As part of our Tool Tuesday series, this week we will be looking at the various options on the market for Measurement While Drilling (MWD) telemetry systems; how we are able to communicate down hole data to surface and beyond. In this weeks article, we will spud with:
- Overview of telemetry systems
- Main system types
- Considerations for each system
In the following weeks, we will take a detailed look at each system, and how the downhole digital data is transmitted to surface for each system; Positive Pulse, Continuous Wave / Mud Siren, Negative Pulse, Electromagnetic, Wired Pipe.
Measurement While Drilling (MWD) systems play a crucial role in the oil and gas industry, providing real-time data transmission from downhole drilling tools to the surface, as the name suggests ‘while drilling’. These systems enable data gathering and transmission for a range of data types, from surveying the well bore, acquiring formation evaluation data, steering the BHA, to optimising the drilling operations, all in real time.
The technology that enables transmission of digital data (ones & zeros) from the bottom hole assembly (BHA) to surface is generally referred to as the ‘telemetry system’, however this may also include the surface systems required to decode the data signal for human consumption. In the coming weeks we will take a look in closer detail at the main systems on the market, however this weeks article will serve as an introduction to MWD telemetry, exploring the different types of systems including their designs, their advantages and disadvantages, as well as considerations when choosing which system is right for your operation.
Positive Pulse telemetry (PP): As the name suggests, this type of telemetry system creates a series of high or ‘positive’ pressure pulses, which travel up the inside of the drill string to the surface pressure transducers. These pressure fluctuations are captured and digitally decoded into usable digital data by the surface systems.
These positive pulse systems rely on a physical movement in the tool that restricts, or modifies the flow path of the drilling medium (e.g. drilling mud), momentarily increasing the pressure to cause a spike or pulse. There are various designs of positive pulse, however typically the pulse is created by a piston system moving against the flow of drilling fluid, towards an orifice which restricts the flow momentarily. This obstruction, or reduction of flow by area, creates a positive increase in pressure (hence the name).
Positive pulse telemetry systems typically offer lower data transfer speeds compared to other telemetry methods. The data rates achievable with Positive Pulse systems can range depending on multiple factors such as pulse repetition rate, pulse duration, and signal compression techniques, however piston and orifice systems are generally slower than other telemetry systems. While Positive Pulse telemetry may not provide the highest data transfer speeds, it is known for its reliability and compatibility with a wide range of drilling environments.
Continuous or Siren systems: A siren system creates a signal by continuously rotating two ‘disks’ with cut outs, called rotors and stators, which overlap each other. The constant change of flow by area (and thus pressure) creates a carrier wave that is adjusted or ‘modulated’ to encode the data signal. Siren systems typically offer faster telemetry speeds than piston type positive pulse telemetry systems, ranging from single figure per second (bps) to several hundred bits per second (Orion II Data Compression Platform | SLB). However, siren systems are generally a low amplitude continuous wave signal which is susceptible to attenuation in mud systems, resulting in lower signal strength in deeper wellbores.
Negative Pulse telemetry (NP): Acting inversely to positive pulse systems, negative pulse telemetry replicates the change in pressure by diverting or venting the drilling fluid, and thus creating a pressure decrease (hence negative) which is as before recoded on the surface pressure transducers. The physical mechanism that allows the pressure drop usually achieves this by ‘venting’ the flow of drilling fluid through the collar body to the annulus of the wellbore, or by rerouting the flow to provide less backpressure through the tools.
Electromagnetic (EM): Electromagnetic or ‘EM’ telemetry relies on transmitting downhole data via electrical impulses typically transmitted via an antenna or transmitter mounted at the top of the MWD system. Once sent, these EM waves propagate outward and are picked up by surface antenna. EM systems can benefit from much improved data transmissions speeds compared to the PP or NP systems, however are restricted in deployment due to various factors such as battery life, signal attenuation due to depth, signal interference due to ferric material in the subsurface blocking the EM waves, or even the ability to physically deploy the system such in offshore wells where running the surface antenna is difficult, time consuming or expensive (although possible). In some cases, signal boosters or repeaters can be used to increase the EM signal strength, however these are application dependent and not always a solution to EM signal issues.
Electromagnetic telemetry systems generally offer higher data transfer speeds compared to Positive Pulse systems. EM telemetry can achieve data rates ranging from several hundred bits per second to several kilobits per second, depending on factors such as frequency band, antenna design, and signal processing algorithms. EM telemetry is favoured for its high-speed data transmission capabilities and resistance to signal attenuation in challenging drilling environments.
