Fluid Selection for Energized Hydraulic Fractures

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 124361, ’Fluid Selection for Energized Hydraulic Fractures,’ by Kyle E. Friehauf and Mukul M. Sharma, SPE, University of Texas at Austin, prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, 4-7 October. Traditional hydraulic-fracture simulators do not take into account the compositional, thermal, and phase-behavior effects that are crucial to the success of energized fractures. Traditional water-based fracturing fluids are not ideally suited for use in tight, depleted, or water-sensitive formations. Gas-energized fracturing fluid addresses the water-trapping problem by creating a high gas saturation in the invaded zone, thereby facilitating gas flowback. A fully compositional fracture simulator was used to evaluate different designs for energized fractures. It showed that energized fluids should be applied to rocks when the drawdown pressure is insufficient to remove the liquid. Use of a compositional fracture simulator allows systematic design of energized-fracture treatments.  Introduction A hydraulic-fracturing process becomes energized with the addition of a compressible, sometimes soluble, gas component into the treatment fluid. Energizing the fluid creates a high gas saturation in the invaded zone, thereby facilitating gas flowback. It is estimated that one-third of the fractures pumped in the US are energized by a gaseous phase. Energized-fluid fracturing is common in reservoirs that are water sensitive or that have a low pore pressure or low permeability. In conducting this study, the hydraulic-fracturing model used compositional and energy balances and coupled them with phase behavior, making this model applicable to energized fluids. The productivity-index (PI) -ratio values were calculated with a PI model that takes into account varying fracture conductivity and the damage around the fracture face caused by fluid invasion. Multiphase-Fracturing-Fluid Properties As with all fluids, the properties of an energized fracturing fluid depend on fluid composition and the range of conditions under which the fluid is applied. Commonly, CO2 or N2 are added to a traditional water-based fluid to energize the fluid. This paper focused on these fluid systems because they are used most often and because their rheology has been studied to a great extent, but methanol is another possible component. This paper discusses commonly used fluid systems containing the following component mixtures: CO2/H2O, N2/H2O, and CO2/methanol/H2O.