Eliminating Slickwater Compromises for Improved Shale Stimulation

摘要

Hydraulic fracturing stimulation is generally accepted as requisite to obtain commercial production from low permeability formations, including oil- and gas-bearing shales. Low viscosity slickwater fluids generate fractures of lesser width and therefore greater fracture length, theoretically increasing the complexity of the created fracture network for better reservoir-to-wellbore connectivity. Unfortunately, slickwater fluid is an inherently poor proppant carrier, necessitating high pump rates to achieve flow velocities sufficient to overcome the tendency of the proppants to settle. Proppant settling within surface equipment or long horizontal laterals can result in premature treatment termination and poor productivity. Crosslinked and linear gel systems have been used to mitigate the proppant settling and placement concerns, but the high viscosity that accomplishes this objective may significantly reduce the desired fracture complexity. A ‘new’ crosslinked polymer system deploys the best attributes of slickwater and conventional crosslinked fluids systems to maximize proppant transport through the surface equipment and long laterals before breaking to create a desirably complex fracture network. The fluid is crosslinked on surface with an ultra-low loading of high- yield guar polymer loading that achieves surface viscosity sufficient to carry proppant from the pumping equipment through the horizontal lateral and perforations. Controllable viscosity degradation converts the fluid to a slickwater viscosity within the reservoir to provide the desired complex fracture network. Gel break-back timing may be used to control the onset of complex network development. For example, it may be desirable to generate a planar fracture for a distance from the wellbore before reverting to the low viscosity-induced complex fracture development. Thus, the system displays the positive attributes of both slickwater and crosslinked gel systems while simultaneously overcoming the negatives associated with each. Fracture modeling illustrates the utility of the fluid performance management and controlling the onset of complex network development. Case histories compare applications of the system to offsets treated with conventional slickwater and crosslinked gel fluids, including well performance and an assessment of the impact upon operational reliability.