Summary
Prediction of formation damage that occurs while drilling horizontal wells
is a critical point for optimizing an oil field’s development. The economic
impact of near-wellbore drilling-induced damage and cleanup efficiency has led
to significant progress being made in both experimental and numerical studies
in order to assess wellbore flow properties during oil production. As a result,
a numerical model has been developed to study the impact of various parameters
related to the properties of drilling fluids on well inflow performance.
This paper describes a numerical approach developed in this model to
simulate near-wellbore formation damage caused by underbalanced drilling (UBD).
It is generally expected that UBD will be of benefit by preventing formation
damage. However, this benefit can be lost for various reasons. For instance, an
overbalanced pressure can be applied on the formation during short periods of
time for various operational reasons and can cause severe formation damage
because of the absence of external cake protection and huge filtrate invasion.
Another possible cause of formation damage in UBD is related to spontaneous
imbibition, which induces waterblocking, which was observed while drilling
tight gas reservoirs.
A methodology for the modeling of possible formation damage during UBD is
presented. Crossflow phenomena caused by spontaneous imbibition is taken into
account to model the filtrate invasion from the well to the porous media, as
the well is in production.
Introduction
It is well known that formation damage caused by drilling fluid has a huge
impact on a well’s oil and gas productivity, especially for openhole completed
horizontal wells. During overbalanced drilling (OBD), mud and mud filtrate
penetrate the near-wellbore formation because of this overbalanced pressure,
altering near-well flow properties. As a result, well productivity is
dramatically reduced. It is generally recognized that UBD can be used to
minimize problems associated with invasive formation damage. When correctly
performed, UBD reduces or eliminates invasive formation damage, improves access
to reserves, and provides potential for reservoir evaluation while drilling.
Additional benefits of UBD are the reduction in drilling time, high rates of
penetration, and increase of bit life.
UBD has recently proven its efficiency in numerous situations in which
serious problems were encountered with classical drilling techniques. For
example, heavy loss formations or depleted zones are ideal candidates for UBD.
During normal UBD, the negative pressure drop between the wellbore and the
formation prevents drilling fluids from entering the formation. However, this
benefit can be lost in at least two situations owing to possible sources of
failure such as temporary overbalanced conditions or spontaneous imbibition.
When drilling underbalanced, the fluid system is not designed to contain
cake-building solids as in the case of overbalanced drilling. Thus, if
underbalanced conditions cannot be maintained 100% of the time, severe losses
can occur, which may result in formation damage. Overbalanced conditions may
result from bit trips, running in, or other situations in which wellbore
intervention is necessary. Spontaneous imbibition is particularly important in
tight gas reservoirs.
Even though UBD has many advantages over OBD, it is necessary to quantify
production improvement and possible formation damage, which is an important
factor in evaluating the economic feasibility of UBD projects (Bennion and
Thomas 1994; Bennion et al. 1994; Bennion et al. 1998; Cade et al. 2003;
Suryanarayana et al. 2003; Xiong and Shan 2003).
This paper presents a numerical model allowing us to calculate well
productivity reduction owing to possible formation damage during UBD, such as
temporary overbalanced drilling or spontaneous imbibition.
The modeling of well performance in OBD has already been presented (Ding et
al. 2002; Ding and Renard 2003). Both internal and external cakes are
considered in the model, and filtrate invasion is calculated using a two-phase
flow equation. Polymer absorption/retention, phase trapping, and wettability
alteration are globally represented using a hysteresis of relative
permeabilities. Nonuniform formation damage along the well is represented by
variable specific skin factors through an optimization procedure. The change of
well performance can therefore be calculated using a flow simulator by taking
into account these variable skin factors.
In this paper, a methodology for the modeling of possible formation damage
during UBD is presented. Formation damage related to temporary overbalanced
pressure is considered. Spontaneous imbibition is modeled using two-phase flow
simulation with capillary pressure. When capillary forces are important,
countercurrent imbibition occurs with the flow of reservoir fluids toward the
well while the filtrate invades the formation. Spontaneous imbibition is
particularly important in tight gas reservoirs. Like the modeling of OBD, a
hysteresis of relative permeabilities is used to represent globally various
physics during drilling and backflow.
The impacts of formation damage on well productivity are simulated by the
numerical model and are compared for both underbalanced and overbalanced
drilling. The sensitivity to various parameters related to drilling conditions
(underbalanced or overbalanced) and reservoir parameters is presented.
Simulations show that formation damage in UBD depends on reservoir and applied
drilling conditions. Damage is sometimes severe because of no filter cake
protection. Therefore, operational precautions should be taken during UBD to
prevent such damage. The proposed numerical model can be used as a selection
guide between OBD and UBD for both well performance predictions and calculation
of the best economic benefits on long-term production, assuming that some
damage can be done to the near-wellbore formation while drilling
underbalanced.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
19 May 2004
- Revised manuscript received:
21 July 2005
- Manuscript approved:
21 July 2005
- Version of record:
20 February 2006
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