Formation evaluation

In petroleum exploration and development, formation evaluation is used to determine whether a potential oil or gas field is commercially viable. Essentially, it the process of "recognizing a commercial well when you drill one".

Only in rare, catastrophic cases and in Hollywood movies, do oil and gas wells 'come in' with a fountain of gushing oil. In real life, that is a 'Blow Out'—and usually also a financial and environmental disaster.

Modern rotary drilling uses a heavy mud as a lubricant and as a means of producing a confining pressure against the formation face in the borehole, preventing blowouts. This is a double edged sword—mud filtrate soaks into the formation around the borehole and a mud cake plasters the sides of the hole. These factors obscure the possible presence of oil or gas in even very porous formations. Further complicating the problem is the widespread occurrence of small amounts of petroleum in the rocks of many sedimentary provinces. In fact, if a sedimentary province is absolutely barren of traces of petroleum, one probably is foolish to drill there.

The formation evaluation problem is a matter of two things:

1. Knowing the lower limits for porosity, permeability and water saturation that work for a particular formation or pay zone, in a particular geographic area, in a particular economic climate.
2. Determining if the formations in the well under consideration exceed these lower limits.

It is complicated by the impossibility of directly examining the formation. It is, in short, the problem of looking at the formation indirectly.

Table of contents 1 Formation evaluation tools: 1.1 Coring: 1.2 Mud logging: 1.3 Wireline logging: 1.4 Electric Logs: 1.5 Porosity Logs: 1.6 Lithology Logs - SP and Gamma Ray: 2 Interpreting the Tools:

Formation evaluation tools:

Tools to detect oil and gas have been evolving for over a century. The simplest and most direct tool is well cuttings examination. Some older oilmen ground the cuttings between their teeth and tasted to see if crude oil was present. Today, a wellsite geologist uses a low powered stereoscopic microscope to determine the lithology of the formation being drilled and to estimate porosity and possible oil staining. A portable ultraviolet light chamber or "Spook Box" is used to examine the cuttings for fluorescence. Fluorescence can be an indication of crude oil staining, or of the presence of fluorescent minerals. They can be differentiated by placing the cuttings in a solvent filled watchglass or dimple dish. The solvent is usually carbon tetrachlorethane Crude oil dissolves and then redeposits as a fluorescent ring when the solvent evaporates. The written strip chart recording of these examinations is called a sample log.

Well cuttings examination is something of a learned skill. During drilling, chips of rock, usually less than about 1/8 inch (6 mm) across, are cut from the bottom of the hole by the bit. Mud, jetting out of holes in the bit under high pressure, washes the cuttings away and up the hole. During their trip to the surface they may circulate around the turning drillpipe, mix with cuttings falling back down the hole, mix with fragments caving from the hole walls and mix with cuttings travelling faster and slower in the same upward direction. They then are screened out of the mudstream by the shale shaker and fall on a pile at its base. Determining the type of rock being drilled at any one time is a matter of knowing the 'lag time' between a chip being cut by the bit and the time it reaches the surface. A sample of the cuttings taken at the proper time will containg the current cuttings in a mixture of previously drilled material. Recognizing them can be very difficult at times, for example after a "bit trip" when a couple of miles of drill pipe has been extracted and returned to the hole in order to replace a dull bit. At such a time there is a flood of foreign material knocked from the borehole walls.

Coring:

One way to get more definite samples of the formation at a certain depth in the well is coring. There are two techniques commonly used at present. The first is the "whole core", a cylinder of rock, usually about 3" to 4" in diameter and, with good luck, up to 50 feet to 60 feet long. It is cut with a "core barrel", a hollow pipe tipped with a ring shaped, diamond chip studded bit that can cut a plug and retain it in a trip to the surface. If no shales or fractures are encountered, the full 60 foot length of the core barrel can be filled. More often the plug breaks while drilling, usually at the aforementioned shales or fractures and the core barrel jams, very slowly grinding the rocks in front of it to powder. This signals the driller to give up on getting a full length core and to pull up the pipe.

Taking a full core is an expensive operation that usually stops or slows drilling for at least the better part of a day. A full core can be invaluable for later reservoir evaluation. One of the tragedies of the oil business is the huge amount of money that has been spent for cores that have been lost because of the high cost of storage. Once a section of well has been drilled, there is, of course, no way to core it without drilling another well.

The other, cheaper, technique for obtaining samples of the formation is "Sidewall Coring". In this method, a steel cylinder—a coring gun—has hollow-point steel bullets mounted along its sides. These bullets are moored to the gun by short steel cables. The coring gun is lowered to the bottom of the well and the bullets are fired individually as the gun is pulled up the hole. The mooring cables ideally pull the hollow bullets and the enclosed plug of formation loose and the gun carries them to the surface. Advantages of this technique are low cost and the ability to sample the formation after it has been drilled. Disadvantages are possible non recovery because of lost or misfired bullets and a slight uncertainty about the sample depth. Sidewall cores are often shot "on the run" without stopping at each core point because of the danger of differential sticking. Most service company personnel are skilled enough to minimize this problem, but it can be significant if depth accuracy is important.

