2.1 Basic Principles
CBM wells must be fracked to increase gas production. The use of nitrogen shake fracturing could minimize the impact of water sensitivity from clay minerals on reservoir. The compressed nitrogen, forced into the formation and wellbore ,inflates the space the moment of flowing back, which greatly increases the velocity of fluid back. It is important for the development of low permeability coal reservoir that the well entering liquid is rapidly discharged to the surface in large quantities. In addition, the large amount of water, used in conventional hydraulic fracturing, causes severe pressure on resources and environment in areas where there are water scarcity issues. Nitrogen fracturing can effectively reduce the dependence of fracturing on water, and has achieved good results in the dry coal of multiple, thin and shallow layers in the central Alberta of Canada.
Nitrogen shock fracturing uses a special tool to immediately release compressed nitrogen in a certain pressure in the perforated casing, forming a vibration and injecting plenty of nitrogen into the fractures. The suddenly released high-energy gas creates micro-fracture network system in the coal matrix and communicates with original fractures to improve the permeability of coal reservoir, removes the block around the well zone to clean the gas-way-out. The fracturing process can achieve the following results:
The compressed nitrogen could keep 60 MPa pressure in the bottom hole. The force created by instantaneous release of such pressure is much higher than the tensile strength of the well surrounding rock .Fractures are made in the formation rock.
The compressed nitrogen has both strong expansion capacity and large elastic energy. At the formation temperature, liquid nitrogen turns into gas phase and acts on the surrounding rock by volume expansion. Fractures are expanded in a certain degree.
Nitrogen shock fracturing can inhibit the clay swelling of water-based fracturing fluid in clay mineral formations, minimize the pollution of fracturing fluid on the formation, and increase the extraction energy after high-pressure nitrogen expansion.
Adsorption displacement. The adsorption capacity of N2 is weaker than that of CH4,thus, the purpose of replacing CH4 cannot be achieved through competitive adsorption. The displacement of adsorbed methane is realized by reducing the partial pressure of free methane to affect its isothermal adsorption curve under constant pressure.
2.2 Technology and equipment
Nitrogen shock fracturing is a kind of multi-layer vibration fracturing in single well, which uses a ball-off device, high pressure, large injection of nitrogen, dragging construction to achieve multiple vibration fracturing of each target layer.
2.2.1 The main vibration part
The core of Nitrogen shock fracturing is a ball-off high-energy gas fracturing device, including the upper packer unit, vibration trigger unit, energy release unit, pressure recording unit, lower packer unit and guide shoe connected successively from top to bottom, as shown in figur1 .
The upper and lower packers together seal the fracturing target layer. Considering the different thickness of the target layer, different length segments can be used between the upper and lower packers to make sure the vibration work completely on the target layer. The upper packer unit connects to the ground fracturing truck through coiled tubing. A tailor-made ball is put into the tubing to the ball seat to form a high-pressure section–the vibration trigger unit. With the ball broken at 50 Mpa ,the pressured nitrogen is released to form a vibration. The high-energy gas nitrogen burst through the energy release unit. The device can satisfy the two main contents of nitrogen vibration and release, and ensure that the released gas can effectively enter the target layer.
2.2.2 Ground equipment
Based on the physical characteristics of nitrogen and the related process of nitrogen shock fracturing technology. The whole fracturing process is divided into five systems : liquid nitrogen tank system ,high-pressure pumping system, conveying pipeline and equipment system, downhole tool string system and flare system. The ground equipment includes 2 liquid nitrogen tank trucks, 2 liquid nitrogen pump trucks, 2 coiled tubing trucks, a set of vent pipeline, a flare torch ,a sump and several control valves at the necessary positions, which is shown in Fig. 2.
2.2.3 Coiled tubing
According to nitrogen shock fracturing process requirements, coiled tubing is required to withstand pressure of 80 MPa, the ID of coiled tubing joints and safety joints (hydraulic releasing tool) is not less than the inner diameter of coiled tubing. All the way of the ball, from ball injector to the end of coiled tubing, should be as smoothly as possible. The coiled tubing parameters is shown in Table 1:
yield strength (MPa)
2.3 The work process
1. System Cleaning
Set up the whole system and do a leak detection ; then drop down the coiled tubing to about 2 m up from well bottom; switch on one liquid nitrogen fracturing pump with low power ; open cycling system valve 4, 7, 9 and 12 ,close vent valve 5 and 10. The high-pressured nitrogen, not in a high speed, is injected to the tubing and annulus access, thus, the air water mixture is blowed out from coiled tubing to the mud pit, which is kind of a gas-lifting.
Throttles are used to control backflow during the process. Open vent valve 11 to flare the gas reflux when the outlet water volume decreases and the air volume increases. When there is no water and debris out, stop pumping nitrogen in the maintenance pipeline system to relieve pressure.
2. Pressure Test
Do a pressure test to the whole system after nitrogen pumping truck is cooled down: Shut off all valves of the system and slowly inject nitrogen to raise pressure, observe the change of pressure and the sealing condition of each joint. If there is no obvious leaking, up the pressure to 70 MPa then stop for 5 minutes. The pressure test is completed after confirming that the pressure is stable without leakage. End the pressure test by a controlled pressure release with operation on valve 4 and 5.
3. Single-layer Nitrogen Shock Fracturing
Start a set of frac pumping truck, open the injecting valves 4 and 7, close valves 5 and 9, and inject the ball into the coiled tubing. Continuously pumped from pumping truck to coiled tubing in low velocity ,the high-pressured nitrogen pushes the ball into the ball seat. The system pressure is increasing until reach the maximum pressure that the ball could subject to. As soon as monitored pressure suddenly dropped with ball crushed, turn on the other set of frac pumping truck. All the nitrogen left is pumped into the well in large displacement, which is not less than 800 m3/min ,by two truck working together. Stopped the trucks when there is no nitrogen left in the tanks. If you want once more shock in the same layer, inject another ball with 300 m3/min after pressure released then repeat steps above.
4. Multi-layer Nitrogen Shock Fracturing
If you want to fracture multi-layer in one well, you should fracture from the lowest layer. After finished the work of first layer, move the cup packers to the position of target layer calculated before work, then repeat the one-layer nitrogen shock fracturing. After that ,continue to move the packers up to the next target and do fracturing until the fracturing of all target layers is completed.