Large-Scale Laboratory Investigation of the Effects of Proppant and Fracturing-Fluid Properties on Transport

Abstract

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. Introduction and Background 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[2]. 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.

Quantifying Proppant Transport in Thin Fluids - Theory and Experiments

“…Brannon et al (2006) presented experimentally-measured settling velocities for ultralightweight proppants, conventional Ottawa sand, and high-density bauxite in thin fluids. The McCabe-Smith correlation (eqn 2) matches the experimental data within 5% over the range of density from 1.25 to 3.57 and viscosity from 1 cP to 10 cP, whereas the simplified expression (eqn 1) results in significant errors for all but the lightest proppants.…”

“…Several other authors have presented slot experiments, including Patankar et al (2002), Brannon et al (2006), Woodworth and Miskimins (2007), and Sahai et al (2014). Most of this work has been directed at determining equilibrium bank heights and critical fluid velocities.…”

Experimental Research on the Proppant Transport Behavior in Nonviscous and Viscous Fluids

“…The traditional high-viscosity, polymer-based guar fluid system has been employed as carrying fluid for the past several decades for its good sand-carrying capability. The guar fluid system can provide sufficient viscosity to suspend and transport proppant into deeper fractures, but polymer residue and unbroken polymer significantly damage the formation permeability and fracture conductivity, , which has led to an evolution of improved fluid systems to mitigate the damage. With the development of unconventional reservoirs (such as shale oil and gas, coalbed methane, and tight gas) in the 21st century, further lower formation damage is required for fracturing fluids, because fluid residue is more likely to block the pores in these unconventional formations due to their much lower porosity and permeability.…”

“…Currently, the single-wing transparent parallel plates experimental devices used at the University of Texas, University of Alberta, University of California, China University of petroleum, Southwest Petroleum University, BJ Service Company, Sinopec Research Institute, and others − do not demonstrate a big difference compared with Kern’s and Babcock’s apparatuses, except for some improvements on the fracture wall and entrance. For the design of fracture entrance, Tong et al injected fluid from the top of the slot by gravity, Alderman and Wendorff and Brannon et al used a trumpet-shaped inlet to distribute fluid and pressure more evenly in the fracture; these two types of entrances did not take the influence of perforations into account. Clark, Fjaestad and Tomac, and Tran et al set the inlet as a small hole at the middle of the height and showed the dynamic flow process of carrying fluid entering the slot through the small hole.…”

A new model for simulating particle transport in a low‐viscosity fluid for fluid‐driven fracturing

“…Kern et al conducted the earliest experiment of proppant transport . After that, several proppant transport experiments were conducted to investigate the effects of fluids and proppant properties on the proppant distribution in a fracture . In recent years, many experiments have been conducted to investigate proppant transport in complex fractures .…”