Summary
Since the earliest fracturing treatments more than 50 years ago, many
different materials have been used including sand, glass beads, walnut hulls,
and metal shot. Today’s commonly used proppants include various sands,
resin-coated sands, intermediate-strength ceramics, and sintered bauxite—each
employed for its ability to cost-effectively withstand the respective reservoir
closure stress environment. As the relative strength of the various
materials increases, so too have the respective particle densities, ranging
from 2.65 g/cm3 for sands to 3.4 g/cm3 for the sintered bauxite.
Unfortunately, increasing particle density leads directly to increasing degree
of difficulty with proppant transport and a reduced propped-fracture volume for
equal amounts of the respective proppant, thereby reducing fracture
conductivity. Intuitively, one expects that a lesser-density proppant
would be easier to transport, allowing for reduced demands on the fracturing
fluids, and if it had sufficient strength, would provide increased width, and
hence, enhanced fracture conductivity.
Previous efforts undertaken to employ lower-density materials as proppant
have generally resulted in failure because of insufficient strength to maintain
fracture conductivity at even the lowest of closure stresses (1,000 psi).
Recent research on material properties has at last led to the development of an
ultralightweight (ULW) material with particle strength more than sufficient for
most hydraulic fracturing applications. The current ULW proppants have apparent
specific gravities of 1.25 and 1.75 g/cm3. Laboratory tests will
demonstrate exceptional fracture conductivity at stresses to 8,000
psi. This paper presents data illustrating the performance of the new ULW
proppant over a broad range of conditions and a discussion of relative
performance in field applications.
Introduction
ULW proppants have been the subject of research efforts for at least a
decade. In general, the stronger a proppant, the greater the density; as
density increases, so too does the difficulty of placing that particle evenly
throughout the created fracture geometry. Excessive settling can often
lead to bridging of the proppant in the formation before the desired
stimulation is achieved. The lower particle density reduces the fluid
velocity required to maintain proppant transport within the fracture, which in
turn provides for a greater amount of the created fracture area to be
propped. Alternatively, reduced-density proppants could be employed to
reduce fracturing-fluid complexity and to minimize proppant-pack damage.
Two different avenues of ULW particle-development research pursued in this
area are presented. The first is a porous ceramic that uses novel resin
technology to coat the outside of the particle without invading the porosity to
effectively encapsulate the air within the porosity of the particle.
Encapsulation of the air provides preservation of the ULW character of the
particles once placed in the transport fluid. Additionally, the resin
coating significantly increases the strength and crush resistance of the ULW
ceramic particle. In the case of natural sands, the resin coat protects
the particle from crushing, helps resist embedment, and prevents the liberation
of fines.
The second avenue of research was directed toward an even lighter particle
that may be described as a resin-impregnated and then coated cellulosic
particle. The cellulosic substrate is sized, ground walnut hull. The
low specific gravity of this particle allows near-neutral buoyancy behavior in
flowing streams of slickwater-type fluid. The application benefits of the
ULW proppant are further enhanced beyond those discussed above. Resin
impregnation and coating provide significantly enhanced strength beyond that
afforded by the unaltered walnut hulls alone.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
9 March 2004
- Revised manuscript received:
5 July 2005
- Manuscript approved:
9 July 2005
- Version of record:
20 May 2006
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