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Abstract

This work considers the analysis of pressure transient data obtained from a restricted-entry well under multiphase flow conditions. In particular, we examine the pseudoskin factor caused by partial penetration in a reservoir with multiple flowing phases.

Only the total skin factor, which is a linear combination of the mechanical skin factor due to damage or stimulation and the pseudoskin factor due to partial penetration, can be obtained directly from semilog analysis of pseudoradial flow pressure data. In order to obtain the true mechanical skin factor, an independent estimate of the pseudoskin factor due to restricted entry is required.

The pseudoskin factor correlations of Yeh and Reynolds and Ding and Reynolds for a restricted-entry well completed in a mutilayer reservoir are extended to obtain a new correlation for computation of the pseudoskin factor caused by a restricted-entry well in a reservoir with multiple flowing phases. An extensive set of two-phase (oil-water, gas-water, oil-gas) and three-phase (oil-gas-water) examples are presented and indicate this correlation yields accurate estimates of the pseudoskin factor.

Because this correlation is dependent upon the analysis of pressure transient data obtained under multiphase flow conditions, conditions under which such analysis can be performed and procedures for that analysis are discussed. A new semi-theoretical procedure for analyzing solution-gas systems in which free gas is produced at the sandface is presented. Numerical results for all of the multiphase systems considered in this study indicate that the common practice of approximating a gas cap or aquifer as a constant pressure boundary cannot be justified.

Introduction

In order to delay water and/or gas coning, wells are frequently completed over only a fraction of the productive zone. Wells completed in this manner are referred to as restricted-entry or partially penetrating wells. Important information sought on restricted-entry wells include procedures for the analysis of pressure transient data, the productivity loss caused by restricted entry and whether the well should be stimulated.

Considerable attention has been given to this type of well completion in the literature. Though the reason for completing wells in this manner is due to the existence of multiple phases (oil, gas and water) in the reservoir, the vast majority of studies on this subject consider only the single-phase flow of a slightly compressible fluid of constant compressibility and constant viscosity, e.g., see Refs. 1-20. In addition, the results of Refs. 7-19 are all restricted to homogeneous single-layer reservoirs.

Refs. 1-6 discuss procedures or solutions that can be used to estimate the pseudoskin factor caused by restricted entry in layered reservoirs; the results of Refs. 5 and 6 are restricted to two-layer reservoirs. If pseudoradial flow occurs in a multiphase reservoir, then the multiphase situation should be analogous to single-phase flow in a layered reservoir. Note that this would restrict the analogy of singlephase flow in a two-layer reservoir to a completely gravity segregated reservoir containing only two-phases. In reality, multiphase reservoirs generally have a transition zone separating segregated phases and in solution-gas systems, vertical and horizontal saturation gradients can develop rapidly within the reservoir. In trying to draw an analogy between multiphase and single phase reservoirs, it would then seem appropriate to draw that analogy with the single-phase multilayer reservoirs. This provides the motivation for extending the single-phase multilayer pseudoskin factor correlations of Refs. 1 and 2 to the multiphase systems considered here.

Refs. 21-36 have presented much material on the analysis of well test data obtained under multiphase flow conditions, though the results of Refs. 27-36 are restricted to one-dimensional radial flow and the results of Ref 25 are for a horizontal well.

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