Summary
Recent advances in guar and crosslinker technologies have resulted in the
development of high-viscosity crosslinked borate-fracturing fluids without
increasing polymer loadings. These low polymer (LP) borate fracturing fluids
are being used successfully in various formations previously believed to be too
hot and or too deep for LP fracturing fluids.
Historically, polymer loadings of 3.6 to 4.2 kg/m³ (30 to 35 lbm/1,000 gal)
were commonly pumped in the Western Canadian Sedimentary basin (WCSB) for
formations deeper than 2500 m and bottomhole temperatures greater than 80°C.
These same formations are now fracture stimulated using the LP fluids with
loadings as low as 1.8 kg/m³ (15 lbm/1,000 gal) with exceptional results.
This paper demonstrates that LP fracture fluids can be used in place of
fluids requiring higher polymer loadings with minimal changes to the overall
design of the fracture treatment. The new fluid can be pumped on-the-fly at
conventional pump rates and proppant concentrations because of the fluid’s
improved shear and temperature stability.
The advantages of using a reduced-polymer fracturing fluid include increased
production, lower treatment costs, and lower frictional pressure loss.
This paper illustrates these advantages as it compares the LP fracture fluid
with HP fracture fluids in more than 200 wells in the WCSB. The formations
where LP fluids were used have depths of up to 3250 m and reservoir
temperatures reaching over 100°C.
Introduction
Low-permeability gas wells in the WCSB often require hydraulic fracturing to
be ecomonic (such fracturing treatments are considered to be a critical step in
the completion process). The choice of fracturing fluid is a major component
and concern when designing optimal fracture stimulations. The primary roles of
the fracturing fluid are to initiate and extend a hydraulic fracture and to
distribute proppant. The majority of fracture treatments use crosslinked
water-based fluids. Fluid viscosity is attained by adding some type of guar
polymer to the water. Many types of guars can be and have been used, including
derivatized guars, xanthan gum, and polyacrylamides. For the LP fluids
discussed in this paper, a nonderivatized guar crosslinked with borate was
used. For the purpose of this paper, LP fluids are described as those with
polymer loadings of 1.8 to 2.4 kg/m³ (15 to 20 lbm/1,000gal). Conversely, HP
fluids are those containing polymer loadings of 3.0 to 4.2 kg/m³ (25 to 35
lbm/1,000gal).
Recent advances in guar processing have made the LP systems achievable.
Historically, guar has been known to contain 8 to 12% residue (Gidley et al.
2001), but newer, more-processed guars available today contain 1 to 5% residue.
This enhanced guar has a higher molecular weight than previous guars (Dawson et
al. 2004), thereby increasing the yield of the polymer that, in turn, increases
viscosity. The shear and temperature stability of the resulting fluid is
superior to other guars, leading to a wide range of applications.
Why Reduce Polymer Loadings?
As stated previously, polymer is required to create enough viscosity for the
fracture fluid to initiate and to extend the fracture, as well as properly
place the proppant along the fracture. Unfortunately, polymer is also known to
be damaging to the proppant pack (Kim and Losacano 1985; Roodhart et al. 1988).
This damage adversely affects proppant-pack conductivity and, therefore,
production potential. There can also be damage from filter-cake buildup on the
formation face, which can also reduce the fracture’s effectiveness (Volk et al.
1983).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
30 June 2006
- Meeting paper published:
15 May 2006
- Revised manuscript received:
3 October 2007
- Manuscript approved:
8 October 2007
- Version of record:
20 March 2008
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