Sampling and Analysis of Drilled Cuttings (Sample Collection and Handling)

Sample Collection and Handling

Collecting and Handling Rotary Cuttings

A good wellsite lithological log requires good samples. This involves the accurate determination of the depth a sample represents, selection of a practical sample interval, careful collection, combination, and processing. In most cases, the well-site geologist will be responsible for supervision of cuttings sampling. However, the actual task of sample collection will commonly be delegated to other wellsite personnel. The job is best performed by the mud logging contractor, who will provide wellsite personnel with some geological training (in the case of the major contractors they may be graduate geologists). These people should be fully briefed on the geological section anticipated, particular zones of interest, and problems requiring special attention. Given adequate preparation, a mudlogging crew can provide a reliable sample series, prepare an initial descriptive log, and set aside individual samples of particular interest.

At the opposite extreme is the situation where sampling is assigned to a member of the drilling crew- usually the most recently hired and least experienced. In this circumstance, some time spent explaining the importance of maintaining a regular sampling schedule may be of value, but the geologist should plan to be present at the wellsite whenever important horizons are to be drilled. The common result of using drilling crews to catch samples is the phenomenon known as "boilerhousing" or "doghousing." At some convenient time, when other duties permit (normally at the beginning or end of the shift), the "roughneck" will bag and store away large numbers of samples, giving him a sufficient quantity to fulfill present, and some future, requirements. That such samples are of little or no geological value is not surprising. Some geologists have dismissed the use of well cuttings altogether as a result of working with such meaningless samples. This, however, is a mistake. Actual rock samples often answer crucial geologic questions; any adequate drilling plan will guarantee, as much as possible, the collection of useful samples.

Sample Lag Time

Well cuttings must travel with the drilling fluid to surface from the depth at which they are cut. Even in shallow holes with high fluid flow rates, this may take many minutes. In deep wells, the lag time may be two hours or more. It is necessary to "lag" samples, so that sample descriptions correspond to the depth of the bit at the time the sample was cut (bit depth will, of course, be somewhat greater by the time the sample reaches surface). The simplest method for estimation of sample lag time is a calculation of the annular volume between the drillstem and the borehole wall, and the time necessary for the circulation system to displace it:
Va = Ch-(Cp + Dp) (1)

(2)

(3)

Where:
V_a = Annular volume (ft³/ft of hole)

C_h = Hole capacity (ft³/ft)

C_p = Pipe capacity (ft³/ft)

D_p = Pipe displacement (ft³/ft)

S_a = Annular velocity (ft³/ft)

O_c = Output of drilling fluid pump (ft³/min)

T₁ = Lag time (min)

D = Depth (ft)

Separate calculations must, of course, be made for individual sections of the drillstem with different sizes of pipe, collars, and the like.

Pipe capacity and displacement may be calculated from simple geometry, but books are available, such as the Halliburton Cementing Tables, or Baker Tech Facts, that provide tables for quick reference of pipe displacements and capacities.

This lag calculation assumes that the internal diameter of the borehole is constant and equal to the bit diameter. This will be true of a cased hole (a wellbore that has been lined with cemented steel pipe) or a borehole in extremely hard, well-consolidated rocks. However, in most sedimentary sections, the weaker, less-consolidated rocks in the borehole wall will cave or spall, resulting in an irregular, overgauge (larger than bit diameter) borehole. A typical borehole will, therefore, have a larger annular volume, hence a longer lag time, than will be calculated. The lag time calculation also assumes a constant pump output and does not take into account times when the pump may be turned off, as, for example, when making a connection.

A more representative lag can be obtained by use of a tracer introduced into the circulation system and detected on its return to surface. It is also advantageous to measure the transit of the tracer through the system not as elapsed time, but by counting the number of strokes of the constant displacement drilling fluid circulation pump. A pump stroke counter will add increments only when the pump is running; its rate thus reflects the true pumping rate.

Any material may be used as a tracer that can be safely introduced at the top of the drillstem and can survive in a recognizable form that can be detected on return to surface. Rice, oats, other grains, or strips of colored cellophane, will pass safely through the jets of a drill bit and can be readily distinguished from cuttings on the shale shaker.

