Horizontal Well Testing of Isolated Segments

Abstract

A horizontal well test in a large reservoir and with small wellbore storage should produce certain distinct flow regimes. These should lead to the estimation of three permeabilities (along, normal and vertical to the well trajectory) the total well skin effect and, perhaps, the effective (open) well length. The objective of this work is to optimize well test design to obtain the desired flow regimes. It is shown that the important early-time flow regimes can be masked by wellbore storage effects. Therefore, afterflow should be minimized by using downhole shut-in. The storage effects can be estimated with empirical equations. These relationships also mark the beginning of the first interprétable flow regime. It is shown that for very thin formations or in situations where the well is drilled close to the top or bottom boundaries the early radial flow regime may not be seen at all. The length of an almost fully laterally penetrating horizontal well may also distort the late-time radial flow. In both of these instances, calculation of the controlling permeabilities becomes difficult to impossible. It may thus be desirable to test smaller segments of the well. Placing an observation gauge in an isolated segment during the flow testing of another segment can provide further information. It is shown that the derivative of the pressure response in the observation gauge exhibits a characteristic hump. When the hump appears, the permeability along the well axis can be calculated. Testing of isolated segments would also deliver a detailed picture of the skin distribution along an extended horizontal well. From such a test valuable information for enhanced stimulation and selective well completion design can be drawn.