The need to reduce formation damage and to avoid differential sticking and
lost circulation in depleted reservoirs favors the use of
underbalanced-drilling (UBD) technology. In highly depleted reservoirs, the
pore pressure can be very low, necessitating the use of extremely low-density
fluid to achieve an equivalent circulating density (ECD) below the pore
pressure. In such situations, the stress redistribution around the wellbore has
to be supported mainly by the rock matrix, and limited support that is provided
by the mud pressure. Therefore, UBD can dramatically increase the risk of
From literature review, it was found that the techniques and methodologies
to carry out wellbore-stability analysis on UBD of horizontal wells are not
well documented. This paper presents a wellbore-stability study that was
conducted to evaluate the feasibility of using UBD technology to drill a
horizontal well in a highly depleted reservoir in Libya.
The study started with geomechanical laboratory tests and mechanical Earth
Model (MEM) construction to evaluate the in-situ stresses, pore pressure, and
mechanical properties of the formations likely to be encountered during the
planned UBD campaign.
Wellbore-stability analysis was subsequently conducted for the planned
horizontal well to be drilled underbalanced using a new practical approach. The
analysis revealed a high probability of extensive and severe breakout within
the weak zones if penetrated underbalanced and the potential for massive
wellbore collapse. Under the guidance of this study, the well plan and drilling
designs were amended to minimize drilling risk.
There is a heavily depleted field in the Sirte basin in Libya, with
reservoir pressure approximately 2,200 psi (equivalent to 4.9-lbm/gal fluid
density) that is depleted from an initial reservoir pressure of approximately
3,800 psi in 1969. Accompanying the reservoir-pressure depletion, the
production of the field dramatically declined. To rejuvenate this field, new
horizontal wells are planned both as producers and injectors.
Because the reservoir pressure is so low, UBD technology was initially
recommended to reduce formation damage and to avoid mud loss and differential
sticking during drilling (Bennion and Thomas 1994; Falk and McDonald 1995;
Doane et al. 1996; Robinson et al. 2000; Sarssam, et al. 2003; Qutob 2004).
Additionally, possible benefits derived from UBD include an increase in rate of
penetration (ROP), an increase of bit life, and early production of
However, UBD could exacerbate potential wellbore failure in comparison with
conventional-overbalanced drilling because the mud pressure support on the
borehole wall is removed (Hawkes et al. 2002). The potential for excessive
wellbore failure during drilling could render UBD technology unfeasible. To
evaluate the potential wellbore-instability risk with UBD, a wellbore stability
study was initiated. The implementation of the UBD technology would be decided
on the basis of wellbore-stability-analysis results.
Toward this end, a data audit was conducted on two offset wells (Well A and
Well B) included in the study of the field. The consistency of these data was
checked, and errors or inaccuracies detected in the data were corrected.
Additionally, a thorough review of drilling events encountered during drilling
of the two wells to gather information that would be used to identify and
characterize the drilling problems. Geomechanical laboratory tests were carried
out to obtain accurate descriptions of rock mechanical properties. By using
these audited data and geomechanical-laboratory-test data, we generated an MEM
for Well B for which complete suites of data were available. The model was
subsequently validated by comparing predicted wellbore instabilities on the
basis of the model with borehole-failure observations. The MEM for Well A was
built subsequently by reusing the stress model developed from the construction
of the MEM for Well B.
Once the MEM of Well A was validated, it was propagated to a
planned-horizontal trajectory in the locality of the well. Wellbore-stability
analysis was conducted for the horizontal well to be drilled underbalanced. A
drill map was created for the planned-horizontal well as the integrated outcome
of wellbore-stability analysis.
© 2008. Society of Petroleum Engineers
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- Original manuscript received:
19 November 2006
- Meeting paper published:
11 March 2007
- Revised manuscript received:
25 November 2007
- Manuscript approved:
14 January 2008
- Version of record:
20 June 2008