Summary
Underbalanced drilling (UBD) is defined as a drilling operation in which the
pressure of the circulating drilling fluid is lower than the pore pressure of
the target formation of interest. The most widely recognized benefit of UBD is
the reduction of formation damage by minimizing the drilling-mud leakoff and
fines migration into the formation. It also facilitates the possibility for
reservoir characterization during drilling. The purpose of this paper is to
present (1) how to use the inflow data for the evaluation of formation
properties, and (2) how to cope with the uncertainty of the results.
An in-house multiphase reservoir simulator is used for the simulation of the
formation reservoir-fluid flow during UBD. The model incorporates discrete
consideration of the well with appropriate time-varying UBD boundary
conditions. Capillary forces, which facilitate countercurrent imbibition of the
drilling mud into the formation, are taken into account.
The production rates during UBD depend on the formation properties and the
drilling conditions. The inflow information is analogous to transient test data
and can be used for the estimation of the reservoir-model parameters. With the
progression of the drilling process, the amount of available inflow data
increases and allows for a sequential history-match procedure. Statistical
analysis of the inverse problem can then be utilized to determine the optimal
level of parameterization that is justified by the quality and quantity of the
measured data.
This paper highlights (1) the use of a reservoir simulator to derive the
inflow of fluids in heterogeneous reservoirs, and (2) its coupling with
gradient-based optimization techniques for determination of unknown reservoir
parameters. The simulation-while-drilling approach proposed is fully automated
and can be set up on a known or totally unknown reservoir model, introducing an
iterative and automatic updating process of the reservoir model.
Introduction
In recent years, UBD has become a popular technology. Originally applied to
prevent drilling problems, its value for reservoir engineering is now gaining
greater importance. Benefits of UBD include the reduction of formation damage
by minimizing drilling-mud leakoff and fines migration into the formation.
Additionally, it allows for reservoir characterization during drilling.
Because the wellbore pressure is lower than that of the reservoir, there is
a permanent inflow of reservoir fluids during UBD and, possibly, an outflow of
drilling mud into the formation because of countercurrent imbibition (Friedel
and Voigt 2004). The inflow rates can be obtained by balancing injected and
produced fluids. This, of course, would assume that in-situ downhole conditions
such as temperature and flowing wellbore pressure are continuously measured
while drilling, which is not always the case.
By applying inverse modeling methods (assuming reasonably accurate data), it
is now possible to determine reservoir parameters, such as permeability and
pore pressure, by extracting the productivity signature of the reservoir from
the production data. Although the short time span of the drilling process can
give only an insight into the nearer wellbore vicinity, valuable information
about the reservoir can be obtained. Productive reservoir zones and formation
properties can be determined, provided there is proper flow monitoring at the
surface. This offers significant benefits in both production optimization and
reservoir characterization and can also be used to justify UBD.
The objective of this paper is to demonstrate the use of a
reservoir-simulation tool, coupled with a method for parameter identification
through automated reservoir characterization during drilling. In particular,
the tasks from the reservoir engineering perspective, tracked here, are
- The detection of the layering,
- The identification of layer permeabilities (or other layer properties) and,
optionally,
- Using statistical methods, it is not only possible to obtain important
reservoir parameters but also to assign the corresponding uncertainties. In the
modeling workflow, this data can be useful to validate the reservoir model and
compare it to the initial geological model.
The workflow of the UBD-reservoir-characterization tool also enables
“simulation while drilling.” Its use is, therefore, not limited to the common
post-drilling evaluation but, potentially, also engaged to optimize control
during the drilling process. Optionally, the tool can be helpful for
investigating reservoir formation damage as described in a previous paper
(Friedel and Voigt 2004).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
17 March 2005
- Meeting paper published:
13 June 2005
- Revised manuscript received:
26 November 2007
- Manuscript approved:
5 January 2008
- Version of record:
25 April 2008
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