Sample collection and description of mud

Sample collection and description
Preparation for collection of cutting sample
The cuttings are physical, tangible pieces of drilled rocks which required the forces of nature millions of years to lay them down and, it cost the oil companies much time and millions of dollars to recover. Drilled cuttings are often referred to as “Ditch Samples”. The expression comes from early drilling rigs that used an earthen ditch to channel mud flow at the surface. The cuttings samples can be in a span of just a few minutes either saved for an eternity or lost for ever. Aside from their immediate value, the cuttings can be saved and re-evaluated in the future using knowledge and techniques that have not been discovered yet. On the other hand cuttings falling into the reserve pit are cuttings gone forever along with the information they contain. An accurate lag, a source of representative samples and close attention to making efficient use of available time are all necessary to good cuttings and mud samples collection.
Shaker Samples
Almost every rig has shakers with vibrating screens for separating the cuttings from the mud as they reach the surface. The shaker screen should be examined to ascertain whether the mesh size is small enough to separate small cuttings from the mud. When the shaker screen is used a board box should be placed at the foot of the screen for the collection of the cuttings through a complete interval of the hole. What is meant here is that the sample taken would be representative of the complete interval(10 feet, 30 feet, etc.) and not just cuttings coming across the shaker at some given time representing a spot check of a couple of inches. The broad box should be cleaned following each sigle sample gathering to avoid the mixing-up of samples of different intevals. The table on the following page will help to determine suitable screen sizes.
Shae Shaker Screen Size Detrimination
Particle type Particle Size in
millimeters
Tyler Screen Mesh
Size
Silt > 1/16 250
Very Fine Grain Sand 1/16 to 1/8 250 - 115
Fine Grain Sand 1/8 to 1/4 115 - 60
Medium Grain Sand 1/4 to 1/2 60 - 30
Coarse Grain Sand 1/2 to 1.0 30 - 16
Sperry-Sun Drilling Services Basic Mud Logging
Middle East Training Center - 9/15/2006 (Prerelease) 135

Settling Box Samples
Although the shaker screen cuttin sample interval metod is the one most often used, another means of collecting samples is the Settling Box. The Settling Box is however, far more satisfactory as a means of collecting cuttings samples than the shaker screen. Such a box is available from Houston stock. This box or a similar box should be rigged up in such a manner that a part of the mud from the flow line is diverted into it ( through a two-inch line for example). The mud flows through the box and over a removable gate in the opposite end and into the mud pit. The reduced velocity of the mud permits the cuttings to settle on the bottom of the box. After collecting a sample the gate must be lifted and the remaining cuttings are scraped and flushed out to prepare the box for collecting a sample from the next interval . The use of such a routine immures that a composite sample is collected for an entire interval of the hole and it affords the surest meant of collecting small cuttings and finely divided sand. Such a box provides practically the only means of catching samples while the shaker is being bypassed. Establishing this sample-catching means beforehand will ensure that no quick improvisation will be encountered in the event that the shaker is bypassed and a uniform sample catching method will be assured throughout the hole.
Hint and Precautions
1. The discharge from the settling box go into the shaker pit or sand ditch.
2. A small stream of water added in the settling box will help settle cuttings in heavy viscous mud. Check with the tool pusher or mud engineer before adding such a stream of water.
3. A rapid change to little or no sample coming over the shaker can mean finely divided sand has been increase in this case.
4. The same problem can mean a salt section has been encountered. Check the salinity of the mud, and watch for deterioration of the mud characteristics.
5. Check the de-sander and de-silter discharges for the possible presence of sample material going through the shaker screens.
COLLECTING CUTTINGS SAMPLES
It is common for the Mudlog to be plotted on the basis of a sample interval of five feet or one meter. Obviously the number of tasks assigned to the logger and the amount of equipment he is required to operate will have a great beating on the frequency with which he will be able to catch the samples and therefore, the intervals at which he will plot the log. The logger should learn to work out a routine for himself which will allow to work at a high level of efficiency and to make provision for the collection of the important information in the time allotted him. When circumstance does not permit the catching of the sample at exactly the set interval, some judgement comes into play, usually backed by experience which allows for a little leeway one way or the other of taking the sample. This results in plotting an interpolated log which still describes the formation encountered. However, this manipulating of the schedule should not lead to the act of merely running back and forth to the shaker screen, catching samples and making entries on the data sheet on the next entry line without paying due attention to the proper correlation with depth. Without proper thought to what he is doing the logger can find himself in the situation of his own diligence not to miss a single sample, causing the depth correlation to become an utter chaos.
