Summary
Because a borehole in a limestone formation is more stable than expected, an
openhole completion without a slotted/perforated liner has become popular
recently. However, the following three items are not clear: (1) Why a borehole
in a limestone formation is so stable, (2) why a borehole in a limestone
formation can be completed without a liner regardless to the formation
strength, and (3) the question of stability after acid treatments. To
answer these questions, two types of laboratory experiments are conducted. One
of them is a series of borehole stability experiments using 1.5- and 2.36- to
2.39-in.-diameter borehole in a 10.5×10.5×17.5-in. limestone blocks with
polyaxial confining pressures simulating a horizontal well with three different
principal in-situ stresses. Two types of limestones are used with and without
borehole acid treatments and two borehole sizes are used to check the size
effect. Another type of experiment is the acid squeezing experiment, in which
15% HCl acid solution is squeezed from one end of a cylindrical core and the
change of porosity, permeability, and hardness are measured throughout the
cores.
The results showed the following new discoveries:
1. The limestones have two distinct failure envelopes. The failure plastic
strain is relatively small for normal shear failure, while it becomes as much
as 10 times larger when a shear failure is induced after pore collapse.
2. One of the limestones used in these experiments has only 1,751 psi UCS,
yet the borehole was unexpectedly stable. The reason was that the borehole
failure is induced by a shear failure after pore collapse. It is well known
that pore collapse is induced within a formation during compaction; however, a
shear failure after pore collapse has never been observed when one boundary is
open like an open hole.
3. The confining stress inducing borehole failure was not significantly
different between hydrostatic and directional loadings. According to the
Kirsch's solution, the directional load should significantly increase the
stress concentration. It is well known that the nonlinearity of rock reduces
the stress concentration induced by directional loading; however, the present
experiments showed that the magnitude of the reduction of stress concentration
was larger than expected.
4. Wormholes stabilize boreholes even though acidizing weakens formation.
Therefore, enhancing wormholes is recommended when a borehole in a limestone
formation is acidized.
Normally, because limestones are relatively strong, open holes are likely
stable; however, the strength must be checked if they need to be completed
without a linear protection. To help a reader applying the laboratory results
to field problems, a guideline to complete an open hole without a liner
protection in limestone reservoirs is provided, with calculation results using
a nonlinear finite-element model.
Introduction
When an openhole completion was selected in a limestone reservoir, the well
used to be completed with a perforated/slotted liner. 1 A perforated/slotted
liner was economical, and so using it as a well protection did not
significantly increase the well completion cost. However, a perforated liner
became an obstacle later when the well was recompleted. Field trials showed
that even if a formation was not significantly strong, an open hole remained
open without collapsing without the support of a perforated liner. 2 On the
other hand, common sense suggests that, if a formation is too weak with respect
to the magnitude of in-situ stress, a borehole cannot remain open. In addition,
it is a common method to use an acid to stimulate a limestone reservoir. Any
acid should weaken a limestone formation. However, field observation seemed to
show that an open hole remains stable even after acid stimulation. We need to
know why an open hole remains stable although we know acids should weaken
formation after acid treatments.
No large-scale laboratory experiments have appeared in the literature on
borehole collapse problems for limestone reservoirs, although many papers have
appeared on wormholes. 3 To answer why a borehole in a limestone formation is
stable, and why acid does not change the borehole stability, two types of
experiments are conducted in this work.
1. First, there are fundamental experiments to know the limestone property:
Complete triaxial stress strain curves are measured for two types of limestone
for several confining pressures. In addition, acidflood experiments are
conducted to measure the porosity change, permeability change, strength change,
and magnitude of limestone dissolution at acid limestone interface.
2. One-half- to one-third-scale models are used for borehole stability
experiments to measure the well collapse condition with and without acid
treatments. Uniform and directional confining pressures are used to simulate
vertical and horizontal wells. The change in borehole diameters is measured at
several points of the borehole while the confining pressure is increased until
the borehole starts collapsing.
The present work is the first publication appearing in the literature to
show a series of large-scale borehole stability experiments for limestones with
and without acid treatments.
Experimental Setup and Procedure
Borehole Stability Test Using a Large Polyaxial Cell.
A large polyaxial pressure cell shown in Fig. 1 equips a flexible square
jacket to hold a rock sample. Unlike other large polyaxial cells, because a
hydrostatic stress is applied through the jacket by the surrounding oil, a high
confining pressure up to 25 kpsi may be applied to the rock sample. After
applying the hydrostatic stress, a polyaxial load may be added to the rock
sample through the loading plates installed at the four faces of the jacket
with loading pistons behind the jacket. A cubic sample (10.5x10.5x17.5-in.)
with a borehole (diameter 1.5 in. and 2.25 to 2.29 in.) is inserted into the
flexible jacket shown in Fig. 1. The three confining pressures (two horizontal
and one vertical), borehole pressure, and the pore pressure may be
independently changed.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
13 January 2003
- Revised manuscript received:
2 July 2004
- Manuscript approved:
9 February 2005
- Version of record:
15 June 2005
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