The Bakken formation is one of the largest unconventional oil plays in the United States, and large-scale hydrocarbon recovery from this formation has only recently become economically feasible as technological advancements have reduced stimulation costs. These low-permeability zones must be fracture-stimulated with large fracture networks for enhanced well production and improved economics. Guar-based fracturing fluids are the most commonly used in stimulation treatments. Cross-linked fluids with low polymer loading and high viscosity are needed to transport proppant efficiently. However, due to the tight shale formation, efficient proppant pack cleanup is critical to minimize polymer residue.
The breaker systems include coated and uncoated oxidative breakers with an enzyme breaker added during the tail-in stages to break down the viscosity and aid in fracture fluid cleanup. In the field, oxidizers were found to perform inconsistently at some temperatures, and the conventional enzymes were inefficient at higher pH and temperatures.
To improve reliability, a novel enzyme breaker was field-tested in early 2010 and proven to function more consistently and enhance post-stimulation productivity. In late 2012 and early 2013, this new enzyme breaker was implemented in multiple fields and consistently yielded similar results as the field trials. The enzyme breaker is a polymer-specific, thermostable mannanohydrolase, genetically modified for increased conformational stability. This enzyme breaker can be used from ambient temperatures to ≥ 250°F and functions well at an elevated pH of 11.0, whereas conventional enzymes are inefficient under these conditions.
Production results from several wells were compared with offsets that had been treated with the conventional fluid system and breaker package. The wells treated with this new enzyme breaker showed higher increase in oil production. This demonstrated persistence of this new enzyme to improve breakdown of the polymer and enhance cleanup of the proppant pack which resulted in enhanced oil production.