It is better to take a few less samples and be positive of their origin than not missing any but ignorant of what depths they all came from. Take as many samples as possible and still make good correlation as to depth.
Collecting “Wet” Samples
This is a sample of unwashed cuttings that is taken for plaleontological and petrological examination in the oil company’s laboratories. It comprises a sample put straight into a fine mesh cloth bag, labelled and left to dry in the sun before tying into bundles and bagging up in labelled sacks or boxes. Care must be taken to adequately fill the sample sack or tin. By operating company’s request, wet samples are sometimes placed into plastic bags prior to being put into cloth sacks or tins. In this event a few drops of a bacteriacide solution such as formaldehyde or zephin chloride must be added to the prevent bacterial decay which can be initiated by some mud materials.
Washing Cuttings From Water-Base Mud
Washing and preparing The cuttings sample to be examined is probably as important as the examination itself. Here again, the technique must be adapted to the type of material and the area. In hard rock areas the cuttings are usually easily cleaned. Washing is usually just a matter of hosing the sample with jet of water to remove the film of the drilling mud from the surface of the cuttings.
 Washing the cuttings from areas of recentgeological ages is, however, a bit more difficult and requires the use of several precautions. As the shales present are soft and dispersible in water, the wash water will tend to wash this shale away. This should be taken into account; the mud logger should remain aware that the shale that would be washed away is a part of the sample and not a foreign material and should be logged accordingly. The sample should be washed no harder than necessary to remove the drilling mud. Cuttings from zones of lost circulation are often intermixed with lost circulation material. This material will usually be floated out of the container. After the cuttings have been washed for mud removal, they should be washed through a 5 mm sieve for the removal of sloughed shale and then into the 80 mesh sieve. Following the sieving a portion of the washed sample is put onto one of the trays provided for the microscopic inspection and drained before use. A larger sample is placed on another tray and dried into an oven before placing in an annotated envelope and boxed for the oil company analysis, The cutting should not be exposed to extra heating inside theoven as the could be burnt loosing the original rock charachters. The tray for immediate examination should contain a single layer of cuttings only. This is important when considering relative percentages of different materials contained.
Cuttings in oil base muds
In the case of oil base mud cuttings are quite representative of the formations as this type of mud prevents sloughing, but at the same time, however, they pose a problem of washing and handling. They cannot be cleaned by washing in water alone, so it is necessary to use a detergent in order to clean themout. For this purpose many of liquid detergents -commercially available- can be utilized.
Procedure:-
Set up two containers such as two halves of a 25 gallon drum. In one a nonfluorescent solvent, vestal or naphtha should be used for first washing the outer coating of oil and mud off the cuttings. In the other container mix solution consisting of one pint of a commercially available detergent to five gallons of water. Wash the cuttings in the detergent solution to remove the solvent, then they can be washed in water as usual. In order to make a good inspect for lithology and staining, the cuttings better be broken crushed.
LITHOLOGICAL EXAMINATIONS OF CUTTINGS
[Sample Description]
1.1 Sample Quality & Examination Techniques
The quality of a sample log is frequently a direct measure of the quality of the samples. Clean, good quality samples are exceptions rather than the rule.
The geologist logging samples must learn to make his interpretations from samples of widely varying quality. Cavings and other contaminants must be recognized and disregarded. Many methods of examining samples are in use throughout the industry. Some geologists pour and examine one sample at a time; others lay out the samples in compartmented trays so that a sequence from five to ten samples may be observed in a single tray.
The following procedure is recommended:
· The samples are laid out in a stack of five-cell trays, with the depths marked on the trays.
· The cuttings should just cover the bottoms of the trays.
· It is sometimes desirable to separate the obvious cavings by either sieving or dry panning.
