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substituting the appropriate thickness, h, and viscosity, µ, values. <br /> Skin Factor <br /> • In theory, wellbore skin is treated as an infinitesimally thin sheath surrounding the <br /> wellbore,through which a pressure drop occurs due to either damage or stimulation. <br /> Industrial injection wells deal with a variety of waste streams that alter the near wellbore <br /> environment due to precipitation, fines migration, ion exchange, bacteriological <br /> processes, and other mechanisms. It is reasonable to expect that this alteration often <br /> exists as a zone surrounding the wellbore and not a skin. Therefore, at least in the case of <br /> industrial injection wells,the assumption that skin exists as a thin sheath is not always <br /> valid. This does not pose a serious problem to the correct interpretation of falloff testing <br /> except in the case of a large zone of alteration,or in the calculation of the flowing <br /> bottoinhole pressure. Region 6 has seen instances in which large zones of alteration were <br /> suspected of being present. <br /> • The skin factor is the measurement of the completion condition of the well. The skin <br /> factor is quantified by a positive value indicating a damaged completion and a negative <br /> value indicating a stimulated completion. <br /> 1. The magnitude of the positive value indicating a damaged completion is dictated <br /> by the transmissibility of the formation. <br /> 2. A negative value of-4 to -6 generally indicates a hydraulically fractured <br /> completion, whereas a negative value of-1 to -3 is typical of an acid stimulation <br /> in a sandstone reservoir. <br /> 3. The skin factor can be used to calculate the effective wellbore radius, rwa also <br /> referred.to the apparent wellbore radius. (See Appendix, page A-13) <br /> 4. The skin factor can also be used to correct the injection pressure for the effects of <br /> wellbore damage to get the actual reservoir pressure from the measured pressure. <br /> • The skin factor is calculated from the following equation: <br /> plhr Pw <br /> s=1.1513 f —log k•t n 2 +3.23 <br /> m(tp +1)•0•U-c, -rw <br /> where, s =skin factor, dimensionless <br /> P,h,=pressure intercept along the semilog straight line at a shut-in time of 1 hour, <br /> psi <br /> P,,,f=measured injection pressure prior to shut-in, psi <br /> µ =appropriate viscosity at reservoir conditions,cp(See Appendix, page A-14) <br /> m = slope of the semilog straight line, psi/cycle <br /> k=permeability, and <br /> =porosity, fraction <br /> ct=total compressibility, psi"' <br /> r,,=wellbore radius, feet <br /> tP= injection time, hours <br /> A-11 <br />