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The emphasis on static reservoir pressures

Formation testing at this stage predominantly consists of openhole wireline pressure testing. The emphasis is on static reservoir pressures, which are used to confirm fluid contacts and fluid density gradients. On that basis, the different hydraulic compartments of the reservoir will be determined and tied into the geological model. Often, field production has already started while additional development wells are being drilled. In the new wells, pressure gradients already may reflect the influence of the production on the reservoir pressure. On those wells, the reservoir simulator, if in place, will be used to predict vertical pressure profiles to be confirmed by the wireline tester measurements. Any differences would be used to refine the geological model and introduce suitable compartments in the dynamic model. This stage is crucial in reservoirs with a large number of stacked layers such as deltaic deposits (which are the most prolific hydrocarbon-producing reservoirs). For these, wireline pressure measurements are an invaluable aid to reservoir dynamics characterization, because there is no other practical way of assessing vertical and lateral communications and the volumetrics of these small individual accumulations. This application contributed to the immediate acceptance of the first wireline testing tool, the RFT* Repeat Formation Tester. Until this tool’s introduction in the mid-1970s, distributed pressure measurements had been unavailable to reservoir engineers. The main objective of conventional testing, if performed on the new development wells, is to measure any skin resulting from formation damage. If skin is absent, the wells can produce at their full potential. If a high skin is detected, it must be corrected before putting the wells on line. Another reason for testing development wells is to prepare them for stimulation operations, which may be necessary to produce them economically. This is common with many reservoirs in low-producibility areas, where the operator needs an early return on investments. When unstimulated production rates would be uneconomical, the operator often spends as much as $1 million or more on extensive stimulation operations, such as hydraulic fracturing. Recovering this investment could take months. Conventional well testing is especially important to assess the productivity gains achieved through measuring skin, determining fracture length and its hydraulic conductivity, and assessing the financial risks.