· Attention should generally be focused on the smaller cuttings with angular shape and fresh appearance.
A standard practice is to scan 100 or more feet of samples, observing the lithological “breaks”. The samples are then re-examined for more detailed study, dry for porosity estimates and wet for all other properties.
Wetting the samples do not only cleans-off the mud and other contamination, but also brings out the rock characteristics that are not apparent in dry samples.
The tray should be dipped in a basin of water, agitated gently to remove any fine  contaminants, and then removed and drained for study, leaving the samples still covered by a film of water. After the cuttings have been logged, they are set aside to dry and then returned to the sample bags. The technique of scanning samples before logging them in detail has many advantages. In addition to helping the examiner to pick the formations’ tops and lithological breaks, it may also aid him in determining the extent of porous and hydrocarbon bearing intervals. However, the principal advantage of this technique is that it provides
the geologist the opportunity to observe and interpret the depositional sequences. When sample intervals are laid out in sequence subtle changes in texture, mineralogy, color and facies often become apparent even before the microscopic examination. Thus the observer is alerted to look for these changes when making the detailed examination. This method of examining samples, encourages geologists to observe and log the lithology rather than the sample  interval units. It eliminates the laborious and time consuming task of routinely describing each sample interval. It increases speed of logging and it invariably helps the geologist make a more meaningful log. Textures in carbonate rock can be clearly observed with the aid of special wetting agents such as mineral oil, glycerin, clove oil, etc.
1.2 Abbreviations
Abbreviations should be used for all descriptions recorded on lithological logs. These terms differ from an exploration company to another, so, it’s of primary importance that the logger should ask for a list of abbreviations the company uses at the begining of every new job. Abbreviations for nouns are designated with capital initial letters; other terms are abbreviated entirely in small letters.
1.3 Order of Written Description
When written descriptions are required, a standardized order of description well help :
(1) reduces the chance of not recording all important properties.
(2) increases the uniformity of description among geologists.
(3) saves time in obtaining specific information from descriptions.
THE FOLLOWING ORDER IS TO BE FOLLOWED:
A. For Clastic Rocks:- Sand/sandstone/siltstone/clay/shale, etc.
First. Color
Second.Grain size, sorting then grain shape
Third. Hardness
Fourth.Cement and/or matrix materials.
Fifth. Fossils and accessories
Sixth. Sedimentary structures.
Seventh.Porosity and oil shows.
Example:
Sst: Lithic, bu-wh , f -med , mod srtd, ang ,occ/ sub ang, hard, arg , mica , pyr, fr intgran por , gd stn , gd cut fluor.
II. For Non-Clastic Rocks:- Evaporites/limestone/dolomite/chert, etc.
First.Color.
Second.Crystallinity
Third.hardness.
Fourth.Cement and/or matrix materials.
Fifth.Fossils and accessories.
Sixth.Porosity and oil shows.
Example:
Ls: Wh, off wh, occ/ mlky wh, cryptoxln-microxln, md hd-hd, sli arg, , dol in pts, rr foss, no vis por, no oil shows.
1.3.1 Rock Types
A proper recording of rock type, consists of two fundamental parts: The basic rock name (underlined): e.g. Dolomite, Limestone, Sandstone, and the proper compositional or textural classification term: e.g., lithic, oolitic grainstone, etc.
1.3.2 Color
Color of rocks may be a mass effect of the colors of the constituent grains, or result from the color of cement or matrix, or staining of these. Colors may occur in combinations and patterns, e.g. mottled, banded, spotted, variegated. It is recommended that colors be described on wet samples under ten-power magnification. It is important to use the same source of light all the time, and use constant magnification for all routine logging. General terms such as dark grey, medium brown, etc., generally suffice. However, if more concise designation is required, rock-color chart may be used. Ferruginous, carbonaceous, siliceous, and calcareous materials are the most important staining or coloring agents. Yellow red or brown shades come from limonite or hematite,. Gray to black color can result from the presence of carbonaceous or phosphatic material, iron sulphide, or manganese. Green coloring comes from.Glauconite, ferrous iron, serpentine, chlorite and epidote. Red or orange mottlings are deriven from surface weathering or subsurface oxidation by the action of circulating waters. The colors of cuttings may be altered, after the samples are caught, by oxidation caused by storage in a damp place, insufficient drying after washing or by overheating. Bit or pipe fragments in samples can rust and stain the samples. Drilling mud additives may also cause staining.
