Coring and Core Analysis

Analyses of rock samples yield data basic to the evaluation of the productive potential of a hydrocarbon reservoir. Bit cuttings are, of course, rock samples; their small size, however, precludes their furnishing more than qualitative information. The desire to obtain and examine larger, unbroken pieces of reservoir rock led to the development of coring techniques, by which relatively large reservoir rock samples are obtained, either from the bottom during drilling, or from the side of the bore hole wall after drilling. The development of coring and core analysis techniques has played a large part in the elevation of petroleum engineering to its current status. All phases of the profession rely to some extent on a knowledge of rock properties and the factors which affect them.

10.1 General Coring Methods and Equipment

Two basic rotary coring methods are applied: coring at the time of drilling (bottom coring) and coring after drilling (side-wall coring). All bottom coring methods utilize some type of open center bit which cuts a doughnut shaped hole, leaving a cylindrical plug or core in the center. As drilling progresses, this central plug rises inside a hollow tube or core barrel above the bit where it is captured and subsequently raised to the surface. Further classification of bottom coring is commonly based on a more specific description of the equipment used:

  1. Conventional Coring a. conventional core head (other than diamond)
  2. diamond core head
  3. Wireline Retrievable Coring

Conventional Coring

Conventional coring equipment requires that the entire drill string be pulled to retrieve the core. This is a disadvantage; however, the corresponding advantage is that large cores, 3 to 5 in. in diameter and 30 to 55 ft long, may be obtained. The 3|-in. diameter core is probably the most common.

Two typical conventional coreheads are shown in Figure 10.1; the fish tail (soft formations) and rolling cutter (hard formation) types. Their usage parallels that of the regular bits from which they have been adapted. Figure 10.2 shows three different conventional diamond core bits as a sample of the numerous types available from different companies.

The core barrels used with diamond heads are generally longer than those used with conventional heads, commonly 55 ft as compared with 30 ft. A typical diamond core barrel is shown in Figure 10.3. Note that it is composed of an outer barrel which acts as a drill collar, and a freely rotating inner barrel which houses the core. The drilling fluid passes outside the inner barrel and is discharged through watercourses at the bit face. Mud trapped above the core is expelled through the check valve atop the inner barrel.

Diamond bits cost much more, but will drill more total footage and when worn out may be returned to their supplier for salvage. This trade-in value is based on the weight of undamaged stones remaining, and may amount to 50% or more of the original cost. The core recovery with diamond heads is generally higher than with conventional heads, particularly in hard rock areas. Rate of penetration may, however, be lower in soft formations than that of the rolling cutter heads.

Wireline coring denotes the method whereby the core (and inner barrel) may be retrieved without -pulling the drill string. This is accomplished with an overshot run down the drill pipe on a wire line. The diamond heads used in this technique have much smaller openings than those shown in Figure 10.2. The core barrels used are

psil

Hard Formation Cutter Soft Formation Cutter

Head Head

Fig. 10.1. Conventional core bits. Courtesy Hughes Tool Company.

somewhat variable, but are basically similar to conventional types. The cores obtained by this method are small, commonly 1| to If in. in diameter and 10 to 20 ft long. The main advantage of this method is the saving in trip time, as mentioned before. The. durability

Oil And Gas Core Inner Barrel Failure
Fig. 10.2. Typical diamond core heads. Courtesy Christensen Diamond Products; and Drilling and Service, Inc.

of the diamond bit coupled with the wireline feature allows a thick section to be cored with no time lost in making trips. This is particularly beneficial in deep wells.

Sidewall Coring

It is often desirable to obtain core samples from a particular zone or zones already drilled. This is commonly accomplished by the use of a device such as that shown in Figure 10.4. A hollow bullet which imbeds itself in the formation wall is fired from an electric control panel at the surface. A flexible steel cable retrieves the bullet and its contained core. Samples of this type are normally f or in. in diameter and

| to 1 in. long. Sidewall coring is widely applied in soft rock areas where hole conditions are not conducive to drill stem testing. The zones to be sampled are normally selected from electric logs.

