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.
New generations of wireline formation testers allow the possibility of obtaining in situ permeabilities by performing mini Drill Stem Tests (DST) and vertical interference tests (VIT), beyond standard probe measurements. The use of straddle packers enables a one-meter interval to be isolated both in open or cased hole and a local production can be individually induced in various different locations in the borehole. The drawdown and buildup pressure measurements are acquired with high-resolution quartz gauges devices, which are located at the packer and at the observation probe while performing a VIT.
A major concern is how the mud filtrate invasion around the wellbore affects the pressure response. When a well is drilled, an invasion process occurs where solute and solid leak through the virgin formation creating an invaded zone. The radius of this invaded zone is inversely proportional to the rock permeability.
Modern wireline formation testers are able to continuously pump and discharge mud filtrate to the wellbore, allowing a representative sampling of the virgin formation fluid and also enabling the acquisition of good quality pressure transient data. As the formation cleans up effective permeability to the formation fluid pressure response is influenced by the viscosity of the fluidscan be reasonably estimated.
The interpretation of the pressure response obtained during the test can lead to good values of reservoir pressure and both vertical and horizontal permeabilities.