Recent tests have shown that the conductivity of a 20–40 sand pack is increased by blending angular sand with the very round product currently used in hydraulic fracturing. The oil and gas industry requires fracturing sand to meet high roundness specifications, according to the Krumbein and Sloss chart, for the purpose of providing the optimum geometric pack in the purpose of providing the optimum geometric pack in the fracture. Indeed this will decrease point pressure on the sand grains and reduce crushing and fines generation in the sand pack, but tests indicate that the optimum geometric pack for crush resistance does not always offer the maximum conductivity through a sand pack. pack. Sand grains with a roundness factor of .5 (the minimum APT standard for frac sand is .6) were mixed in 50/50, 62.5/37.5, 75/25, 87.5/12.5 and 100/0 blends with sand grains having a roundness factor of .8. These combinations were placed in a conductivity cell and tested at closure stresses as high as 10,000 psi. Several blends proved more conductive than the 100% highly rounded grains used as a control sample. This paper will show how the increased void space of a round-angular sand combination overcomes its susceptibility to crushing and provides increased conductivity, the most reliable measurement of a sand's overall performance.
In developing a study on 20-40 fracturing conductivity it is important to review three current principles generally accepted by the well stimulation industry.
The key to a successful hydraulic fracturing treatment is to increase conductivity in the producing formation while minimizing permeability damage.
The most important function of a propping agent in a hydraulic fracturing treatment is to create and sustain fracture conductivity.
In using silica sand as a propping agent fracture conductivity can best be created with a very round and spherical product.
We agree with the first principle. Indeed the industry's search for additional conductivity has prompted this and many other studies. We also agree with the second principle and believe that a reliable analysis of a proppant's fracture conductivity under simulated reservoir conditions would provide the most significant measure of its overall performance.
The third principle we contend is not true all of the time. The American Petroleum Institute (APT) has recommended roundness and sphericity numbers for what would be considered an acceptable silica sand for hydraulic fracturing (Roundness is the smoothness of a sand grain's surface or the lack of rough edges. Sphericity is the degree a sand grain resembles a sphere or circle.). Those numbers are .6 for both roundness and sphericity according to Krumbein and Sloss (Fig. 1). We do believe that a high sphericity must be maintained because this eliminates sand grains with elongated shapes or protrusions and prevents their fitting into a natural void space and restricting conductivity. In the case of roundness, however, we do not feel that it is always advantageous for all of the sand grains to be perfectly round or smooth. We feel the same about the reverse situation of 100% angular grains for even though angular blends have the most porosity in a loose pack they possess surfaces that will break off or crush easily with any significant closure stress because of uneven pressure distribution on the surface points that touch each other.
But if a certain percentage of angular grains are interspersed in a very round, spherical product the benefit of a highly porous angular sand can negate its prohibitive properties of easy crushing and produce prohibitive properties of easy crushing and produce more conductivity than either of the pure round or angular blends. This will be demonstrated with conductivity test results.
The results of these findings could have a significant technological and economical impact on the usage of 20–40 silica sand for hydraulic fracturing.