Practicalities of Detecting Faults From Buildup Testing

The influence of a sealing fault (and other linear discontinuities) on transient well tests is a widely discussed subject. The topic first was presented by Horner for pressure buildup testing. Fig, 1 illustrates ideal pressure behavior in a tested well near a fault. This figure is a schematic semilog plot that applies to any kind of single-well transient test; doubling of the straight-line slope is the key to the existence of a fault. The material presented in this paper applies to falloff and buildup tests. It does not paper applies to falloff and buildup tests. It does not apply to flowing-only tests, such as drawdown, injectivity, and multiple-rate flow tests. The information in this paper assumes horizontal radial flow in a homogeneous, unfractured reservoir and assumes that wellbore storage effects are unimporant. There an pitfalls in detecting faults (used here to mean any linear discontinuity) from pressure buildup and falloff tests that generally are not recognized. The reason is that flow time determines how much reservoir a test investigates. So, for a pressure buildup test, the flow time before shut-in determines if it will be possible to recognize a fault from buildup data. If flow time is long enough, then the shut-in time need only be long enough to obtain data that show the dope doubling. A shortflow time cannot be compensated for by a long shut-in time. Interestingly enough, the pressure behavior caused by a fault is such that if shut-in time is not long enough to show the double slope, it is possible to interpret the data incorrectly as indicating a small closed system rather than a large system with a nearby fault. Flow times too short to detect faults during buildup testing are particularly apt to occur in two common types of well testing. One is drillstem testing, which generally has a short flow period. The second is the testing of a gas well where no pipeline facilities are available, so a short flow period is used while the shut-in period may last weeks or months. The theory describing pressure behavior in a well near a fault is relatively simple. Fig. 1 shows pressure buildup curve shape and indicates that the pressure buildup curve shape and indicates that the second slope, m2, is twice the first slope, m1. From the first slope, one calculates skin factor and permeability. permeability. The doubling of the slope is a strict theoretical requirement and not an approximation.

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