Hydraulic fracturing is a common stimulation technique used to enhance production of fluids from subterranean formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant material is injected into the formation at a pressure sufficiently high enough to cause the formation or enlargement of fractures in the reservoir. During a typical fracturing treatment, proppant material is deposited in a fracture, where it remains after the treatment is completed. After deposition, the proppant material serves to hold the fracture open, thereby enhancing the ability of fluids to migrate from the formation to the well bore through the fracture. Because fractured well productivity depends on the ability of a fracture to conduct fluids from a formation to a wellbore, fracture conductivity is an important parameter in determining the degree of success of a hydraulic fracturing treatment.
Hydraulic fracturing treatments commonly employ proppant materials that are placed downhole with a gelled carrier fluid such as aqueous-based fluid such as gelled brine. Gelling agents for proppant carrier fluids may provide a source of proppant pack and/or formation damage, and settling of proppant may interfere with proper placement downhole. Formation damage may also be caused by gelled carrier fluids used to place particulates downhole for purposes such as for sand control, such as gravel packs, frac packs, and similar materials. Formulation of gelled carrier fluids usually requires equipment and mixing steps designed for this purpose.
Hydraulic fracturing treatments may also employ proppant materials that are placed downhole with non-aqueous-based fluids, such as liquid CO2 and liquid CO2/N2 systems. Proppants commonly employed with such non-aqueous-based fluids tend to settle in the system.
Many different materials have been used as proppants including sand, glass beads, walnut hulls, and metal shot. Commonly used proppants today include various sands, resin-coated sands, intermediate strength ceramics, and sintered bauxite; each employed for their 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/cc for sands to 3.4 g/cc 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, reducing fracture conductivity. Previous efforts undertaken to employ lower density materials as proppant have generally resulted in failure due to insufficient strength to maintain fracture conductivity at even the lowest of closure stresses (1,000 psi).
Recently, deformable particles have been developed. Such deformable particles for sand flowback control are significantly lighter than conventional proppants, and exhibit high compressive strength Such deformable materials include polystyrene divinylbenzene (PSDVB) deformable beads. Such beads, however, have not been entirely successful primarily due to limitations of the base material. While PSDVB beads offered excellent deformability and elasticity, they lacked the structural integrity to withstand high closure stresses and temperatures.
The first successful path to generate functional deformable particles was the usage of modified ground walnut hulls. Walnut hulls in their natural state have been used as proppants, fluid loss agents and lost circulation materials for many years with greater or lesser degrees of success in each respective task. As a proppant, natural walnut hulls have very limited applicability, because they deform fairly readily upon application of closure stress. This deformation drastically reduces conductivity and limits utility of the natural material to relatively low-closure environments.
Walnut hull based ultra-lightweight (UCW) proppants may be manufactured in a two-step process by using closely sized walnut particles (i.e. 20/30 US mesh), and impregnating them with strong epoxy or other resins. These impregnated walnut hull particles are then coated with phenolic or other resins in a fashion similar to most resin coated proppants (RCP). Such walnut hull based ULW proppants have a bulk density of 0.85 grams/cc and withstand up to 6,000 psi (41.4 MPa) closure stress at 175° F. (79° C.).
Generally speaking, 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.
ULW proppants which allow for optimization of fracturing treatment with improved fracture length and well productivity are therefore desired.