The primary function of fracturing fluids is to provide the means and media for the transport and placement of a conductive proppant pack in the created fracture such that resident hydrocarbons may be more easily produced. In recent years significant effort and expense has been invested to develop an ideal fracturing fluid system. Such efforts have been often been akin to the proverbial dog chasing his tail, rather than on the addressing the engineering objective to place a conductive propped fracture. Development focus has been primarily on optimization of fluid rheological stability to get the treatment pumped and secondarily to mitigating any damage caused by new fluid system. Post-frac production analysis frequently demonstrates less than anticipated fracture area, suggesting excessive proppant-pack damage or that the proppant was not placed in designated areal location due to inadequate proppant transport.
Recent testing was conducted in a large-scale slot apparatus at the Well Construction Technology Center in Oklahoma to evaluate the relative effects of proppant slurry component characteristics and the proppant transport capability. The effects of various fluid specific gravities, fluid viscosities, proppant specific gravities, proppant sizes, slurry flow rates, and slot widths were investigated. Testing included fluids from slickwater to gelled, weighted brines, proppants from 40/70 Ottawa sand to 14/30 ultra-lightweight proppants, pump rates from 0.1 to 1.0 bbl/ft/min, and slot widths from 0.25 to 0.5″. Evaluation of the proppant transport testing data and the comparative abilities of current fracturing slurry system technologies to achieve placement of a productive propped fracture will be discussed.
Hydraulic fracturing may be characterized as a complex process involving pumping highly pressurized fluid into a well to create fractures in a subterranean formation[1–3]. The resultant fractures provide flow pathways radiating laterally away from the wellbore. Proppant is placed in the created fractures to ensure that they remain open once the treating pressure is relieved, thus providing the desired highly conductive pathways to increase the productivity of an oil or gas well completion.
Optimization of conductive fracture area is among the principal tenets of fracturing design engineering. The conductive fracture area is defined by the propped fracture height and the effective fracture length. The productive intervals are typically bounded by relatively non-productive rock and thus, the potential for maximizing the conductive area via fracture height is limited to placement of proppant across the height of the productive interval. Thus, the key design parameter over which fracturing treatment design engineers may have influence is the effective fracture length.
Fracturing Fluids. The industry has focused great effort on the development of products and application techniques to facilitate proppant transport in efforts to maximize effective fracture length. Highly viscous, crosslinked polymer-based fluids and/or relatively high fracture flow velocity have historically been employed to properly place the proppant throughout the fracture area. In the late 1980's it was recognized that the residues of commonly used crosslinked guar-based fracturing fluids often cause greater than 80% damage to proppant-pack conductivity, leading to the rapid evolution of improved breaker systems to mitigate the damage. The past decade has seen much advancement in these areas, including the introduction of crosslinked fluids having reduced polymer concentrations and viscoelastic surfactant-gelled fluid systems.