Wireline logging: tuning into missed data, lost value and unforeseen risks

Filling the gaps with wireline logging

Wireline logging services comprise a versatile and powerful suite of downhole geophysics that allow more information to be collected from each borehole than through drilling and physical sampling alone.

Surface geophysical investigations can provide huge value when used in early investigation stages as a site screening tool – effectively providing a view of the bigger picture by filling in the gaps (spatially) between point borehole positions. In a similar sense, wireline logging can provide better representation of soil/rock properties and characteristics as well as hydrogeological parameters particularly where core recovery is very poor, such as in heavily weathered or fractured ground. In these situations, wireline logging data can be indispensable, salvaging the return on the cost of that borehole.

Wireline logging also offers advantages over oriented coring, the traditional method for collecting information about geological discontinuity structures in hard rock. Oriented coring can be prone to errors and cannot be applied in boreholes with poor core recovery. In contrast, borehole imaging with wireline provides continuous, high resolution and oriented information for detailed and viable data about bedding, fracturing and coring.

Where does wireline logging fit within the site investigation?

For many decades, wireline logging has been used to help provide a complete picture of the subsurface for formation evaluation, technical checking and supervision in hydrocarbon exploration and production wells. Given the risks to construction engineering, it is not surprising that technology and lessons learnt from the exploration industry can be applied effectively to geotechnical and hydrogeological site characterisation campaigns.

A wireline logging survey usually accounts for a very small fraction of the SI budget - around 3 % to 5 % of the overall cost. Wireline investigations are also relatively quick to perform with minimal disruption to other investigations or the overall programme.

What does wireline logging tell us?

Modern wireline logging can deliver a wide range of invaluable data for geotechnics, tunnelling, mining, and groundwater exploration, such as:

• Structural analysis of discontinuity planes

• Lithological profiles and crosshole correlation

• Evaluation of petrophysical parameters including formation density, porosity, resistivity, magnetic susceptibility, velocity of elastic waves and elastic moduli

• 3D survey of subsurface cavities

• Evaluation of rock stress orientation

• Mineral identification

• Identification of aquifers and evaluation of hydraulic parameters

• Technical check of water wells

• Wireline data + consultative expertise = maximised value

Deploying the right combination of downhole geophysical methods to achieve comprehensive information about the subsurface is only part of the story. Multi-disciplinary geophysical, geotechnical and consultant engineering expertise is needed to accurately extract the full value of the acquired data.

Geo-data specialists at Fugro can integrate drillcore, wireline logging, in situ testing and laboratory data into composite logs to facilitate robust and representative interpretation and understanding of ground conditions – feeding into ground risk and design programmes.

A well-designed and expertly delivered wireline investigation can deliver many benefits:

• High resolution data on formation properties along the full length of the borehole

• When core returns for drilling are inadequate, wireline logging can fill in with representative information

• Wireline logging data can be used to better position packers for hydraulic or geo-mechanical downhole tests so that results are more representative of the (hydro) geology and geotechnical rock condition

• Provides better insight into karst conditions to avert water migration issues for tunnel structures

• Provides a fully digital dataset that can be easily accessed in later project phases

Unlocking the bigger picture with wireline

As part of Fugro’s ‘Triple A’ approach (acquisition, analysis, advice), wireline information helps deliver the best value ground model to reduce uncertainty and increase confidence in making reliable design and construction decisions.

With better risk screening, more accurate ground performance predictions, and greater detail within the 3D subsurface model, the tunnel driller and foundation engineer have a much clearer geological and geotechnical understanding of what to expect.

Wireline geophysics intelligence – don’t leave site without it

Today’s construction technology, engineering know-how and industry practice are sharply focused on achieving better risk management and cost control of new developments. Even so, foreseeable and preventable problems are all too common, wreaking untold disruption on construction schedules and costs, or potentially causing major closures or downtime for repairs - even catastrophic failure - further on in the asset lifecycle.

With a relatively narrow window for borehole investigations, there is a tendency to ‘turn the tap off’ too soon without fully exploiting the added value that can be derived through wireline geophysics and other in situ testing.

Did you know?

• Fugro’s new mobile logging unit immediately reduces CO2 emissions by between 20 – 25 kg each day it’s on site.

About the author

Wolfram Felfer is a Wireline Geophysics Lead (EUAF)