10.2 Operational Procedures

While this section is not offered as an operating manual, there are certain general operational considerations which warrant discussion. The following are recommendations which apply to conventional coring.1'2

  1. Every precaution should be taken to insure that the hole is junk free; small pieces of steel (bit teeth, tong dies, etc.) will quickly ruin a core bit, whether diamond or conventional. Running a subtype junk basket with the last' two or three bits will usually provide safe conditions.
  2. Just as in normal drilling, sufficient driy collars should be run to furnish the bit weight. Stabilizers have been successfully applied in some cases to prevent drill string wobbling. The core barrel itself should be inspected for straightness. A crooked inner barrel will cause eccentric action on bottom.
  3. Core heads should be run into the hole at a safe speed to avoid plugging or damage from hitting a bridge or dog-leg.
  4. During coring the weight should be fed off smoothly and uniformly — not in bunches. This requires the full attention of a crew member at the brake, unless an automatic feed control is available.
  5. Coring should begin at light bit weight and low rotary speed; these may be increased as soon as cutting action is established. Normally the applied bit weights and table speeds should be held within the limits given in Tables 10.1 and 10.2, unless specific experience in the area dictates otherwise. Circulating volumes for conventional core bits approach those of regular bits of the same size. Diamond bits require less fluid volume, and may actually be pumped and bounced off bottom by excessive circulating rates. Also, severe erosion of the water courses and bit matrix may occur. Table 10.2 includes recommended circulating rates for diamond coring.
  6. Pump pressure should be closely watched during diamond coring as an indication of whether drilling fluid is passing over the face of the bit. With the bit on bottom, pressure should be higher than when the bit is off-bottom. This is essential to bit cleaning and performance. A sudden pump pressure increase not alleviated by raising the bit off bottom, may mean that the core barrel is plugged by trash in the mud; if this happens, it should be pulled for inspection.

Fig. 10.1. Conventional core bits. Courtesy Hughes Tool Company.

Conventional Core
  1. 10.2 (cont.).
  2. A sudden decrease in penetration rate which cannot be attributed to a formation change may mean that the inner barrel is jammed or plugged and the assembly should be pulled for inspection. Also, the barrel may be full, unless it is certain that no errors have been made in measurement.
  3. The drill string should be retrieved slowly to avoid excessive pulling suction which may suck the core out of the barrel. This is particularly important in a reduced diameter or rat-hole section. These have been general operating recommendations that may be altered to fit particular cases. Experience

TABLE 10.1

Recommended Practices for Conventional (Non-Diamond) Core Bit Use (Courtesy Hughes Tool Company)

Maximum

Size range, in.

Maximum weight recommended, lb

10,000 12,000 15,000 18,000

Formation Cutter head

Soft shale, sand Soft formation gumbo

Soft with hard Soft formation streaks — chalk

Broken formation, Hard formation sulphur bearing rock

Pump Speed Fast as necessary to prevent balling up Fast as necessary to prevent balling up Medium

Rotary speed 25-35

45 50 60 60

Weight required on bit Light weight

Anhydrite, chert, hard lime, granite, quartzite

Hard formation 'Full

25-35 Light to medium

25-35 Light weight

30-40 Heavy weight

Remarks Too much weight will "ball up" bit Use light weight in soft formation Heavy weight and high speed breaks up core

On granite, and such rocks, more hole per cutter head can be obtained with slow rotation

TABLE 10.2 Recommended Diamond Coring Practices (Courtesy Christensen Diamond Products)

Drilling weight

Rotary speed

Circulation raie

TABLE 10.2 Recommended Diamond Coring Practices (Courtesy Christensen Diamond Products)

Drilling weight

Rotary speed

Circulation raie

Bit size

Recommended

Recommended

Recommended

Recommended

Recommended

Recommended

Recommended

range

starting

minimum

Recommended

starting

minimum

drilling

minimum

maximum

OD, in.

weight, lb

drlg. wt., lb

drlg. wt., lb

rpm

rpm

rpm

gal/min

gal/min

4-5

4,000

4,000

8,000

50

30

125

100

250

5-6

4,000

4,000

10,000

50

30

125

125

300

6-7

4,000

4,000

12,000

50

30

100

125

300

7-8

8,000

8,000

15,000

40

20

100

150

350

8-9

8,000

8,000

20,000

40

20

80

150

400

9-10

8,000

8,000

25,000

40

20

80

200

450

10 and

450

larger

8,000

8,000

30,000

40

20

60

200

in an area is always the best guide to successful coring. Complete core recovery is the rule rather than the exception in hard, non-fractured formations, but it becomes successively more difficult to achieve in fractured and/or less consolidated formations. Poor operating procedures are one hazard that can be avoided.

10.3 Handling and Sampling of Core Recovery

The ultimate purpose of coring is usually the quantitative analysis of the physical properties and fluid content of the recovered core. (Some samples, however, may be evaluated by visual inspection — such as solid shale — while some are cut for lithological information only.) Consequently, it is extremely important that proper care be exercised to insure that the core reaches the laboratory in the best possible condition. The following are recommended handling and sampling procedures which have been furnished by Core Laboratories, Inc.3 Field Sampling

1. Field check list

A log sheet is provided for the field engineer on which a record is kept of the depths of the interval cored, coring time for each foot, lithologic description of the core, sample number and depth, fractures and any other notable features of the core, and type and properties of drilling fluid.

