Tortuosity has been one of the biggest challenges for shale and tight gas hydraulic fracturing treatments, leading to high near-wellbore friction pressures, premature screen-outs, reduced treating rates and poor production results. Several solutions have been applied with varying levels of success, but often result in nothing more than a frustrating attempt.

A new solution has been developed that has demonstrated substantial success in overcoming particularly stubborn tortuosity problems. The method builds upon the use of a non-damaging, single-stage phosphonic/hydrofluoric acid system (SAS) that has been well established around the globe for matrix stimulation treatments in a single stage (without an acid pre-flush). These encouraging results in matrix treatments encouraged us to consider the SAS for hydraulic fracturing applications.

An opportunity arose in a tight gas fracturing campaign in Northeast Brazil, where severe tortuosity prevented us from achieving the required pump rate while staying below the surface treating pressure limitation. Conventional tortuosity-mitigation methods such as proppant slugs, viscous fluids, re-perforating and sand-jetting all failed to solve the problem. The novel technique was tested (to the authors’ knowledge for the first time anywhere), whereby a high-strength SAS was injected at fracturing rates and pressures, without any pre-flush, in order to clean up the tortuosity and improve injectivity. The tight gas well was subsequently hydraulically fractured in six perforated intervals, with a SAS pre-frac performed prior to each stage. Injection rates in the main fracturing treatments were improved by 25 to 100 percent.

This paper describes the SAS technique, including acid properties, treatment design overview, field results and measured tortuosity improvements.

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