Summary
A model is described that is capable of simulating in detail the time
variation of formation pressures measured while drilling, in situations where
supercharging is significant. Simulation results illustrate the variation of
supercharging pressures with formation permeability, drilling-fluid-filtration
properties, and drilling-fluid hydraulics. The model is used to explore how
drilling operations influence the levels of supercharging when drilling two
formations, widely separated along the well trajectory, and of significantly
different permeabilities. The forward-simulation capability presented is
believed to be a useful aid to the planning, understanding, and interpretation
of formation-pressure measurements while drilling.
Introduction
The new formation-pressure-while-drilling tools will bring great benefits
for improving well placement, navigating narrow mud-weight windows, and
generally avoiding hazards (Pop et al. 2005). To best use these new tools, it
is essential to understand the effects of drilling on formation pressure, and
to exploit this understanding to plan jobs so as to ensure that fit-for-purpose
data are obtained. One reason this is necessary is that supercharging
[elevation of near-wellbore formation pressure because of
drilling-fluid-filtrate leakoff (Pelissier-Combescure et al. 1979; Phelps et
al. 1984)] can be a problem for formation-pressure-while-drilling measurements.
The problem occurs because active mud circulation limits filter-cake growth, so
leakoff rates are larger than when the mud is static. Also, because the
formation-pressure-while-drilling tools may test the formation shortly after it
has first been drilled, there can be comparatively little time for elevated
pressures to relax.
So, when do we need to worry about supercharging? The usual answer is “in
low-permeability formations.” But the drilling-fluid-filtration properties play
a controlling role too, and the sequence and timing of drilling operations are
also important. Furthermore, the use to which the measurements will ultimately
be put dictates whether any particular level of supercharging is
acceptable.
Hence, to answer the question properly, it is necessary to calculate
supercharging pressures, taking account of the influential factors and details
of the particular situation, and to assess the results in light of the intended
use of the measurements. This paper demonstrates a simulation model with these
capabilities.
We first outline the basic ideas behind the models used to simulate
drilling-fluid-filtrate leakoff, formation pressure, and supercharging. Some
example results illustrate how supercharging varies with formation permeability
and drilling-fluid-filtration characteristics. The simulation results are used
to motivate a simple approximation, which is used to map out the ranges of
formation permeability and the filtrate-leakoff rate over which significant
supercharging is to be expected. A further set of simulations is then used to
show how the sequence of drilling operations affects the relative levels of
supercharging in two formations of different permeabilities, separated by
approximately 1 day of drilling time. “What-if” simulations allow operational
sequences to be found that give low supercharging in both zones.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
16 May 2005
- Meeting paper published:
23 February 2005
- Revised manuscript received:
25 May 2007
- Manuscript approved:
29 September 2007
- Version of record:
25 February 2008
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