The Process of Extracting Shale Gas
in Lancashire England.
Shale gas extraction is a sophisticated process, involving a large volume of skilled and experienced labour, specialist suppliers and costly capital equipment.
This report provides a very basic summary of the procedures and timelines
involved in drilling, fracturing and then harvesting shale gas. The process will differ to some extent between an exploration well and full commercial extraction, although the basic principles remain the same. We comment on these differences below, which revolve primarily on the degree of testing and analysis that is required and amount of fracturing.
Preparation of the Pad
The area where a well is drilled is referred to as the well pad. A typical test well pad is approximately 7,000 sqm in size and provides enough space for the drilling rig equipment, piping and storage, and other site facilities such as mobile portacabins for offices and worker restrooms. A test well pad of this size could typically support up to 10 wells spaced out across the site area, although for the purposes of test well extraction a single well will be drilled from each pad.
The main works included in the process are the insertion of a impermeable membrane and surface drainage ponds (as a safety precaution for groundwater supplies in the unlikely event of a spillage on the surface of fracking fluid or seepage of hydrocarbons), hard standing, and other ground works including landscaping the site to reduce the visual impact of the pad.
The time taken to prepare a pad can range from 30-60 days as the works are very weather dependent. The task of securing planning permission, which includes the necessary ground condition surveys etc, would precede the pad preparation process.
Drilling the Well
Prior to the main drilling rig arriving on site, water-well drilling techniques are used to remove the gravel deposits below the surface. Once the top soil and gravel deposits have been drilled, a cement collar is inserted. Following insertion of the cement collar, the main drilling rig is brought on-site and takes one week to assemble. Once assembled, the main drilling programme lasts for approximately eight weeks.
Key steps in the drilling process are as follows:
A drill bit is mounted on the end of a drill pipe. As the bit grinds away a mixture of water and additives called ‘mud’ is poured into the hole to cool the bit and to flush the deposits cut to the surface. The mud also coats the walls of the well providing greater resilience to the well wall. Once at the required depth past the freshwater zone (aquifers) the drill pipe and bit are removed from the bore hole.
Surface casing is then inserted into the well hole to isolate the freshwater zone the bore has passed through, and to act as a foundation for the blow out preventer (a safety device which connects the rig to the well bore). Cement is pumped down the casing and out through the bottom of the casing, and forced up between the casing and the bore hole sealing off the well from the freshwater zone preventing contamination of the water supplies.
The piping and drill bit is reinserted into the bore hole and drills through the plug and cement to carry on drilling down to the required depth. Hi-tech drill bits are then utilised to develop the angled curve to allow the bore hole to stretch out from beneath the main pad area out in to the shale.
Once the bore has reached the required distance, the drill pipe and bit are removed from the well. Production casing is then inserted in to the full length of the well. This process permanently secures the well bore, preventing hydrocarbons and other fluids from seeping out in to the geological formation. Testing is carried out at this stage to monitor the well and to ensure that it is safe.
The drilling rig is the most costly piece of capital equipment required for the drilling operations, and manned by a specialist crew. A range of specialist sub-contractors, providing items such as drilling fluids, tubular casings, drill bits etc are integral to the process.
Throughout the drilling process extensive daily deliveries and removals are required, providing essential bulk inputs (cement etc) and removing drilling debris from the site. Drill sites typically also require 24-hour security personnel.
A schematic of a Bowland Shale well is provided below.
Perforation, Fracturing & Testing
The next stage in the test well process is the perforation and hydraulic fracturing of the well to release the shale gas. This process typically starts about 3-4 weeks after drilling has been completed. It takes 2-3 weeks for a test well, longer for full commercial extraction wells. The key steps in this process are:
A perforating gun is lowered in to the well to the targeted section. An electrical current is sent to the perforation gun, and sets of the charge which perforates the casing and cement, as well as a short distance out into the shale formation.
The hole is then hydraulically fractured, or fracked, by pumping a mixture of water, sand and additives (99.85% water) in to the well bore and casing under extremely high pressure. The mixture is forced out of the perforations and into the surrounding rock; the pressure causes the shale to fracture. This process widens the fractures in the rock and makes these pathways wider for the gas to flow more easily from the rock to the well. The process of perforation and fracturing can be repeated several times to cover the distance of the well.
Once fracturing has been completed a period of up to 8 weeks is typically taken to test and monitor the well. Plugs are inserted and gas levels are monitored, both for production levels but also for health and safety reasons. Once the main period of testing has been completed the plugs can be drilled out to allow the gas to flow up the well bore. A permanent well head is then installed and the test well is closed until production commences.
Harvesting the Gas and On-going Maintenance
Once production commences infrastructure is required to feed the gas into the mainstream energy supply. The two main options are (i) additional pipelines to connect into the main UK gas pipeline network and (ii) on site electricity generation which is then connected into the national grid. Under either approach a substantial body of additional labour and equipment is required to put in place the necessary infrastructure, which will grow in scale as the number of wells in any one location increases. It may well be the case that in Lancashire a
hybrid solution is put in place which combines gas pipelines at some wells and electricity generation at others.
An on-going maintenance resource will be required to oversee the well field once it is in full production phase. This resource will carry out routine and scheduled maintenance on wells and also respond to any emerging issues. Figure 3-3 provides an image of what a well pad would look like after all drilling and fracturing has been completed.
A full cycle of test well pad preparation, drilling and fracturing can take up to half a year to complete (see Figure 3-4 over page). Drill kit will be freed up and available for other wells in the area part way through the process, allowing two test wells to proceed with just one drill rig. As and when full commercial extraction occurs certain elements of the process can be completed more quickly and efficiencies will emerge as a result of a greater critical mass of activity.
Figure 3-4: Overview