Wired Pipe systems (WP): A less common method of MWD telemetry, wired pipe systems use physical cables embedded within the body of drill string to transmit data electronically in real-time. This direct electrical connection between downhole sensors and surface equipment offers several advantages, mainly two way communication that greatly exceeds the data transmission rates achieved by PP, Siren, NP or EM above. Wired pipe systems can achieve extremely high data transmission rates, similar to wireline logging, allowing for much higher data density formation evaluation data, much improved monitoring of drilling conditions, and allow precise directional drilling control. WP also offers immunity to signal interference since data is transmitted through physical cables, this system is less susceptible to signal interference from surrounding environments and is independent from drilling fluid limitations or considerations. However, Wired Pipe systems have drawbacks such as increased complexity, higher installation costs, and limited deployment flexibility due to the requirement of continuous physical connection between downhole and surface equipment.
Wired Pipe telemetry provides the highest data transfer rates, up to 57,600 bits per second for IntelliServ Wired Drill Pipe | NOV, depending on the communication protocol, cable design, and signal processing techniques. Wired Pipe telemetry offers robust and reliable data transmission, particularly in deep or complex wells where other telemetry methods may be limited.
Technical Advisory
When planning your operation(s), evaluate the different types of Measurement While Drilling (MWD) telemetry systems, clients and end users should consider the following factors:
Positive Pulse (PP) Telemetry:
- Data Rate: Generally lower data transfer speeds; must consider if the rate suffices for the operation’s data requirements.
- Reliability: Known for being reliable; evaluate its historical performance in similar drilling environments and fluid systems.
- Signal strength: Positive pulse telemetry typically produces stronger and more pronounced pulses, and thus is effective at greater depths than other systems.
- Cost effective: As an established technology, positive pulse systems are readily available, robust and cost-effective options for the majority of applications.
Continuous Wave / Mud Siren Telemetry:
- Speed: Offers faster telemetry speeds than PP; assess whether the operation requires or benefits from higher data rates.
- Signal Strength: Susceptible to attenuation; consider well depth and mud system to ensure adequate signal transmission.
Negative Pulse Telemetry:
- Signal strength: Vented pressure creates a pronounced, crisp pressure decrease, effective at higher data rates at increases well depths, able to overcome signal attenuation that might impact other systems.
- LCM tolerance: Negative pulse systems are typically highly resilient to lost circulation material, and are capable of functioning after cement slurries have been pumped through the system. If wellbore instability and higher mud weights are possible, then NP systems may become ideal candidates.
Electromagnetic (EM) Telemetry:
- Data Transmission Speeds: Generally higher rates; consider if the operation requires these speeds for efficiency.
- Deployment Limitations: Evaluate the well’s depth, subsurface materials, and deployment challenges, such as offshore environments.
- Battery Life: Consider power requirements and battery life, especially for extended use.
- Signal Boosters: Determine if boosters or repeaters are necessary and feasible for the application.
Wired Pipe (WP) Telemetry:
- Data Transfer Rates: Extremely high; assess the need for such rates versus the cost and complexity.
- Bi-Directional Communication: Benefit from real-time, two-way data flow; consider the value this brings to the operation. Nice to have versus a requirement.
- Signal Interference: Offers immunity; useful in environments where other telemetry signals may be compromised.
- System Costs: Higher costs and complexity; balance against the benefits of improved data quality and drilling control. Offset costs against greater drilling efficiency with increased logging speeds, and reduced survey times.
- Deployment Flexibility: Limited due to the need for a physical connection(s); consider the well’s logistics and project’s scalability, potentially not suitable for remote locations with minimal repair facilities should the wired pipe become damaged.
Closing Remarks
With the range of telemetry systems on the market, and advances in technology which are resulting in faster signal speeds, stronger downhole signal generation and more robust data transfer options, careful consideration should be taken when evaluating which option is best for your well. Clients and the data end users must weigh the trade-offs between data transfer rates, signal reliability, environmental (fluid) compatibility, and cost when selecting an MWD telemetry system. Each system has its specific advantages and limitations, which should be aligned with the operational requirements, well conditions, and overall project objectives.
If you have upcoming operations and are unsure what system would best suit your needs, get in touch for an informed, impartial discussion on the options available to you.
In the meantime, keep that bit turning and that data pulsing!
