Pressure Drawdown in a Well With Limited Flow Entry

Abstract

Pressure response is given for a well of limited flow entry producing from Pressure response is given for a well of limited flow entry producing from (1) an oil reservoir with a gas cap, (2) an oil reservoir with a gas cap in a zone of low permeability, and (3) an oil reservoir with impermeable top and bottom boundaries. The pseudoskin due to limited flow entry is derived for these types of reservoirs and is presented graphically. Introduction Flow tests conducted to estimate reservoir properties often require calculations of unsteady-state pressure distribution in wells of limited flow entry. Deformation of the flow pattern due to the limited flow entry causes a resistance superimposed on that of a uniform radial flow and results in additional pressure losses; the magnitudes of these losses depend on the geometric and hydraulic properties of the system.Hydrocarbon reservoirs often consist of either formations containing oil or formations containing oil with a gas cap. The gas/oil contact may be in zones of different permeability, because reservoirs are seldom homogeneous. Although there have been many studies of unsteady- and steady-state behavior of partially penetrating wells, the literature is still lacking a unified approach for determining the effect of limited flow entry on well productivity in various types of reservoirs.Nisle and then Brons and Marting investigated the effect of partial penetration on the pressure drawdown and buildup of oil wells in an isotropic layer with no-flow boundary conditions at the top and bottom of the layer, the flow entry of the well being adjacent to either the top or bottom boundary. Since, as Brons and Marting noted, the function of partial penetration cannot be expressed analytically, partial penetration cannot be expressed analytically, numerical calculations of the function have been given as a pseudodamage or pseudoskin, presented in graphical form. This graph has also been given by Earlougher, though without indication of the limits of its application. The effect of anisotropy on pressure transient analysis for wells of restricted flow pressure transient analysis for wells of restricted flow entry of arbitrary location has been studied by Seth and Kazemi and Seth on the basis of the Hantush solutions for a partially penetrating well. For this case again, no-flow boundary conditions were assumed at the top and bottom boundaries of the formation with a uniform flow along the producing section of the wellbore. A steady-state solution for an oil well with limited entry to flow in an isotropic formation has been given by Odeh, the solution later being developed by Odeh to include the effect of an altered zone around the wellbore, the top and bottom boundaries of the formation being assumed impermeable to flow. Estimation of the skin effect for damaged wells of partial penetration has been given by Jones and Watts.The combined effect of wellbore damage, storage, and partial penetration on well test analysis has been considered by Bilharts and Ramey. The reservoir was assumed to be closed to the over- and underburden. A comprehensive review of previous studies of transient flow to partially penetrating wells has been given by Gringarten and Ramey. JPT P. 1469