What is Petrophysics?
Spindletop was a PR disaster for the oil industry – not just because of the environmental damage, but also because it created a false impression that finding oil was easy. When the Lucas Gusher came in on 10 January 1910, photographs went around the world and a myth was born: if you drill through an oil accumulation, oil will flow to the surface. This myth is still believed by the majority of the general public.
The reality is horribly different. It is perfectly possible to drill through an accumulation of oil or gas and not realise it – particularly if the drillers are keeping the pressure in the wellbore higher than the formation pressure (‘overbalanced’). A quick thought experiment shows the problem – you are peering down a hole that is about a metre in diameter at the surface, but only 15 to 20 cm across at the bottom. The big problem is that the hole could be anything from 2,000m to 5,000m long, and you have very little information: just the rate that the drill bit cut through the formations at various times, and the cuttings – gravel-sized chips of rock that have been flushed out of the wellbore by the drilling mud. It is like standing at Piccadilly Circus looking into a pipe that ends at King’s Cross Station and trying to count the number of people in the concourse.
So how do you determine what’s down there? The blisteringly simple idea that the Schlumberger brothers came up with was to lower an electrical instrument down the wellbore and take regular readings of some physical property of the rock. If this was done at regular intervals, then the changes in properties would give a clue as to changing rock layers in the subsurface. On 5 September 1927, the first wireline electric log was run on the Diefenbach 2905 well in Alsace and the science of petrophysics was born. That first log measured the potential difference between each layer and the surface.
Today, Schlumberger and the other service companies such as Baker Hughes and Halliburton have a bewildering variety of tools that can measure natural radioactivity, porosity, density, resistivity, and the dip of layering. Most importantly, these data can be analysed to give us the fluid content of the rock. Petrophysics research in industry and academia is leading to the development of new tools and also to new ways of analysing and interpreting the data. It is a global, multi-billion dollar business.
What Does a Petrophysicist Do?
Petrophysics is defined as the study of the physical and chemical properties of rocks and their interaction with fluids, but what do petrophysicists in the oil and gas industry actually do? Petrophysics provides the link between sub-surface disciplines. While the reservoir architecture is defined by geophysics and geology, the reservoir properties (porosity, permeability and fluid distributions) are determined by petrophysical interpretation based on well data. Petrophysicists plan and supervise log and core data acquisition, ensure data quality, assemble databases of all types of well data and make interpretations to address the needs of their sub-surface colleagues, through all stages of oil and gas exploration and production. They provide real time operational support and interpret wells to support exploration prospect evaluation.
In field appraisal and development, they integrate all types of data (including log and core data, fluid sample analyses, formation pressure data, well test and production data) to populate static and dynamic models of the reservoir and determine volumes of hydrocarbons in place. Petrophysicists also advise on alternative production development options.
During the development and production phases, petrophysicists plan new wells and interpret logs acquired in producing wells to monitor production and the movement of fluid contacts, supporting reservoir management.
Where Do They Come From?
There is only one problem – there are not enough petrophysicists being trained to support this sophisticated enterprise.
Traditionally, petrophysics has been taught as a small part of postgraduate degree programmes in petroleum geology or engineering, but this is not enough to develop a real specialism in the subject. Working petrophysicists are recruited from a wide range of technical backgrounds: geology, electronics, mechanical engineering, physics, and mathematics. Practitioners commonly start their careers in field operations as logging engineers or mud-engineers before joining operating companies. Petrophysical expertise is then built on the job by a combination of work experience and short courses. Such diversity of experience and technical backgrounds works well to address the wide range of activities and knowledge encompassed by petrophysics and formation evaluation. However the development of experienced petrophysicists this way takes anything between three and eight years, depending on the variety of work experience and peer support that is available.
A fresh approach to training new-start petrophysicists from diverse technical backgrounds is being launched as an MSc programme by the University of Aberdeen’s School of Geosciences and Senergy in partnership. The aim is to provide a development route for petrophysicists, where individuals already working in industry will acquire a broad range of skills, knowledge and expertise over a period of two to three years. Candidates will be assessed via assignments, examinations and a dissertation.
Written and interpersonal communication is a key requirement for petrophysicists; this will be emphasised in the programme. Since most of the petrophysical interpretation is performed on software which allows integration and visualisation of data and facilitates mathematical and statistical modelling of reservoir properties, some skill in simple algebra and statistics is required.
The Future of Petrophysics
Specialist petrophysics may involve development of new logging tools or interpretation methods; support for geophysical studies of rock properties; and rock mechanical studies for optimisation of drilling. Such research problems are being tackled by industry- academia partnerships around the world and the lab in Aberdeen University is a good example.
The guiding principle for the research projects in the Aberdeen University Petrophysics Laboratory is to use rock samples from outcrop analogues or core, and integrate the petrophysical measurements with quantitative, predictive subsurface models. Recent projects in the laboratory have addressed:
- anisotropy of permeability in faulted sandstones, using samples from Scotland, and petrophysical variations in carbonates across normal fault zones, with data from Malta. Both projects were funded by Total E&P UK and BG International;
- the petrophysical characterisation of reservoir quality in fold and thrust belt carbonates in Kurdistan, funded by the Ministry of Higher Education and Research, Iraq;
- the seismic anisotropy of evaporites as a function of mineralogy and crystallographic fabric with samples from Nova Scotia; a project funded by the National Board of Science and Technology and Ministry of Energy, Mexico.
Ph.D. students working on these projects have been trained in experimental measurement methods, statistical data analysis and rock physics modelling. All of their work is either submitted for publication in international peer-reviewed journals, or in preparation for submission.
Future projects scheduled for 2014 include a study of the rock physics of fractured and altered basement reservoir analogues, an investigation into coupled poroelasticity and permeability of fault zones in sandstones and limestones, and analysis of spatial and temporal evolution of porosity around fault zones in granular rocks.
While the inherent length-scale related limitations of core plug datasets are widely known, we believe that repeatable, quantitative laboratory measurements of rock properties provide useful knowledge in our quest to understand, model and predict the behaviour of rocks and fluids in the subsurface. In addition, the practical acquisition and analysis of these data is in itself a valuable training exercise for our students.
In the Petrophysics Laboratory there is equipment for helium porosimetry on core plugs and mercury injection porosimetry on rock chips and powders up to 15 cm3 in volume, as well as a range of permeameters, using either nitrogen or water as the permeant, and pressure vessels capable of confining pressures up to 250 MPa (equivalent to about 8 km depth). It is possible to measure ultrasonic acoustic velocities, for both P- and S-waves, in dry core plugs. The lab is commissioning a new system to provide fluid-saturated ultrasonic velocity data from up to five 1.5 inch diameter core plugs at confining pressures up to 70 MPa (about 3 km depth). A recent expansion in capability in the laboratory has been funded through Joint Industry Projects (JIPs) with Total and BG, and generous donations from the Aberdeen Formation Evaluation Society (AFES) – a chapter of the Society of Petrophysicists and Well Log Analysts.
An immediate challenge is to fund and purchase a new state-of-the-art scanning electron microscope (SEM) to replace the existing machine. This would be a high vacuum chamber capable of handling large sample sizes (e.g. 10 cm x 10 cm), complete with detectors for cathodoluminescence (CL), electron backscatter diffraction (EBSD) and energy dispersive X-ray analysis (EDX). Integrating laboratory petrophysical measurements with data from the new SEM will produce a step change in the capacity of the Aberdeen petrophysicists to quantify rock properties, with applications from reservoir quality, to rock physics and geomechanics.