Skin Factor And Flow Efficiency In Wells Producing Under Solution Gas Drive

Abstract

Abstract It is argued that the skin factor is an inappropriate measure of damage around the well bore for wells producing from solution gas drive reservoirs in which the pressure has fallen below the bubble point. This is because the value of the skin factor is influenced by the high gas saturations close to the well which lead to reduced effective permeabilities to oil. The curvature exhibited by the inflow performance relationships of Vogel and Fetkovich is a reflection of the increased gas saturations in the vicinity of a producing well so that a value of flow efficiency, based on a comparison of actual performance with the "type-curve" IPR, should give a truer measure of skin damage than that given by the skin factor. Formulas are presented which enable the value of the flow efficiency to be determined from a two-point rate/ flowing RHP test together with knowledge of the static pressure at the well in question. An estimate of the production rate increase, resulting from remedial work to improve the flow efficiency of a particular well, is readily obtainable. Introduction In many producing wells the value of the formation's permeability close to the well bore is different from its value further back in the reservoir. There are several possible reasons for this: well completion techniques, for example, may result in formation damage due to the mechanical disturbance of the sand grains, or to the invasion of the pores by mud filtrate, or to resistance to flow created by a gravel pack or by inadequate casing perforation techniques. On the other hand, bullet perforation of the casing and cement may create channels around the well bore that improve the effective permeability, while acidization or fracturing jobs are designed to improve that permeability. When production takes place at a flowing BHP that is below the bubble point, the presence of free gas in the pressure sink centred on the well reduces the ease with which oil flows in this region, that is, alters the effective permeabilities. It is useful in the field to have a method for determining the extent to which the average permeability around the well bore differs from that of the formation in order to assess and improve completion practices and to judge the necessity for and the profitability of possible remedial work at some stage in a well's life-perhaps an acid wash, an oil backflush, or a reperforation. Skin Factor The most commonly used measure of the degree of damage around the well is the skin factor, the value of which is routinely determined as part of a pressure build-up survey. Referring to Figures 1 and 2, and using standard symbols, a dimensionless number S is defined by Equations (available in full paper) Of the various factors appearing in Eq. 4 some uncertainty surrounds the value of (k/ µ), although this may be determined with reasonable accuracy from m (Eq. 3) provided that the formation thickness, h, can be adequately defined;