Coring and Core Analysis (Bottomhole Coring (Coring While Drilling))

Bottomhole Coring (Coring While Drilling)

Coring Devices

Bottomhole Goring: An Overview

The core bit and core barrel used in bottomhole coring are installed below a conventional, but slightly shorter and lighter, string of drill collars and stabilizers. Connecting the core barrel and bottom collar is a safety joint or "back-off" sub ( Figure 1 , Conventional core barrel and diamond core bit). This is a coarse-threaded connector that allows the drillstem to be unscrewed (backed-off) and retrieved, should the core barrel become stuck in the hole.

 

Conventional Core Assembly

Commonly, only a single size coring assembly and only one or two core bit types (based on formation hardness) will be available at the wellsite. This is mainly because the high cost and intermittent usage of coring equipment economically precludes maintaining a full range of equipment. A core bit having the same diameter as the smallest drill bit that will be used can be run into any hole of this, or larger, size (the cored hole may later be enlarged using a hole opener).

In order to achieve good core recovery, it is necessary to use a bit-barrel combination that cuts a relatively thin kerf, i.e., a large diameter core relative to the diameter of the bit. If a range of core bit sizes is used, then a compatible range of core barrels, capable of accepting various core sizes, must also be available. Coring costs may be reduced by standardizing on a single core bit-barrel combination. The most commonly used is an 8 1/2-inch "nominal" diameter bit (actual diameter is, in fact, 8 15/32-inch, since the bit cuts a slightly overgauge hole) and a 3 1/2-inch (inside diameter) core barrel. This results in an optimal kerf of 2 1/2-inch annular radius.

The core barrel consists of two main parts: a core-retaining inner barrel, and a protective outer barrel. Both are approximately 30 ft long. Up to three barrel "joints" may be combined to allow a 90 ft long core to be cut. The outer barrel provides connection between the drill collars and the core bit, and has stabilizer blades to prevent tilting, bending, or flexing, which may break or jam the core.

When the coring assembly is run into the borehole, drilling fluid is free to flow through the inner barrel ( Figure 2 (a), Core barrel running into the hole). When the core bit reaches bottom, the kelly is attached and fluid is circulated through the inner barrel for a short time in order to flush from it any debris that may have accumulated during the trip to bottom ( Figure 2 (b), Core barrel circulating on bottom).

A steel ball is then introduced into the drillstem and pumped down to the top of the core barrel, where it seats in a check valve in the inner barrel ( Figure 2 (c), Core barrel coring).

Circulation is now diverted through circulation ports into the interbarrel annulus (the space between inner and outer barrels), and to discharge ports in the face of the bit. Coring is commenced by the careful application and progressive increase of weight on bit as rotation proceeds.

As the core enters the inner barrel, it pushes the "rabbit" upward, displacing drilling fluid and wiping the walls of the barrel. The core catcher has spring-loaded fingers that prevent the core from moving downward and out of the barrel. Nevertheless, the greatest care is required when lifting, lowering, stopping, or starting rotation. Any jarring may result in the core slipping and being lost from the barrel.

Special care is needed both when a connection is made and when the drill-stem is finally picked up off bottom at the completion of coring. Some overpull (resistance to lifting) will occur initially; but if the core catcher operates correctly, the core will break cleanly and remain in the barrel. Sometimes, slippage may occur and the core catcher fail to latch. The barrel must then be lowered back to bottom and the pick-up attempted again.

Coring is necessarily complete, in that the rate of penetration falls to zero, either when the core barrel is full or it becomes jammed with core debris. The latter occurs most commonly in extremely hard, brittle rocks, which may split sub-vertically inside the core barrel to form wedge-shaped fragments that jam in the barrel and block further core entry.

Softer, less well-consolidated rocks do not break up inside the barrel this way. They may, however, be eroded by circulating drilling fluid entering the core catcher. This erosion within the inner barrel can reduce the diameter of the core, allowing it to slip through the core catcher and fall. Once coring is completed, the drillstem should be tripped out of the hole as quickly as possible -but also gently, to avoid jolts or sudden accelerations that could cause the core to be lost.

Retrievable Core Barrel

Occasionally, it may be necessary to core a larger formation interval than the 90 ft maximum allowed by a conventional core barrel. In this case, it will be necessary to make several trips out of the borehole to recover core whenever the barrel is full.

An alternative method utilizes a retrievable inner core barrel. This can be pulled back through the drillstem to surface on a wireline, or pumped back up by reverse circulation of the drilling fluid (down the annulus and up the drillstem). A replacement inner barrel can then be lowered down to bottom, and coring continued with minimum delay.

