Fracturing products and additives

Hydraulic fracturing treatments create conductive cracks or fractures in producing zones. These very deep-penetrating, high-permeability pathways help reservoir fluids enter wellbores by changing formation flow from radial to linear. Fluids pumped at pressures significantly above minimum in situ principal rock stress actually split formations apart. Temporary, artificially high pressures cause target zones to separate along maximum stress planes. The resulting opposing fracture "wings" propagate away from wellbores perpendicular to the minimum stress axis.

Propping agents (proppants), including naturally occurring sand, man-made intermediate and high-strength ceramics (sintered bauxite) and resin-coated sand, added to fluid systems as sandstone formations are treated, prevent induced fractures from closing completely after pressure is released at the end of a job. Proppants are not needed in limestones or dolomites if these formation types can be differentially etched by acid fracturing treatments.

Fractures reduce radial flow pressure drop, and a linear flow pattern accelerates reservoir recovery rate. Improved recovery rate has a significant impact on investment rate of return, especially in low permeability "tight" reservoirs. Fracturing treatments can also be used to improve productivity in severely damaged wells and accelerate recovery even in higher permeability reservoirs.

Treatment Schedules

Fracturing stimulation treatment pumping schedules involve injection of prepad, pad, proppant-laden fracturing and flush fluids.

Prepad. Thin, low-viscosity base fluids like oil, water or foam, with low gel concentrations or friction-reducing agents, fluid loss additives and surfactants or potassium chloride (KCl) to prevent formation damage, can be pumped ahead of main treatment volumes to help initiate fractures. Low-viscosity prepad stages penetrate rock matrix more easily and cool formations to reduce high-temperature gelling agent degradation. Prepad stages are not critical at low to moderate temperatures and fracture gradients.

Pad. Viscous fracturing fluid without proppants is pumped to generate dynamic fracture width and length, and prepare fractures for proppant-laden fluid stages. Higher viscosity fluids reduce fluid leakoff to formations. Pad volumes need to be sufficient to avoid 100% leakoff before total fracture length and width have been generated and proppant has been placed. Chance of premature pumping treatment screenout can be reduced by increasing injection rate, pad volume or fluid system efficiency. Pad volume is usually reported as a percentage of total viscous fracturing fluid (pad and proppant-laden stages). Typical pad volumes are 25 to 45%, but may be higher in screen-out-prone areas like naturally fractured and vugular zones. However, larger pad volumes increase load water recovery requirements, treatment cost and formation damage risk.

Proppant stages. Proppant transporting stages propagate fracture wings away from the wellbore, continue width and length generation, and carry proppants into the fracture. Maximum proppant concentrations depend on formation characteristics, fluid system type and gelling agent concentration. Efficient treatments are designed to place maximum amounts of proppant with minimum fracturing fluid volumes for optimized, cost-effective stimulations.

Flush. Less viscous base fluids, like prepad stages, with low friction-loss characteristics, are used to displace proppant-laden fluid stages through wellbore tubulars. Pumping schedules should not overdisplace the last proppant stages away from the near-wellbore region.

How To Use This Reference

Products are subdivided into groups that have similar function and performance within each functional category. Where applicable, performance groups have been subdivided to reflect significant differences in additive or proppant chemical nature to emphasize uniqueness in company product lines. What cannot be reflected is differences in product performance resulting from different material sources around the world, and from fluid or formation compatibility effects. Users are cautioned that it is generally not possible or prudent to make direct "translations" of fracturing or stimulation treatment fluid system formulations between service companies based solely on additive descriptions.

Products and additives are grouped into 22 functional fluid additive and two proppant categories:

		Water-base polymers			Defoamers
		Friction reducers			Oil gelling additives
		Fluid-loss additives (FLAs)			Biocides
		Breakers			Acid-based gel systems
		Emulsifiers			Water-based gel systems
		Clay stabilizers			Crosslinked gel systems
		Surfactants			Alcohol/water systems
		Non-emulsifiers			Oil-based systems
		pH control additives			Polymer plugs
		Crosslinkers			Continuous mix gel concentrates
		Foamers			Resin-coated proppants
		Gel stabilizers			Intermediate-to-high-strength ceramics