Well testing has long been a valuable tool for the petroleum industry. The practice continues to be widely used today, but increasingly more situations arise in which conventional well tests can be impractical due to cost, logistical or environmental constraints. For instance, weather conditions may dictate a time window beyond which operations have to cease as in arctic conditions. In such cases a wireline formation tester (WFT) test (commonly known as mini-DST) may present a viable alternative to acquire formation fluid samples and pressure transient data.
A WFT test or mini-DST uses a probe or a straddle packer to test a selected reservoir interval. Downhole pumps are used to clean up mud filtrate invasion and flow formation fluids at a stable rate. Downhole fluid analysis (DFA) is used simultaneously to monitor clean up and measure fluid properties such as fluid color, density, and Gas-Oil ratio (GOR) in real time. As part of the sequence, one or more pressure build up periods may be performed on each mini-DST station. Similar to classic well test analysis, transient pressure interpretation of the drawdown and buildup responses is used to derive the mobility thickness product and skin relevant to the rock volume investigated by the test. The permeability of that flow unit can be then calculated using the thickness of the flow unit and viscosity of the formation fluid as inputs.
In this paper, we present field data from the Caspian region in which WFT mini-DST results are compared with conventional well testing using drillstem test (DST) results. We show that, as long as the scale of measurement is taken into account, reservoir parameters obtained are in close agreement over the same interval. In addition, the mini-DST reveals differences in permeability between individual flow units.
Two case studies are presented to compare independently derived productivity parameters of the reservoirs. Permeability, thickness and skin from mini-DST results are used to construct reservoir models to represent individual zones.
In the first field case, we have exhibited where mini-DSTs are able to provide detail that a DST is unable to, and show an example of how Nuclear Magnetic Resonance permeability can be used to upscale mini-DST results to estimate the total permeability-thickness of a reservoir.
In the second field case, we have shown that with adequate sampling using mini-DSTs, it is possible to estimate the total permeability-thickness across several reservoir sands. Additionally, nodal analysis is used to predict downhole flow rates. Individual zone parameters are used to predict zonal contributions and hence construct a composite inflow profile response for commingling selective zones. These have then been compared to actual results from production logs performed during the well test.
This integrated approach can be used to complement and calibrate well testing (DST) results or to acquire sufficient reservoir information when a full well test is not feasible and/or not required. The advantages and limitations of this approach are discussed to assist the proper selection of test types depending on desired objectives.