1.3.3 Texture
Texture is a function of the size, shape and arrangement of the component elements of a rock.
1. Grain or crystal sizes:
Size grades and sorting of sediments are important attributes. They have a direct bearing on porosity and permeability, and may be a reflection of the environment in which a sediment was deposited. The microscopist should better record size grades with reference to some standard comparator of mounted sieved sand grains or photographs of these. Another simple and useful tool is a photographic grid of halfmillimeter squares which may be fixed on the bottom of a sample examination tray.
2. Grain Shape
Shape of grains has long been used to indicate the history of a deposit of which the grains are a part. Shape involves both sphericity and roundness.
A) Sphericity:
Refers to a comparison of the surface area of a sphere of the same volume as the grain, with the surface area of the grain itself. For practical purposes, distinction is usually made in large particles, on the basis of axial ratios and in grains.
B) Roundness
Roundness, which refers to the sharpness of the edges and corners of a fragment, is an important characteristic that deserves careful attention in detailed logging. Five degrees of rounding may be distinguished as follows :-
Angular :edges and corners sharp; little or no evidence of wear.
Subangular :faces untouched, but edges and corners rounded.
Subrounded :edges and corners rounded to smooth curves; areas of original faces rounded.
Rounded :original faces almost completely destroyed, but some comparatively flat faces may be present; all original edges and corners smoothed off to rather board curves.
Well Rounded :no original faces, edges, or corners remain; entire surface consists of broad curves. Flat areas are absent.
3. Sorting
Sorting is a measure of dispertion of the size frequency distribution of grains in a sediment or rock. It involves shape, roundness, specific gravity and mineral composition as well as size. A classification given by Payne (1942) that can be applied to these factor is:
Good : 90% in 1 or 2 size classes
Fair : 90% in 3 or 4 size classes
Poor : 90% in 5 or more size classes
The terms; Well, modrately and ill sorted can be aso used to identify
the above mentiond.
1.3.4. Cement and Matrix
Cement: is a chemical precipitate deposited around the grains and in the interstices of a sediment as aggregates of crystals or as growths on grains of the same composition.
Matrix consists of small individual grains that fill intersections between the larger grains. Cement is deposited chemically and matrix mechanically. The order of precipitation of cement depends on the type of solution, number of ions in solution and the general geochemical environment. Several different cements, or generations of cement, may occur in a given rock, separately or overgrown on or replacing one another.
Chemical cement is uncommon in sandstone which has a clay matrix.
The most common cementing materials are silica and calcite.
Silica cement is common in nearly all quartz sandstones. This cement generally occurs as secondary crystal overgrowth deposited in optical continuity with detrital quartz grains. Opal, chalcedony and chert are other form of siliceous cement.
Dolomite and calcite are deposited as crystals in the interstices and as aggregates in the voids. Dolomite and calcite may be indigenous to the sandstone, the sands having been a mixture of quartz and dolomite or calcite grains, or the carbonate may have been precipitated as a coating around the sand grains before they were lithified.
Calcite in the form of clear spar may be present as vug, or other void
filling in carbonate rocks.
Anhydrite and Gypsum cements are more commonly associated with dolomite and silica than with calcite. Additional cementing materials, usually of minor importance, include
Pyrite, generally as small crystals, Siderite, Hematite, Limonite,
Zeolites and Phosphatic material.
Silt acts as a matrix, hastening cementation by filling interstices, thus decreasing the size of interstitial spaces.
Clay is a common matrix material, which may cause loss of porosity, either by compaction, or by swelling when water is introduced into the formation.
