Cuttings Analysis

8/11/2019 Cuttings Analysis

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SAMPLE DESCRIPTION BASICSThe wellsite geologist is responsible for inspection of rock samples as they are gatheredand preparation of the sample log. He may delegate this to the mud loggers if they are onsite. The wellsite geologist also is responsible for a myriad of other tasks, including orderinggeological supplies, picking the location of cores and tests, packing and shipping recoveredcores and fluid samples, calling for and supervising wireline logging operations, velocity

surveys, mud logging crews, and other survey personnel, liaison with drilling crews andhome office, and above all, must be a good cribbage or gin rummy player.

The sample log, often called the stratigraphic log, strat log, or geology log, is a record of therock samples retrieved from the drilling mud, and is one of the primary sources of rock andfluid descriptions for the well. It consists of a verbal description of the rock type as well asqualitative or interpretive data concerning evidence of the fluid content of the rock. Sampledescription is sometimes called formation evaluation, but this term usually covers a broaderscope, including drill stem test a and log analysis.

Cuttings are collected, washed, and described at 5 to 20 foot (2 to 5 meter) intervals unlessextremely rapid penetration rate makes this impractical. In such cases, they would then becollected as often as possible, but no less than once for each time drilling is resumed after apipe connection. A constant sampling rate is chosen, depending on the lithology expected.

Sample chips are quite small, so large scale features such as fractures, bedding planes, andfossils often cannot be seen. Samples can be contaminated by rock sloughing from above,or may be lost due to lost circulation, pulverization, or careless well site procedures.


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Sample descriptions will include anestimate of visual porosity, porositytype, grain size, sorting, roundness,and colour, as well as shaledistribution, brittleness, laminations,and colour.

Primary porosity is the porositydeveloped by the originalsedimentation process by which therock was created. In reports, it isoften referred to in terms ofpercentages, while in calculations itis always a decimal fraction.

Secondary porosity is created byprocesses other than primarycementation and compaction of thesediments. An example of secondaryporosity can be found in the solution

of limestone or dolomite by groundwaters, a process which createsvugs or caverns. Fracturing alsocreates secondary porosity.Dolomitization results in theshrinking of solid rock volume asthe material transforms from calciteto dolomite, giving a correspondingincrease in porosity.

Porosity Types: a. intergranulas, b.

Sucrosic,

c. Moldic or Ool i t ic, d. Matrix or

Chalky,

e. Moldic or Vug gy, e. Fracture

porosi ty .

Intergranular porosi ty is prim ary, the

balance are secondary.

For details,See Also:Shale BasicsPorosity Basics

Oil show indicators are found by examination of the rock samples for oil stain, bleeding,fluorescence, or cut. Stain is the trace of asphaltic material left behind on drill cuttings afterthe oil has been washed off during drilling. Stain left by high quality oil has a typical

iridescent sheen, visible in normal light. Bleeding is the exudation of oil from the pores dueto pressure release as the sample is brought to the surface.

Fluorescence represents oil's distinctive ability of emitting light in the visible range whenexposed to ultraviolet light. Unfortunately, quite a number of minerals and many refinedproducts are fluorescent, so there is a certain amount of technique involved indistinguishing between primary hydrocarbons and refined products or fluorescent minerals.

A simple chemical test may be carried out to determine whether fluorescence in drill
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cuttings is a result of oil or some fluorescing mineral. This is easily and quickly establishedby immersing some of the drill cuttings in a petroleum solvent (chlorothene, trichlorothene,ether, or acetone). If the fluorescence is derived from mineral sources, the minerals will notdissolve in the solvent and the solvent will remain colorless under ultraviolet light.

However, if hydrocarbons are present in the rock, they will disseminate into the solvent,giving the entire solvent a distinctive color under ultraviolet light. This sheen under UV lightis called cut and the colour of the cut indicates the quality of the oil. Pale blue-white is highgravity (light) oil, yellow is medium gravity, and orange-brown for low gravity (heavy) oil.

Lightly washed wet samples should also be examined, and not dried, because light oils andcondensates may evaporate. Under UV light, a differentiation should be made betweenmineral fluorescence, natural hydrocarbon fluorescence, and fluorescence from introducedoils and grease. Natural hydrocarbon fluorescence will usually be lithology specific, whileintroduced hydrocarbon fluorescence will be associated with all lithologies. A note ofpercentage of cuttings exhibiting natural hydrocarbon fluorescence and the color andintensity should be made. Mineral fluorescence is determined by the test for cut.

Under ordinary light, the oil stain or oil bleeding from the sample may be visible. It should benoted for its volume and its intensity, and efforts made to distinguish introduced oil.Bleeding often indicates low permeability. Light oils are more prone to exhibit irridescence,while dark stains tend to indicate heavier crudes.

