Summary
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 because of 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 testing (WFT) [commonly known
as a mini-drillstem test (mini-DST)] may present a viable alternative to
acquire formation-fluid samples and pressure-transient data.
WFT a or mini-DST uses a probe or a straddle packer to test a selected
reservoir interval. Downhole pumps are used to cleanup 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-buildup 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 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 shown where mini-DSTs are able to provide
detail that a DST cannot, and we 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 with 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.
© 2012. Society of Petroleum Engineers
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History
- Original manuscript received:
10 December 2010
- Meeting paper published:
9 November 2010
- Revised manuscript received:
2 November 2011
- Manuscript approved:
17 January 2012
- Published online:
30 May 2012
- Version of record:
12 June 2012
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