Summary
This paper presents a wellbore-stability study of the San Andreas Fault
Observatory at Depth (SAFOD) research borehole located near Parkfield,
California, USA. In the summer of 2005, the SAFOD borehole was drilled
successfully through the active trace of the San Andreas Fault (SAF) in an area
characterized by fault creep and frequent microearthquakes. In this study, we
report how the analysis of wellbore failures in the upper part of the hole,
geophysical logs, and a model for stress gradients in the vicinity of the fault
were used to estimate the mud weights required to drill through the fault
successfully. Because logging-while-drilling (LWD) acoustic caliper data and
real-time hole-volume calculations both showed that relatively little failure
occurred while drilling through the SAF, the predicted mud weight was
successful in drilling a stable borehole. However, a six-arm caliper log, run
after drilling was completed, indicates that there was deterioration of the
borehole with time, which appears to be caused by fluid penetration around the
borehole. The LWD-resistivity measurements show that essentially no fluid
penetration occurred as the hole was being drilled. Because of this, the mud
weight used was capable of maintaining a stable wellbore. However, the
resistivity data obtained after drilling show appreciable fluid penetration
with time, thus negating the effectiveness of the mud weight and leading to
time-dependent wellbore failure. Using finite-element modeling (FEM), we show
that mud penetration into the fractured medium around the borehole causes
failure with time.
Introduction
Drilling perturbs the stress state around a well, and wellbore-stability
problems can occur when the near-wellbore stresses substantially exceed the
strength of the rock. Excessive instability around a wellbore can be suppressed
by choosing an optimally stable borehole orientation and sufficiently high mud
weight. Some types of wellbore-wall failure, such as key seating, usually do
not cause instability in the borehole but can exacerbate failure in an already
unstable borehole.
As described below in the paper, drilling through the SAF in the SAFOD
project was conducted in various phases. In this paper, we discuss an analysis
of wellbore failures after the first phase of drilling in order to predict the
mud weight required to drill successfully through the SAF during the second
phase of the project. The main challenges during the SAFOD drilling were the
unknown stress field and rock strength along the planned drilling trajectory.
Hence, to estimate the mud weight to be used for drilling the SAF zone, we
first calibrated a theoretical-stress model (Chéry et al. 2004) for the SAF
with the observed borehole failures and minifrac-test data of the first phase
of drilling. Then, we estimated and calibrated a uniaxial-compressive-strength
(UCS) profile for the rocks encountered by modeling the severity of borehole
failures, allowing for a wide range of expected rock strengths because of the
possibility of extensive damage to the rocks caused by earthquakes on the
SAF.
As will be shown, LWD-caliper data during the second phase of drilling show
successful drilling through the SAF using the estimated mud weights inferred
from the geomechanical model. However, wireline calipers were enlarged at the
top section of the borehole, indicating severe failure with time. We use FEM to
show the borehole failure with time. Block theory (Goodman 1989; Goodman and
Shi 1985), used for the stability of underground openings, suggests that only
the removable blocks of the top section fail because of gravity, but the lower
section remains in gauge despite having removable blocks. This explains why the
time-dependent enlargement of the SAFOD borehole is restricted to only the top
section of the hole.
© 2008. Society of Petroleum Engineers
View full textPDF
(
7,219 KB
)
History
- Original manuscript received:
22 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
20 May 2008
- Manuscript approved:
15 June 2008
- Version of record:
10 December 2008
View Cart