To test for hydrocarbon cut, a small sample is placed in a spot plate, the solvent isintroduced, and the color, intensity, and rate of cut are observed in ordinary and ultravioletlight. The sample is crushed and the test is repeated. If there is no cut at this stage, anyfluorescence is probably mineral derived. Generally, the heavier the oil, the greater the cut;however, asphaltic oils show a greater cut than paraffinic oils of the same gravity.

The chromatography of the oil should be noted. A drop sample of hydrocarboncontaminated solvent should be placed on filter paper and observed under ordinary andultraviolet light. The characteristics of any separation should be noted, including the numberof rings formed.

Larger samples, or a number of smaller samples, may be placed in a test tube and examinedin a similar manner as in a spot plate; the sample should be crushed and the solvent

introduced. A standard method should be employed so comparisons can be made withnearby wells, ie., 1 cc of crushed cuttings in 2 cc of solvent in a standard 10 cc test tube.

A control test tube of 3 cc solvent should also be set up, and a comparison against whitepaper is then observed. Again, a drop of hydrocarbon contaminated solvent should betested for chromatography as before.

To test for gas in the cuttings, place an unwashed sample in a blender or food processor,add water, and analyze the resulting gas. This is done by running a sample of the headspace


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gas into the gas chromatograph operated by the mud logging crew.

Sample containing carbonates, either as a main constituent or as a cement, can be testedfurther for oil by adding a small amount of hydrochloric acid. Hydrocarbons present in asample, either natural or introduced, will cause carbon dioxide bubbles, released by theaction of hydrochloric acid on carbonates, to enlarge by forming an oil film around thebubbles. Thus, the reaction is more prone to frothing. This test is very sensitive, but it doesnot differentiate between natural and introduced hydrocarbons.

The sample log can take many forms: a written narrative, a graph versus depth with aschematic drawing (along with abbreviated verbal descriptions), or a mud log, whichdescribes the rock samples, as well as fluids, recovered in the mud. Drilling data such aspenetration rate may also be included.

Example of a sample descr ip t ion log

Below is a a sample of the colours and symbols in use for lithology descriptions on modernsample description logs.


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Colour codes and sym bols used to plot l i tho logy and accessory features on a sample

descr ip t ion log

All sample log results are reported daily by telephone, FAX, or email and recorded on astandard report form for distribution within the oil company. Hydrocarbon shows areespecially desirable; a show is defined as any indication of hydrocarbons, ie., stain,fluorescence, or cut, or any increase in drilling mud gas of reasonable percentage abovebackground.

LAG TIMEDepth information is obtained from the driller's log, which records depth versus the time ofday. However, these depths cannot be used directly. We wish the mud log data to bepresented at the depth of the drill bit, but the mud log measurements are made at the

surface. The time it takes for the mud to move from the bit to the surface must be accountedfor in positioning samples and gas kick data on the log. This time is called the lag time anddepends on the velocity of the mud in the annulus between the drill pipe and the rock. Thisin turn depends on the mud pump speed and displacement, which are usually constant forreasonable periods of time.

The lag time can vary from a few minutes in an air drilled hole, to hours in a deep mud filledhole. If lag time is much shorter than expected or multiple lags are found, it usually means aleak in the drill pipe which must be repaired immediately. The most reliable method of


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establishing the lag time is to use a tracing material such as oats, corn, paint, or calciumcarbide. Carbide will produce a bubble of acetylene gas. Typically, a sample of tracingmaterial is introduced into the drill pipe during a connection and circulated down throughthe bit jets and back up the annulus. The use of calcium carbide as a lag tracer has asecondary benefit. It permits verification that the entire gas detection system is functioning.Since it is necessary for the gas detector to extract, pump to the logging unit, and sense theacetylene gas, it verifies the integrity of the entire system.

This is only part of the story, as the time it takes the tracer to go down the inside of the drillpipe must first be calculated from the pump displacement, pump speed, pipe diameter, andpipe length. The calculated downward time is deducted from the total measured time to findthe lag time.

SAMPLE DESCRIPTION LOG EXAMPLESBelow are examples of sample description logs from various eras.


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One of the f i rs t wel l logs in Western Canada from Proceedings and Transact ions of the

Royal

Society of Canada for the Year 1886 Volume IV. Glenbow Arch ives


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A hand drawn samp le log typ ical of the 1940 to 1970 era.


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A computer drawn sample log of the 1980's . Hand draf ted logs could be drawn as neat ly asth is using le tter ing guides.


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Modern sample log using co lour and computer graphics symb ols.

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Cuttings Analysis