8/11/2019 Downhole Vision

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Logging tools using real-time transmissionsystems are now capable of imaging the wellboreas it is drilled.

A lthough, logging and measurement while drilling(LWD/MWD) tools have been available for many years, it is only recently that advances in data transmission and interpretation have progressed togenerate accurate images of the wellbore. These imagesare based on real time data and offer insight into what isreally happening downhole.

Typically, a high quality image is drawn from detailed,three-dimensional resistivity data. This data is suppliedby a resistivity tool similar to a logging formation micro-imager, which is run on wireline. The resistivity tool iscapable of identifying wellbore features andcharacterizing faults, cementation changes and threadedor spiraling caused by bit whirl. Software transforms theresistivity data into images of 3-D wellbores that are

viewable at all angles with simple mouse movements.The resistivity measurements are transformed into 56azimuthal sectors around the circumference of thewellbore to provide extremely detailed images.

Schlumberger drilling engineer Chris Lenamond said,Gone are the days of just more data dumps that need

processing and advanced interpretation expertise beforedecisions can be made. The technology has now arrivedwhere we can improve geological steering while drilling.Todays software and MWD telemetry systems providereal-time answers about 3-D wellbore features whiledrilling, well placement within the reservoir, wellborestability issues, formation dip and structuralconfigurations.

The combination of resistivity and density servicesbased on real-time logging images and geo-steeringtechniques is likely to enable operators to reduce risk andovercome geological uncertainties commonly associatedwith complex wells.

Ultra high telemetry rates (12 bits per second) havebeen successfully used to optimize horizontal well placement as well as warn of wellbore stability issuesbefore they become serious enough to jeopardizeoperations or impact drilling costs.

Augusto Silva, Schlumberger measurement engineer,said, The ability to transmit high quality real-timeazimuthal and image log data is possible even in caseswhere penetration rates are high. The resistivity imagesshown in Figure 1 are transmitted uphole presenting thewellbore in four quadrants. This information can bewrapped into a 3-D image of the wellbore, which assists inoptimizing well placement using geological markers.

Adjustments to the wellbore trajectory relative togeological bedding planes or faults can be quickly

determined and steering changes are made while drilling.Wellbore stability problems are detected with

ultrasonic calipers from density logging while drilling(LWD) tools. Hole enlargement or washouts can beidentified while drilling or during subsequent trips. This isbeneficial as it helps monitor wellbore stability andallows adjustments to be made to mud weights or effective circulating density as required. Wellborestability problems are confirmed using vision technologyincorporating Azimuthal Density/Neutron viewer software, which provides density image and caliper data while drilling. The software also generates 3-D imagesand caliper logs. Together, these offer easier methods of understanding well bore conditions during drillingoperations. Additionally, the 3-D density images andultrasonic caliper allow wellbore instability mechanismsto be better characterized, and when necessary, resolved.This is particularly important in completions wheregravel packs or expandable screens are required. Theultrasonic and density caliper information gatheredduring drilling can indicate whether hole quality is goodenough to permit specialized completions to proceed.While an up log obtained on a subsequent wiper tripallows visualization of the hole enlargement and stressfailures after drilling.

Specialized software uses a recorded mode to gather real-time dip information, provided by the LWD resistivityimaging tools. This information is harnessed to viewgeological structures and reduce the uncertainties in pre-existent geological models. Analysis of surface seismiccan also help indicate the possibility of erosion surfaces,which can jeopardize well navigation. Specialized data transmission from the rig site allows continuousobservation of the wellbore to anticipate changes in thebedding plane and structure behavior of the reservoir.

The software also allows structural dip picking fromimages, which can be used in combination with the real-time data for structural interpretation. Bed dips and layer thickness are also characterized permitting the

evaluation of structural cross-sections. The reduction inrisk and geological uncertainty will make the advent of wellbore imaging hard to resist for productioncompanies.

WELL CONSTRUCTION

Downhole vision

WAJID RASHEED,Editor, Latin America, can be reached via e-mail at wrasheed@chemweek.com.

Representation of down and up dip. (Image courtesy of Schlumberger)

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