Superior Transport Capabilities of Neutrally Buoyant Proppants in Slickwater Fluids Deliver Step-Change Increase in the Conductive Fracture Area of Unconventional Wells
Authors: Brannon Harold 
Published: October 2018

Technological advancements have recently been directed toward development and optimization of horizontal completions in unconventional reservoirs, with the ultimate objective of increasing asset performance and value. Unconventional plays are being completed with ever-longer laterals, tighter stage spacing, and high rate slickwater applications designed with increasingly larger volumes of sand to create increased reservoir contact area for greater hydrocarbon recovery. Success is predicated upon overcoming the limited transport capabilities of slickwater. The benefit of higher injection rates employed to enhance proppant transport is soon lost as the lateral velocity declines exponentially with distance from the wellbore, allowing the sand to fall rapidly to the bottom of fractures, resulting in propping only a fraction of the created fracture area. While there are advantages to the use of slickwater and sand for unconventional applications, the transport characteristics inherent to slickwater/sand slurries suggest significant limitations to step-changes in hydrocarbon recovery.

Near-neutrally buoyant, ultra-lightweight proppant is a proven solution to make productive the otherwise non-propped area. Several previous studies in parallel plate slot flow models have shown ULWP-1.05 is transported well in slickwater, whereas sand settles rapidly to form a dune even at high flow rates. Such behavior is intuitive given the near-neutrally buoyant ULWP has an Apparent Specific Gravity of 1.05, in contrast to the 2.65 ASG of sand and the 1.0 ASG of water.

Two new proppant transport models have recently been introduced, including a slot with multiple fracture branches and, a 3D complex network flow model designed to imitate flow through a lateral wellbore into a complex fracture network. In both, the ULWP-1.05 was observed to be transported near-homogeneously with the fluid to the extremities of the apparatus. Conversely, small mesh sand tended to stay in the lower sections of the models and to deposit prior to reaching the extremities.

As a prelude to ULWP-1.05 field application in Permian Basin extended length horizontal wells, proppant transport and fracture conductivity data for the near-neutrally buoyant ULWP-1.05 were used in fracture models to optimize proppant placement for maximizing conductive fracture area, with iterations to optimize well performance in production simulations. A desired outcome of this endeavor is the development and validation of an optimized stimulation design exhibiting materially enhanced well performance.

This paper includes analyses and observations from the proppant transport testing, fracture conductivity testing, discussion of the subsequent fracture designs and production simulations, and comparison of the production simulations with production experienced in field applications. Performance of slickwater fracs with sand alone and, with both sand and near neutrally buoyant ULWP are compared. Lessons learned may be used to substantially increase the conductive fracture area of unconventional wells, optimizing production performance and stimulated reservoir recovery efficiency.