BASIC ROCK PHYSICS
      Rocks, Oil, Gas, Water,  and Other Valuables

All of my 50+ year career has been involved with the science of Petrophysics, literally the physics of rocks, in some way or another. Petrophysics is a branch of Geoscience and intimately linked to geology, geophysics, and petroleum / mining engineering. There is no degree granted in pure petrophysics, so people in this field are often graduates of a closely related specialty and are self-taught from there.

Petrophysics is mainly used in petroleum exploitation, but also in defining mining and ground water resources.

To understand petrophysics, you need to understand rocks and the fluids they contain, how the earth's surface and subsurface change shape, and how pressure, temperature, and chemical reactions change rocks and fluids over eons of time. That's a tall order.

Microphotograph of a rock -- black colour is the porosity where
 oil, gas, and water can be held inside the rock
 

Rocks are formed in several ways, but usually end up as moderately flat layers, at least initially (mountain building comes later). As successive layers are laid on top of each other, the Earth builds a sequence of rocks with varying physical properties. Some layers will have open spaces, called pores or porosity, that contain fluids (water, oil, or gas). A rock on Earth with porosity cannot be "empty" -- they must contain something, even if it is only air.

Think of a porous rock as similar to a huge sponge full of holes that can soak up fluids. Although we often talk about "oil pools", these are not tanks of oil underground -- they are porous rocks. The porosity, or quantity of open space relative to the total rock volume, can range from near zero to as much as 40%. Obviously, higher values of this physical property of a rock are good news.

Some rocks have very little porosity and do not hold much in the way of fluids. These are often called "tight" rocks. Both tight and porous rocks can contain animal and plant residue that are ultimately transformed into hydrocarbons such as coal, oil, or natural gas that we can extract and use to power vehicles and heat our homes. As the plant and animal residues mature into oil or gas, they may migrate through porosity or natural fractures in the rock until trapped by a non-porous rock structure. Sometimes a rock only sources itself or an adjacent porous rock, so little migration occurs.
 

An anticline, the simplest form of petroleum trap

Rocks that are capable of holding  hydrocarbons in economic quantities are called reservoir rocks. Rocks in which the plant and animal residue has not been fully converted to useful hydrocarbons are called source rocks. Some rocks are both source and reservoir: others are barren of hydrocarbons, and some others may act as the trapping mechanism that keeps hydrocarbons from migrating to the surface and escaping.

A trap is what keeps oil and gas in the rocks until we drill wells to extract the hydrocarbons. Coal, being a solid, doesn't need a trap to be kept in place.

Reservoirs that contain oil or gas also contain water. The quantity of water relative to the porosity is called the water saturation. In the illustrations, the brown colour is solid rock grains and the space around the grains is the porosity. The black colour is the hydrocarbon and the white is the water, which forms a thin film coating the surfaces of each rock grain. This is a water-wet reservoir (left). In an oil-wet reservoir, the black and white colours are reversed (right).

Finding and evaluating the economics of such reservoirs is the job of teams of geoscientists and engineers in petroleum and mining companies. A petrophysicist, or someone playing this role, will be part of that team.

Once a useful accumulation has been found, drilling, completion, and production engineers take over to put wells on stream. Oil production may initially flow to surface due to the pressure in the reservoir. Some oil pools do not have enough pressure to do this and need to be pumped. Depending on the reservoir drive mechanism, some wells that start flowing will later need to be pumped. Water may be produced with the oil. It is separated and disposed of by re-injection into a nearby unproductive reservoir layer. You can't just dump the water in the nearest swamp.

