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Abstract

This paper presents methods, materials, and procedures that can enable operators to prepare safe and effective fracturing fluids from returned fracturing fluid (commonly called “flowback”) and produced formation water. Reuse of frac fluid and formation water especially focused on high volume and rate (HVR) fracturing requires establishment of a number of interrelated chemical and geochemical solutions. Specifically addressed are environmental attributes of the chemical additive components and whole fluid, geochemical precipitates, scale, microbially induced biogeochemical interactions, water analysis, friction reducer compatibilities, and saline content.

Materials discussed in the paper include:

• Friction reducer chemistry and attributes that enable reuse of frac water in actual flowbacks from shale reservoirs.
• Solutions and need for complexing iron and converting to insoluble precipitates that can damage fracture conductivity, reduce production potential, and negatively impact scale inhibition.
• Methods for protection against an assemblage of naturally occurring geochemical precipitates.
• A solution for environmentally sound quick-kill of microorganisms responsible for producing damaging downhole biogeochemical byproducts, including hydrogen sulfide.

Fracture-stimulation of shale-gas wells requires an enormous volume of frac fluid, which traditionally has been developed from fresh water purchased from sources near the drilling location or municipalities. Along with the chemicals introduced by the frac fluid, the flowback may contain a wide variety of dissolved constituents such as salts and metal ions. The constituents can make wastewater disposal environmentally prohibitive, difficult, and expensive, and may impair gas production by placing damaging precipitates within the fracture, perforations, and wellbore. The paper presents and discusses practical, cost-effective remediation of frac fluid to minimum standards required to achieve environmental, technical, and economic goals. Keys to successful reuse of frac fluids are chemical additive developments and processes involving analysis of water chemistry, flow-loop testing, and geochemical modeling in the establishment of treatment and/or dilution standards to enable fracturing fluid reuse.

Introduction

Shale reservoirs are characterized by extremely low-permeability rock that has a number of unique attributes, including high organic content, high clay content, extremely fine grain size, plate-like microporosity, little to no macroporosity, and fickian vs. darcy flow through the rock matrix. This combination of traits has led to the evolution of hydraulic-fracture stimulation involving high rates, low viscosities, and large volumes of proppant. Production from shale is dependent upon many variables, including hydrocarbon content, total organic carbon, shale maturity, porosity, permeability, kerogen content, formation pressure, and net thickness.

Improvements in drilling and completion techniques, such as landing a horizontal borehole strategically and creating a series of multiple-staged hydraulic fractures, have improved production rates greatly in recent years. The extremely low permeability of shale requires a complex fracture to create primary induced fractures, reactivate and/or intercept more naturally occurring fractures or parting planes, and ultimately expose more surface area to enhance gas desorption and diffusion from the impermeable shale matrix. Early increased production is dependent on the number of natural fractures intercepted and long-term production is dependent on the amount of surface area exposed in the fracture network. Total improved production depends on complex fracture geometry, which is influenced by many factors: stress contrasts, fluid leakoff, natural fractures, layering, weak planes, brittleness, fracture height growth, differing critical stress, post-fracture retention of connectivity to the created frac network, and mechanical stratigraphy, which controls frac network creation.