Threading the formation ‘needle’s eye’

Navigating Becoming Less Tricky

American Association of Petroleum Geologists (AAPG)

Drilling wells that often penetrate a number of miles deep into the earth and, in many instances, angling the wellbore to ultimately attain a horizontal position is now so commonplace it's become kind of a ho-hum event.

Unless you're the party-in-charge.

Indeed, wellbore placement can challenge even the best of the best.

It's a critical – and tedious – process, depending heavily on an in-depth understanding of the subsurface environment. It's not just a matter of staying on target and steering the drill bit through what may be a really thin reservoir, it's also crucial to do this as quickly as possible, especially offshore where rig time can be particularly costly.

Advanced reservoir navigation and geosteering techniques are key to successfully drill and complete horizontal and multi-lateral wells with enhanced accuracy and with maximum pay exposure.

These techniques enable detection of remote boundaries in real time to allow the drilling team to upgrade the models that describe and predict the environment laterally from the wellbore.

For so long, it's been difficult to determine whether a bed boundary being approached is above, below or even alongside the drilling assembly.

This can be blamed on limits in the types of resistivity measurements available for reservoir navigation.

For instance, the deep-reading omni-directional measurements failed to yield information to tie an approaching boundary, i.e., reservoir roof or oil-water contact, to a specific azimuth.

On the other hand, high-resolution resistivity images provided a 360-degree view of the wellbore in detail – but the depth of investigation was limited to only a few inches. Consequently, the value of these data in reservoir navigation applications ordinarily was limited.

Downhole GPS?

The AziTrak Deep Azimuthal Resistivity offering from Baker Hughes INTEQ is a new tool designed to resolve this problem. With its far-reaching depth of investigation, the multi-functioning AziTrak can detect, measure and visualize bed boundaries and oil-water contact zones long before they can be discerned with conventional sensors, according to Eric Hart, product champion resistivity and gamma ray services at INTEQ.

He noted the tool essentially is a GPS system for subsurface navigation.

The tool's surface system captures the data from the downhole MWD/LWD electronics package to immediately display navigational data and memory quality data for on-the-scene analysis by reservoir navigation experts.

"It's a bed boundary detection device," Hart noted. "As you're drilling horizontally through maybe a sand reservoir and you have shale encroaching from above or possibly water from below, and using forward modeling and resistivity responses we know how this should react but we lacked any directionality.

"What this tool does is it gives directionality so we can say, yes, there's a shale coming in from above us – even though it may be 15, 18 feet away.

"The well path can be altered to maintain the sweet spot in the reservoir," he said, "whether to the right, left, above or below."

'A Better Look'

Hart ticked off a few of the successful case studies using the tool:

  • A leading international oil and gas producer in the North Sea utilized real-time deep-image and distance-to-boundary calculations from AziTrak to optimize its horizontal wellbore placement in an area if high seismic uncertainty by steering the well close to the roof of the reservoir, avoiding waterflood zones below.

Indications are that an added one million barrels were accessed.

  • A major NOC in the Middle East used AziTrak to help drill more than 25,000 feet in three months, producing more than 90 percent net pay in the targeted reservoir sections.
  • Real-time visualization of the deep-reading AziTrak data enabled a Middle East client to make on-the-spot decisions 150 feet prior to what would have been doable with conventional logging tools.

All of these case studies occurred during the development stage of AziTrak, which made its official industry debut this past June.

"It's been a proof-of-concept project for about four years," Hart said, "so we've been collecting all kinds of information and data.

"Its primary function is to optimize extended reach and horizontal well paths," he added. "It gives the geologist, reservoir engineer and drilling department a better look to see where in the reservoir do I really want to be.

"They want to stay in the reservoir and maximize production," he said, "with a minimal amount of unproductive wellbore."  

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