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Since its introduction in 1948, hydraulic fracturing has become a popular method for the stimulation of oil and gas well production.
Oil and gas well stimulation involves fracture initiation and propagation from the wellbore, which is often cased, perforated and inclined with respect to the in situ principal stress directions. Consequently, fractures generally are complex, three-dimensional, nonplanar features that change shape and orientation as they propagate away from the wellbore. As propagation continues, fracture interaction and coalescence occurs, often producing the classical bi-wing shape.
Hydraulic fracturing demand has shown a steady increase over the past decade as a method to increase production from oil and gas wells. It routinely is used in newly completed wells to increase drainage rates and maximize field development. Also, favorable economics in today's markets have caused many producers to fracture stimulate wells in developed or mature fields in order to get the most out of existing infrastructure and maintain production rates.
With the increasing use of hydraulic fracturing, the associated costs have also increased dramatically. A typical fracturing job can cost from US $50,000 to $500,000.
The type of fluid is important as it must not only fracture the rock, but also carry proppant into the fracture. Newtonian fluids, such as water, and non-Newtonian fluids, such as complex polymers and gels, are common in hydraulic fracturing. General differences between Newtonian and polymer fluids are well-known. Newtonian fluids have a linear shear stress vs. strain rate relationship while power-law fluids exhibit shear thinning as the shear rate increases. Therefore, one must account for the mechanics of the various fluids, as well as the mechanics of the rock, and their interaction when modeling the hydraulic fracturing process. Complex polymers and gels have become the fluids of choice in the industry.
When properly performed, fracture stimulation can create a network of highly permeable flow channels that effectively increase drainage of the production zone. However the full potential of fracture stimulation is sometimes not realized because gelling agents used in the frac fluids cause formation damage. Residual polymer gelling agents cause damage by decreasing permeability and blocking flow from producing zones.
Severe damage from unsuccessful frac jobs can kill the well, as seen in two wells fracture-stimulated using guar-based frac fluids. Well A is a 12,000ft (3,660m) gas well in Crockett County, Texas (Figure 1). It was fractured with an acid stimulation fluid incorporating a guar copolymer. After stimulation, production could not be restored. Well B is an oil and gas well in the Tia Juana region of Venezuela (Figure 2). Production declined after fracture stimulation and the well plugged off.
Micro-Bac worked with the operators to diagnose the problems and recommended that treatment with Micro-Bac's Gum-Bac biological stimulation fluid would be a safe and effective solution for removing polymer damage and restoring well productivity. Well treatments were designed to repair the formation damage caused by the polymer, restore flow and enhance the effectiveness of the fracture stimulations.
Well A was producing about 100 Mcf/d to 200 Mcf/d before fracture stimulation. After fracturing with a copolymer, production could not be restored. To repair the damage caused by the polymer and restore productivity, the well was treated with Micro-Bac's Gum-Bac biological stimulation fluid to degrade the copolymer. When the well was opened after treatment, the operator reported a large amount of viscous material was removed from the well by swabbing. The well came back on production with 500 Mcf/d to 750 Mcf/d.
After fracture stimulation, Well B produced 60 b/d to 80 b/d for less than 2 months and then plugged off. A treatment of Gum-Bac biological stimulation fluid was injected into the well to remove polymer damage and restore flow. When the well was reopened, production climbed to 100 b/d and remained steady. More than 15,000 additional barrels of oil were produced from the well in the 6 months following treatment.