With the introduction of horizontal drilling and fracture methods, smaller environmental footprints for oil and natural gas production have now become possible. Instead of spreading several wells across a broad landscape, multiple well heads are now being consolidated in a restricted area.
In this manner, the different facets required for the production lifecycle—separation equipment, wellheads, and tank storage, together with production monitoring controls and instruments—are situated in a geographically isolated landscape, thus minimizing the ecological and physical footprint.
Most often, in newly drilled wells, the water, oil, and gas flow freely to the surface via the casing pipe of the well, requiring little or no wellbore augmentation. When the liquid or gas flow drops to a point called the Critical Flow Rate, it is usual to set up a second pipe in the well called the production tubing—a concentric pipe within the casing pipe.
Due to the narrower production tube, the flow velocity is increased above a critical level needed to preserve a free-flowing well. But, with time, the natural pressure inside the well reservoir starts to fall. As a result, production reduces and follows a well-established downward curve.
Backpressure ensuing from the accumulation of liquids and oil in the tubing decelerates production and can ultimately stop the flow.
Prior to this occurrence, or when there is a rise in oil prices, companies are likely to implement one or more improvement method to enhance production. A sampling of these methods would comprise the following.
A main valve entirely stops the flow via the production tubing, allowing the adequate buildup of natural pressure at a certain point to offset the liquid backpressure. As soon as the valve is opened, the flow continues and the liquids are lifted out of the wellbore. The flow will go on until the accumulation of liquid causes natural pressure to decrease again, reducing the rate of flow.
Plunger Lift Systems
A piston-like plunger provided at the base of the well tubing acts as the interface seal between the natural gas pressure below and the liquid buildup within the tube. Once again, the main valve stops all flow via the tube until the natural gas pressure beneath builds to a level that is sufficient to lift the plunger to the surface. This plunger brings the water and liquid oil with it, and thus relieves the liquid back pressure to allow the flow of gas.
Numerous methods introduce gas to the base of the well casing. The natural pressure is supplemented by the gas and this helps in bringing the liquids and/or plungers in the tube to the surface.
These units are placed at the base of the well and usually powered by electricity. They increase gas production by artificially lifting liquids to the surface.
Figure 1. Wellpad with multiple wells, shared processing and storage, and automation controls.
Within the intermittent and plunger lift systems, engineers can automate the operation of wells using instruments.
The differential pressure between the well casing and the production tubing is tracked by pressure transmitters. Control software integrated into a control device can automatically close or open the main valve, when conditions command such an action to take place. The well automatically cycles on and off at a speed that improves production.
As stated above, with numerous (10 or more) wells located inside a wellpad, engineers can further extend the instrument functions to improve the entire production of the wellpad. Functions like these can include oil storage tanks, the associated separator, flow and level transmitters, water pits, alarming, gas piping, and emergency shutdown of the entire wellpad or individual wells.
The ABB solution
The ABB Totalflow XRC fourth-generation Remote Terminal Unit (RTU) has the potential to automate the operation of a whole wellpad or an individual well.
This RTU can be specified by operators or engineers with integrated, field-proven applications for optimization of Intermit or Plunger Lift wells or automation of a site. Alternatively, users can customize system operation by programming the PLC software function blocks.
A choice of Total flow input-output (TFIO) modules not only offers digital, analog, and pulse IO but also provides communication ports and valve interface functions. Basic units can be scaled up by adding micro-controller boards in a master-slave relationship. The boards provide excellent memory capacity and processing speed, and can work under the Windows® CE operating system.
A built-in Ethernet port allows complete networking capabilities. A combination of USB host and device ports allows data archiving, local configuration, and firmware upgrades. In addition, the front panel display can be a simple LED readout and/or an advanced Human Machine Interface (HMI) to display wellpad data in a wide range of graphical formats.
Figure 2. Totalflow XRC RTUs scale up to accommodate additional well heads, PLC processing functions, and IO. New HMI permits monitoring of multiple well heads in a variety of graphical formats. Totalflow IO modules mount on DIN rails and interface with valves and communications.
The ABB RTU can be easily configured to determine and control almost all the functions on the pad. Based on simple to intricate algorithms, the RTU turns the wells on and off according to users’ comfort level, to improve production.
When linked to suitable sensors, the RTU computes the gas, oil, and water generated. It also determines and regulates the temperatures of separator and tracks water and oil tank levels. During emergencies, it shuts down the entire wellpad or individual wells. Moreover, communications for both measurement and control operations can be either wireless or directly wired. Typically powered by solar charging and batteries, and with the system sized correctly, operation can go on autonomously for many days.
Apart from these pre-engineered lift applications for the oil and gas sector, ABB Totalflow RTUs help in creating customized applications through generic IEC-61131 PLC programming tools.
The solution supports the following six tools:
- Instruction List
- Function Block Diagram
- Structured Text
- ISaGRAF Flow Chart
- Sequential Function Chart
- Ladder Diagram
In this manner, users can create customized applications to measure and automate operations based on their specific programming experience and skills.
TFIO module hardware designed for the ABB RTUs features DIN-mountable enclosures that use Phoenix contact technology for field wiring. In addition, the modules interconnect with one another to offer the required interface and power signals along their bus. Typically, the capacity of these IO modules is only limited by the size of the enclosure.
The modular and flexible nature of ABB Totalflow XRC RTUs offers complete customized or pre-engineered solutions for the oil and gas sector. System scalability enables perfect adaptation for retrofit, reinforcement, and upgrades of wellpad.
This information has been sourced, reviewed and adapted from materials provided by ABB Measurement & Analytics.
For more information on this source, please visit ABB Measurement & Analytics.