Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Fracturing-fluid additives serve two purposes: to enhance fracturecreation and proppant-carrying capacity and to minimize formation damage. Additives that assist fracture creation include viscosifiers, temperaturestabilizers, ph-control agents, and fluid-loss-control materials. Those used tominimize formation damage are gel breakers, biocides, surfactants, claystabilizers, and gases. This paper discusses the qualities and applications ofeach of these additives.
Most wells drilled today are fracture-stimulated as part of the completionprocess; if not fractured, these wells would not sustain a commercialproduction rate. Fracturing fluids are injected into a subterranean formation(1) to create a conductive path from the wellbore extending into the formationand (2) to carry proppant material into the fracture to create a conductivepath for produced fluids. To create a fracture hydraulically, a fluid must bepumped at high enough pressure to break down the rock and at high enough rateto cause fracture extension. Although oil-based fluids were the earliest used, more than 90% of today's fluids are water-based. Aqueous fluids are economicaland can provide control of a broad range of physical properties as a result ofadditives developed over the years. Additives for fracturing fluids serve twopurposes: to enhance fracture creation and proppant-carrying capability and tominimize formation damage. Additives that assist fracture creation includeviscosifiers, such as polymers and crosslinking agents, temperaturestabilizers, ph-control agents, and fluid-loss-control materials. Formationdamage is reduced with such additives as gel breakers, biocides, surfactants, clay stabilizers, and gases.
Much of the concern about fracture design in modern computer simulatorscenters on the viscous nature of the fluid. Viscosity affects both fracturegeometry and proppant transport. More viscous fluids generate wider andsometimes higher fractures and allow less proppant settling during placement inthe formation. Water-soluble polymers from natural sources, guar and cellulose, are used most often to prepare viscous fracturing fluids. Chemical modificationof these polymers has allowed a broad range of physical properties to beattained. Derivatives, such as hydroxypropyl guar, carboxymethyl hydroxypropylguar, or carboxymethyl hydroxyethyl cellulose provide viscosity for fracturingwells with formation temperatures from 60 to more than 400 degrees F [16 to 204degrees C].
Wells with formation temperatures below about 150 degrees F [66 degrees C]are successfully fractured with aqueous solutions of the above polymers, oftencalled base gel solutions. Sufficient viscosity is generated from polymerconcentrations of 30 to 50 lbm/1,000 gal [3.6 to 6 kg/M3] to place low tomoderate proppant concentrations. For placement of high concentrations ofproppant, or for use in higher-temperature wells, additional viscosity isneeded. Higher viscosity fracture width in vertical fractures and improvesproppant transport. Higher viscosity is also needed to offsettemperature-thinning effects of polymer solutions at elevated temperature.
High viscosity may be attained by increasing polymer concentration in thebase gel solution or by crosslinking the base gel. Increasing the polymerconcentration is generally not cost-effective and creates operational problems, such as sand wetting and high pumping pressures. Crosslinking the base gelpolymer, usually with a transition-metal cation, provides aviscosity-multiplying effect at much lower cost than adding more base polymer. Various crosslinking agents, such as aluminum, antimony, borate, titanium, andzirconium, are selected for compatibility with a particular base polymer andits pH and thermal limitations. Profiles of fracturing-fluid viscosity vs.increasing fluid temperature are shown in Fig. 1 for both base-gel-type fluidsand typical delayed-action crosslinked gelled fluids. Crosslinking a polymersolution changes its nature from a viscous fluid (like honey) to a viscoelasticfluid (like gelatin). Viscoelastic fluids may be degraded by pumping at highlyshearing conditions and are shear-history dependent. Techniques have beendeveloped to delay chemically or thermally the onset of the crosslinkingreaction until the high-shear portion of the treatment in the tubing string hasbeen passed by the fluid. The viscosity increase resulting from crosslinkingmay then occur when the fluid enters the low-shear portion of the treatment inthe fracture itself.