Since the GasGun generates high pressure gases at a very rapid rate, adequate perforation area is necessary so that the gas can exit the pipe without putting undue pressure on the casing. Hole size and density determine the exit area.
Experience has shown that a perforation density of 6 shots per foot with a 0.375″ entry hole diameter or larger is a recommended minimum for the 3.375″ and 4″ OD GasGun systems. A perforation density of 6 shots per foot with a 0.25″ entry hole or larger is the recommended minimum for the 2″ OD GasGun system. Larger diameters are preferred if available from your wireline company.
Since the advent of our hollow steel carrier system (HSC) in early 2005 we’ve had no reports of casing damage. However, one must recognize that the GasGun is a very powerful tool and produces large quantities of gas at high pressure. The potential for damage exists and even though the risks are very small, they must be acceptable to the operator.
Obviously, casing of decent quality will help ensure its integrity during stimulation. Also, the wireline company fielding the tool must take care to place the GasGun carefully in the zone that has been perforated. The well owner’s representative should work carefully with the wireline crew to confirm that the zones are perforated and treated correctly. If these conditions are met, and guidelines for perforation size and density are followed, the chances of casing damage are extremely low.
The amount of debris left in the well after a GasGun propellant stimulation is negligible. The 3.375″ and 4″ high-strength hollow steel carriers have either aluminum or plastic port plugs along their length. These port plugs provide a water tight seal for the propellant inside the carrier. Once the propellant is ignited the port plugs blow out of the carrier and into the well. This amounts to only a handful of material and to our knowledge has not interfered with any well operations. In cases where an entirely debris free system is required we can supply steel port plugs that can be retrieved after the stimulation with a magnet. The other option would be to use our 2″ OD GasGun system which is completely debris free.
Put simply, the GasGun fractures rock – of any type. If the formation in question has oil or gas present at sufficient pressure and there is some impediment to the flow of these fluids to the wellbore, then a stimulation may be indicated. The impediment could simply be low permeability or formation damage of some kind. But whether the rock is sandstone, limestone, dolomite, shale, or coal, the GasGun will create multiple fractures and improve the ability of the formation to move fluids through it.
In many shallow wells, less than say 1500 feet, fluid is often ejected for a few seconds immediately after igniting a GasGun tool. In some wells, regardless of depth, a second ejection of fluid occurs anywhere from 1 to 30 minutes after ignition.
This second blow results from the residual low-pressure gas bubble that can form after fracturing is complete. A low permeability formation may not bleed off all the gas volume generated and the gases may migrate out of the fractures and back to the borehole. There the gas bubble may have enough pressure to lift the fluid column. The lift may occur very slowly at first, but as fluid slowly emerges from the well at the surface, the column gets lighter and velocity increases. The second blow can eject fluids as high as 30 feet or more, but will normally only last a few seconds.
To contain the energy of the GasGun, a fluid tamp of 300 to 16,000 feet is employed. This fluid column acts as a large mass that resists movement during the 10 milliseconds or so that the tool’s solid propellant is expended. While a 300-foot fluid column can be forced uphole at relatively high speed after a GasGun shot, this occurs long after the fracturing is complete. Dynamic calculations show that approximately one percent of the propellant energy is lost when fielded under only 300 feet of water.
On the down side, the smaller fluid tamp and resulting higher velocity uphole increases the chance of kinking or damaging wireline. When possible, a fluid column of at least 1000 feet is recommended to minimize the chance of wireline damage.
Yes, a lubricator may be used at the surface, but care must be taken to maintain a minimum 100 foot air cushion below the wellhead to act as a shock absorber. If the well is filled to the surface and then closed tight with pressure control equipment, the pressure wave from the GasGun will be transmitted directly to the surface. The water hammer effect will more than likely damage surface equipment. In very shallow wells it may be more desirable to fill the well to the surface with fluid that will do no harm if expelled and then allow it to escape.
The 3.375″ OD and 4″ OD GasGun systems comes in lengths from 1 to 10 feet in one foot increments. The 2″ OD GasGun system comes in lengths of 1 to 20 feet in one foot increments. We also make the GasGun in metric lengths for our international customers. Producing zones greater than the maximum gun length available are stimulated by making separate trips into the well with wireline. Long intervals are most commonly stimulated via tubing (TCP). There are no gun length limitations when conveying the GasGun on tubing.
