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Steady-state radial flow in reservoirs

The measurement commonly known as “reservoir pressure” is a measurement of the pore fluid pressure pp in a porous reservoir. The reservoir pore fluid pressure is the fraction of the overburden pressure that is supported by the fluid system. The other fraction, the effective stress σv′ is supported by the rock. The overburden stress (σv) is caused by the weight of the fluid and rocks in the lithostatic column above the measured point. Pore pressure is linked to rock stresses with the relation (1) The static pressure measurement always results from some form of transient test, where a specific volume of fluid is withdrawn from the well before the pressures are allowed to stabilize. The efficiency of wireline testing, where static pressures can be acquired at the rate of possibly one measurement every few minutes, results from the small volume of the fluid sample. Conversely, in conventional well testing, static pressures take much longer to stabilize only because of the larger amount of fluid withdrawn, which creates pressure disturbances observable at much greater distances into the reservoir. The term “sandface pressure” refers to the value of the pressure existing at the boundary between the reservoir and the wellbore, whether the reservoir flows (drawdown or flow tests) or not (shut-in or buildup tests). Ideally the sandface pressure would be the pressure measured by a wireline tester (considering the probe penetration to be nil) or the well pressure—static or flowing—measured at depth in the well by a hanging pressure gauge. To illustrate the relationship between the reservoir pressure and the reservoir dynamic properties, this book will review the essentials of steady-state (or stabilized) flow in reservoirs and the propagation of pressure in reservoirs under the effect of transient flow conditions. Steady-state radial flow in reservoirs Flow through a homogeneous reservoir into a wellbore is considered radial when the flowlines are horizontal, parallel and converge toward the wellbore axis. Infiniteacting radial flow (IARF) is a special case of transient flow regime illustrated in Fig. 4. In IARF, an idealized cylindrical model can be used to calculate flow rates and describe the pressure distribution away from the wellbore.