Logon T.L., Mavor M.J., Khodaverdian M.
Optimizing and Evaluation of Open-Hole Cavity Completion Techniques for Coal Gas Wells. / 9346. -
The 1993 International Coalbed Methane Symposium (May 17-21, 1993, Birmingham-Jefferson Civic Center, Birmingham, Alabama, U.S.A). Volume 2. - Alabama: University of Alabama, 1993. - pp.609-622.
ÊÎÍÑÏÅÊÒ: Êëþ÷åâîå ñëîâî - self-propped fractures (ñàìî-ïîääåðæèâàþùèéñÿ ïðîöåññ ðàçðóøåíèÿ).
ABSTRACT
An innovative completion technique that has been used to
effectively complete coal gas wells is an open-hole
completion in which the coal is encouraged to slough into
the wellbore. During the completion process, the wellbore
is effectively linked to the reservoir due to the creation of
numerous multi-directional self-propped fractures. The
technique has been commonly called an "open-hole cavity
completion." However, the cavity is a by-product of the
process and not the primary objective of the completion. A
more suitable terminology for the technique is "dynamic
open-hole completion." The majority of the dynamic open-
hole completions have been performed in the Fruitland
Formation of the San Juan Basin, Colorado and New
Mexico. Dynamic open-hole completed coal gas wells in
some areas, but not all, produce at significantly greater
rates than wells completed using other techniques such as
hydraulic fracturing. Because of the success in the San
Juan Basin, dynamic open-hole completions have been
attempted in other basins including the Piceance, Powder
River, Arkoma, Uinta, and Black Warrior. This paper
presents a conceptual model and a working hypothesis
concerning what takes place in the coal reservoir during
the dynamic open-hole completion process. Based upon
this model and hypothesis, techniques to: i) optimize the
completion, ii) evaluate the effectiveness of the technique,
and iii) how to determine when to terminate completion
operations will be presented.
INTRODUCTION
Coal natural gas wells typically require stimulation
resulting in effective wellbore to reservoir linkage to
achieve economic gas production rates. The objective of a
dynamic open-hole completion is to: i) effectively link the
open-hole wellbore with the undamaged reservoir, ii)
create multi-directional self-propped fractures in the
reservoir, and iii) to intersect the natural fracture systems
within the coal. A by-product of the dynamic open-hole
completion procedure is an enlarged wellbore caused by
multiple pressure surges that encourage the friable and
relatively low strength coal to slough into the wellbore. In
this process, near wellbore damage is removed, multi-
directional self propped fractures
are created, and the
enlarged wellbore may become linked to the natural
fracture system within the reservoir. During the open-hole
completion process, it is hypothesized that failure occurs
in the coal due to shear and tensile stresses creating
numerous multi-directional tensile, shear and extension
fractures. These fractures stimulate production by
effectively linking the wellbore to a large pre-existing
natural fracture surface area within the coal gas reservoir.
/p.609/
The tensile failure mechanism is a result of the injection
process where the wellbore pressure is increased to a
pressure greater than the minimum wellbore stress
concentration near the well and the minimum principal
stress away from the wellbore in the coal. The orientation
of the tensile failure zone is parallel to the maximum
horizontal stress direction and the zone contains
numerous connected parallel fractures. These tensile
failure induced fractures may become self-propped.
Also during the injection period, the increase in pore pressure
near the wellbore causes the wellbore to decrease in size.
The decrease in the wellbore diameter may result in a
stress reduction away from the wellbore and cause tensile
failure initiation away from the well in multiple
directions. [11] It is hypothesized that these tensile
fractures may extend 30 to 60 m (100 to 200 feet) from the
wellbore on each side of the wellbore. This hypothesis is
based upon limited laboratory data. Additional laboratory
and field research are needed for confirmation.