Use of visual tracers has one important disadvantage-someone must wait at the shale shaker for the first arrival to be seen. A better method is available if a mudlogging unit is being used. This unit is equipped with a digital pump stroke counter, as well as various gas detectors that draw samples from the returning drilling fluid (one of the main functions of mudlogging is the analysis of hydrocarbon gases in the drilling mud). A gas that does not naturally occur in the sedimentary section may be used as a tracer and its arrival automatically detected at the mud logging unit.

The most commonly used tracer is calcium carbide, a crystalline solid that can be conveniently introduced at the top of the drillpipe when a connection is made. Inside the drillstem it reacts with water to produce acetylene, a hydrocarbon gas that does not occur naturally.

A gas lag is acceptable for solid samples because the density and gel strength of drilling fluid prevent gas from rising and cuttings from settling in the annulus. Gas and cuttings lags are therefore synchronous when the drilling fluid is in good condition.

Sample Interval Selection

A representative sample log requires sampling at uniform intervals throughout the section. However, the sample interval must be governed by the rate of penetration. A sample depth interval should be selected that allows the personnel to collect and process samples properly while carrying out what other duties they may be assigned. Attempting to reduce the time available for sample catching and preparation will result in degradation of the quality of all samples.

Commonly, a sample interval of 30 ft is suitable in quickly drilled, shallow formations. As the borehole is deepened and the rate of penetration slows, the interval may be reduced, commonly to 10 ft. However, formation boundaries and thin zones of interest cannot be guaranteed to coincide with a fixed sample interval. Special samples should be caught when changes in the rate, or character, of penetration occur. This may be a "drilling break" (a sharp increase in rate of penetration indicating greater porosity), a reverse break (a sharp decrease), or a change in the way a certain rate of penetration varies over a short time (uniform or irregular), reflecting formation consistency and cohesion. The driller's rotary torque gauge is also a good indicator of changes in formation fabric.

When formation boundaries do occur, even if the lag time is accurately known, samples should be taken slightly earlier and slightly later than the expected arrival time. No matter how sharp a drilling break, lithology changes seen in well cuttings will always appear to be transitional. This is due to the small amount of sorting of cuttings by size, shape, and density that occurs in transit to surface. By "bracketing" the drilling break with samples, it is possible to obtain a true picture of the change taking place. Individual samples may be misleading, but a formation boundary can be recognized by the first appearance of a new lithology type, even if it represents only a small proportion of the total sample.

Sample Collection

The sorting by shape and density of cuttings occurs to a much greater extent in the drilling fluid conditioning equipment that removes solid material: the shale shaker, desander, desilter, and centrifuge. A totally representative sample requires the compilation of material from each of these. However, the desilter and centrifuge are only used intermittently to remove extremely fine solids. Such material, commonly known as drilling solids, consists of insoluble drilling fluid additives and finely abraded and unidentifiable cuttings debris. Sampling this material is rarely useful. The shale shaker and desander must be sampled and the samples combined at every selected sample point.

A geologist must always know what type of shale shaker is in use, when screens are changed, and the mesh size of the screen or screens used. He or she should also regularly check operation of the desander and request adjustment or replacement of the liner when required. If a desander is performing correctly, it will exhaust a stream of clean water and fine solids. If the exhaust is discolored with drilling fluid, repair or adjustment of flow rates is required.

If a conventional "rhumba" shale shaker is used ( Figure 1 , Traditional "rhumba", 6-screen shale shaker ), the screen nearest to the drilling fluid return flow line will have a greater proportion of large-sized cuttings. The second and third screens will have progressively smaller cuttings. Samples should be taken from all three screens and combined. On a modern double-decker shale shaker ( Figure 2 , Modern "double-decker" shale shaker ), material from the upper (coarse) and lower (fine) screens should be combined.

When drilling is slow, or when drilling is done with a small diameter bit, only a small volume of cuttings will be present on the shale shaker screen at any time. It will be necessary to make several visits to the shaker in order to accumulate sufficient material for a single sample. Remember that a single sample point must represent the formations cut between that depth and the previous sample point. A bucket or board placed beneath the shale shaker screens can be used to accumulate well cuttings between sample points. This is not a substitute for regular sample collection, but an acceptable alternative if a busy schedule requires it. If a catching bucket or board is used, then the geologist should instruct the drilling crew that the board or bucket is to be washed clean immediately after a sample is caught and at no other time!

