Drilling and the Hydraulic Fracturing (Fracking) Process
How is natural gas extracted from the ground?
Oil and natural gas reserves can be safely and efficiently delivered to customers using highly advanced modern drilling techniques.
A typical drilling process has the following stages:
Preparing a drilling site involves ensuring that it can be properly accessed and that the area where the rig and other equipment will be placed has been properly graded. Drilling pads and roads must be built and maintained which includes the spreading of stone on an impermeable liner to prevent impacts from any spills but also to allow any rain to drain properly.
A hole is drilled straight down into the ground. A special drilling mud is used to cool the drill bit, carry the rock cuttings back to the surface and also to provide stability to the walls of the borehole. Once the hole extends past the deepest freshwater aquifer (typically 300 – 1000ft), the drill pipe is removed and replaced with steel pipe, called ‘surface casing’.
Next, cement is pumped down the casing and then back up between the casing and the borehole wall, where it sets. This cement provides a bond which prevents any fluids moving between the casing and the hole. In doing so, this creates an essential, impermeable protective barrier between the well bore and any freshwater sources. At this stage, rigorous tests are carried out to make sure that the bond is completely impermeable before any more drilling takes place.
Typically, depending on the geology of the area and the depth of the well, extra casing sections are put in, much like surface casing, and then cemented in place to ensure that there can be no movement of fluids or gas between those layers and the groundwater sources.
What makes drilling for hydrocarbons in a shale formation unique is that it is necessary to drill horizontally. A well is drilled vertically to the right depth called the ‘kick off point’, and at this point, the well bore begins curving to become horizontal.
One of the advantages of horizontal drilling is that it’s possible to drill several laterals from only one point on the surface (surface drilling pad) which minimises the impact and scale of activity on the above ground.
When the target distance is reached, the drill pipe is removed and more steel casing is inserted through the full length of the well bore and, once again, the casing is cemented in place.
Once the drilling is finished and the final casing has been installed, the drilling rig is removed and preparations are made for the next steps, well completion.
The first step in completing a well is to create a connection between the final casing and the rock which is holding the oil and gas.
A special tool, called a perforating gun, is lowered to the rock layer. This perforating gun is then fired, creating holes through the casing and the cement and into the targeted rock. These perforating holes connect the rock holding the oil and gas and the well bore.
Since these perforations are only a few inches long and are performed more than a mile underground, no activity is detectable on the surface. The perforation gun is then removed before for the next step, hydraulic fracturing (fracking).
Hydraulic Fracturing (fracking)
Stimulation fluid, which is a mixture of over 90% water and sand, plus a few chemical additives, is pumped under controlled conditions into deep, underground reservoir formations. The chemicals are used for lubrication and to keep bacteria from forming and to carry the sand.
These chemicals are typically non-hazardous and range in concentrations from 0.1% to 0.5% by volume and are needed to help improve the performance and efficiency of the hydraulic fracturing.
This stimulation fluid is pumped at high pressure out through the perforations made by the perforating gun. This process creates fractures in the shale rock which contains the oil and natural gas.
The sand is left in the fractures in the rock in order to keep them open when the pump pressure is relieved. This crucially allows previously trapped oil or natural gas to flow to the well bore more easily.
Once the hydraulic fracturing process is completed, production can begin, oil and gas can flow from the well bore and the fracturing fluid can be recovered.
During initial production of the well, about 25-75% of the fracturing fluid is recovered. This is either recycled for use on other fracturing operations, or safely disposed of according to government and environmental regulations. This whole process of developing a well typically takes from 3-5 months and includes the following activity:
- a few weeks to prepare the site
- eight to twelve weeks to drill the well
- one to three months of completion activities including between one and seven days of stimulation.
This initial three- to five-month investment has the potential to deliver a well that will produce oil or natural gas for 20 to 40 years, or more.
When all of the oil or natural gas that can be recovered economically from a reservoir has been produced, the land is returned to the way it was before the drilling operations started.
Wells will be filled with cement and pipes cut off 3-6 feet below ground level. All surface equipment will be removed and all pads filled in with earth or replanted. The land can then be used again by the landowner for other activities, and there will be virtually no sign that a well was once there.