 

  Aquifer Drive - Before and After Production        Gas Cap Drive        Gas Expansion Drive

An aquifer drive mechanism usually maintains the reservoir pressure for some time but may drop off gradually. Recovery factors vary from 30 to 80% of the oil in place. The oil water contact rises as production depletes the oil. A gas cap drive pushes oil out as the gas expands. Recovery factor is similar to aquifer drive. There may or may not be some aquifer support. the gas oil contact drops as the oil is depleted. Gas expansion reservoirs do not have aquifer or gas cap support. Gas dissolved in the oil expels oil into the well bore because the pressure at the well bore is below the reservoir pressure. Recovery factor is awful - usually less than 10%, but this can be improved to maybe 20% by injecting water nearby to increase or maintain the reservoir pressure. Water floods, carbon dioxide injection, and re-injection of produced gas or water can be used in nearly any reservoir to improve recovery efficiency.

Gas wells do not need pumps, but if they also produce water, a special process called artificial lift is used to get the water out. That water is also disposed of legally.

The economics of a reservoir varies with improving technology. Bypassed reservoirs, discovered and ignored years ago, are now economic due to technical improvements in drilling practices and reservoir stimulation techniques. Horizontal wells and deep water drilling are now common. The use of heat or steam to assist production of heavy oil or bitumen, and multi-stage hydraulic fracturing to stimulate production in tighter reservoirs are relatively new techniques and relatively economic today. Obviously the specific price of oil or gas after delivery to the customer plays an important role in how much effort can be expended to recover oil and gas from underground.

There is controversy, of course, about new technology. Just as the Luddites resisted the weaving machines in the early 1800's, modern Luddites insist that the old ways of oil and gas extraction are best, while at the same time complaining loudly about the price of gasoline at the pumps or the cost of electricity for their air conditioners. You can't have low-cost and low-tech at the same time.

BASIC PETROPHYSICAL CONCEPTS
"Last week, I couldn't spell Petrophysicist. Now I are one." That describes me in 1962 as I moved from Montreal to Red Deer, Alberta to run well logs for a company called Schlumberger. The word petrophysics had been coined 12 years earlier by a geologist named Gus Archie and it wasn't used much back in the day. Lately it has attained a certain cachet, denoting a professional level career path.

Illustration of a wireline logging job: logging truck with computer cabin, cable and winch (right), cable strung from winch into drilling rig derrick and lowered into bore hole, with logging tool at the end of the cable. Logs are recording while pulling the tool up the hole. Logs can also be run with special tools located at the bottom of the drilling string, or conventional tools can be conveyed on coiled tubing.

What is a "well log" you ask. It is a record of measurements of physical properties of rocks taken in a well bore, usually drilled for oil or gas, but possibly for ground water or minerals. Think of a ship's log. The first record of such a log dates back to 1846 when Lord Kelvin measured temperature versus depth in water wells in England, from which he deduced that the Earth was 7000 years old. The fact that he was wrong is not important. Log analysis is an imperfect science.

The first logs for oil field investigation were run by the Schlumberger brothers, Marcel and Conrad,  in 1928 in Pechebron, France. Soon, the service migrated to North and South America, Russia, and other locations in Asia. At that time, the only measurement that could be made was of the electrical resistivity of the rocks. High resistivity meant porous rock with oil or gas, or porous rock with fresh water, or tight rock with very low porosity. Low resistivity meant porous rock with salty water or shale. Take your pick. Local knowledge helped.

One virtue of the well log was that the top and bottom of each rock layer could be defined quite accurately. When the log and depths were compared to the rock sample chips created by the drilling process, a reasonable geological interpretation might be possible, but was far from infallible. 

By 1932, the spontaneous potential (SP) measurement was added. The analysis rules expanded: low SP meant shale, or tight rock, or fresh water. High values meant salt water with or without oil or gas in a porous rock. The resistivity could then be used to decide on water versus hydrocarbons. Perfect. Except there were lots of shades of grey and the SP was not always capable of defining anything.

Logs from 1932 in Oil City-Titusville area, Pennsylvania, the location of Edwin Drake's "First Oil Well" (in the USA - 6 other countries had oil wells predating this one). His well was only 69 feet deep, so it penetrated just to the top of these logs, which found deeper and more prolific reservoirs. Each pair of curves represents the measured data versus depth for one well. The SP is the left hand curve of each pair; deflections to the left (shaded) show porous rock. The resistivity is the curve on the right of each pair. Deflections to the right (shaded) show high resistivity, and when combined with a good SP deflection, indicate oil zones. Some good quality rocks in this example do not have high resistivity and are most likely water bearing.

The gamma ray log appeared in 1936. The rules were easy: low value equaled porous reservoir or tight rocks. High values were shale. It said nothing about fluid content.

By 1942, Gus Archie had defined a couple of quantitative methods that turned analysis into a mathematical game, instead of just some simple rules of thumb. His major work established a relationship between resistivity, water saturation, and porosity. If we knew porosity from rock samples measured in the lab, and a few other parameters, we could calculate water saturation from the resistivity log values. This was really new news.

He even attempted to calculate porosity from the resistivity log. This worked in high quality (high porosity) reservoirs but had problems in low quality rocks or heavy oil.
 

This is an example of a modern sonic log with gamma ray and caliper curves (far left), shear and compressional sonic travel time curves (middle) and sonic waveform image log (right). Depths are shown in the narrow track next to the gamma ray curve.

Just after 1945, a method that investigated the response of rocks to neutron bombardment became available. The neutron log was the first porosity indicating well log. High values meant low porosity or high porosity with gas. Low values meant high porosity with oil or water, or shale. Add the gamma ray log, SP, and resistivity and again the world was perfect, except for all those shades of grey. Calibrating the response to porosity depended on a lot of well bore environmental parameters (hole size, mud weight, temperature) so it was not terribly accurate.

It wasn't until 1958 that the measurement of the velocity (or travel time) of sound through rocks in a well bore was achieved. It turned out that the travel time was a linear function of porosity and a few other factors.

 

Shortly after 1960, another porosity indicating log appeared that measured the apparent density of the rocks. Porosity was a linear function of density -- higher density meant lower porosity.

 

Both sonic travel time and density as measured by these logs could be transformed into moderately accurate porosity values, using the gamma ray to discount shale, and the resistivity to distinguish between salty water and oil. Fresh water was still a problem and gas zones could only be located if a neutron log was also run.

This was the state of petrophysics when I entered the scene in 1962. The rules were obvious, the math was easy. And running the logging tools into the well bore meant lots of travel. I loved the job. There were no computers on every desk, calculators were bigger and heavier than typewriters, so the quantitative work was done with penciland paper or sliderule. Anybody know what a sliderule is?

Later, with sidetracks into seismic data processing, reservoir engineering, project management, and seismic data center management, I finally noticed that petrophysics was the underlying foundation of much of geology, geophysics, and reservoir engineering.

That realization led me to my consulting and teaching career. I got to see a lot of the world, wrote a dozen or more software packages, analyzed the log data from thousands of wells, and saw even more more of the world,

 

This may be the only editorial cartoon ever published in a newspaper (Calgary Herald, circa 1974 - 75) concerning petrophysical analysis. That`s me peering down a borehole on Melville Island NWT, estimating the gas reserves to be "four trillion cubic feet". The final value was closer to 17 trillion. I was the log analyst and logging supervisor on about 140 wells in the Canadian Arctic across a 10 year period. We didn`t use our eyeballs to look into the wellbores directly, of course; we used well logs and calcualtions based on those measurements
to do what our eyes could not.  

 

Mainframe computers and dumb terminals were really unfriendly environments. It was apparent that some portable form of computer was needed to do the math and make pretty images of our results to show to management and team members. Five years before the IBM-PC, the HP9825 calculator became a computer and LOG/MATE, "The Friendly Log Analysis System", was born (1976). Today, far more sophisticated and powrerful systems are common, but LOG/MATE was the first.

 

    
Advertisemenys for two of my forays into the software business:
LOG/MATE (1976I and META/LOG (1986)

We now call the business "Integrated Petrophysics" because we use much more than log data to get our answers. Lab data from core analysis, such as porosity, permeability and grain density, are critical input parameters used to calibrate our work. More exotic lab measurements have become more common as we move into unconventional reservoir types like shale gas and tight oil prospects.

TYPES and USES OF WELL LOG and LABORATORY DATA
The table below might not mean too much to someone who is not in the oil and gas business, but it will give everyone an idea of the scope of work, wealth of data types, and the multiplicity of uses that petrophysical data  can be applied to.

DATA USES -  General Outline     Petrophysical Analysis     Geophysical Applications     Geological Applications     Drilling Applications     Engineering Applications     Completion Applications     Production Applications DATA USES - Petrophysical Analysis     Shale Content     Porosity     Lithology     Water Saturation     Movable Hydrocarbon     Irreducible Water Saturation     Water Cut / Relative Permeability     Permeability / Productivity     Fracture Intensity / Orientation     Fluid Contacts - Original and Dated     Productive Intervals     Swept Zones     Pore Volume / Hydrocarbon Pore Volume     Flow Capacity     "Net Pay"     Where Are The Reserves?     How Much Does This Well Contribute? DATA USES - Geophysical Applications     Velocity and Density     Seismic Modelling ?     Synthetic Seismograms     Editing Logs for Seismic        Bad Hole Condition        Invasion        Missing Log Data     Modeling Hypothetical Rock Sequences     Modeling Hypothetical Fluid Content     Vertical Seismic Profiles     Seismic While Drilling     Calibrating Seismic Inversion     Calibrating Seismic Attributes     Amplitude versus Offset Models     Is the Seismic Interpretation Realistic? DATA USES - Geological Applications     Reservoir Description     Structure and Stratigraphy     Dip and Direction     Sedimentary Models     Sequence Stratigraphy     Bedding Type / Orientation     Mineralogy     Depositional Environment     Tectonic Structures     Sedimentary Structures     Multi-well Analysis     Cross Sections / Fence Diagrams     3-D Visualization     Correlation and Mapping     Geostatistics        Conventional        Fractal      What Are the Geologic Risks? DATA USES - Drilling Applications     Designing Vertical Wells     Designing Deviated Wells     Designing Horizontal Wells     Drilling Prognosis     Overpressure     Stress Regimes / Fractures     Borehole Stability     Bit Selection     Cost Estimates     Where Are The Drilling Risks? DATA USES - Engineering Applications     Calculating Reserves     Calculating Productivity     Calculating Cash Flow     Reservoir Simulation / Modeling     History Matching     Production Prediction     Production Optimization     Economic Analysis     Is The Well/Pool/Project Any Good?  DATA USES - Completion Applications     Perforating Interval     Stress Regime / Orientation     Hydraulic Fracture Design     Acidizing / Other Treatments     Sand Control     Maximize Productivity     Are There More Targets?     Is production maximized? DATA USES - Production Applications     Through Casing Reservoir Description     Fluid Identification     Cement Evaluation     Casing Inspection     Flow and Production Analysis     Gas Leak Detection     How Do We Repair The Well?

DATA TYPES ­ General Outline     Seismic     Magnetics     Gravity     Radiometrics     Air / Satellite Images     Well History        Tops, Tests, Cores, Perfs, Logs, Status     Logs - Many Variations     Cores - Many Types of Analyses     Data Gathering Considerations     Data Digitizing     Project Planning     Quality Control   DATA TYPES - ENGINEERING     Fluid Properties     Pressure Transient     Wellhead / Bottomhole Pressures     Production History     Injection History     Completion Diagram     Facilities In Place / Needed     Economics / Costs / Prices DATA TYPES ­ While Drilling     Sample Descriptions     Drilling Records     Mud Logs     Core Descriptions     Measurements While Drilling     Logging While Drilling     Seismic While Drilling DATA TYPES ­ After Drilling     Conventional Open Hole Logs     Image Logs     Thin Bed Tools and Processing     Petrophysical Analysis Results     Geological Correlations / Maps     Seismic Analysis / VSP     Test Results     Core Analysis Results DATA TYPES - Open Hole Logs     Resistivity and Resistivity Imaging     Acoustic and Full Wave Acoustic     Natural and Spectral Gamma Ray     Formation Density and Litho Density     Neutron Porosity     Dipmeter and Deviation Surveys     Formation Imager and Televiewer     Electromagnetic     Nuclear Magnetic Resonance     Induced Gamma Ray Spectroscopy     Pulsed Neutron and Activation     Pressure Profiles / Sample Taker     Sidewall Cores DATA TYPES ­ After Completion     Cased Hole Logging     Reservoir Description Logs     Production Logs     Casing / Cement Evaluation Logs     Bottom Hole Pressure Survey     Well Test Results     Initial Production / AOF / IPR DATA TYPES ­ Special Cases     Horizontal / Deviated Wells     Logging Through Drill Pipe     Coiled Tubing Logging DATA TYPES ­ Core Data     Conventional Core Analysis        Permeability, Porosity, Saturation        Grain Density Lithology Description     Special Core Analysis        Electrical Properties        Capillary Pressure        Relative Permeability        Thin Section Petrography        Scanning Electron Micrographs        X-Ray Diffraction        Infra-red Mineralogy     Core Imaging        White Light        Ultra Violet Light        X-Ray        CT Scans DATA TYPES ­ Fluid Properties     Density, Viscosity     Water Resistivity, Chemical Analysis     Oil / Gas Analyses DATA TYPES ­ Pressure Transient     Pressure versus Time     Buildup or Drawdown     Horner / Ramey Plots     PBU Modeling / Curve Fitting     Static Wellhead Pressure     Static Bottom Hole Pressure DATA TYPES ­ Production Data     Oil / Gas / Water Rates     Changes With Time     Completion History     Well / Pool / Reservoir Summaries     Deliverability Analysis Results

BASIC VISUAL LOG ANALYSIS
I have been teaching the practical application of petrophysics since 1967. The seminars always start with "What is a log?" and "What do we do with them?". The first question was answered in the previous section. Here, I'll try to provide an answer to the second, just as it s done in the seminar. We use the rules as developed over the last 80 years and apply them to the individual log curves as we see them on paper or on a computer screen.

The step by step procedure using Crain's Rules will reduce the complexcity considerably and give you a straight forward path toward your goal. The illustration below is to give you a few of the basic rules in one single illustration. Further on there is a more detailed coverage of the Rules.

 

Lets start with just 3 curves - the gamma ray (GR), resistivity, and a porosity indicating log (a sonic in this example). The GR is at the far left and the sonic is the left edge of the red shading. The resistivity and sonic have been overlaid to make it easier to see the shape of the two curves relative to each other.

 

Basic Rule "A": When GR (or SP) deflect to the left, the zone is clean and might be a reservoir quality rock. When GR deflects to the right, the zone is usually shale (not a reservoir quality rock). There are exceptions to this rule, of course.

 

Basic Rule "B": Porosity logs are scaled to show higher porosity to the left and lower porosity to the right. Clean and porous is good, so compare the GR to the porosity log and mark clean+porous zones.

 

Basic Rule "C": Resistivity logs are scaled to show higher resistivity toward the right. Higher resistivities mean hydrocarbons or low porosity. Low resistivity means shale or water zones. So clean+porous+high resistivity are good. There are exceptions to this rule too.

 

The exceptions are what makes the job interesting. There are low resistivity pay zones, radioactive (high GR) pay zones, gas shales, oil shales, coal bed methane, and low porosity zones that produce for years.
 

To learn more, go to Crain's Petrophysical Handbook