Applying petrophysics to geoscience challenges

Before we get into this topic, it is important to note how I am defining petrophysics. For a full explanation see my previous blog, but in a nutshell I use petrophysics as “a term to express and explain the physical responses of particular rocks and sediment types”. 

Straight off the bat it should be obvious that petrophysics and the associated insights it brings are applicable to a wide variety of geoscience challenges, particularly in the understanding of the subsurface. I would like to offer a few examples below but I should also mention that this article is not exhaustive. These are just a few example applications of data, primarily from the International Ocean Discovery Program (IODP) and its precursor programs, where downhole (well) log data is routinely collected and has been used to answer the scientific challenges of its science plan. 

Evidence for the wide applicability of petrophysical data is its role in the IODP science plan 2013-2023. The science plan is intended to guide multidisciplinary international collaboration by outlining the breadth of questions that IODP aim to tackle. Petrophysical analysis plays a part in answering all these questions covering the fields of: 

1. Global climate past and present
2. Deep life and the biosphere
3. Planetary dynamics and tectonics
4. Geohazards

Many of the strengths and applications of petrophysical data derive from the nature of its acquisition. Well log data provide an ‘investigation area’ that is relatively unique in the fields of geoscience investigation techniques. Sediment, rock and core samples and their analysis are commonplace, and investigate the nano-to-centimetre scale. Whilst seismic profiling provides basin-scale architecture, at its very best the data have a resolution of 10 metres. Log data covers the 0.1-100 metre scale area of investigation and is one of the few technologies specifically designed to do so. Added to the fact that logs capture continuous data and the in situ properties and, for these reasons, I think logging data should be more commonly utilised and considered when generating, and ground-truthing, geological models. 

Geoscientists can benefit greatly from integrating petrophysical data with their other data generated from direct sampling or observation of sediments or rocks. Petrophysics is geared towards answering why certain rock types exhibit the physical responses that they do, and in doing so producing quantifiable information on chemistry, mineralogy and fluid content (where possible). Invaluable information for sedimentology and petrology. 

The continuous downhole (or more accurately, up-hole) recording of data also makes it ideal for capturing and analysing stratigraphy, cyclicity and other trends. This again can be valuable information for all geoscience disciplines but really add value to geoscientists researching palaeoclimates, paleoenvironments and paleoceanography, including those interested in sediment source. 

Logging tools have different purposes with some aiming to characterise different aspects of the formation than others. Many tools such as electrical resistivity have deep areas of investigation (typically 1.5-2 metres). These tools can provide information on formation structure and fluid content rather than mineralogy and chemistry. Other tools can have extremely shallow depths of investigation such as the various imaging tools (millimetres). Borehole images can be used to analyse millimetre-scale textures and sedimentary structures within rocks and sediments as well as to examine fault orientation, dip and dip direction. 

Imaging tools are deployed as standard during IODP expeditions and are becoming increasingly common elsewhere. The examples of borehole images below cover most of the spectrum. The left image was recorded during IODP Expedition 364 using a slimline acoustic imager ^[1]. The image has clearly recorded the coarse grained nature of the granite, but has also captured two generations of intrusion and measurable fractures comparable with the corresponding core from this depth. In the right image, wireline electrical resistivity images and Logging While Drilling (LWD) images have captured cross bedding in deltaic sandstones 1600 feet (~490 metres) below the surface in an oil well in Oklahoma ^[2]. 

The discipline of petrophysics is closely related to geophysics and integration of petrophysical measurements with other types of geophysics data can be of great benefit. It could be argued that the most useful data are those generated by the sonic tool. The sonic log is a continuous, usually high-resolution record of compressional velocity along the well path ^[5]. These data can ground-truth seismic surveys in the area by establishing the time-depth relationship and, importantly, linking well log to seismic profile and ultimately core to seismic. Morgan et al. ^[3] demonstrate this in their scientific expedition drilling in the Chixculub impact crater peak-ring. Here the seismic P-wave velocity (km/s) obtained from sonic wireline logging data confirmed that the predominantly coarse-grained, granitic rocks of the peak ring were indeed characterised by the low densities and low seismic velocities suggested by geophysical models based on seismic refraction data. 

When the sonic log is combined with a density log it also becomes possible to calculate acoustic impedance (a property of rock layers and their boundaries that govern acoustic reflection coefficients). Combining these petrophysical log data allows for creation of a synthetic seismogram. Further insights can be made into seismic profiles if shear slowness logs are generated as these can advise on formation fluids. 

Access to fresh water is already one of society’s greatest challenges and will be an increasing concern in to our future. Lofi et al. ^[4] used a range of data from IODP Expedition 313 including lithology, 2-D seismic profiles, pore-water salinity measurements, porosity measurements, density measured from core, thorium content (from downhole spectral gamma-ray logs) and sonic velocities from downhole logs to determine the geological heterogeneities affecting groundwater exchanges on the New Jersey shelf. Their work revealed evidence for a multi-layered reservoir/aquifer where waters with very low salinities (<3 g/L) were encountered at depths below sea floor exceeding 400 m and fresh and/or brackish-water intervals alternate vertically with salty water intervals on this passive margin.

It is also worth mentioning borehole gravity surveys, and here I must admit that I am a bit out of my area of expertise. Suffice to say though they are to gravity surveys what sonic logs are to seismic surveys [5]. For more information on borehole gravity surveys and their relationship to surface gravity surveys see Martin Kennedy’s book Practical Petrophysics – Chapter 14: Geophysical Applications.

Further applications:

Downhole tools are becoming increasingly versatile with tools for magnetic susceptibility, fluid sampling, magnetisation and borehole imaging. Core-based petrophysics is also a rapidly expanding field with increasing commonality of chemical analysis such as XRF, hyperspectral imaging and near-infrared spectroscopy. Understanding of the data produced by these new tools is of increasing importance to academia and industry. 