The GasGun does have a 2″ OD thru-tubing system available. This patented tool, released in 2014 allows for use in 2 7/8″ tubing or larger.
No the GasGun does not carry any proppant, but the GasGun fractures do not simply close back up after the stimulation. The original research that was performed in the 70’s at Sandia National Laboratories showed that the fracture propping characteristics of solid propellant stimulations occur from two sources. The violence of the event produces some debris that is propelled into the fractures, plus there is some degree of “self propping” that occurs from shear motions on fractures that are not aligned with the principal in situ stresses. The self-propping mechanism from shear motions is a bit difficult to describe if you are not familiar with states of stress in the earth. Hydraulic fractures orient themselves perpendicular to the least principal stress, taking the path of least resistance. That plane will have no shear stress on it, so the fracture opens and closes without any lateral shift. Solid propellant fracturing, when formulated correctly, produces multiple fractures, some of which are oriented at angles to the principal stress directions. These fracture planes have shear stresses acting on them, meaning that the fractures will shift slightly sideways while they are open. That way, as they close, the “jigsaw puzzle” does not fit back together neatly, and the fractures will remain partly open.
No. If the second GasGun tool is placed at a different location from the first, the fluid tamp between the zones will force the energy to be expended on the formation immediately adjacent to the second tool. Fracturing is restricted to within a few feet vertically of where the tool is placed (see energy loss calculations).
For the same reason as described above there is no need to isolate each zone you wish to stimulate with GasGun through the use of packers or bridge plugs. If a GasGun tool is placed at a different location from the first, the fluid tamp between the zones will force the energy to be expended on the formation immediately adjacent to the tool. Fracturing is restricted to within a few feet vertically of where the tool is placed (see energy loss calculations).
Based on research conducted by Sandia National Laboratories, fractures are expected to grow radially from 10 to 50 feet out into the formation, but no more than 1 to 3 feet above or below the zone treated. GasGun propellant stimulations are not like hydraulic fracturing which is conducted so slowly that the treatment has time to find the path of least resistance. The time of pressurization for the GasGun is only tens of milliseconds and the pressures reached overpower the earth’s internal stresses. This forces the fracturing to be confined to within a few feet vertically of the zone in question. However, unique downhole conditions make it impossible to make any absolute guarantees.
A GasGun tool can be shot above a bridge plug or underneath a packer with proper pre-job planning. In general we recommend shooting the GasGun no closer than 15 feet from plug back total depth (PBTD). It is recommended that a minimum of 20 feet of cement be placed on top of a bridge plug prior to shooting a GasGun. It is also recommended that a minimum distance of 80 feet be maintained between a GasGun and a packer.
The GasGun is most commonly shot in water or brine, but some formations, such as shale, can be damaged by the introduction of these fluids. The GasGun can be shot under any compatible fluid with the formation such as diesel, oil, acid, methanol, etc.
The types of gases produced are N2, CO2, H2O, H2, CO, N2O, and a few others in trace amounts.
The maximum pressure rating of 8000 psi is hydrostatic pressure. Formation pressures can be much higher.
There are no specific operational risks associated with performing a GasGun stimulation that are not already present when performing standard well operations. Service companies that are familiar with the use and operation of explosives will already have the necessary training to safely transport, handle, and perform a GasGun stimulation. We recommend that companies who perform GasGun services follow the guidelines contained in the American Petroleum Institute’s Recommended Practice for Oilfield Explosive Safety (API RP-67) and the Material Safety Data Sheet to ensure that the GasGun is handled safely.
Note: The GasGun and Kraken fracturing tools do have a maximum temperature rating that is lower than many other common oilfield explosives. Maximum 1-hour temperature rating is 280 F (138 C). Maximum 10-hour temperature rating is 260 F (127 C). **Warning: Exceeding the maximum temperature ratings can result in unintentional detonation.**
The GasGun can be applied in horizontal and/or multilateral wells. In order to stimulate a horizontal well one would need to run the GasGun on conventional tubing, coiled tubing, well tractor, etc. and ignite it with a pressure activated firing head.