In contrast, shear failure is a result of active loading when
the wellbore pressure is depressed, as during the
production period of the completion operations or during
under-hydrostatically balanced drilling operations. In this
case, active shear failure zones are created that are
oriented perpendicular to the maximum horizontal stress
direction and perpendicular to the tensile failure zone
orientation. The concept of active shear failure is similar to
that for wellbore breakout which has been extensively
researched and documented in the literature. [12,13,14]
The fractures in the shear zone may also become self-
propped. Based upon laboratory measurements in
sandstone rock types, the shear zones may extend several
wellbore diameters or upwards of 7.5 m (25 feet) from the
wellbore. The orientation of the tensile and shear failure
zones is illustrated in Figure 1.
/p.611/
Techniques need to be developed to clean out the
wellbore more effectively and rapidly to reduce completion
costs. One technique that may increase the cleaning
efficiency is a side jetting tool placed in the drill string, as
shown on Figure 10. The objective of the jetting tool is to
create turbulent flow in the cavities where coal has
accumulated. During conventional clean out operations, air
and small slugs of water are circulated down the drill string
and up the annulus. The flow direction in the annulus is
vertical with the high velocities in the in-gauge wellbore
and low velocities in the cavities.
When the velocity of the
fluid decreases, rock particles transported by the fluid are
dropped at the base of the cavities. If the base of a cavity
is in a zone of high permeability, the cavity will be self
cleaning due to the inflow of water and gas from the
reservoir. However, if the base of the cavity is not in a
zone of high permeability, coal will accumulate in the
bottom of the cavity. This accumulation can block the
lower portion of the wellbore, as is illustrated in Figure 10.
In addition, if the base of the cavity deteriorates, wellbore
cleaning and bridges at the base of the cavity become
more of a problem and will require additional time and
expense to remove. Therefore, the completion operations
should be terminated as soon as expected fluid production
rates are achieved and the wellbore is stabilized.
Extended operations typically result in increased wellbore
problems and costs, without enhancing fluid production.
/p.615/
SUMMARY
• The objective of a dynamic open-hole completion is to:
i) effectively link the open-hole wellbore with the
undamaged reservoir, ii) create multi-directional self-
propped fractures in the reservoir due to tensile and
shear failure, and iii) to intersect the natural fracture
system within the coal.
• The enlarged wellbore "cavity" is a by-product of the
operation and is not the primary cause of the increased
production rates observed.
• The agreement between pre- and post-completion
permeability estimates has given us confidence in the
potential for estimating the applicability of dynamic
open-hole completions and the prediction of the post-
completion well performance based upon pre-
completion tests. Analysis of data that results in
estimates of the absolute permeability greater than 20
md suggests that dynamic open-hole completion
procedures can be used.
• Although dynamic open-hole completions have been
successful in some areas, the technique can be
improved, thereby enhancing the productivity, reducing
completion costs, and potentially broadening the
reservoir types where this technique can be
successfully applied.
• One technique that may be used to complete multiple
coal seams is to increase the pore pressure and thus
the near wellbore stresses in the zone that is accepting
the air by initially injecting water ahead of the air
injection volume. The increase in the near wellbore
stress inhibits continued injection into the zone and
increases the ability to inject into other coal zones that
have not been affected by past injection/surges.
• The injected volume for each pressure/surge, should
be determined based upon the reservoir volume
required to i) pressurize the wellbore to create shear
failure, and ii) pressurize the fractured zones to
propagate the fractures. As the completion continues,
the fluid volume required increases due to the
increasing wellbore volume as the result of coal
sloughing.
• The average total time required to complete a dynamic
open-hole completion operation is approximately 10
days at an average daily cost of approximately $8,000
($US) per day. During the completion operations, the
majority (65.2%) of the operation is spent cleaning out
the wellbore while only 22.3% of the total time is spent
conducting the injection/surges. Techniques need to be
developed to more effectively and timely clean out the
wellbore to reduce completion costs. One technique
that may increase the cleaning efficiency is a side
jetting tool placed in the drill string.
• Generally, completion operations are discontinued
when successive injection/surge operations do not
result in increased fluid production rates. A more
quantitative approach is based upon estimates of the
fluid productivity of the reservoir once the wellbore and
the natural fracture system are linked.
/p.616/
© 05.02.2006 Øåñòîïàëîâ À.Â.