As the industry looks to develop wells in more technically and operationally demanding environments, the potential challenges increase.

The result is that the need to more fully understand what is happening downhole throughout the well construction process is critical. An effective telemetry system that can reliably provide valuable data from downhole to the surface is therefore an important tool during well construction activities.

Current data telemetry platforms such as mud pulse, wired drillpipe (WDP) or electromagnetic (EM) systems are subject to significant limitations. These could be operating restrictions, relatively low bandwidth and complicated installation with associated high implementation costs, all of which lead to increased barriers to deployment.

WDP might represent the most comprehensive platform for information acquisition via telemetry because of the high levels of data that can be achieved. However, this process requires a dedicated string through wired components, and its integration can be costly, lengthy and operationally complex.

EM platforms often support a niche land-based market as their data acquisition rate is relatively low, but they are formation-dependent and difficult to implement in an offshore environment.

Mud pulse telemetry platforms are the most common method of downhole data acquisition, working by restricting fluid flow to send pressure pulses up through the fluid to surface. However, they deliver a relatively low data rate, rely on fluid flow, are depth-limited and create a constriction in the bore of the drillpipe. This eliminates the ability to implement distributed measurements along the drillstring, drop balls or pass wiper darts. In addition, they are only employed during drilling operations, which can make up as little as 15% of total well construction time, and they therefore are not an option for the completion installation process.

Sub-optimal offshore completions
Research from global operators gathered by XACT Downhole Telemetry suggests that the practice of completing wells with limited downhole data in real time has encountered consistent challenges related to understanding downhole conditions during critical operations.

Subject matter experts have repeatedly suggested that up to 50% of all offshore wells have sub-optimal completions, while 20% result in material nonproductive time due to a lack of downhole visibility, with an average remediation period of two weeks to three weeks per failure—assuming the well is recoverable.

Previous technological limitations have meant that operators have had to accept that there are often substantial differences between their surface measurements and models and what is actually occurring downhole. This leads to the potential for nonproductive downtime and costly delays.

Improved real-time downhole visibility is therefore desirable to drive structural cost changes to the well construction process. A better data telemetry platform that operates in all environments and through all aspects of the well construction process will enable operators to structurally reduce such costs.

Acoustically linked nodes
An alternative to existing telemetry systems that enhances the existing telemetry market and expands into new, previously inaccessible areas in the well construction process has been developed by XACT.

The company’s acoustic telemetry platform delivers real-time data in these previously unavailable environments with no change to the drillstring, surface equipment or any rig component, unrestricted by depth, fluid flow or formation constraints.

The system is based on the installation of acoustically linked downhole measurement nodes in-line with the drillstring, which form a robust telemetry data and sensor network.

The nodes are typically placed about 1,524 m (5,000 ft) apart in the vertical sections of the well and 914 m to 1,219 m (3,000 ft to 4,000 ft) apart in horizontal sections to provide optimum signal strength and transmission range, depending on the angle of the hole. All nodes are fully through-bore, which allows the deployment of wireline-conveyed tools as well as wiper darts, third-party activation balls or lost circulation material and enables use in both cementing and completion installation environments.

Collar-based design
The collar-based design of the nodes provides sufficient space for lithium batteries, sensors, electronic boards and the piezoelectric stack that allows the acoustic transmission of data back to surface.

In addition, mechanical parameters such as pressure, weight, torque, bending and temperature are recorded downhole and transmitted acoustically in real time via the steel body of what has now become essentially a smart drillstring.

A benefit over the common mud pulse systems, which can only take measurements at the bottom of the string, is the ability to gather measurements from a range of locations via the multiple spaced nodes offering a real-time snapshot of the conditions along the entire wellbore and drillstring. Distributed measurements coupled with the integrated strain and pressure measurement capabilities and greater data rates than mud pulse and EM technologies offer better understanding of weight and torque transfer and
segmented equivalent circulating density or annular frictional pressure along the complete drillstring regardless of circulation rate.

In drilling applications these along-string measurements can greatly enhance the understanding of effective weight or mechanic specific energy transfer, hole cleaning, the onset of stuck pipe and early kick detection. With this higher resolution data, both in time and spatially along the string, more challenging wells can be drilled and completed with the right information at the right time and from the right places.

Data are sent to a wireless receiver at surface and from there to a laptop for further transmission.

XACT’s patented Electronic Acoustic Receiver (EAR) system was installed on a frack head offline, meaning zero rig operation time was required to mobilize it for a deepwater completion in the GoM. The wireless receiver enabled real-time downhole data at surface throughout the operation. (Source: XACT)

The EAR device is compatible with a range of pipe diameters and can be installed on either the top drive or frack head, allowing the downhole acoustic data packets to be received and forwarded to the decoding and display unit on surface in real time. (Source: XACT)

BP early adopter
The telemetry platform, first deployed successfully in underbalanced and managed pressure wells onshore in North America, drew the attention of BP Ventures’ upstream technology group in 2012. It decided to support the system’s further development for use in a wider range of complex applications.

BP’s support enabled XACT to develop a greater understanding of operator requirements for offshore wells from a mechanical perspective and the ability to deploy the platform in the well construction process in a range of deepwater applications.

A progressive development program agreed upon between both companies has seen the telemetry platform since deployed multiple times for BP in a range of deepwater wells in the Gulf of Mexico (GoM) since year-end 2014. BP and other major operators have now
deployed the technology in deepwater applications including liner running, cementing, formation integrity tests, casing and completion operations.

The applications chosen by BP were seen as systematic steps toward the goal of providing real-time downhole data during installation of a full deepwater sand control completion, a goal successfully achieved by year-end 2015.

Sand control completion in GoM
A 6,919-m (22,700-ft) complex “S-shaped” deepwater well in the GoM was selected by BP for the project.

Key objectives included safe and efficient operations, the demonstration of acoustic communication from a significant depth (set at greater than 6,096 m [20,000 ft]), confirmation of set-down weights in real time on a crossover tool, and ensuring XACT’s real-time data integrated with BP’s own data network and those of third-party service companies.

All criteria were successfully met by the acoustic telemetry platform. Six downhole nodes, measuring tension, torque, and outer and inside diameter pressure, were placed on the string and delivered updates every 35 seconds, allowing data acquisition while tripping
for pipe fill, confirmation of the set-down weight and the monitoring of loads during the sand control completion operations. These measurements were transmitted during flow and no-flow conditions.

The acoustic telemetry platform showed there was a significant difference between the estimated and actual set-down weight on the crossover tool, from an original estimation of 70,000 lb at surface to what was actually happening downhole with a weight of 92,000 lb.

Additionally, during the pumping of cooler fluids down the string it could be seen that more than 70,000 lb of weight came off the packer due to drillstring contraction. The ability to view and therefore maintain the weight differential downhole in real time meant that modifications could be made rapidly at surface.

By monitoring tension and weight and continually examining the differences between the bore and annular pressure, the acoustic telemetry platform also was able to confirm the reverse-out position, thereby creating an additional benefit for operators in completion construction.

As a result of successfully completing this project, XACT’s acoustic telemetry platform is now being scheduled for further completion installations in the near future.

Better visibility ahead
The characteristics of wells are becoming increasingly more challenging to navigate. Many of today’s obstacles are the result of attempting to tackle issues that the industry has so far been unable to see.

Better downhole visibility, delivered in real time, will have an increasingly important role to play in reducing well costs, from completions to drilling to cementing and beyond. A versatile, easy-to-install acoustic telemetry system, which can replace the practice of having to work with limited real-time downhole data, has the potential to save the industry millions of dollars in reducing flat time, avoiding well remediation or even coping with a well loss.

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