Completion Workover And Packer Fluids

COMPLETION AND WORKOVER FLUIDS The Skin Effect

Reservoir engineers have calculated from pressure draw-down data that many wells produce at less than their potential.1 2 A barrier, or skin, appears to exist around the wellbore, as shown in Figure 10-1. This skin is caused by a zone of reduced permeability around the wellbore, and results from contamination by mud particles or filtrate. In some wells, poor completion practices—such as too widely-spaced perforations, or insufficient penetration into the reservoir—also contribute to the observed skin.3

Although the contaminated zone extends at most only a few feet into the reservoir, it can cause quite a large reduction in well productivity, because the flow is radial, and therefore pressure drop is proportional to 1 n r/rw, where rn is the radius of the well, and r is the radius of interest. Muskat4 gives the ratio of productivity of a damaged well to that of an undamaged well as;

Where Q is the undamaged productivity, Qd is the damaged productivity, re is the radius of the drainage area, k and kd are the virgin and the damaged permeabilities, and rd is the radius of the damaged zone.

* This chapter co-authored with Keith Sharp.

Wellbore

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