2. Removal of core from core barrel and handling prior to preservation a. Whole core analysis

The core should be removed from the barrel in segments as long as possible and care should be taken to prevent excessive breaking up of the core. Jarring and hammering on the core barrel is often necessary, but this should be done as carefully as possible to avoid crushing the core or opening fractures. Each piece should be wiped clean with dry rags (not washed) as soon as it is removed from the barrel, and laid out on the pipe rack and marked as to top and bottom. After all the core is removed from the barrel, the core is measured with a tape and marked off into feet. Any lost core is logged at the bottom of the cored interval. If more core is measured than was supposedly cut, the discrepancy is resolved by the operator.

b. For conventional plug type core analysis

The procedure for removal of core from the core barrel for conventional analysis is generally the same as for whole core analysis. In this case short pieces of core are used for analysis, and the extra precautions to recover long pieces are not necessary.

c. For sidewall core analysis

Due to the normally fragile condition of sidewall cores, care should be exercised in removing them from the coring instrument. It is recommended that they be securely sealed in small containers immediately upon removal from the coring instrument.

3. Frequency of sampling a. For whole core analysis

Frequency of sampling is no problem in whole core analysis; all the core recovered from any section to be studied should be analyzed.

b. For conventional core analysis

In plug type analysis, one sample per foot is ordinarily taken. Formation which is obviously nonproductive, such as solid shale, is not sampled. If the section to be analyzed is heterogeneous, samples may be taken closer than one per foot. Taking fewer samples than one per foot is not recommended. Sufficient samples should always be taken to define net productive thickness, transitional zones, and contacts.

c. For sidewall core analysis

Sample frequency in sidewall cores is normally beyond the control of the core analyst.

4. Preservation of core

If the core samples selected for analysis are to be analyzed for fluid content, it is necessary that the samples be preserved for transportation to the laboratory to prevent the evaporation of the liquids. This is

Fig. 10.3. Conventional diamond core barrel. Courtesy Christensen Diamond Products.

normally done by freezing with dry ice. It has been shown that samples frozen with dry ice can be stored for long periods of time without their fluid content or other properties being affected. Alternatively, one can wrap the samples tightly in plastic bags so as to exclude air. Care must be taken to prevent puncturing the bag or

Conventional Coring Operation
Fig. 10.4. Sidewell coring device. Courtesy Schlumberger Well Surveying Corporation.

exposing it to extremes of temperature. Samples of both whole core and conventional analysis are preserved by the above procedures. A third method that is sometimes used for conventional samples is to wrap them tightly with foil and coat them with paraffin. Sidewall samples are usually kept in the bottles supplied by the coring service.

5. Core handling in laboratory

After the samples arrive in the laboratory, they are placed in order of depth and sample number. If frozen, they are allowed to thaw until they can be handled. They are wiped clean again and an ultraviolet examination and a visual (microscopic) description are made and recorded. A detailed notation of fractures and vugs is also made at this time. Whole diamond core sections are frequently photographed to permit later detailed study of fractures and vugs.

10.4 Routine Core Analysis

The core which reaches the laboratory has undergone an extreme environmental change from its original undisturbed state in the reservoir. First it has been subjected to flushing and contamination by the drilling fluid; then the confining pressure and temperature have

  1. 10.5. Environmental changes undergone by core between the reservoir and laboratory. After Clark and Shearin,4 courtesy AIME.
  2. Fluid saturation: the fraction of the pore volume occupied by a particular fluid:
  3. 10.5. Environmental changes undergone by core between the reservoir and laboratory. After Clark and Shearin,4 courtesy AIME.

been reduced when it was brought to the surface. Thus gas has evolved from solution in the oil. The expansion of this gas expels some part of the liquid content. Evaporative or weathering losses occur during surface handling before the core is finally frozen or sealed. Therefore it is obvious that the fluid content as determined in the laboratory cannot be the original content.

Fortunately, the porosity and absolute permeability are not appreciably altered by these factors in most cases, provided the sample is properly cleaned. It is possible, however, that invasion of the core by drilling fluid solids may affect these values to a measurable extent. Also, the mud filtrate may, in some cases, permanently alter or cause movement of interstitial clay particles within the rock. Normally, no specific attention is given to these possibilities in routine analyses. These factors will be discussed further in the next chapter when we consider the general topic of formation damage. Figure 10.5 illustrates the conditions and factors which affect the reservoir to laboratory alteration of core samples.4

Routine core analysis, as discussed here, will include the measurement of porosity, absolute permeability, and fluid saturations. These quantities were defined in Chapter 2; however, a brief summary is in order.

