Gel Placement in Production Wells

Abstract

Summary Straightforward applications of fractional-flow theory and material-balance calculations demonstrate that, if zones are not isolated during gel placement in production wells, gelant can penetrate significantly into all open zones, not just those with penetrate significantly into all open zones, not just those with high water saturations. Unless oil saturations in the oil-productive zones are extremely high, oil productivity will be damaged even if the gel reduces water permeability without affecting oil permeability. Also, in field applications, capillary pressure will not prevent gelant penetration into oil-productive pressure will not prevent gelant penetration into oil-productive zones. An explanation is provided for the occurrence of successful applications of gels in fractured wells produced by bottomwater drive. With the right properties, gels could significantly increase the critical rate for waterinflux in fractured wells. Introduction Coping with excess water production is always a challenging task for field operators. The cost of handling and disposing produced water can significantly shorten the economic producing life of a well. The hydrostatic pressure created by high fluid levels in the well also is detrimental to oil production. The two major sources of excess water production are coning and channeling. Water coning is common when a reservoir is produced by a bottomwater drive mechanism. Fractures and high-permeability streaks are the common causes of premature water breakthrough during waterfloods. Polymer gels have been applied to many wells to reduce excess water production without adversely affecting oil production. Moffitt production without adversely affecting oil production. Moffitt reported that polymer gels are particularly effective in suppressing water coning. In many cases, however, gel treatments have not been successful. Part of the reason for the sporadic success was problems with gel placement. During gel placement in production wells, much of the gel formulation will enter zones production wells, much of the gel formulation will enter zones responsible for the excess water production. However, some of this fluid may enter and damage oil-productive strata. The objectives of this study are to model gel placement in production wells mathematically and to examine the potential effect production wells mathematically and to examine the potential effect of gelant invasion into oil-producing zones. Particular attention is paid to the importance of two phenomena. The first is hysteresis of oil/water relative permeability curves during the "pump-in, pump-out" sequenceused during gel placement in production wells. pump-out" sequence used during gel placement in production wells. The second phenomenon is that gels(or polymers) can reduce the relative permeability to water more than to oil. Sensitivity studies covering most known field and laboratory applications are discussed. In particular, we study permeability contrasts from 1 to 1000, oil/water viscosity ratios ranging from 0.1 to 100, endpoint water relative permeabilities ranging from 0.1 to 0.7, water saturations ranging from 0.2 to0.8, and fractured and unfractured wells. Therefore, our conclusions should be applicable to most field applications of gels in production wells. Examples are provided to illustrate and contrast situations where gels are or are not expected to damage oil productivity. In these examples, we assume that the gels reduce water permeability without affecting oil permeability. This assumption is made as a best-case scenario to demonstrate that oil productivity can be damaged even if the gels do not affect oil permeability. If gels reduce oil permeability, then further oil productivity reduction should be expected. Our equations and analyses are general and can be applied readily for any degree of oil permeability reduction caused by the gel. Several terms should be deemed." Gelant" and "gelling agent" refer to the liquid formulation before gelation. Resistance factor, Fr, is defined as water mobility divided by gelant mobility. It is equivalent to the effective gelant viscosity in porous media relative to that of water. Residual resistance factor, Frrw, is defined as water mobility in the absence of gel divided by water mobility in the presence of gel and is a measure of the permeability reduction caused by gel. permeability reduction caused by gel. Theoretical Model The first objective of this analysis is to develop a theoretical model for gel placement in production wells. Fractional-flow theory is applied to model mathematically the degree of gelant penetration into zones with different permeabilities during penetration into zones with different permeabilities during unrestricted injection. Basic Assumptions. In examining the placement of gels in production wells, we made these assumptions. production wells, we made these assumptions. 1. All fluids are incompressible and Newtonian. 2. Gelant formulations are miscible with water. 3. The gelation reaction is slow relative to the placement process. placement process. 4. Dispersion, retention, and inaccessible PV are negligible. 5. Fr is independent of permeability. 6. There is no mass transfer between phases. 7. Gravity and capillary effects are negligible. 8. Darcy's law applies, and no fingering occurs during displacement. 9. Each layer ishomogeneous, isotropic, and isothermal. 10. The reservoir consists of a number of horizontal, noncommunicating layers. 11. All layers have the same areal dimensions and share the same injector and producer. (The layers can have different thicknesses.) For simplicity, we assume that the water and oilrelative permeabilities are functions of water saturation orgy. Eqs. 1 and permeabilities are functions of water saturation orgy. Eqs. 1 and 2 are used throughout this analysis for relative permeability calculations. (1) and (2) Linear Flow. For near-wellbore gel treatments in production wells, the gelation reaction often is slow relative to the placement process. Thus, the fluid flow in a porous medium during the placement process. Thus, the fluidflow in a porous medium during the placement of aqueous gelants ran be assumed to be the same as that of aqueous polymer solutions during the polymer-flooding process. Fig. 1 is a schematic of the saturation profile in Layer i at a certain instant during the placement process. In linear flow, the instantaneous pressure drop in Layer i between the producer and the injector is (3) PD is defined as the ratio of the pressure drop between Lpm and the injection well to the pressure drop between the production well and Lpm just before the injection of any gelants (see Ref. 9 for a more detailed discussion). The average water saturation behind the gelant front, S wi, is determined with the Welge integration procedure. Eq. 3 is simply a Darcy equation. Consider the case in which all layers share the same injector and producer and all fluids involved are incompressible. SPEPF P. 276