The desander should be sampled by placing a sieve under the solids discharge ( Figure 3 , Desander operation in hydroclone ).

 

 

 

 

 

 

 

 

Sample Processing

After the well cuttings have been caught and accumulated for a sample interval, the cuttings must be split and prepared into several separate samples.

Unwashed Sample

Samples for paleontologic or geochemical analysis should be sealed in plastic bags or metal cans exactly as they come from the shale shaker, without any washing or other treatment (sometimes they may require the addition of a bactericide to the container before sealing). Geochemistry samples may need the combination of a drilling fluid sample with the cuttings sample, or a separate sample of each.

Containers for these and other samples should be clearly marked in waterproof ink with:

Oil company name;

Well name;

Well location and coordinates;

Sample depth interval (from/to).

On a "tight hole"(a high security exploration well), it may be necessary to report some or all of this information using a prearranged substitution code.

Archival samples are collected in labeled cloth sample sacks, about one pint in each. These may be shipped untreated, or may be lightly rinsed, to remove drilling fluid, and hung out to air dry. At least one set of archival samples is required on all wells for future reference and analyses. Extra sets may be necessary for partners in the well, for trade with other oil companies, or for government agencies (Geological Survey, Ministry of Petroleum, National Oil Company, and so forth). When numerous archival sample sets are required, collecting them can be close to a full-time job. Remember that members of the drilling crew cannot be relied upon for this. The mudlogging crew can collect one or two sets of archival samples as part of their normal duties, but if larger numbers are needed, they must take time from their other, more important work-gas, oil, and cuttings analyses. In such circumstances, additional temporary personnel whose only duty is the catching and bagging of samples under the direction of the geologist, or mudlogger, should be provided.

Washed and Dried Samples

At each sample point, approximately one pint of well cuttings (in addition to that required for archival samples) should be caught at the shale shaker, along with one quart of drilling fluid from the flow line, and about one minute's worth of desander effluent caught in a 170-mesh sieve (the actual volume of solids caught will depend upon the concentration of unconsolidated material in the drilling fluid).

The desander sample is lightly rinsed and set to one side for later examination. If the desander is operating correctly, this material should already be cleaned of drilling fluid.

The procedure for washing well cuttings samples should be agreed upon before the well is drilled so that later misunderstanding can be avoided. When drilling through an unconsolidated clay section, a lightly rinsed sample, after drying, will result in a geologically useless clay "brick." On the other hand, a sample that has been vigorously washed to remove all clay may later be misinterpreted as representing 100% of what are, in fact, only trace constituents. This is a common cause of disagreement between wellsite and laboratory personnel. The following procedure is intended as a guideline to avoid such problems.

The cuttings sample should be placed in the top of a sieve stack (8-mesh over 80-mesh over 170-mesh is usually adequate) and lightly rinsed to remove drilling fluid only. Contents of the 8-mesh sieve will be predominantly cavings from the borehole wall and can be set aside for later inspection. If the sample contains unconsolidated clay, a portion should also be set aside at this time before further washing.

From the material in the 80-mesh sieve, take about 100 cc and disaggregate it by blending with 600 cc of water. After blending, inspect the water surface for oil droplets, sheen, or petroleum odor (a mud logger will also take a gas sample from the blender jar). Decant the water and set aside the solid residue for later inspection.

If oil indications are seen, then repeat the blender test with 400 cc of drilling fluid and 300 cc of water. Small portions of unwashed cuttings and drilling fluid should also be checked under ultraviolet light for oil fluorescence (if none is immediately seen, try stirring the sample with a little fresh water). Record any oil observations; these will be included with the results of later oil tests.

If a "rinsed only," dried sample is required, remove sufficient material from the 80-mesh sieve for drying.

Wash the sample remaining in the 80-mesh sieve, manipulating it with the fingers in order to evaluate the clay content and consistency. Set aside a portion of the sieve contents for examination. Dry and bag the remainder of the sample for later reference.

