Fracturing Fluids and Breaker Systems - A Review of the State-of-the-Art (1996)

Abstract Hydraulic fracturing, as a means to improve the productivity of oil and gas wells, has been applied since the late 1940's. Since those early days, major improvements have been made in the properties of the fluids used to initiate and propagate the fracture and to carry the proppants that hold the fracture open after pump pressure is released. Fracturing fluids have been described by one notable industry figure as "the ultimate schizophrenic fluids." This rather colorful description is not without some merit. The ideal fracturing fluid should be thin and easy to handle on surface but should have sufficient viscosity to transport proppant from the surface equipment to the well tubulars. In the course of transiting the tubulars, it should develop sufficient additional viscosity to transport the proppant through the tortuous near-wellbore region and to support the proppant in the relatively low shear environment of the fracture. This additional viscosity is also needed to minimise fluid leak-off from the fracture so that it will remain open and able to accept proppant without the requirement for excessive volumes of fluid and unreasonably high pump rates. However, the viscosity development cannot occur too soon since this would produce very high friction pressures in the tubulars, leading to higher surface pressures and horsepower requirements. Finally, after successfully transporting proppant into and along the fracture, the fluid must "break," reverting to a thin, water-like condition for easy clean-up from the well. This "thin-thick-thin again" behaviour can today be achieved by clever chemical manipulation of a select few natural polymers and their derivatives. This paper seeks to "de-mystify" the current state-of-the-art in fracturing fluids whilst putting it within the historical perspective of its early beginnings. Historical The first recorded hydraulic fracturing operation was in the Hugoton gas field in westem Kansas in 1947. The first fracturing fluids were oil-based and their use arose from the existence of large quantities of surplus "napalm" after World War II. Napalm is a hydrocarbon (usually gasoline or kerosene), gelled with an aluminium soap. This rather hazardous fracturing formulation was soon superseded by viscous refined oils and subsequently by gelled lease crudes and emulsions. These oil-based fracturing fluids were considered to be essential to prevent damage to water-sensitive formations. Such formations contain pore-filling clays that may swell, or become detached and migrate, to cause drastic reductions in rock permeability. By the mid-1960's, however, it was clear that water-based fluids offered a cheaper and safer alternative to these oil-based fluids. This was made possible by the work of a number of authors who showed that the majority of water-sensitive formations could be protected by the incorporation of salts, like potassium or calcium chloride, in the treating fluid. Credit should also go to those field engineers who questioned established dogma and pumped water-based systems with great success, thereby forcing a re-evaluation of accepted practices. Gellants for Aqueous Systems In order to thicken these newer aqueous systems, natural gums (like guar and locust bean gum), starches and cellulose derivatives, were found to provide acceptable properties. These natural polymers swell in water to produce a viscous liquid or, so-called, linear gel which provides some degree of proppant transport and leak-off-control. The introduction of crosslinkers further enhanced the performance of these aqueous fluids, allowing reductions in gellant concentration and extending the temperature range of these systems by the mechanisms discussed below. Because of its competitive price, wide availability and overall performance flexibility, guar (and its derivatives) is the most common material in use today as a fracturing gellant. Guar is a natural polysaccharide extracted from the endosperm of guar beans, a crop grown extensively in India, Pakistan and certain areas of the southern United States. Chemically, the guar molecule consists of long chains of mannose with pendant galactose residues and the average molecular weight of the polymer lies in the range 200,000 – 2,000,000 Daltons. Addition of borate, or certain transition metal salts, under certain pH conditions, produces intermolecular crosslinking between cis-hydroxyl groups on the galactomannan polymer chain. P. 237