The present invention relates generally to production of hydrocarbons from a borehole. More particularly, the present invention is a method and apparatus for perforating and fracturing a formation surrounding a borehole and propagating that fracture to increase hydrocarbon production from the borehole.
Techniques for perforating and fracturing a formation surrounding a borehole are known in the art. The most common technique for perforating and fracturing a formation to stimulate production includes the steps of: 1) penetrating a production zone with a projectile; and 2) hydraulically pressurizing the borehole to expand or propagate the fractures initiated by the projectile. This technique proves to be extremely expensive due to the preparation required for pressurizing a portion of a borehole. Typically, pressure around a production zone in the borehole is increased by pumping fluids into that portion of the well to obtain the high pressures necessary to expand the fracture in the production zones. This operation is generally time intensive and costly making these techniques unattractive for either multiple zone wells or wells with a low rate of production.
Other less expensive techniques using gas propellants have been implemented in place of hydraulic fracture propagation. The resulting procedure is similar to that discussed above. First, a projectile is fired to penetrate the production zone. Second, a propellant device is ignited to pressurize the zone of interest and propagate the fracture.
Godfrey et al., U.S. Pat. No. 4,039,030, describes a method using a propellant to maintain the pressure caused by a high explosive charge over a longer period. The high explosives are used to generate fractures while the role of the propellant is to extend these fractures. In accordance with this technique, the casing must be perforated prior to ignition of the high explosives and propellant as the high explosives are used exclusively to fracture the formation but not to perforate the casing.
Ford et al., U.S. Pat. No. 4,391,337, describes an integrated perforation and fracturing device in which a high velocity penetrating jet is instantaneously followed by a high pressure gas propellant. In essence, a tool including propellant gas generating materials and shaped charges is positioned in a desired zone in the borehole. The penetrating shaped charges and propellant material are ignited simultaneously. The high pressure propellant material amplifies and propagates the fractures initiated by the shaped charges.
Dees et al., U.S. Pat. No. 5,131,472, is an overbalance perforating and stimulation method which employs conveyed tubing to pressurize a portion of the borehole. Liquid is pumped downhole until the pressure in the tubing reaches a pressure above the fracture pressure of the formation. A perforating gun is then fired to perforate the casing. After the pressure has dropped, pumping of liquid downhole is resumed to open the fractures initiated by the perforation gun.
In Hill, U.S. Pat. No. 4,823,875, the well casing is filled with a compressible hydraulic fracturing fluid comprising a mixture of liquid, compressed gas, and proppant material. The pressure is raised to about 1000 psi greater than the fracture extension pressure of the zone to be fractured by pumping fluid downhole. The gas generating units are simultaneously ignited to generate combustion gasses and perforate the well casing. The perforated zone is fractured by the rapid outflow of an initial charge of sand-free combustion gas at the compression pressure followed by a charge of fracturing fluid laden with proppant material and then a second charge of combustion gas.
Although the prior art suggests downhole generators for use in fracturing operations, these techniques have not proven attractive from an economical or technical point of view. In conventional hydraulic fracturing, even with the use of downhole propellant gas generators, a substantial amount of hydraulic power capability must be maintained at the surface. None of the techniques have provided an economical process for perforating and fracturing as part of a single highly efficient operation.