Enhancement of Productivity in Oil and Gas-Condensate Wells by the Artificial Alteration of Wettability

Abstract

Abstract Operating experience shows significant declines of actual productivity in oil and gas-condensate wells during exploitation. The decline of production rates in oil reservoirs occurs when the bottom-hole pressure (Pp) is lower than the bubble-point pressure (Pbp). The production decline in gas-condensate wells is caused predominantly by precipitation of retrograde condensate in the near-wellbore zones. This effect is significant in reservoirs with a high initial condensate content and, especially, in reservoirs with residual oil. The experimental results of this study demonstrate that the decline of production rate occurs in reservoirs in which the pore surfaces are initially partly non-water wet (hydrophobic), or become hydrophobic during filtration of oil and condensate in the porous medium. The procedure used for restoration of productivity in actual wells is described in general terms. The method is based on treatment of the reservoir with various chemicals to artificially alter the wettability of pore surfaces from completely or partially nonwater wet (hydrophobic) to water wet (hydrophilic). The process is called hydrophilization, and results in the restoration of the initial productivity, reducing the probability of water coning by an active aquifer or of water encroachment during water flooding operations. Results of practical, field application of hydrophilization are presented. Introduction Actual productivities of many oil and gas-condensate wells are lower than calculated rates, even though the calculations are based on correct values of hydro-conductivity, formation pressure (Pf), and producing bottom-hole pressure (Pp). The discrepancies are especially significant in two cases:Oil reservoirs in which the producing bottom-hole pressure (Pp) is lower than the bubble-point pressure (Pbp); and,Gas-condensate reservoirs with high initial condensate/gas ratios (CGRo >100 cm3/m3), especially when residual oil is present in the gas saturated part of the reservoir. By definition, the productivity of an oil well is the flow rate at a constant drawdown pressure. The productivity of a gas well is the flow rate at constant Pf2 - Pp2. A decline of well productivity has been reported in some oil and gas-condensate reservoirs(2, 3, 5, 9). This is especially characteristic of deep-seated (>3,000 m) oil and gas-condensate reservoirs as well as of oil reservoirs saturated by oil with polar hydrocarbons, especially asphaltenes. Wells in some gas-condensate reservoirs with very high initial condensate/gas ratios (CGRo greater than 300 cm3/m3) exhibit a sharp decline in productivity, to a third or less of the initial production rate, shortly after being put on stream. Reservoirs with lower CGRo values also exhibit productivity declines, but only after longer-term production. The declines are accompanied by changes in the chemical-physical properties of the produced condensates, particularly reductions in density, and by unstable rates of gas and condensate at constant Pf2 - Pp2 It is difficult to determine the optimum operational drawdown pressures in the late stages of gas-condensate field development, since large drawdown pressures are often accompanied by a reduction in condensate production.