The oil and gas value chain is long, and at the beginning of the extraction process is drilling. After a site has been identified and explored, this step in the upstream stage uses a series of industrial machinery to bore through the geological formations and safely remove crude oil or natural gas from the ground. The equipment includes:
These pieces of equipment operate under high-pressure conditions with heavy vibration and pulsation. Onshore, dust and extreme ambient temperatures are a challenge for machinery. Offshore drilling exposes machinery to salt water and sprays. Every component used in the drilling scenario benefits from reliable sensors that are heavy-duty enough to perform under these demanding conditions.
Drilling rigs vary widely in size and are located either offshore or on land, ranging from compact mobile units to permanent structures the size of skyscrapers. A rotary table or, alternatively, a top drive rotates the entire drill string with the drill bit positioned at the bottom of that string. Regardless of the rig’s size, location, or type, these machines have one purpose: to bore deep into an oil fields’ reservoir in order to extract hydrocarbons.
For safe and efficient operations, every drilling rig requires a myriad of sensors. These range from the temperature-compensated tension/compression force transducers in the crown block assembly to level and pressure sensors in drilling fluid equipment. Resistance thermometers, with thin-film technology to withstand heavy vibrations and pulsation, help monitor the condition of the many pumps and motors that make well drilling possible.
As the drill bit and drill pipe dig through the earth, one or more mud pumps send a slurry down the borehole at extremely high pressure – over 7,000 psi (483 bar) in deep-hole drilling. This drilling mud serves several functions:
This drilling fluid then returns to the surface via the annulus, i.e., the space between the drill string and the wellbore.
A mud pump has a power end with a motor and a fluid end with ports. A hammer union pressure transmitter with a thick corrosion-resistant measuring element is paired with an IS (intrinsically safe) barrier to monitor the process pressure at the suction and discharge ports. Also, an pressure gauge with a diaphragm seal provides local indication of the pump’s discharge pressure. In some mud pump configurations, an pressure gauge is used to monitor the pressure of the oil that lubricates the machinery’s bearings and seals.
Blowouts are when formation fluids, including highly flammable hydrocarbons, flow uncontrollably out of the well. Blowout preventers (BOPs) and BOP closing units head off this dangerous situation.
The BOP, mounted securely on top of a well, is a stack of valves designed to control and – when necessary – seal off an oil and gas well in the event of a kick (unexpected influx of fluids in a wellbore). Pressure gauges, along with intrinsically safe or explosion-proof pressure transmitters, are a key part of this well control system.
A closing unit uses pre-charged nitrogen bladder accumulator bottles, hydraulic fluid, and electric and air-based pumps to generate pressure to operate multiple functions on the BOP stack. A complement of pressure gauges, pressure transmitters, and level sensors ensure that this safety mechanism works properly when needed.
Cementing plays an important role in drilling. After reaching the desired depth, the drill string is removed and the casing string is inserted. Primary cementing seals off the various formation zones – oil, gas, water – from the wellbore. It also fills the annulus to hold the casing securely in place, prevents it from corroding, and protects it from underground pressure. Secondary cementing is to repair or correct a primary cementing job, or to plug a depleted well.
A cementing unit on a drilling rig, mounted on either a skid or semitrailer, receives water, dry cement, mud, and special additives from storage tanks. It then mixes the components into a lead slurry and a tail slurry, then injects them into the casing and out into the annulus. A control panel on the cementing unit allows an operator to regulate the flow and density of the slurries, monitor pressure, and control other parameters. Integral to the monitoring of pressure is the hammer union pressure transmitter, which is designed for extremely high pressures and can withstand the abrasive quality of the cement mixture.
Mud tanks are large multi-segmented vessels for treating, mixing, and storing the customised fluid that a mud pump circulates in and out of a well during drilling. After returning to the surface, the used mud is pumped through various compartments in the active tank:
After cleaning, the drilling mud is ready for reuse. Reserve tanks store excess mud and as well as heavy mud for kicks and other emergencies.
Level sensors allow operators to monitor the fluid in each of these compartments. But more importantly, level measurement can reveal issues during drilling operations:
Any level sensor used in mud tanks must be suitable for hazardous areas. The instrument should also be heavy-duty, as drilling mud is often corrosive, viscous, and abrasive with suspended solids. Many types of level sensors can be used in mud tanks, but the most robust and maintenance-free option is an intrinsically safe submersible pressure transmitter that can be used to measure both the fluid density and hydrostatic pressure. An anti-clog attachment provides stability and extra protection against sludge, turbulence, and physical damage.
At a drill site, a mud pump injects a fluid down a borehole to lubricate and cool the drill bit, to flush the well of drill cuttings, and to provide the equalising pressure that prevents formation fluids from entering the well. This specialised mixture is either water-based or oil-based, depending on the geological formation, economic considerations, environmental regulations, desired purpose of the mud, and other factors.
Regardless of the liquid used as the continuous phase, water is almost always a component – in varying percentages – of drilling mud. Water is stored in massive tanks, ready to be piped to mud tanks and mixed with other components.
Submersible pressure sensors accurately and reliably measure the level in a drill site’s water tanks, ensuring that the water does not overflow and the tank does not run dry. A WIKA LevelGuardTM offers extra stability for the pressure sensor and further protects the instrument from turbulence and strain.
Choke manifolds are an important part of a drilling rig’s well control system. During a kick, the blowout preventer (BOP) is closed while the formation fluid and drilling mud flowing from the annulus is diverted to the choke line and through the manifold’s assemblage of pipes, fittings, and valves. The choke manifold bleeds off the lighter drilling mud to stay below the MAASP (maximum allowable annulus surface pressure), while heavy mud from the reserve mud tank is pumped down the hole to regain control of the well. Alternate paths in the choke manifold allow operators to redirect the flow as needed by closing and opening valves. Some chokes are operated manually, while others are controlled hydraulically (remotely).
Pressure gauges and pressure transmitters – integral parts of a choke manifold system – are located at several locations, including:
To ensure reliability and safety, the pressure sensors mounted on a choke manifold should meet the following requirements: