I have been fortunate during most of my career to have worked for a major oil company (Shell) which has been at the forefront of petrophysics.
I am therefore grateful to Shell for providing the training and environment that has made this book possible.
Toby Darling was educated at The Pilgrim’s School and Winchester College before gaining a scholarship to Wadham College, Oxford, where he graduated with a degree in physics in 1982.
For readers who do not have a mathematics, engineering, or physics degree, some of the basic mathematical principles assumed in this book may be problematic.
Therefore, this Appendix is designed to provide a fuller explanation of some of the theoretical derivations used in the chapters.
The trajectory of a deviated well may be described in terms of its inclination, depth, and azimuth. The inclination of a well at a given depth is the angle (in degrees) between the local vertical and the tangent to the wellbore axis at that depth (Figure 13.1.1).
The convention is that 0 degrees is vertical and 90 degrees is horizontal. Parts of a degree are given in decimals, rather than minutes and seconds. Gravity varies with latitude, and its direction may be influenced by local features such as mineral deposits and mountains, as well as the Earth’s rotation.
Homing-in techniques are used when the position of one borehole relative to another needs to be known. Reasons why relative positions may be important are:
The principal methodology available for homing in comprises electromagnetic and magnetostatic techniques. While there has been research in the past on acoustic homing-in techniques, these have not been found to be successful.
Consider a gas for which the behavior is as shown in Figure 11.1.1. The line dividing the regions where the substance is liquid or gas is called the vapor pressure line. Hence, crossing the line from left to right, the liquid boils and becomes a gas; and in crossing from right to left, the gas condenses to form a liquid.
Above the critical point (defined by Pc and Tc), there exists only a fluid phase with gas indistinguishable from liquid.
The purpose of this chapter is to provide readers with a convenient reference for production geology as it relates to the petrophysicist’s daily tasks, though not intended to be a comprehensive guide to the wider discipline.
The interface between the petrophysicist and production geologist is crucial in ensuring that:
Equity determination becomes necessary when part of an accumulation extends across a boundary and becomes subject to different conditions.
Such a boundary may be a result of (a) different ownership of the acreage, such as occurs in the North Sea, where different groups of companies control different blocks, or (b) international boundaries.
In either of these cases, it becomes necessary to determine the relevant amounts of hydrocarbons lying on either side of the boundary. Typically, following an equity determination there will be a unitization agreement, whereby a single commercial unit is formed with the aim of optimizing the total recovery of the field.
It is important for petrophysicists to have a feel for the economic impact of the work they are performing and whether or not the cost of running a certain log is really justified in view of the economic benefit it will ultimately bring. In this chapter, some considerations and tools for assessing this will be explained.
While rock mechanics can be a very complicated subject, there are a few basics that all petrophysicists will need in their day-to-day work, which will be covered here.
In a normal reservoir, the formation rock is subject to greatest stress from the overburden. This stress arises from the weight of rock above and can be measured by integrating the density log to surface.
Since density logs are not usually run to surface, a common working assumption is that the overburden stress is approximately 1 psi/ft.
While one part of petrophysics is concerned with the interface with geologists and reservoir engineers in order to produce reservoir models, a further interface exists with the seismologists to ensure that the well data are used to help calibrate and understand seismic properties.
While the preparation of synthetic seismograms to tie log-formation tops with seismic horizons is a long-established technique, recent advances in far-offset seismic processing, combined with the ability to measure shear-sonic transit times, have opened up a lot of new possibilities for facies and fluid determination from seismic data.
In this chapter some of these techniques will be discussed.
Shales can cause complications for the petrophysicist because they are generally conductive and may therefore mask the high resistance characteristic of hydrocarbons.
Clay crystals attract water that is adsorbed onto the surface, as well as cations (e.g., sodium) that are themselves surrounded by hydration water.
This gives rise to an excess conductivity compared with rock, in which clay crystals are not present and this space might otherwise be filled with hydrocarbon.