Stimulating naturally fractured carbonate formations

In naturally fractured carbonate formations like the Bluell Formation in North Dakota, inadequate stimulation reduces recovery. An operator drilling in the Bluell formation needed to stimulate a 4,800-ft lateral well, but ensuring good stimulation in these formations can be problematic because drilling mud leakoff damages the natural fractures. In addition, after the well is stimulated, a temporary plug is required to divert the treatment acid to the main formation, which is normally less permeable but contains most of the hydrocarbons. The problem is worse when fracturing and diverting a long openhole section with a higher differential pressure.

An operator drilling in the Bluell Formation needed to stimulate a 4,800-ft [1,463-m] lateral well. It contacted Schlumberger to stimulate the well and divert the acid away from the natural fractures and toward the low-permeability matrix.

Degradable diverting technology

MaxCO₃ Acid system and SXE acid were selected for the treatment. The MaxCO₃ Acid system stimulates natural fractures and high-permeability zones first and then creates a temporary restriction to steer the treatment acid for a more even fluid distribution. The system combines nondamaging viscosity-based viscoelastic fluid with degradable fibrous particulates. The carrier fluid reacts with the carbonate rock to develop viscosity. The particulates create a fiber network to strengthen the diversion capability and, in turn, provide a very robust diversion system.

The final treatment design for this job incorporated the MaxCO₃ Acid system to divert the treatment across the large lateral section of the wellbore to maximize recovery. The main acid treatment containing emulsified acid was pumped in five stages to create fractures. Four stages of MaxCO₃ Acid stimulation fluid were pumped to create diversion, to navigate the main treatment, and to create multiple fractures throughout the lateral.

Production increase of 500%

Following the treatment, actual production was monitored and evaluated, showing an increase of 500%. Oil production increased from 50 bbl/d to an average rate of more than 300 bbl/d. With results exceeding its expectations, the operator plans to treat 10 more wells in the area using the MaxCO₃ Acid system.