News

I’m excited to share that the borehole image (BHI) interpretation I completed for Gold Hydrogen, in collaboration with Andy Wilson of ImageStrat, is now publicly available as an open-file report.

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This work represents research-grade image analysis delivered on commercial timelines, and we are immensely proud of the outcome. The report can be accessed directly, either as a standalone document or within the end-of-well report with enclosures.

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Andy and I have been working together to advance BHI interpretation methodologies by combining our contrasting backgrounds in sedimentology and structural geology, and by actively challenging long-standing assumptions. One of our key areas of focus has been the quantification of secondary porosity.

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To our knowledge, this open-file report is the first publicly available case to directly compare the Luthi and Souhaité (1990) method for modelling fracture aperture from excess resistivity with manual fracture-width picking on borehole images. While neither method perfectly captures secondary porosity, our results show that manual width quantification corresponds more closely with independent indicators of permeability than excess resistivity modelling.

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The Luthi and Souhaité (1990) method is often applied to micro-resistivity borehole image data, likely because it is built into many commercial image analysis software packages. Our results indicate that further investigation into the validity of this approach is required, particularly in hard-rock environments. Importantly, our findings align with the original caveats noted by Luthi and Souhaité. There are two key lessons here: (1) Even when a method is easily executed and readily available through software, it may not yield useful results and (2) we should always refer to the primary literature.  

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Andy and I are interested in advancing this line of investigation. Get in touch if you are a researcher working on secondary porosity quantification from borehole images or are interested in hearing more about our interpretation services. 

I've been invited by SPWLA to join this impressive lineup next week to talk about geomechanics in the energy transition.

I’ll discuss how geomechanics fits into the resource evaluation process for conventional geothermal systems. There are no silver bullets but we’ve found that geomechanical modelling is an important piece of the puzzle—particularly in deep circulation systems and volcanic systems with steep permeability gradients. As we seek to develop geothermal systems with more complex permeability patterns, we must raise the bar on the geoscience used to evaluate them.

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The core of this talk is the workflow I have developed for structural geology and geomechanics in geothermal. You can download a free, high resolution version of this workflow here. 

Register for the SPWLA event here. 

Today I'm giving an invited talk at the Australian Natural Hydrogen Conference with Dr Andy Wilson (ImageStrat). Over the last two years, we have done research-grade borehole image interpretations for two of the leading movers in this exciting new energy: Gold Hydrogen and HyTerra. Looking forward to sharing our experience with others in the natural hydrogen industry and supporting their subsurface characterization. 

Deep diving into data using specialised plots helps us to move from basic observation to resource insight.

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Over the last few years, I've developed unique approaches to displaying borehole image data that improve interpretation and reduce the impact of bias on our results. In this post, I discuss geometric sample bias. In the next post, we will look at the importance of quantifying fracture width/aperture and using it when plotting data.

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These lower hemisphere stereonets plot the poles to planes for data picked from two borehole image logs acquired at the Fish Lake Geothermal System. This figure is adapted from my 2023 paper (download here). 

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I include contours of geometric sample bias on these stereonets. Fractures and beds that are perpendicular to your well have a high probability of being intersected. In contrast, those that are parallel or near-parallel to the well have an extremely low probability of being intersected and cause a data gap (the blind zone, highlighted yellow).

FL-1 is a vertical well. This means that the absence of high-angle fractures from this borehole image does not guarantee that high-angle fractures are absent from the reservoir. Instead, this absence may just be a function of the geometric sample bias.

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Key takeaway: Understanding and visualising sample bias prevents us from over-interpreting data gaps.

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The deviated well FL-3 intersected numerous high-angle fractures and some of these occur within the blind zone. Fractures with the orientation indicated by the red arrow are sufficiently numerous in the reservoir to balance the diminishingly small probability that they will be intersected. The grey squares indicated by the arrow are the top and bottom surface of a fault gouge. This is likely to be an important structural trend.

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Key takeaway: Considering data frequency in the context of sample bias and geologic processes enables us to determine relative importance of fracture clusters.

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The theory of geometric sample bias was first developed by Ruth Terzaghi ("Sources of Error in Joint Surveys". Geotechnique, 15(3), 287–304, 1965). Her proposed correction is built into most borehole image log software. That correction weights the available fracture data based on the angle between that fracture and the borehole trajectory. Because this correction weights the fractures observed in the borehole image, it cannot fully correct for the blind zone (c.f., the FL-1 example). This is why it's important to plot and consider the blind zone, even if a Terzaghi Correction is applied.

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Key takeaway: When applying corrections to data, it is important to understand how they operate and their limitations.

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If you would like to include contours of geometric sample bias on your stereonets, check out my open-source Python library fractoolbox.

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[click on the image to expand]

I was recently invited to give a talk to the borehole image special interest group of the Society of Petrophysics and Wireline Log Analysists (SPWLA). I took the opportunity to challenge peers to find ways to better characterise fracture width and aperture. For those like me who work in fractured reservoirs, these data are critical for identifying the key structural features and determining which of them may influence fluid flux. Our current methods are flawed, especially the automated modelling of fracture aperture. We need more validation studies and research into width/aperture characterisation for all host rock types. My talk is available on the SPWLA YouTube channel. 

I've been developing a document that helps geoscientists and managers navigate the path from data to insight about what controls permeability in a geothermal resource. This new, updated version can be downloaded here. Free to use with acknowledgement.  

I've launched a new, expanded Cubic Earth website to highlight projects and services. Get in touch to let me know what you think.