The retrievable core barrel offers a substantial time saving over conventional coring in the case where a longer core section is required. However, because the inner barrel must be free to move through all drillstem components, the cores obtained by this method are substantially smaller in diameter (11/2 inch or less) than regular samples.

Rubber-Sleeved Core Barrel

Although a high rate of drilling penetration is an encouraging sign of good porosity and permeability, it can be very worrisome to the geologist during coring. The only formations that core quickly are those that are extremely weak, and almost totally unconsolidated. Cores of such sediments, therefore, can be readily lost from the core barrel. Even if successfully recovered to surface, they may collapse into loose debris when removed from the barrel.

The rubber-sleeved core barrel offers a solution to this problem. It is similar in design to a conventional core barrel, but incorporates a shrink-fit rubber tube. This is drawn into the inner barrel by the rabbit as the core enters. The whole core is, therefore, contained in a tight rubber sheath.

On recovery to surface, the whole sheathed core may be removed in a single piece, and cut into convenient lengths for shipping or analysis. One resulting disadvantage, however, is that in order to visually inspect the core, a wellsite geologist only has access to the cut ends of the core lengths. Later, the core may be frozen or artificially consolidated by injection of plastic gel. The rubber sheath can then be removed to allow complete core evaluation.

In recent years, the use of fiberglass or aluminum inner barrels has effectively replaced rubber-sleeve coring methods in fractured or unconsolidated rock (Skopec, 1994)

Unconsolidated samples can be recovered in a rubber-sleeve core barrel. Once encapsulated in this special, tough, rubber-sleeved tool during the coring process, the core remains there during removal from the coring device and on the trip to the laboratory. Care must be taken when removing the sleeve from the core barrel; this is done by pulling the sleeve onto a V-shaped tray to prevent twisting or bending of the core.

Foam-Lined Core Barrel

One of the purposes of coring and core analysis is to obtain an estimate of the type and relative saturations of water, oil, and gas in the formation. Unfortunately, the decrease in confining pressure as the core is brought to surface will change original saturations. As the core barrel is pulled from the hole, the confining pressure on the fluids in the pore space of the core will drop from bottomhole hydrostatic pressure down to atmospheric pressure. Depending on the cored depth, this drop can be substantial. As confining pressure declines, dissolved gas will escape from solution, while free gas will expand, flushing oil and water from the core. Therefore, measured saturations at surface will be higher in gas and lower in oil and water than the original values.

A simple solution is provided by the foam-lined core barrel. Fluids expelled from the core on recovery are absorbed into the porous plastic foam lining of the inner core barrel. After the core has been removed, this foam lining is stripped from the barrel and analyzed separately for oil and water content.

Pressurized Core Barrel

A more complete, but also more expensive, solution to the problem of gas expansion is provided by the pressurized core barrel. Using automatic valves, this apparatus seals the upper and lower ends of the inner barrel after coring is completed, so that original pressures are maintained.

At surface, the mud is removed from between the inner and outer core barrels by displacement with a gelled diesel oil and the barrels are immersed in dry ice for several hours to freeze the core. Once the core is frozen, the inner barrel may be disconnected, removed, cut into four- or five-foot lengths and shipped to the laboratory for analysis (one disadvantage of this procedure is that the wellsite geologist has little chance, if any, to examine the core.

During shipment and until analysis, the barrel length must be kept frozen. This samples are then prepared and thawed in the laboratory where the oil that would normally escape with pressure reduction can be measured. When Wilhirtz and Charlson (1978) compared the results of conventional versus pressure coring, they found that approximately 50% of the residual oil in a San Andres core was lost during the pressure drop associated with non-pressured core recovery.

The primary advantage of the pressurized core barrel is that fluids do not escape as the core is brought to surface. Thus, it provides additional information on residual oil saturation valves (Hagedorn and Blackwell, 1972). Although the barrel has been available for years, it achieved special prominence when enhanced oil recovery became a matter of special interest. While this device does not prevent flushing of the core, improvements in coring fluids and bits have assisted in reducing flushing.

It is often desirable to know the quantity of oil remaining after waterflood or in a natural water-encroached zone. At the time a core is taken in a water-flushed zone, the oil exists as an immobile, trapped phase. By using a coring fluid with a water filtrate and maintaining a low sandface pressure differential, the immobile residual oil is not displaced and, in this case, flushing is not detrimental.

The pressure coring process is complex and relatively expensive, but is considered by some to be one of the better techniques for defining reservoir residual oil saturation (Hensel, Jr. 1984; Sparks, 1982).

Oriented Core Barrel

The major advantages of the core sample over well cuttings is that it offers a coherent 'mini-exposure" of the selected interval, and therefore offers nearly the same basic range of crucial geologic information as a fresh road-cut outcrop. Unfortunately, a conventional core is not oriented, and important data regarding directional features can not be determined. Even if the orientation of the inner barrel is known, this may drift during the coring operation. Even dip amounts may be uncertain, if the borehole itself is deviated from the vertical.