Argillaceous material can be evenly distributed in siliciclastic or carbonate rocks, or have laminated, lenticular detrital or nodular form. Compaction and the presence of varying amounts of secondary quartz, secondary carbonate and interstitial clay are the main factors affecting pore space in siliciclastic rocks. While there is a general reduction of porosity with depth due to secondary cementation and compaction, ranges of porosity vary considerably due primarily to extreme variations in amounts of secondary cement. For instance, coarse grained sandstones have greater permeability than finer ones, when the same amount of cementing material is available to both. However, the same thickness of cement will form around the grains regardless of their size, therefor the similar intersections which occur in finer grained sandstones, will be cemented earliest.
1.3.5. Fossils and Accessories
Microfossils and some small macrofossils, or even fragments of fossils, are used for correlation and may also be environment indicators. For aid in correlation, anyone making sample logs should familiarize himself with at least a few diagnostic fossils. The worldwide Cretaceous foraminiferal marker,  Globotruncana, for example, should be in everyone’s geologic “vocabulary.” Any geologist who examines samples, should be able to distinguish such form as foraminifera, ostracods, chara, bryozoa, corals, algae, crinoids, brachiopods, pelecypods and gastropods, so as to record their presence and relative abundance in the samples being examined. More detailed identification will probably have to be made with the aid of the literature, and/or the advise and assistance of a paleontologist. An excellent reference for the identification of the more common microfossils is “Recognition of Invertebrate Fossil Fragments in Rocks and Thin Sections”, by O. P. Majewske (1969). Fossils may aid the sample examiner in judging what part of the cuttings is in place and what part is caved. For example, in the Gulf Coast region, fresh, shiny foraminifera, especially with buff or white color, are usually confined to Tertiary beds; their occurrence in samples from any depth below the top of the Cretaceous is an indication of the presence of caved material. It would be helpful to each sample-logger to have available one or more slides or photographs illustrating the principle microfossils which might be expected to occur in each formation he will be logging.
Accessory constituents
Although constituting only a minor percentage of the bulk of a rock, may be significant indicators of the environment of deposition, as well as clues to correlation. The most common accessories are glauconite, pyrite, feldspar, mica, siderite, carbonized plant remains, heavy minerals, chert and sand-sized rock fragments.
1.3.6. Sedimentary Structures
Most sedimentary structures are not discernible in cuttings. On the other hand; one or more of them can always be found in any core and they should be reported in the description thereof.
Structures involve the relationship of masses or aggregates of rock components. They are conditioned by time and space changes; e.g. stratification may result from discrete vertical (time) change in composition, as well as changes in grain sizes or of fabric. In time of origin, they are formed either contemporaneously  with deposition (syngenetic), or after deposition and burial (epigenetic). Syngenetic structures are often very important indicators of the environments of deposition of sediments.



1.3.7. Porosity
Among the most important observations made in the course of sample examination, are those relating to porosity. Porosity is a measure of the volume of void spaces in a rock. The ability to estimate an accurate porosity, comes through practice and experience in examining samples. Although the magnification of about 10x is adequate to detect porosity, higher magnification is often necessary. Pores are easier to recognize in dry samples than in wet ones. If porosity of any category is observed, it should be thoroughly described, and additional comments should be made in the remarks column. Samples with porosity, should always be checked for hydrocarbons, regardless of whether or not staining is observed, high gravity oils may leave little or no staining on the rock surface. In siliciclastic rocks, three types of porosity are common: intergranular, moldic and fracture. The intergranular is the most common type and most rapidly seen in cuttings, others are difficult to recognize in cuttings. In carbonate rocks, porosity is always classified in one of the following categories: interparticle, intercrystal, vuggy, moldic and fracture. A number of classifications considering various aspects of carbonate porosity and permeability have been developed, including those by P. W. Choquette and L. C. Pray (1970) and by G. E. Archie
(1952).
1.3.8. Hydrocarbon Shows
The primary objective of any exploratory well is to discover oil or gas in commercial quantities. Therefore, it is vital to continuously analyze both the drilling fluids and sample cuttings for hydrocarbons. The geologist and mud Iogger are required to make a detailed lithological description of the drilled section, which is used to evaluate the area for future drilling, particular emphasis is placed on the analysis or hydrocarbon shows. NB: A show is an indication of hydrocarbons in a formation. Porosity and productivity is only a factor in how good the show is

 

posted by Geology on 07:29

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