1. Porosity: the fractional void space within a rock.

Vp Vb where = porosity

Vb = bulk volume

V, = solid or grain volume

Vp = pore volume

2. Permeability: the ability of a rock to transmit fluids. The discussion to this time has considered only the absolute permeability which is purely a rock property and independent of fluid content. This was defined analytically by the Darcy equations.

Vju vp

where Sw, S0, Sg = water, oil, and gas saturations, respectively

Vw, Vo, Vg = water, oil, and gas volumes in the rock

Since many measurements of rock properties require a clean dry sample, let us first consider the cleaning, drying, and other preparational procedures.

10.41 Sample Preparation

It is generally desirable to perform tests ort. samples as large as possible. This is particularly important when the formation is quite heterogeneous, as is the case with many limes and dolomites, or any fractured rocks. These are best analyzed by using the entire core, hence the common name, whole core analysis. Reasonably homogeneous rocks having intergranular porosity are normally analyzed by selection of small, representative samples at frequent intervals through the section of interest. This is commonly referred to as conventional or plug analysis. Porosity and permeability valúes are measured from small cylindrical or cubical plugs in conventional work, or from a length of the full diameter core in whole core analysis.

In plug analysis, it is customary to cut samples parallel to the bedding planes, so that measurements of horizontal permeability may be made. It is often desirable to measure the permeability perpendicular to bedding planes (vertical permeability) as well. Cubical samples are used when directional measurements are desired. This also requires that the individual cubes be carefully marked when cut so that no orientation mixup results.

After the method of sampling has been decided upon, the samples to be used for permeability (and, normally, for porosity measurements also) are thoroughly cleaned of all interstitial fluids and dried. The cleaning process is commonly performed in extraction apparatus such as that indicated in Figure 10.6. The samples are placed in the extractor with a solvent (pentane, naphtha, toluene, carbon tetrachloride, etc.) and boiled for several hours. This method of cleaning is not entirely satisfactory, and other methods are often employed. Centrifugal extractors which cycle clean solvent through the sample have several obvious advantages in that they produce a cleaner sample in much less time. After extraction, the samples are oven-dried at 250°F, or less if alteration of

Cold water

Condenser

Plug samples

■ Porous thimble

Fig. 10.6. Soxhlet extractor for plug cleaning.

SOLVENT

SOLVENT

CORES

C09 CYLINDER

interstitial clays is anticipated. After the sample has cooled, it is ready for the necessary testing.

A special method of cleaning whole core samples is shown in Figure 10.7.6 The cores are placed in the core chamber through the open end. An O-ring cap is screwed in place and the chamber is then filled with gas to a pressure equal to that of the gas dissolved in the solvent. Next, this gas is displaced at constant pressure by the solvent-gas mixture. Then, the chamber is pressured up by means of the hydraulic pump to approximately four or five times the solvent-gas pressure. When liquid flow into the cores ceases, the core chamber is depressured rapidly to atmospheric pressure; the cores are left submerged in solvent until most of the gas has flowed from the cores. The solvent is then drained off and the cycle repeated. Data showing the number of cycles necessary to clean four different types of formations are shown here. The core samples reported were cleaned and dried and their porosity determined. They were then subjected to additional cleaning cycles, and dried each time until the porosity showed no increase.

10.42 Porosity Measurements

It may be deduced from Eq. (2.1) that the porosity of a sample can be determined from a knowledge of any two of the factors Vb, V„ or Vp. There are several ways by which these may be measured with satisfactory accuracy.6 The choice of method depends largely on the type of sample, subsequent tests to be made, and the preference of the individual laboratory. Only a few techniques will be discussed here.

Grain and/or Pore Volume Measurements

1. Boyle's law porosimeters: the operation of these devices is based on the gas law. The two-cell type

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Responses

  • Cottar
    What is coring in petroleum engineering?
    2 years ago
  • leah mauer
    What is the disadvantage of drilling mud on core barrel?
    2 years ago
  • BERND
    What is coring and core analysis.?
    2 years ago
  • Maarit
    What is conventional coring?
    2 years ago
  • nuguse
    What are the types of coring rotary eqvipment as diamond coring, wireline retriveble coring?
    2 years ago
  • J
    What is coring analysis in petroleum Engineering?
    1 year ago
  • Barbara Fischer
    How a core is made in petroleum engineering?
    12 months ago
  • callisto
    What is core Analysis in petroleum engineering?
    11 months ago

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