A more serious problem with cores is the change they undergo as they are brought to the surface. It might seem that cuttings and cores are very direct samples but the problem is whether the formation AT DEPTH will produce oil or gas. Sidewall cores are deformed and compacted and fractured by the bullet impact. Most full cores that are taken from any significant depth expand and fracture as they are brought to the surface and removed from the core barrel. Both types of core can be invaded or even flushed by mud, making the evaluation of formation fluids difficult. In one core, taken by the author in a driving rainstorm, the analysis later indicated the core contained fresh water! The formation analyst has to remember that, without going down the well himself, all tools give indirect data.

Mud logging:

Mud logging is a process in which a small sample of the drilling mud that has returned from its trip down and out through the bit and then back up the hole is automatically analysed for hydrocarbon gases dissolved in the mud when an oil or gas zone is drilled. A gas chromatograph[?] is the instrument used in this analysis. The chromatograph record is usually accompanied by a sample log and a drilling time log which includes drill bit changes. This ensemble of information is called a Mud Log.

Wireline logging:

Electric Logs:

In 1928, the Schlumberger brothers in France developed the workhorse of all formation evaluation tools: the electric log. Electric logs have been improved to a high degree of precision and sophistication since that time, but the basic principle has not changed. Most underground formations contain water, often salt water, in their pores. The resistance to electric current of the total formation—rock and fluids—around the borehole is the sum of the volumetric proportions of mineral grains and conductive water-filled pore space. If the pores are partially filled with gas or oil, which are resistant to the passage of electrical current, the bulk formation resistance is higher than for water filled pores. For the sake of a convenient comparison from measurement to measurement, the electrical logging tools measure the resistance of a cubic meter of formation. This measurement is called resistivity.

Modern electric logs fall into several families of tools, with varying names, depending on the company providing the logging services. Conceptually, they can be divided into two groups: electric logs and induction logs. The first group includes tools such as Normal logs, Lateral logs and Guard logs. These all have electrodes on a long rubber covered sonde, 20' to 50' long. This is lowered to the bottom of the well on a steel cable. As the sonde is pulled back up the well, measurements are made by producing an electric voltage on the positive and negative electrodes of the sonde. This voltage causes a current to flow away from the negative electrode, through the mud, out into the formation and back to the positive electrode. Most tools use the amount of voltage needed to maintain a constant current as the measure of resistivity. This is presented as a traced line on a stripchart. The stripchart or electric log, shows resistivity versus depth. These instruments work well in fresh water muds.

In salt muds, which are very conductive, the current never gets into the formation but stays in the borehole. In oil-based muds or in air drilling, the current cannot get to the formation at all. In these situations, the second group of electric logs, the Induction logs, are used. These logs use a stong, momentary, electric current within the sonde to "induce" a secondary horizontal circular current in the formation around the borehole. When the exciting current is shut off, the magnetic field of the secondary current persists for a short time. This, in turn, induces a voltage in the wiring inside the sonde. This can be related to the resistivity of the formation. There is never a physical path between the sonde and the formation. The connection is entirely electromagnetic.

Until the late 1950s electric logs, mud logs and sample logs comprised most of the oilman's armamentarium. Logging tools to measure porosity and permeability began to be used at that time. The first was the microlog. This was a minature electric log with two sets of electrodes. One measured the formation resistivity about 1/2" deep and the other about 1"-2" deep. The purpose of this seemingly pointless measurement was to detect permeability. Permeable sections of a borehole wall develop a thick layer of mudcake during drilling. Mud liquids, called filtrate, soak into the formation, leaving the mud solids behind to -ideally- seal the wall and stop the filtrate "invasion" or soaking. The short depth electrode of the mudlog sees mudcake in permeable sections. The deeper 1" electrode sees filtrate invaded formation. In nonpermeable sections both tools read alike and the traces fall on top of each other on the stripchart log. In permeable sections they separate.

Also in the late 1950s porosity measuring logs were being developed. The two main types are: Sonic logs and radioactivity logs.

Porosity Logs:

Sonic logs use a pinger and microphone arrangement to measure the velocity of sound in the formation from one end of the sonde to the other. For a given type of rock, acoustic velocity varies indirectly with porosity.

Radioactivity logs are of two main types: Density logs and Neutron logs. Density logs use a gamma ray source to irradiate the sides of the borehole. Electrons in the material of the formation absorb some of the gamma rays and reemit them at a characteristic frequency. The number of gamma rays reemitted is directly proportional to the electron density of the formation. That is very nearly the same as the actual density or specific gravity of the formation. If one knows the density of the minerals in the solid part of the formation, it is a simple algebra problem to determine the amount of water or oil filled porosity. Gas filled porosity reads a little high.

