Abstract
Lightweight, intermediate-strength proppants have been developed that are intermediate in cost between sand and bauxite. A wide variety of proppant materials is characterized and compared in a laboratory fracture conductivity study. Consistent sample preparation, test, and data reduction procedures were practiced, which allow a relative comparison of the conductivity of various proppants at intermediate and high stresses. Specific gravity, proppants at intermediate and high stresses. Specific gravity, corrosion resistance, and crush resistance of each proppant also were determined. proppant also were determined. Fracture conductivity was measured to a laminar flow of deaerated, deionized water over a closure stress range of 6.9 to 96.5 MPa [1,000 to 14,000 psi] in 6.9-MPa [1,000-psi] increments. Testing was performed at a constant 50 degrees C [122 degrees F] temperature. Results of the testing are compared with values from the literature and analyzed to determine proppant acceptability in the intermediate and high closure stress regions. Fracture strengths for porous and solid proppants agree well with calculated values. Several oxide ceramics were found to have acceptable conductivity at closure stresses to 96.5 MPa [14,000 psi]. Resin-coated proppants have lower conductivities than uncoated, intermediate-strength oxide proppants when similar size distributions are tested. Recommendations are made for obtaining valid conductivity data for use in proppant selection and economic analyses. proppant selection and economic analyses.
Introduction
Massive hydraulic fracturing (MHF) is used to increase the productivity of gas wells in low-permeability reservoirs by creating deeply penetrating fractures in the producing formation surrounding the well. Traditionally, producing formation surrounding the well. Traditionally, high-purity silica sand has been pumped into the created fracture to prop it open and maintain gas permeability after completing the stimulation. The relatively low cost, abundance, sphericity, and low specific gravity of high-quality sands (e.g., Jordan, St. Peters, and Brady formation silica sands) have made sand a good proppant for most hydraulic fracturing treatments. The closure stress on the proppants increases with depth, and even for selected high-quality sands the fracture conductivity has been found to deteriorate rapidly when closure stresses exceed approximately 48 MPa [7,000 psi]. Several higher-strength proppants have been developed to withstand the increased closure stress of deeper wells. Sintered bauxite, fused zirconia, and resin-coated sands have been the most successful higher-strength proppants introduced. These proppants have improved proppants introduced. These proppants have improved crush resistance and have been used successfully in MHF treatments. The higher cost of these materials as compared to sand has been the largest single factor inhibiting their widespread use. The higher specific gravity of bauxite and zirconia proppants not only increases the volume cost differential compared to sand but also enhances proppant settling.
Lower-specific-gravity proppants not only are more cost effective but also have the potential to improve proppant transport. Novotny showed the effect of proppant diameter on settling velocity in non-Newtonian fluids and concluded that proppant settling may determine the success or failure of a hydraulic fracturing treatment. By using the same proppant settling equation as Novotny, the settling velocity of 20/40 mesh proppants is calculated for four different specific gravities and shown as a function of fluid shear rate in Fig. 1. The specific gravity of bauxite is 3.65 and sand is 2.65; therefore, bauxite is 37.7 % more dense than sand. The settling velocity for bauxite, as shown in Fig. 1, however, is approximately 65 % higher than sand. Work on proppants with specific gravities lower than bauxite was initiated to improve the transport characteristics of the proppant during placement. It has been demonstrated that vertical propagation of the fracture can be limited by reducing the fracturing fluid pressure. The viscosity range of existing fracturing pressure. The viscosity range of existing fracturing fluids makes minimizing fluid viscosity a much more effective method of controlling pressure than lowering the pumping rate. A potential advantage of decreasing the pumping rate. A potential advantage of decreasing the specific gravity of the proppant is that identical proppant transport to that currently achievable can take place in lower-viscosity fluids. (Alternatively, higher volumes of proppant can be pumped in a given amount of a proppant can be pumped in a given amount of a high-viscosity fracturing fluid.)
Not only are low-viscosity fluids capable of allowing better fracture control, they are also less expensive. More importantly, it recently was shown that the conductivity of a created hydraulic fracture in the Wamsutter area is about one-tenth of that predicted by laboratory conductivity tests.
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Recent Advances in Proppant Embedment and Fracture Conductivity after Hydraulic Fracturing
