Hydrophobically coated sand, when combined with gas injection into an appropriate fluid, provides a significant improvement in proppant transport compared to uncoated sand. This supports the expectation that gas bubbles adhering to sand particles improve transport properties. In addition, the increase of nitrogen gas volume fraction within the fracturing fluid significantly enhances proppant suspension, particularly for smaller size proppant. Proppant transport occurs by two mechanisms, suspended transport and bed transport.
This study aims at determining the optimum conditions and composition for a fracturing fluid, comprised of a gas dispersed into an aqueous liquid (but not as a foam), to transport hydrophobically coated proppant. The aqueous phase is comprised of water with a friction reducer, to enhance suspension performance. The impacts of the gas volume fraction of the fluid and of the proppant particle size are studied.
The experimental setup is designed to compare the proppant transport efficiency of different gaseous and/or liquid fracturing fluids. In the present case, high pressure nitrogen (13.8 MPa - 2000 psi) is dispersed within an aqueous fluid. Proppant is then injected at the entrance of a straight pipe section. This central section is equipped with several collectors where the sand can accumulate. After a specific time, the system is shut down and depressurized. Sand collection along the section provides a quantitative measurement of the proppant transport efficiency. Additionally, direct observation through sight glasses allows qualitative assessment of the transport method.
The main impact of increasing the volume fraction of gas is to increase suspended transport, but we also observed a moderate improvement in bed transport. Thus, overall we observed a significant improvement in proppant transport as the gas volume fraction was increased from 10% to 30%. This effect was greatest at the smallest particle size tested, i.e. 40/70 U.S. mesh sand, but was also observed with 30/50 and 20/40 U.S. mesh sands. The upper limit on gas volume fraction was imposed by the nature of the aqueous fluid, which is not designed to operate as a foam. Phase separation effects become dominant at high gas volume fractions. The effect of particle size is attributed to the increasing number of particles present at constant loading by mass.
Here we present data to support the selection of gas volume fraction for use with hydrophobically coated proppant, in order to optimize proppant transport in practical fracturing operations. These data are based directly on proppant transport measurement under high pressure conditions, which enable proper consideration of elasticity and proppant suspension effects of added gas.