Utilizing Mini-DST for Formation Evaluation

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Abstract Well tests have been widely used for several decades in the oil industry for evaluation of the well productivity and formation damage, estimation of reservoir characteristics such as initial pressure, fluid type, effective permeability and identification of reservoir barriers or boundaries close to the wellbore. Based on the well test objectives and the regulatory constraints, several techniques may be applied: –a single wireline formation test to obtain a single-point static reservoir pressure, used for determining reservoir pressure gradients;–a wireline formation test to obtain the reservoir pressure and a reservoir fluid sample;–a mini-DST, using a wireline formation tester (WFT) to obtain some reservoir parameters in the near wellbore domain in addition to the fluid sample;–a vertical interference test (VIT) using a wireline formation tester with a vertical observation probe to estimate the near wellbore vertical transmissibility;–a closed chamber test to obtain reservoir parameters around the wellbore region while obtaining fluid samples;–a drill stem test (DST) to obtain average reservoir parameters, well productivity, PVT fluid samples and verify possible barriers and/or heterogeneities in the investigated region;–an interference test to check continuity and estimate average reservoir properties between wells;–an extended well test (EWT) to evaluate the production performance of well/reservoir in addition to other various objectives. Due to the high costs involved in well test operations, especially in offshore exploration wells, the test type must be carefully chosen and properly designed in order to meet its required objectives at the lowest cost. The mini-DST is one of the cheapest options and may successfully be applied to obtain the reservoir static pressure and collect a reservoir fluid sample. In certain circumstances, the vertical and the horizontal permeabilities may be obtained. The WFT commercial tools may test the reservoir with a shorter investigation than the conventional DST, investigating the reservoir in the order of tens of meters. Regardless of the type of test adopted, all the operations must be conducted in accordance with strict environmental standards. This work presents the mini-DSTs currently performed at Petrobras, indicating the operational and interpretation difficulties in order to highlight their actual applicability.