Fracturing-Fluid Crosslinking at Low Polymer Concentration

Abstract

Proposal Crosslinking water-soluble fracturing fluids has been one of the most cost effective technologies developed for hydraulic fracturing. It has allowed these fluids to be used in hostile conditions that would have otherwise required impossibly high polymer concentrations. A great deal of work has been done on crosslinking chemistry and crosslinked fluid rheology and this has led to a better understanding of these fluids. Higher prices for hydrocarbons have taken exploration into more difficult areas and increased the importance of production from existing fields, both of which will require better technologies to economically exploit these reserves. This means that new efforts need to be focused on fracturing fluid improvements Low polymer concentration crosslinked systems have proven to be extremely effective for both initial and restimulation. The first of these fluid systems were based on zirconium crosslinked carboxymethyl guar. These fluids were effective at transporting proppant and, because of the low polymer concentration, provided low levels of formation damage and quick clean-up. In order to develop the next generation of cost-effective fracturing fluids, a better understanding of the controlling mechanisms for low-polymer concentration crosslinking must be developed. This paper will describe techniques for characterizing the crosslinking process and crosslinked fluids that will be needed to develop this next generation fluid. We will clearly demonstrate the relationship between the critical crosslinking concentration (minimum polymer concentration where the fluid is crosslinked) and the critical overlap concentration (the minimum concentration for polymer-polymer interaction). In addition, this paper will demonstrate the relationship between crosslinking agent and polymer type to crosslinked gel properties. Most water-soluble polymers exhibit two transitions in the viscosity-concentration curve. The first is the critical overlap concentration and the second is generally recognized as the transition from semi-dilute to concentrated. Concentrations necessary to produce a useful low-polymer concentration crosslinked system are generally between these two transitions and above the critical crosslinking concentration. The useful concentration depends strongly on the polymer and not as much on the crosslinker. All of this information will be used to provide guidelines for the development of future crosslinked systems.