What is the GasGun?
The GasGun uses solid propellant, often referred to as a low explosive, to generate high-pressure gas at a rapid rate. The rate is tailored to the formation characteristics to be rapid enough to create multiple fractures radiating 10 to 100 feet from the wellbore, but not so rapid as to pulverize and compact the rock as is experienced with classic high explosives, such as nitroglycerine. The star-shaped pattern of multiple fractures removes wellbore damage or blockage and increases the formation permeability near the wellbore.
The propellant used is similar to that used in large-bore military guns. While the concept of using solid propellants to stimulate oil and gas wells is not entirely new, the GasGun incorporates a vastly improved design with progressively-burning propellants that have been proven by independent research to be many times more effective in creating fractures and increasing formation permeability.
The GasGun is simple and easy to field. The propellant is conveyed to the formation in a pressure-tight rubber canister under a fluid column of 300-4000 feet. The fluid column tamps the charge and assures that the energy is restricted to the pay zone. The fluid can be almost anything compatible with the formation; such as fresh water, brine, oil, or solvent.
The tool is usually fielded by wireline. It attaches to the wireline much like a perforating gun, and can be shot with or without a casing collar locator. The tool is ignited while being suspended at the correct depth. Pressure control equipment, such as a lubricator, can be used when needed. Typically little or no cleanup is required, and the well can usually be put back on production immediately after the stimulation.
The GasGun has been engineered in two distinct formulations, one for open-hole and one for cased-hole completions. Perforated casing must be of good quality and have perforation density at least four shots per foot with a 0.5" entry hole diameter or six shots per foot with a 0.375" entry hole to allow sufficient area for the high pressure gas to exit the pipe. The GasGun comes in a standard diameter of 3¼ inches and in lengths of 2, 4, 6, 8 and 10 feet. Producing zones that are more than 10 feet thick are stimulated by connecting multiple tools. Up to 50 feet can be stimulated with a single run.
High explosives, such as nitroglycerine or gelatin, detonate and create a shock wave. Pressures created are extremely high but last only a few microseconds (Figure 1).
Figure 1. Pressure-Time Profiles of Three Stimulation Methods
Extensive research has shown that the pressure pulse created by high explosives enlarges the wellbore by crushing and compacting the rock.1 The enlarged wellbore is left with a zone of residual compressive stress. These residual stresses and compacted rock can actually reduce permeability near the wellbore. Extensive cavings often fill the wellbore with debris that require days, even weeks, to clean up. High explosives are limited to open-hole completions.
The solid propellant used in the GasGun does not actually detonate; it deflagrates. Deflagration is basically a burning process that takes place without any outside source of oxygen. Gas pressures in the range of 20,000 psi are produced that last approximately 10 milliseconds. No shock wave is produced, the rock is split rather than compacted, and multiple fractures are created. The GasGun produces large volumes of gas under high pressure that rapidly extend the fractures into the formation. There are little or no cavings, the integrity of the wellbore is maintained, and cleanup is usually minimal. The well can normally be put back on production immediately. The GasGun can be used in both open hole and perforated pipe completions.
Updated February 2003