Obviously a modified method for analyzing and interpreting the data must be employed to account for the effect of length of the fracture on the pressure response during the infinite-acting flow period. DRAWDOWN LABS SKINThis will result in a calculated flow capacity kh which is too large, an erroneous average pressure, and a skin factor which is too small. However, the slope of the buildup curve becomes progressively smaller with increasing fracture penetrations. For fractures of small penetration, the slope of the buildup curve is only slightly less than that for the unfractured “radial flow” case. If the fracture penetration ratio x f/ x e is defined as the ratio of the fracture half-length x f to the half-length x e of a closed square-drainage area, then Figure 1.82 shows the effects of fracture penetration on the slope of the buildup curve. This dependency of the pressure response behavior on the fracture length is illustrated by the theoretical Horner buildup curves given by Russell and Truitt and shown in Figure 1.81. As noted by Russell and Truitt (1964), the slope obtained from the traditional well test analysis of a fractured well is erroneously too small and the calculated value of the slope progressively decreases with increasing fracture length. For relatively short fractures, the flow is radial but becomes linear as the fracture length increases as it reaches the drainage radius. The pressure behavior during this infinite-acting period is very dependent on the fracture length. It should be pointed out that the external boundary can distort the semilog straight line if the fracture half-length is greater than one-third of the drainage radius. The usual procedures are Back Pressure test (Flow after Flow), Isochronal and Modified Isochronal tests. Gas well test: specific testing methods are used to evaluate the deliverability of gas wells (Absolute Open Flow Potential, AOFP) and the possibility of non-Darcy flow condition (rate dependent skin factor S′). DRAWDOWN LABS SERIESWith pulse tests, the active well is produced with a series of short flow / shut-in periods and the resulting pressure oscillations in the observation well are analyzed. Interference tests are designed to evaluate communication between wells. Interference test and pulse testing: the bottom hole pressure is monitored in a shut-in observation well some distance away from the producer. The properties of the injected fluid are in general different from that of the reservoir fluid, interpretation of injection and fall-off tests requires more attention to detail than for producers. Injection test / fall-off test: when fluid is injected into the reservoir, the bottom hole pressure increases and, after shut-in, it drops during the fall-off period. It is for this reason build up tests should be performed. During shut-in periods, the flow rate is accurately controlled (zero). Before the build-up test, the well must have been flowing long enough to reach stabilized rate. The analysis of flowing periods (drawdown) is frequently difficult and inaccurate.īuild-up test: the increase of bottom hole pressure after shut-in is used for analysis. Ideally, the well should be producing at constant rate but in practice, this is difficult to achieve and drawdown pressure data is erratic. In Handbook of Petroleum Exploration and Production, 2002 Test procedureĭrawdown test: the flowing bottom hole pressure is used for analysis. The main advantage of drawdown testing is the possibility for estimating reservoir volume. It is also recommended for a well where there are uncertainties in the pressure build-up interpretations. If a well has been closed for some reason, a drawdown test may also be done. Drawdown tests are normally recommended for new wells. It may take a few hours to several days, depending on the objectives of the test. Figure 6-1 shows the production and pressure history during a drawdown test. When a constant flow rate is attained, the pressure measuring equipment is lowered into the well. It is completed by producing the well at a constant flow rate while continuously recording bottom-hole pressure. In a drawdown test, a well, now static, stable, and shut-in, is open to flow. Moreover, the well is shut-in until it reaches a constant reservoir pressure before testing. Generally, the well is closed in earlier to the flow test period of time because it is necessary to allow the pressure to become equal throughout the formation. Many traditional analysis techniques are derived using the drawdown test as a basis. Pressure drawdown tests can be defined as a series of bottom-hole pressure measurements completed during a period of flow at a constant producing rate.
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