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Note that the term tp/(tp+At), where At=1 hr, appears in the log term. This term is usually assumed to result in a negligible contribution and typically is taken as 1 for large t. However, for relatively short injection periods, as in the case of a drill stem test (DST), this term can be significant. Radius of Investigation • The radius of investigation, ri, is the distance the pressure transient has moved into a formation following a rate change in a well. • There are several equations that exist to calculate the radius of investigation. All the equations are square root equations based on cylindrical geometry, but each has its own coefficient that results in slightly different results, (See Oil and Gas Journal, Van Poollen, 1964). • Use of the appropriate time is necessary to obtain a useful value of ri. For a falloff time shorter than the injection period, use Agarwal equivalent time function, At., at the end of the falloff as the length of the injection period preceding the shut-in to calculate ri. • The following two equivalent equations for calculating ri were taken from SPE Monograph 1, (Equation 11.2) and Well Testing by Lee (Equation 1.47), respectively: r; = 0.00105 k t _ k t 0.p.c, �948-0.p.c, Effective Wellbore Radius • The effective wellbore radius relates the wellbore radius and skin factor to show the effects of skin on wellbore size and consequently, injectivity. • The effective wellbore radius is calculated from the following: rW, = r,,e -s • A negative skin will result in a larger effective wellbore radius and therefore a lower injection pressure. Reservoir Injection Pressure Corrected for Skin Effects • The pressure correction for wellbore skin effects, AP,k,,,, is calculated by the following: OPsk,,, = 0.868•m•s where, in = slope of the semilog straight line, psi/cycle s=wellbore skin, dimensionless • The adjusted injection pressure, P,,,fa is calculated by subtracting the AP,kin from the measured injection pressure prior to shut-in, P,,,f. This adjusted pressure is the calculated reservoir pressure prior to shutting in the well, At=O, and is determined by the following: A-12