Transient, Semisteady-State, and Steady-State Flow

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Transient Flow The pressure behavior at a well producing at constant flow rate is shown inFig. During early producing times, the pressure behavior is essentially thesame as that in an infinite reservoir. This is the transient flow period. Thepressure at the well can be described by the following equation for most timesof practical interests: well pressure = initial pressure - A [log (producing time)-B], where A and Bare constants. During this period, the plot of well pressure vs. the logarithm of time willbe a straight line. On Cartesian coordinates (Fig. 1) the well pressure willdecline rapidly at first, then less rapidly with time. Semisteady-State Flow Curves of pressure in the reservoir vs. radial distance are shown in Fig. 2. If there is no flow across the drainage boundary as more producing timeelapses, the pressure behavior will begin to deviate producing time elapses, the pressure behavior will begin to deviate from the infinite-reservoir case. At a time shown as "late transient" in Fig. 2, the pressure at points insidethe drainage boundary will begin to decline at a rate higher than that in aninfinite reservoir. Finally, as shown in Fig. 2, if compressibility is smalland constant, the rate of pressure decline becomes equal through the drainageboundary. This is the semisteady-state period. All pressures in the drainagearea of the well now decrease by the same pressures in the drainage area of thewell now decrease by the same amount in a given time. The difference betweenaverage reservoir pressure and wellbore pressure remains constant during thisperiod. pressure and wellbore pressure remains constant during this period. JPT P. 385