In an extreme example, consider a core cut in a borehole that deviates at 45° from the vertical. In this example, bedding planes are observed to dip at 45° to the long ("vertical") axis of the core. Viewed from one borehole orientation, these beds are horizontal (45 - 45 = 0), but from the opposite orientation they are vertical (45 + 45 = 90). Unless the orientation of the core, relative to the borehole, is known, one can only say that true dip is 45°, plus or minus 45°

A conventional core barrel may be adapted for oriented coring by the addition of two devices that mark the core with respect to the barrel, and the core barrel with respect to the outside world.

Orientation of the core is performed by adding an "orienting shoe" to the core catcher. This has blades that scribe grooves onto the core as it passes into the barrel. By aligning these grooves, it is possible to orient the entire segmented core relative to itself and to the core barrel.

The latter, meanwhile, is oriented by rigidly attaching a "multishot" survey tool to the top of the inner core barrel. This tool records a series of measurements of inclination and azimuth (dip and strike) at fixed time intervals. By comparing these timed measurements with the drilling record, it is possible to determine the core barrel orientation foot by foot as the core was cut. Comparison with measured dips at measured depths on the oriented core gives the true dip and strike of structures and grain textures.

Core Ejector Bit

This is not a real alternative to conventional bottom hole coring, but rather a means of obtaining usable size cuttings when drilling with a diamond bit.

The core ejector bit is similar to a conventional diamond bit, but has a central orifice like that in a core bit (although much smaller). When the bit drills, a small diameter core is cut and passes through this orifice into the drillstem -there is no core barrel or catcher. Immediately above the bit is a "core catcher" sub-a short section of drill collar-with an eccentric inside diameter. As this sub rotates, its inner wall will periodically strike the core and break it.

Some fragments of core will be retained inside the sub and recovered on its return to surface; others will fall out of the bit orifice when the bit is picked up off bottom and will be carried to surface by drilling fluid circulation.

Preparation

When extensive coring and core analysis are planned, a specialized crew of core technicians may be assigned to the wellsite from a core analysis service company. In most other circumstances, the mudlogging crew and the wellsite geologist will be responsible for core handling and evaluation.

In either case, the wellsite geologist must determine what procedures are to be followed in handling and processing the core, and what supplies will be required to accomplish this. After taking inventory at the wellsite, he or she should order sufficient quantities of the necessary supplies, leaving sufficient time for delivery.

As supplies arrive, they should be checked against the inventory and stored in a clean, dry, safe place. Wood is a precious commodity on an offshore rig. Unless locked away, wooden core boxes and lids will tend to disappear.

If the drilling crew are unfamiliar with coring, a meeting should be arranged so that the geologist or mud logger can brief them on what will be expected of them during core retrieval. It should be explained where they can be of help and where they cannot. For example, warn them against washing the core with a hosepipe! This may damage the core and will render it useless for saturation measurements.

Core Point Selection

Picking a core point is a normal job of stratigraphic correlation. The approximate depth of the zone of interest will often be already known from seismic profiles. Using logs from other wells, it is possible to identify marker horizons immediately above the zone. The sonic log is usually best for this, since it is strongly responsive to changes in formation rock strength and porosity. These are the same characteristics that affect the rate at which a bit will drill the formation. For this reason, the sonic log from a previous nearby well will show good correlation with a rate of penetration log from the present well. Even better correlation is given by 'normalized drilling exponents" (dx, NX, , etc.). These are mathematical treatments used to standardize and cancel the influence of certain mechanical variables (e.g., weight on bit, rotary speed, cutting structure design) on rate of penetration, in order to derive a parameter that is predominantly controlled by rock strength and porosity ( Figure 1 , Correlation of sonic and drilling exponent logs in order to select a coring point).

If a core barrel is run into the hole too early, before the selected interval has been reached, rig time and operating costs are wasted. If the zone of interest is recognized too late, the core can no longer be cut and a major objective of the well may be lost. Ideally, a clear, prominent marker horizon should be found that occurs immediately above the core point. This, of course, is not always possible, and on remote exploration wells (rank wildcats) it may be necessary, when approaching the zone of interest, to adopt a more time-consuming, but failsafe, procedure.

This involves halting the drilling periodically, or when a drilling break occurs; continuing to circulate for the lag time, inspecting the well cuttings from the drilled interval, and then deciding whether to drill further or commence coring.

Cutting the Core

There is little that the geologist can do to obtain useful information during the coring process itself. Rate of penetration is slow and unresponsive to lithologic changes. Well cuttings will be of the same poor quality as those from a diamond drill bit. There will also be a much smaller volume of cuttings, since only an annular kerf of formation is being cut.

