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Production and injection well tests

In the production phase, the objectives of testing shift from flow evaluation to reservoir monitoring, data collection for history matching of reservoir simulators, and productivity tests to assess the need for stimulation. Well tests are performed to check for skin resulting from migration of fines to the near-wellbore region, and to assess the need for acidizing to remove those fines. Partial completion effects are diagnosed and remedied by reperforating or by extending perforation intervals. Gravel-packed wells are tested to evaluate the gravelpack skin (which is often very large) and decide on any subsequent treatment. Most of these tests are performed using a pressure gauge suspended from an electric cable or a slickline, or using a production logging tool that will provide complementary measurements such as downhole flow rate. The complexity of tests will depend on the well and may vary from a simple buildup test to a series of step-rate tests intended to analyze the dynamic performance of complete multilayer systems. Cased hole wireline testing also may be performed to obtain formation pressures in layered sequences, collect samples of bypassed oil detected by saturation measurements and, in some cases, measure permeability. Productivity well tests versus descriptive reservoir tests The interpretation of well tests is one of the most widespread sources of dynamic reservoir data. Tests on oil and gas wells are performed at various stages of drilling, completion and production. The test objectives range from simple measurements of reservoir pressure to characterization of complex reservoir features. Well tests can be classified either as single-well productivity tests or descriptive reservoir tests. ■ Single-well productivity tests are conducted to – determine well deliverability – characterize formation damage and other sources of skin – identify produced fluids and determine their respective volume ratios – measure reservoir pressure and temperature – obtain representative fluid samples suitable for PVT analysis – evaluate completion efficiency – evaluate workover or stimulation treatments.