13-08-2012, 12:24 PM
Optimizing Reservoir Characterization During Underbalanced Drilling: Tools, Analysis Methods and Results
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Abstract
Underbalanced drilling (UBD) provides engineers with the
opportunity to quantitatively describe and evaluate each
geologic zone as it is penetrated. This provides them with the
flexibility to adjust a drilling program still in progress if
warranted by confirmed reservoir characteristics. UBD is
different from conventional drilling in that all reservoir
intervals are tested automatically while drilling, whether or not
they are believed to contain commercial volumes of
hydrocarbons at the outset. By drilling in a UB mode and by
maximizing the discrete characterization of each formation,
many instances have occurred in which zones previously
thought to be uneconomic—or missed altogether by available
modeling and planning data—have been found to contain
reserves adequate to justify completion.
Introduction
This paper describes the basis of analysis for this reservoir
evaluation process, the analytical methods that have been
developed and used in the field to determine reservoir
properties such as permeability, reservoir pressure, skin
damage - if any and reservoir extents (extent, will depend
upon whether there is sufficient time to see influence of
boundaries) for each zone drilled based upon history
matching. A key enabler to performing analysis while drilling
has been the ability to obtain the mathematical solution to the
moving boundary condition problem. In conventional well
testing, the length of the wellbore open to flow is constant,
since it is assumed that the well has already been drilled; in
UB drilling, however, obtaining a solution is more
complicated, because the length of the wellbore open for flow
increases as the well is being drilled. This problem has been
solved and has been implemented in software for easy
application. A case history will be described comparing
conventional and UBD results as well as well testing while
drilling.
Underbalanced Drilling Candidates
Proper candidate selection is critical to the success of UBD
projects, especially when these projects focus on the reservoir.
It is essential that the main project objectives are identified at
the beginning of the project to ensure that they can be
achieved.
The candidate selection process consists of analyzing the
drilling, geological, reservoir, geomechanical and petrophysical
information to determine whether a particular well
and/or reservoir is a potential candidate evaluating some of the
main reservoir and wellbore characteristics.
Reservoir Characterization Candidates
To perform reservoir characterization while drilling, it is
important that the system remain underbalanced throughout
the operation. It is important to note that while it is possible to
perform reservoir characterization while drilling underbalanced
with some short periods of overbalance, it is not
possible to do this with overbalanced drilling or managed
pressure drilling (MPD) because of the deliberate suppression
of reservoir influx during these operations. Even if the system
is kept in an at-balance condition, the pressures would be
fluctuating above and below the formation pressures, making
it difficult to clearly identify and characterize the properties of
the productive intervals; This is due to the fact that it would
not be possible to determine what section in the open hole
being drilled is producing when and in response to what
corresponding drawdown pressure.
Development
Reservoir characterization while drilling began to gain
recognition as a testing option in the 1990s as rate and
downhole pressure data were observed to be of sufficient
quality for transient analysis. Several investigators looked at
different means to perform this type of analysis including
Kardolus, van Kruisjdijk, Larsen, Nielsen, Kniessel, Rester,
Hunt, Biswas, Vefring et al.1-6 Reservoir characterizationwhile-
drilling methods and software development described
and used in this paper began in earnest in 1998 with the
development of a model for the moving boundary condition
seen while drilling and implementation and testing of the
model.3,7
Analysis Methods
The reservoir characterization-while-drilling process
begins several months prior to the operation in conjunction
with the engineering design and job preparation phase. At this
time, offset data and information are obtained from the field’s
geologists, reservoir, and drilling engineers to build a model of
the reservoir and estimate expected reservoir behavior while
drilling underbalanced for different possible scenarios. As
mentioned above, wellbore hydraulics will also be modeled to
ensure that underbalanced conditions and hole cleaning can be
maintained with the expected influx. Then, the testing
procedures are developed for all productive formations and
discussed with the project team to ensure that objectives will
be met within the time and operational constraints of the
project.
Conclusions
A method using a multilayer reservoir model to analyze
flow-rate data acquired during underbalanced drilling has been
presented. This process was demonstrated on an example
dataset based on actual data from a field case.
Using the multilayer reservoir model to analyze data
acquired during the underbalanced drilling process provides a
more complete reservoir characterization than would have
been possible using conventional drilling and formation
evaluation techniques. The characterization is further
enhanced when integrated with existing geological,
geophysical, petrophysical and engineering data.