Here is a list of frequently asked questions about the GasGun. This is by no means a complete list. If you need further information please call (877) 557- 1370 or e-mail: firstname.lastname@example.org.
- How many perforations do I need to shoot the GasGun in a cased well?
- Is there any chance of damaging my casing?
- Is any debris left in the hole after a GasGun propellant stimulation?
- Which rock formations are best suited for a GasGun propellant stimulation?
- Will fluids be ejected from my well during a GasGun propellant stimulation?
- Doesn’t this fluid ejection indicate that energy was lost uphole?
- Can I shoot the GasGun with a lubricator or other pressure control equipment?
- What if I have a pay zone thicker than 10 feet?
- Can I shoot the GasGun through tubing?
- Does the GasGun carry proppant? If not, won’t the fractures close up?
- If I stimulate a second formation on a separate run, won’t the second shot simply re-inflate the fractures created by the first shot?
- Do I need to isolate each zone I wish to stimulate with the GasGun?
- My formation is really close to water. Will the GasGun fractures have vertical growth that could “bring in the ocean”?
- Can I shoot on top of a bridge plug or packer?
- I don’t want to put any fluid on my formation. Can I still use the GasGun?
Q: How many perforations do I need to shoot the GasGun in a cased well?
A: 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. Perforation penetration depth is of no consequence, since the GasGun will be providing the penetration into the formation.
Experience has shown that a perforation density of 6 shots per foot with a 0.375″ entry hole diameter is a recommended minimum. Larger diameters are preferred if available from your wireline company.
Q: Is there any chance of damaging my casing?
A: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 must be of good quality, and the cement job must be competent. Also, the wireline company fielding the tool must take care to place the GasGun carefully at the zone that has been perforated. The well owner’s representative should work carefully with the wireline crew to confirm that the zones perfed and treated are correct. If these conditions are met, and guidelines for perforation size and density are followed, the chances of casing damage are extremely low.
Q: Is any debris left in the hole after a GasGun shot?
A: The amount of debris left in the well after a GasGun propellant stimulation is negligible. The high-strength hollow steel carrier has small plastic port plugs along its 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. Essentially the GasGun is a debris free stimulation.
Q: Which rock formations are best suited for a GasGun propellant stimulation?
A: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.
Q: Will fluids be ejected from my well during a GasGun propellant stimulation?
A: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.
Q: Doesn’t this fluid ejection indicate that energy was lost uphole?
A: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.
Q: Can I shoot the GasGun with a lubricator or other pressure control equipment?
A:Yes, a lubricator may be used at the surface, but care must be taken to provide a few hundred feet of air cushion on top of the fluid tamp 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.
Q: What if I have a pay zone thicker than 10 feet?
A:The GasGun comes in a standard diameter of 3.375 inches and in lengths from 1 to 10 feet in one foot increments. We also make the GasGun in metric lengths for our international customers. Producing zones that are more than 10 feet in length are stimulated by making separate trips into the well with wireline. Lengths longer than 10 feet have been successfully shot on tubing.
Q: Can I shoot the GasGun through tubing?
A:At this time the GasGun comes in a standard diameter of 3.375 inches and we have released a 4 inch gun which is available in select markets. We frequently get requests to provide a smaller diameter tool that can fit through tubing. While it may be possible for us to develop such a tool, we are reluctant to do so. The GasGun has been proven to be an effective propellant stimulation tool primarily because of its progressive burning characteristics and the amount of propellant put in every tool. We would have to significantly reduce the propellant volume in order to make a tool that could fit through tubing. The reduced energy delivered to the formation would severely restrict the effectiveness of the stimulation. While we are eager to sell as many GasGuns as possible, we don’t want to provide a product to customers that isn’t likely to provide them with a return on their investment.
Q: Does the GasGun carry proppant? If not, won’t the fractures close up?
A: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.
Q: If I stimulate a second formation on a separate run, won’t the second shot simply re-inflate the fractures created by the first shot?
A: 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).
If a second GasGun tool is placed in the same location as the first one, then the fractures created by the first shot would simply be re-inflated by the second. No benefit should be expected by shooting the same zone twice.
Q: Do I need to isolate each zone I wish to stimulate with the GasGun?
A: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).
Q: My formation is really close to water. Will the GasGun fractures have vertical growth that could “bring in the ocean”?
A: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 2 to 5 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.
Q: Can I shoot on top of a bridge plug or packer?
A: Several situations have required that a GasGun tool be shot above a bridge plug. Most of these were completed without a problem, but a few cases caused the plug to fail and be driven to the bottom of the well. While it is difficult to generalize, the evidence suggests that a bridge plug will be more likely to survive the shot if there is fluid below it as well as above. This situation allows the pressure wave to pass through the bridge plug while restricting its motion.
We do not recommend shooting the GasGun when a packer is in a well. It is very likely that the GasGun will unseat the packer and possibly damage it.
Q: I don’t want to put any fluid on my formation. Can I still use the GasGun?
A:Possibly. Some formations, such as shale, can be damaged by the introduction of any fluid to the well, be it water, brine, diesel, or acid. In special cases we have fielded the GasGun in air. Please contact us for further information.