A combination of propellant and underbalanced tubing-conveyed perforating enhanced the hydraulic fracturing treatment of a well in the foothills of western Alberta.
Petromet Resources Ltd., Calgary, Wild River No. 6-20 well had previously been perforated in another zone.
Canadian Completion Services (CCS), Calgary, says most wells in the area prior to fracturing are typically pretreated by extreme overbalance perforating (EOB).
EOB uses a nitrogen column and a small amount of liquid in the tubing, pressured to well in excess of the formation fracture gradient. When the perforating gun fires, the fluid and nitrogen rush toward the formation at a very high velocity, surging open all of the perforations and initiating a small fracture.
CCS indicates that because the Petromet Wild River well's existing perforations above the zone of interest made the EOB technique impractical, it suggested the alternative Stimgun method. CCS licenses Stimgun technology from Marathon Oil Co. The process involves an oxidizer, potassium perchlorate that is molded into a tube fitted over the perforating gun. When bound with a resin, the potassium perchlorate is known as a propellant.
CCS describes the perforating process as follows:
- When the perforating gun detonates, the resulting heat and shock ignite the propellant sleeve, generating huge CO2 gas volumes at very high pressures.
- The fluid's hydrostatic pressure directs the bulk of the gas into the perforations. This gas pulse can generate pressures as high as 27,000 psi for 10-15 ms.
- The result is, in most cases, instantaneous perforation breakdown and the initiation of a fracture near the wellbore.
- The spent gas can then be used to flow back debris from immediately around the wellbore.
CCS says the uniqueness of the Petromet project was the fact that the Stimgun technique was combined with underbalanced tubing-conveyed perforating (TCP).
In the assembly (Fig. 1), CCS included a surge tool to keep a predetermined fluid level in the tubing in order to create a 3,000-psi drawdown on the formation. It described the work in the well as follows:
- The process involved logging the bottomhole assembly into position, setting the packer, installing the wellhead, and then dropping a firing bar from surface to detonate the perforating gun. The firing bar also breaks a glass disc in the surge tool allowing communication between the tubing and the annulus below the packer.
- When the gun fired, the propellant ignited, surging the perforations and initiating a bi-wing fracture.
- Once the initial surge from the Stimgun sleeve had dissipated, the high underbalance pressure from the reservoir backsurged into the wellbore, further helping to clean up near-wellbore perforating and drilling damage.
- A split second later, the gun disconnected itself from the bottomhole assembly leaving full-bore tubing for the hydraulic fracture.
CCS says the end result of the Stimgun treatment on this well was a near-classic hydraulic fracture. It indicates typical breakdown pressures in this field may exceed the formation fracture gradient by 3,000-4,000 psi before the formation breaks and the actual hydraulic fracture begins to take place.
It found that in the Wild River No. 6-20 well, no formation break was noted, and sand was pumped into the zone at fracture pressure, with the end result of requiring 30% less pumping horsepower for the treatment.
It says the calculated fracture gradient of the well was 55 MPa (7,900 psi) and pumping operation began at 55 MPa and continuing until screen out, with a total of 25,000 kg of carbo-prop placed in the formation.