producing rate, and the permeability ratio, well spacing, producing rate, and the impermeable shale barriers have very little influence on both the water-gas ratio response and the ultimate recovery, Generalized analytical solutions that are developed in graphical form, using a numerical coning simulator, allow a multitude of calculations. These calculations include predicting a well's behavior with or without a production predicting a well's behavior with or without a production history, determining an economical with drawal rate, generating a rate-time profile, and estimating a breakthrough time. The type curves could also be used as a diagnostic tool to identify possible completion problems. Introduction Water coning in bottom water-driven gas reservoirs is a complex phenomenon that merits careful attention. Because water coning may affect the ultimate gas recovery, an operator is often confronted with the dilemma of determining a well's completion interval and its producing rate. A low with drawal rate, below the critical coning rate, would produce water-free gas over a long period of time, while a high producing rate may considerably reduce a well's effective producing life. The latter case could be economically attractive. Muskat and Wyckoff show that the critical coning rate is a strong function of the penetration ratio and the formation thickness for oil-water systems. Thus, completion strategy would largely depend on the desirability of producing water-free gas. However, the ultimate gas producing water-free gas. However, the ultimate gas recovery is insensitive to the penetration ratio - a fact that is demonstrated in this work. Water coning in gas wells located in bottom water-drive reservoirs lacks a thorough treatment in the literature. Two papers address thick formations (100 ft +) and one papers address thick formations (100 ft +) and one reports on productive intervals that are characterized by 20 to 124 ft of gas pay. This limited investigation presumably stems from economic considerations when compared to oil wells for which numerous articles have been published. However, studies on edge water-drive gas reservoirs have appeared in the literature. Although the literature contains scanty information on the water coning in gas reservoirs, a great deal could be learned by studying the water coning behavior in oil wells. Because of the similarities between a well with a high oil mobility and a typical gas well, we may expect similar trends in their coning performance; thus, we see fit to explore the literature dealing with the water coning problem in oil wells. Early works of Muskat and Wyckoff and Arthur present simplified analytical models for homogeneous present simplified analytical models for homogeneous formations. They developed graphical solutions for critical coning rates for various formation thicknesses and penetration ratios, and reservoir and fluid properties. penetration ratios, and reservoir and fluid properties.