Hydraulic fracturing, or fracking, is the process of creating fractures in underground rock formations to allow natural gas and oil to flow more freely into wellbores. It is a proven mechanical technique that has been employed in the oil and gas industry since 1947 and in recent years advanced technology has led to a rapid rise in its use for the production of natural gas from dense shale rock.
Shale formations have fine grains with few interconnected pores, resulting in low porosity and permeability. In order for natural gas or oil to be produced economically, individual molecules trapped in pores smaller than the width of a hair must find their way to the well. Hydraulic fracturing creates a network of small fractures in the formation rock, allowing molecules a long distance away to migrate to the fracture and travel quickly to the well.
Perforating gun
The production casing of a newly drilled well is lined with steel and cement and is capable of withstanding high pressures. A tool called a perforating gun is lowered through the production casing and punches small holes in the well casing, cement and rock. Water under high pressure is pumped down the well and through the perforations. The rate at which fluid is pumped must be fast enough to maintain the pressure needed to propagate the fractures. This is known as the propagation pressure. This initial volume of fluid is termed the pad and typically comprises 20% of the total fluid volume. The actual fracturing generally takes place over several kilometres below groundwater levels.
Proppant keeps the fracture open
As the fractures continue to propagate, sand is added to the fluid, wedging the fractures open. This is known as the proppant. When pumping is stopped and the excess pressure is removed, the fracture attempts to close. The proppant keeps the fracture open, allowing fluids to flow more readily. The fracturing fluid is typically 99% water and sand, with the remainder made up of chemical additives which help reduce friction and prevent bacterial growth and scale from blocking the flow of gas or oil.
The well is fractured in stages and plugged between each stage to maintain the highest water pressure possible and get maximum fracturing in the rock. After all the stages in the well are fractured the plugs are drilled out, the water pressure is reduced and fluids are carried up the wellbore for disposal or treatment and re-use, The sand is left to prop open the fractures and allow gas and oil to flow up through the well to begin production. The result is a highly sophisticated process that optimises the network of fractures and keeps them safely contained within the deep shale formation.
A short process
Fracture treatments can usually be completed within a day and are normally only performed once during the life of a well. Once a well is drilled and fractured it is ready for production. Everything is dismantled except for a well-head and connecting gas lines running to the distribution network. A natural gas well can produce gas for up to four decades and is monitored continuously to ensure well integrity.
The mechanics of fracking
The highly developed and regulated hydraulic fracturing process utilises state-of-the-art fracture mechanics and poro-elastic theory as an integral part of the design and construction of the well. The most important parameters are the in-situ stress, Poisson’s ratio and Young’s modulus.
Fracturing occurs when the fluid pressure within the rock exceeds the minor principal stress plus the tensile strength of the rock. Fractures extend along the path of least resistance. At any point the rock will have three stresses acting upon it: a vertical stress primarily due to the weight of the rock that lies above the target zone, and two horizontal stresses. The fracture is created by using fluid pressure to push back against the least of these three stresses. At the depths of typical shale gas formations, the lowest stress will be one of the horizontal stresses as the weight of the rock above exceeds any of the forces pushing from the sides. The fracture will always propagate in the direction normal to the smallest principal stress in the formation. Two symmetric fracture wings develop 180° apart, perpendicular to the least principal stress.
In the vertical direction, the fractures will extend until they reach a more ductile rock material which is more difficult to fracture than brittle shale rocks. These layers provide the containment and cause the remaining fracture to travel horizontally within the more brittle layer. The fracture will extend laterally as long as the fluid pressure within the fracture exceeds the lowest stress pressure.
Equipment
The process of hydraulic fracturing is very equipment intensive but only for a short period of time. When hydraulic fracturing is completed at one site, equipment is moved to the next, so it needs to be mobile. Various surface facilities and mobile equipment including fracture fluid storage tanks, sand storage units, chemical trucks, blending equipment and pumping equipment, surround the wellhead. A combination unit which has the fracturing equipment fitted on a truck can be used for multiple stage fracturing, where one fracturing crew performs several fractures in a single day. Compared to conventional fracturing setups, the combo unit provides significant savings through increased efficiency of equipment utilisation and related services. The unit leaves a small environmental footprint and no fluid waste.
The hydraulic fracturing process is monitored from a single truck called the data monitoring van. This tracks and records the rate and pressure at which the fracturing fluid is pumped down the wellbore, the rates of the necessary additives present in the fracturing fluid and proppant concentration. Pressure transmitters are a critical part of the system.
The fracturing pressure must be greater than the stress that geological forces apply to the reservoir rock, the tectonic stress, but within the pressure rating of the well and fracturing equipment. Fracturing equipment operates over a range of pressures and injection rates, and can reach up to 100 MPa and 265 litres per second. The pumping power requirement in kW is the pressure in MPa multiplied by the flow rate in m³/sec. The equipment utilises the latest in electronics and motor and pump technology to ensure that it is reliable and effective in placing fractures as designed.
A wide variety of pumps are used in the fracturing stage. Progressive cavity pumps are used to mix solid proppant with water. Metering pumps, especially diaphragm or piston pumps, are used for adding chemicals and other additives to the hydraulic fluid, which is then pumped to the frac pump. Frac pumps are large, engineered reciprocating pumps, typically powerful triplex, or quintiplex pumps, that can move hydraulic fluid at very high pressures. Because of the high pressure requirements, many frac pumps are connected through a manifold. To keep the frac pump running, smaller pumps are employed. For example, gear and vane pumps are used for lubrication and fuel transfer.
Pumps used in these operations experience harsh conditions, and failure of any one component may disrupt the drilling, completion and production process. As a result, the key consideration for pumps used in shale gas applications is reliability.
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