Summary
The recent success of coiled-tubing fracturing in shallow wells has
increased interest in using coiled tubing for fracturing deeper and hotter
wells. Industry efforts now need to focus on understanding what properties a
fracturing fluid must possess to carry proppant successfully at high rates
through coiled tubing, frac ports, and perforations and into these deeper
formations. The key performance requirements of a coiled-tubing fracturing
fluid for deeper wells are low frictional pressure loss and adequate
proppant-carrying capability after exposure to high-shear zones and high
temperatures.
This paper summarizes the results of pilot- and field-scale testing that led
to the development of an optimized coiled-tubing fracturing fluid. Results show
that polymer-based fracturing fluids can be controllably delayed to have low
frictional pressure loss through the curved coiled-tubing unit and straight
tubing. However, results also show that fluid stability can be reduced
significantly when the fluid is pumped through small-diameter tubing followed
by high-shear zones such as frac ports and then by perforations. Results
demonstrate that correct fluid choice and fluid optimization are required to
meet proppant-transport requirements. For coiled-tubing fracturing to be
successful, the fluid and treatment-design recommendations should balance
frictional-pressure-loss limitations with fluid-stability limitations.
Introduction
A recent review of the literature revealed that minimal information is
available on how a fracturing fluid is affected by pumping at high rates
through long lengths of small-diameter tubing followed by the high-shear zone
in the frac ports and perforations. Several papers have been written describing
the adverse effect that pumping fracturing fluids at high shear rates has on
fluid stability (Goel et al. 1997; Shah et al. 1997).Other papers have
presented data on the adverse effects on fluid stability caused by pumping
fracturing fluids at low rates through coiled tubing (Shah and Subramanian
1997),but little information is available on the adverse effects of pumping at
high rates. Still others have described the erosive effects of sand when pumped
through small-diameter coiled tubing and through isolation tools, but the
effect of these high-shear environments on fluids has been largely neglected
(Gavin 2000; McLaury et al. 1997).Because of the lack of information on fluids
used in coiled-tubing applications, particularly in deep-well coiled-tubing
applications, it was necessary to conduct pilot- and field-scale testing to
determine how a fracturing fluid is affected under these conditions and how its
fluid properties can be optimized specifically for deep-well coiled-tubing
fracturing applications. Crosslinked fracturing fluids may show ideal
properties in the laboratory but may exhibit completely different behavior
under actual field conditions. Therefore, the properties of crosslinked fluids
must be determined and optimized in laboratory testing that simulates as
closely as possible actual field and downhole conditions (Goel et al.
1997).
The first step in the development of a fluid optimized for deep-well
coiled-tubing fracturing applications was to set product performance
specifications based on targeted job requirements. These targeted job
requirements involved pumping through 1,000 ft of 2-in. coiled tubing and then
down 7,500 ft of straight tubing at pump rates of 8 to 10 bbl/min.Jobs would
treat small zones with 25,000 lbm of proppant with proppant loadings up to 4
lbm proppant added (ppa). Product specifications required the product to have
minimal frictional pressure loss to allow pumping through the coiled tubing and
then through long lengths of straight tubing at high pump rates. The fluid
crosslink time should beat least three-quarters of the transit time through the
tubing length. The fluid also should have at least 1-hour stability at 250°F
and must also have adequate viscosity to carry proppant at high flow rates
through the frac portsand then immediately through the perforations.
The development of the improved coiled-tubing fracturing fluid for deep-well
application was carried out in two testing phases. Test Phase 1 involved
screening of potential fluids in a pilot-scale friction test loop. Test Phase 2
involved field-scale testing of the best candidate fluid from Test Phase 1
through a coiled-tubing unit and a straight-tubing test loop.
© 2007. Society of Petroleum Engineers
View full textPDF
(
1,758 KB
)
History
- Original manuscript received:
16 June 2004
- Revised manuscript received:
17 January 2006
- Manuscript approved:
20 March 2006
- Version of record:
20 February 2007
View Cart