Petrophysics and its techniques can also aid in the fields of: 

1. Contamination
2. Remote sensing
3. Soil and sediment science
4. Geochemistry
5. Hydrology and hydrogeology
6. Geotechnical measurements

And finally, what of geological models? Martin Kennedy makes a great point about this in his book Practical Petrophysics. I’ll let his words speak for themselves: 

“The increasing use of software to build detailed 3D geological models [of reservoirs] has meant that petrophysics has to be properly integrated with the other sub-surface disciplines. The model builder needs to know what assumptions have gone into the creation of the petrophysical property curves and the petrophysicist needs to know that their results are being used appropriately. Consequently a working knowledge of practical petrophysics is no longer just a ‘nice to have’.”

For ‘of reservoirs’ read any sedimentary basin, aquifer, impact crater, passive margin, mid-ocean ridge, obducted ophiolite, subduction zone, slow-slip zone – the opportunities are limitless. So if this has given you pause for thought and you are interested in knowing more, why not have a look into how petrophysics can benefit your science?

References:

1. Lofi, et al. 2017. Scientific Drilling, 24, pp.1-13.
2. Ritter, et al.2004. SPWLA 45th Annual LoggingSymposium. Society ofPetrophysicists and Well-Log Analysts.
3. Morgan, et al. 2016. Science 354 (6314),878-882.
4. Lofi, et al. 2013 Geosphere; August 2013;v. 9; no. 4; p. 1009–1024
5. Kennedy, 2015. Practicalpetrophysics (Vol. 62). Elsevier.

Petrophysics is about much more than oil

“Petrophysics” is a term not widely used in academic circles (at least in my experience), but it is one that is quite extensively used within the language of the oil and gas industry. So what is petrophysics exactly and what does it mean in an academic context? The summary that I most commonly come across goes like this:

“Petrophysics is the study of the physical (and chemical) properties of rocks and their interactions with fluids, and integrates downhole insitu data from logs with core and seismic data”

This is, in my view, a good definition. But I wanted to take it a step further by exploring a little history.

Petrophysics is a term generally linked to downhole (well) log measurements and their analysis (by petrophysicists) to evaluate rock properties. The Schlumberger brothers ran the first well log (or something close to it) in 1927 when they lowered an electric sonde down a well in Pechelbronn, Alsace, France to measure electrical resistivity. This was the first “down hole” measurement of rock properties using technology that Conrad Schlumberger had been developing since 1911. For a great history of the first well log and the road travelled by the Schlumberger brothers to start the international company we know today, see the Schlumberger website (definitely worth it for the pictures alone).

The first well log, September 5th 1927, Pechelbronn

So began the relationship between the rise of petrophysical analysis techniques and the growth of the oil and gas industry. Technological developments and new techniques have since stemmed from the needs of the industry and petrophysics remains a tool most commonly used for describing and analysing all aspects of the hydrocarbon system. In turn, this created a bias in the available technologies, with the majority of tools (at least originally) being designed for describing porous media and the quantities and nature of the fluids they contain.

The Schlumberger brothers

However. I would argue that, despite this ‘tool development’ the Schlumberger brothers are not the fathers of petrophysics. I would argue that this title belongs to Gus Archie, the author of two of the top 10 landmark papers in petrophysics and formation evaluation - including the famous Archie equation for determining water saturation ^[1]. The Schlumberger brothers were the first to develop and implement the technology, but it was Archie who was the first to understand the data. In his book “Practical Petrophysics”, Martin Kennedy discusses the history of the technique, stating that before Archie, petrophysical data were primarily used for qualitative interpretation of the sub-surface, such as identifying sands and sometimes distinguishing water and oil in pore space ^[2]. It was Archie who, in 1938, was charged by Shell's Texas-Gulf area production manager, D. B. Collins, with the task of understanding electrical logs ^[3]. And it was through this venture that Archie’s now-famous equation appeared in 1942 followed by the Archie’s first published use of the term “petrophysics” shortly thereafter in 1950 ^[4].

Gustave Erdman Archie (source)

In his 1950 paper Introduction to Petrophysics of Reservoir Rocks, Archie describes petrophysics as: “A term to express the physics of rocks. The term should be related to petrology as much as geophysics is related to geology. ‘Petrophysics’ is suggested as the term pertaining to the physics of particular rock types” ^[4].

It’s worth noting that this term may have been already used informally at the time, but as the first published example, I believe Archie should be credited with the definition.

So how does this definition differ? Martin Kennedy expands the definition with an explanation: “As a pure science its [petrophysics’] objective would probably be to explain why rocks have the properties they do. In particular how the relative amounts and arrangements of the minerals that comprise them determine their physical properties.” ^[2]

It is within this ‘why‘ that I think academic petrophysics can thrive. Petrophysics has its roots in understanding why rocks exhibit the physics that they do, and this is not limited to sands and mudstones (shales). While the majority of downhole tools are still biased toward characterising reservoir (sandstones) and cap rocks (mudstones) for hydrocarbon prospecting there are so many other useful tools, data and applications out there where petrophysical analysis can make a major contribution (more on that in the next blog).

All of the statements above are my own opinion.

Laurence Phillpot

1. Archie, G. E., 1942. The electricalresistivity log as an aid in determining some reservoir characteristics, Trans.AIME, 146, 54–67.
2. Kennedy, M.,2015. Practical petrophysics (Vol. 62). Elsevier.
3. Thomas, E.C.,1992. 50th Anniversary of the Archie Equation: Archie Left More Than Just anEquation. Log Analyst May–June, 199-205.
4. Archie, G.E.,1950. Introduction to petrophysics of reservoir rocks. AAPG bulletin, 34(5),pp.943-961.