Redesigning Fracturing Fluids for Improving Reliability and Well Performance in Horizontal Tight Gas Shale Applications

摘要

Fracture stimulation is necessary in tight gas and shale formations to ensure commercial production volumes. Variations of slickwater are the predominant fluid used in shale stimulation because their low viscosity and relatively low costs. These stimulation treatments commonly use ceramic proppants or sand to create, develop and maintain the fracture network connectivity to the wellbore. Low viscosity fluids generate fractures of more narrow width and thereby, increased complexity of the created fracture network, which is thought to provide better stimulation of tight gas shales. However, proppant settling in low-viscosity fluid can reduce the chance of achieving optimal fracture conductivities. Friction reducers allow surface equipment to achieve high pump rates and fluid velocities that can overcome some proppant settling. High proppant concentrations during slickwater treatments tend to create settling and banking problems in surface equipment and in the horizontal wellbore. Conventional crosslink systems have been used to improve placement of the proppant by minimizing proppant settling. However, in addition to potential conductivity damage imparted by the conventional crosslinked fluids, the inherent high viscosity of such systems may significantly reduce the amount of desired fracture complexity created in the tight gas formations. Recent advances in polymer refining offer the capability of pumping an instant-crosslink system with polymer loadings as low as 8 pptg. This ultra-low polymer crosslink system provides a surface viscosity that maintains effective proppant transport through the surface equipment, perforations, and into the near-wellbore area. Additionally, programmed viscosity degradation converts the fluid to a low viscosity within minutes of introduction to the reservoir to provide a desired complexity to the created fracture network. The low polymer loadings minimize polymer volumes introduced to the formation, and therefore formation damage. This paper will detail the effects of new system's viscosity on proppant transport through the surface equipment and horizontal wellbore, and the development of a viscosity break profile that can deliver a low viscosity, near-Newtonian fluid that is ideal for developing the intricate fracture system to optimize production from tight gas formations.