Low-viscosity slickwater treatments are a popular hydraulic fracturing technique in low permeability reservoirs. Slickwater treatments can provide adequate conductivity in tight gas sand operations at comparatively low costs, and wells treated with low-viscosity slickwater often produce better results than those treated with cross-linked fluids in low permeability situations. Theoretically, proppant transport is poor in low-viscosity slickwater type fluids. Improving the understanding of proppant transport capabilities of slickwater would be beneficial to many operators if the cost or performance were not endangered. Improved proppant transport would result in longer propped fracture half-lengths and more favorable conductivity.
Laboratory experiments performed by STIM-LAB, Inc.'s Proppant Consortium show proppant falls from suspension and builds a proppant mound before any form of proppant transport takes place. Clean fluid stages pumped between sand-laden stages were shown to erode proppant from the proppant mound. These results formed the basis for the development of power and bi-power laws to describe the transport. These laws and the results of the laboratory experiments were used to perform sensitivity analysis to determine the relative effects of fluid viscosity, fluid density, pump rate, proppant diameter, proppant density, proppant concentration, and fracture width on slickwater treatments in the field.
Using the power and bi-power laws, the resulting sensitivity analysis, and the laboratory observations, experimental slickwater schedules were designed and field tested. A total of five experimental slickwater fracturing treatments were performed. Production data from each experimental slickwater treatment and well were compared to offset data to determine any possible effects from improved proppant transport. Production results from the field trials, including both initial production (IP) rates and early cumulative production totals, indicate significant improvement when compared to offset wells.
Over time, many different names have been used to describe slickwater-type fracturing treatments including river fracs, pit fracs, water fracs, treated water fracs, low-proppant fracs, and friction-reduced fracs. The fluid in slickwater treatments consists of water-only or water with a low linear gel concentration. Low proppant concentrations are typically used (0–2 ppg) and are often ramped. Some water fracs are pumped with no proppant at all. Pump rates can vary widely. There are many reasons that may explain why low-viscosity slickwater treatments are successful including: minimal leak-off in low permeability formations, the absence of gel residue, reactivation of natural discontinuities, optimal dimensionless fracture conductivity, and overall economics.1 Variations of slickwater treatments include hybrids and sweep stage slickwater jobs. Hybrid fracture treatments are defined as a low-viscosity pad pumped to initiate the fracture followed by a heavier, higher viscosity fluid laden with proppant.2 Multiple stage slickwater treatments,3 where stages alternate between clean fluid and proppant laden fluid, are defined as sweep stage type slickwater jobs.
The overall objective of the project described in this paper is to improve the understanding of proppant transport mechanisms in low-viscosity slickwater hydraulic fracture treatments. Improved proppant transport may result in longer propped fracture half-lengths, thereby resulting in improved fracture conductivity, and higher production rates. Power and bi-power law equations, developed to describe this type of transport, were used to simulate various slickwater treatment scenarios. Observations from these experiments along with the simulations are used to predict what the resulting proppant mound may look like in a slickwater hydraulic fracture and how it is affected by pumping techniques.
A sensitivity analysis was also performed to show the sensitivity of certain variables on proppant mound growth. These variables include: fluid density, proppant density, proppant diameter, fluid viscosity, fracture width, fluid flow rate, and proppant concentration.