High Strength, Ultralightweight Proppant Lends New Dimensions to Hydraulic Fracturing Applications
- Allan R. Rickards (BJ Services Company) | Harold D. Brannon (BJ Services Co. USA) | William D. Wood (BJ Services Co. USA)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- May 2006
- Document Type
- Journal Paper
- 212 - 221
- 2006. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 2.5.1 Fracture design and containment, 2.2.2 Perforating, 1.8 Formation Damage, 5.2 Reservoir Fluid Dynamics, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.4 Scale
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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.
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.
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