Neutron logs irradiate the borehole walls with neutrons. They tend to pass through most minerals, but to be captured by hydrogen nucleii. When that happens a gamma ray is emitted: a gamma ray of capture. The number of these are proportional to the number of hydrogen nucleii present in the formation. For most lithologies, hydrogen is restricted to the oil or water in the pores. Gas, opposite to its effect on density logs, makes the neutron log read low. The Neutron-Density combination logs take advantage of this by presenting both curves on on log. Where they overlay one another, the pore fluid is water or oil, where they separate, it is gas. Density logs and Neutron logs are usually run together on the same sonde. A very common modern logging configuration is the 'Triple Combo' an Induction log run on the same sonde with a Density-Neutron log. This ideally provides all the parameters to evaluate a suspected pay section with only one logging run.

Both sonic and density-neutron logs give porosity as their primary information. Sonic logs read farther away from the borehole so they are more useful where sections of the borehole are caved. Because they read deeper, they also tend to average more formation than the density-neutron logs do. Modern sonic configurations with pingers and microphones at both ends of the log, combined with computer analysis, minimize the averaging somewhat. Averaging is an advantage when the formation is being evaluated for seismic parameters, a different area of formation evaluation. A special log, the Long Spaced Sonic, is sometimes used for this purpose. Seismic signals average tens to hundreds of feet of formation, so an averaged sonic log is more directly comparable to a seismic waveform.

Density-neutron logs read the formation within about four inches of the borehole wall. This is an advantage in resolving thin beds. It is a disadvantage when the hole is badly caved. Corrections can be made automatically if the cave is no more than a few inches deep. A caliper arm on the sonde measures the profile of the borehole and a correction is calculated and incorporated in the porosity reading. However if the cave is much more that four inches deep, the density-neutron log is reading little more than drilling mud.

Lithology Logs - SP and Gamma Ray:

There are two other tools, the SP log and the Gamma Ray log, one or both of which are almost always used when logging wells. Their output is usually presented along with the electric and porosity logs described above. They are indispensible as additional guides to the nature of the rock around the borehole.

The SP log, known variously as a "Spontaneous Potential", "Self Potential" or "Shale Potential" log is simply a voltmeter measurement of the voltage or electrical potential difference between the mud in the hole at a particular depth and a copper ground stake driven into the surface of the earth a short distance from the borehole. A salinity difference between the drilling mud and the formation water acts as a sort of natural battery and will cause several voltage effects. This "battery" causes a movement of charged ions between the hole and the formation water where there is enough permeability in the rock. The most important voltage is set up as a permeable formation permits ion movement, reducing the voltage between the formation water and the mud. Sections of the borehole where this occurs then have a voltage difference with other nonpermeable sections where ion movement is restricted. Vertical ion movement in the mud column occurs much more slowly because the mud is stagnant, not being circulated while the drill pipe is out of the hole. The copper surface stake provides a reference point against which the SP voltage is measured for each part of the borehole. There can also be several other minor voltages, due for example to mud filtrate streaming into the formation under the effect of an overbalanced mud system. This flow carried ions and is a voltage generating current. These other voltages are secondary in importance to the voltage resulting from the salinity contrast between mud and formation water.

The nuances of the SP log are still being researched. It is a simple tool but unlike the other logging tools, nature designs the measurement and we can only make it. In theory, almost all porous rocks contain water. Some pores are completely filled with water. Others have a thin layer of water molecules wetting the surface of the rock, with gas or oil filling the rest of the pore. In sandstones and porous limestones there is a continuous layer of water throughout the formation. If there is even a little permeability to water, ions can move through the rock and decrease the voltage difference with the mud nearby. Shales do not allow water or ion movement. Although they may have a large water content, it is bound to the surface of the flat clay crystals comprising the shale. Thus mud opposite shale sections maintains its voltage difference with the surrounding rock. As the SP logging tool is drawn up the hole it measures the voltage difference between the reference stake and the mud opposite shale and sandstone or limestone sections. The resulting log curve reflects the permeabilty of the rocks and, indirectly, their lithology. SP curves degrade over time, as the ions diffuse up and down the mud column. It also can suffer from stray voltages caused by other logging tools that are run with it. Older, simpler logs often have better SP curves than more modern logs for this reason. With experience in an area, a good SP curve can even allow a skilled interpreter to infer sedimentary environments such as deltas, point bars or offshore tidal deposits.

The Gamma Ray log is simply a measurement of naturally occuring gamma radiation from the walls of the borehole. The basic idea is that sandstones are largely nonradioactive quartz and limestones are nonradioactive calcite. Shales however may have a high content of radioactive potassium isotopes. Thus the presence or absence of gamma rays in a section of borehole is an indication of the amount of shale or clay in the surrounding formation. The Gamma Ray log is useful in holes drilled with air or with oil based muds. These wells can have no SP voltage. Even in waterbased muds, the gamma ray and SP logs are often run together. They comprise a check on each other and can indicate some unusual shale sections which may either not be radioactive, or may have an abnormal ionic chemistry. The gamma ray log is also useful to detect coal beds (which tend to concentrate small amounts of uranium). Finally, the gamma ray log will work inside a steel casing making it essential when a cased well must be evaluated.

Interpreting the Tools:

The immediate questions that have to be answered in deciding to complete a well or to plug and abandon it are: - Do any zones in the well contain producible hydrocarbons? - How much? - How much, if any, water will be produced with them?

The elementary approach to answering these questions uses the Archie Equation: