Zeta Potential Altering System for Increased Fluid Recovery, Production, and Fines Control

Abstract

Abstract This paper presents a new zeta potential altering system that is based on an inner salt developed to enhance the water flowback recovery on borate and slick-water frac jobs. The system also aids in the control of fines. The mechanism of this system is to modify the zeta potential on particles such as frac sand from -50 mV, or coal from -28 mV, to more neutral values. This modification helps increase the potential for particle agglomeration and changes in the sand pack in the fracture to increase fluid recovery and production. Results from lab studies have shown that the flow rate ratio for a 2% KCl solution through sand packs (treated/not treated) increased up to 45% when treated with 6.0 gal of chemical per 1,000 lb of frac sand. Similar results were obtained on conductivity tests using ceramic proppant, improving conductivity from 7,150 mD-ft to 11,387 mD-ft at 2,000 psi closure stress. This new system does not interfere with fracturing fluid rheological profiles for borate systems and improves friction reducing characteristics in slick-water systems. The system was tested in the field on a slick-water job, where the additive was mixed in the blender tub. Introduction Several factors may affect the productivity of a well.1–5 Invasion of fines into proppant pack affects the permeability, resulting in early decline in well productivity. Besides this, conductivity of fractured wells under closure stress of 3,000 psi or more in 20/40-mesh Badger sand could be significantly crushed and decrease the conductivity of the wells. Therefore formations produced from wells completed with hydraulic fracturing, frac-packing, and gravel packing are susceptible to pack invasion of fines and subsequent permeability or conductivity losses. Different proppant coating systems have been designed to control the movement of fines generated from crushed proppant: phenolic resins, furan resins, epoxy resins, and urea-formaldehyde resin among others. When consolidated, these also prevent proppant flowback.6,7 Fiber-based products can also be used to decrease the amount of proppant flowed back, especially where aggressive well-cleaning procedures are applied. Although all these methods are shown to be effective in the control of fines from crushed proppant, they do not help control fines produced from some formations when fluid salinity or interfacial tension changes to cause fines to migrate with the producing fluid. In these particular cases, chemical stabilizers such as cationic and water-soluble polymers are often used to minimize fines production, especially from soft formations.8 Zeta Potential Altering System (ZPAS) is a new chemical system that is thermally stable up to 450°F. It was developed to help increase the conductivity in sand and ceramic pack, control movement of fines generated during proppant crushing, or control fines produced from the formation. This unique chemical system is insoluble but dispersible in oil and water; it can be applied as liquid additive to water- and oil-based fluids. It coats the proppant or any metal oxide surface immediately. This paper shows the mechanism and lab results by which changing the zeta potential of different substrates increases the degree of aggregation of sand particles, changing the density of sand settling for better distribution of sand particles. Tests results show how movement of fines, such as the movement of silica flour, is controlled throughout treated sand packs with this new system without a decrease in permeability. The paper also discusses how ZPAS helps improve load recovery of treatment fluids. Zeta Potential The zeta potential is defined by the charge that develops at the interface in the boundary of hydrodynamic shear between solid surfaces as a product of the electrostatic repulsion and the attractive forces related to the Van der Waals' forces. Zeta potential is therefore a function of the surface charge of the particle, any adsorbed layer at the interface, and the nature and composition of the surrounding suspension medium. In other words, zeta potential can be affected by changes in pH, conductivity of the medium (salinity and kind of salt), and concentration of particular additives (polymer, non-ionic surfactants, etc.). Zeta potential values (−20 and 20 mV) show that the effective charge is low enough in the particles that the repulsion between them is lowered to one point where flocculation, coagulation, and aggregation could occur.9,10