Finally, rinse the material remaining in the 170-mesh sieve and decant any solids contained in the rinse water.

At the end of this process, several separate samples will be available for microscopic examination and evaluation (not all of them will always be required):

unwashed cuttings;

drilling fluid;

unwashed cuttings and drilling fluid diluted with fresh water;

8-mesh cavings;

80-mesh lightly rinsed cuttings;

80-mesh well washed cuttings;

170-mesh well washed cuttings;

170-mesh decanted washings;

desander effluent;

blender-decanted residue.

Cuttings Sample Examination: General Requirements

Evaluation of the prepared sample may be as sophisticated a procedure as available time and equipment allow. For routine wellsite sample description, the following is the minimum equipment required:

binocular microscope and illuminator;

ultraviolet light inspection enclosure;

sample inspection and drying trays;

sample reference trays;

tweezers;

steel probe or blade;

porcelain spot plate;

syringes or dropping bottles;

grain size comparison chart or graticule;

10% hydrochloric acid;

chlorothene (or other safe organic solvent);

phenol phthalein solution;

barium chloride solution;

silver nitrate solution.

Microscopic Examination: An Overview

At the microscope, the various samples should be arrayed in a single sample tray as shown in Figure 1 (Samples for microscopic examination ) , with a small quantity of each available to the microscope. A reference fray containing 80-mesh fractions from the previous 50 or 100 ft of drilling should be kept beside the microscope to assist the recognition of progressive changes in rock texture or color.

Notice that Figure 1 shows cuttings spread a single layer deep in the trays. If a representative sample has been prepared, there is no benefit to using larger quantities. Thick layers of sample in the tray will cause focusing problems when the microscope is used to track across the sample. They also complicate the selection and testing of individual cuttings by tweezers or sample probe.

 

 

 

Initial cuttings evaluation should include the following characteristics:

rock type;

color;

hardness or induration;

grain size;

grain shape;

grain sorting;

luster;

cementation or matrix;

porosity (amount and type) and oil shows;

fossils, accessories, and inclusions.

These are commonly described in the order shown, although the last two may be reversed. Although "rock type" is the first item on this list, it should be the last determination and is, in fact, among the least important. The first and most important responsibility of the wellsite evaluator is to provide a complete and graphic description of the sample in terms that are detailed and comprehensive enough to allow their recognition and correlation when the same rocks are encountered in other drilled wells. The following are two sample descriptions:

Sst: bu.-wh., wI. ind., med.-crs., wI. rndd., wI. srt., sill. cmt., gd. intgran. por., gd.

Stn., gd. cut Fluor.

Ls: Pkst., brn., crs., fr. intpar. por., fr. cut Fluor:, sli. crinal., arg.

Abbreviations such as these are commonly used for on-site evaluation. Properly written out these descriptions would read:

Sandstone: buff-white, well indurated, medium-coarse grained, well rounded, well sorted, silica cement, good inter-granular porosity, good stain, good cut and fluorescence.

Limestone: packstone, brown, coarse grained, fair interparticle porosity, fair cut and fluorescence, slightly crinoidal, argillaceous.

Following the sample description, the wellsite evaluator must identify the stratigraphic unit of which a single sample represents only a small part. Also, it should be remembered that formation boundaries, no matter how sharp in reality, will appear transitional in well cuttings, due to the sorting of cuttings in the return annulus. It is necessary to review several samples before a valid decision can be made regarding the rock type and the true boundaries of a section. For example, a series of samples that, taken individually, might be described as silt-stones and argillaceous sandstones, might, when considered together and with reference to changes in rate of penetration and rotary torque, be interpreted to represent a uniform, massive shale with thin, clean sandstone intercalations.

Finally, in naming rock type, the wellsite geologist should beware of being too specific, possibly to the extent of implying mineralogical components or variations that are indeterminate at the well-site. An unjustified conclusion that is later disproved may result in doubt being cast on the whole sample log. A detailed and graphic description of a nonspecific "mudstone" or an unidentified "accessory mineral" will provide sufficient information and will draw the attention of specialist personnel during postdrillling studies, while avoiding unnecessary errors.