Although visual examination of these cuttings is virtually useless, the geologist should have the mudlogger or sample catcher catch as much of them as possible for the archival sample sacks. These may be dumped later when successful core recovery is confirmed. If, on the other hand, all or part of the core is lost, these archival samples will be the only sample material available, for better or worse!

While coring, even the depth of the well becomes uncertain, due to the low weight on bit used. The total depth of a well, during drilling, is determined as the simple sum of the measured lengths of all components of the drillstem below the rotary kelly bushing (RKB), which is used as the datum of zero-point for depth measurement. Thus, the depth at any time is taken to be the length of the drill bit, drillstring, and as much of the kelly as has been drilled down below the RKB.

When drilling, the drill collar will be in compression, i.e., shortened by weight on bit, and the drillpipe will be in tension, i.e., stretched. These two effects will introduce small but opposing errors into the depth calculation: the net error will be very small-a few feet in several thousands of feet-and will remain consistent as drilling proceeds.

When a core barrel is run, the bottom-hole assembly will contain fewer drill collars, and the length of the drillstem will be compensated by the addition of extra drillpipe. Less weight on bit will be applied and, therefore, the relative lengths of shortened or stretched pipe will be changed. The net depth error will remain small, relative to total depth, but will differ from that measured during normal drilling. The effect is that total depth will appear to have changed after the core barrel is run into the hole. Although the difference of a few feet is relatively insignificant in terms of the total depth, it is important with respect to the core for two reasons. First, it affects how the core's length is estimated. Second, a few feet can become extremely important when the zone of interest is itself short, say 25 ft or less.

Nothing can be done to solve this problem entirely. However, the geologist, engineer, and drilling foreman can work together to have the drillstring accurately remeasured before it goes into the hole. The kelly is measured when weight is first applied to the core bit on bottom. A consensus start depth for the core should be decided on-one that can be used for all further reports and calculations.

 

Retrieving and Handling the Core

Set-Up

As soon as the core barrel is picked up off bottom, and the trip out of the hole begun, geological personnel should begin their preparations to receive, process, and package the core. This will require a period of nonstop, hectic activity-especially if the core barrel is to be returned to bottom for a second or third coring run. Work areas should be prepared, and materials made ready, in order to facilitate these activities.

Work Area

Cores will be collected, and eventually shipped, in boxes that are approximately 6 inches in height and width, and 3 feet long. An area must be found where the whole core can be laid out, walked around, and worked on by one or two people. Ideally, a bench should be used for laying down the core boxes so that the geologist may work in a comfortable standing or sitting position. The area should be under cover and well lit (cores might be recovered at night or in bad weather). It should have electrical outlets for the microscope illuminator, ultraviolet inspection lamp, and other devices. Running water is a convenience, but not essential, since the core should not be washed.

In addition to the equipment required for microscopic examination of cuttings, the following special supplies should be available in the work area:

· sufficient clean core boxes to receive the expected core footage (plus a few spares);

supply of clean rags for cleaning the core and stuffing core boxes;

measuring tape;

geological hammer (or hammer and cold chisel);

hand lens;

waterproof marker pens in red, black, and other colors (with spares);

spare work gloves;

worksheets, note pads, and pencils; wrapping, sealing, and canning supplies;

labeled archival and dried sample sacks;

copy of the core sampling and shipping instructions.

Rig Floor

About one hour before the core barrel is due on surface, the geologist should begin preparing the area of the rig floor that has been designated as the core-retrieval area. The materials needed include:

· catching set of core boxes, labeled and stacked as shown in Figure 1 (Labeling and stack order of core boxes):

geological hammer;

soft metal hammer;

broom;

work gloves;

supply of clean gloves;

notepad and pencils on clipboard.

The catching set is used to keep the core in correct order, both while it is being retrieved and later, after it has been removed to the work area. After use, the boxes may be washed and reused. They should be clearly labeled with top, bottom, and box number, and stacked in an orderly manner to avoid confusion even on a crowded, poorly lit rig floor at night.

Notice that boxes are numbered according to the order of the core as it comes out of the barrel: box #1 holds the first (bottom) section of core, box #2, the second section, etc. This numbering scheme also reflects the age progression of the rocks sampled, box #1 holding the oldest sediments and so on.

Enough boxes should be available to accommodate the whole length of core. Each will hold about two feet of core (it will, in fact, hold almost three feet, but the core cannot be expected to come from the barrel in convenient lengths, and should not be broken until after initial inspection).

A nearby area should be cleared for stacking the filled core boxes and drilling crew members delegated to help transport them to the work area. If the filled boxes are to be lifted down from the rig floor by crane, then a pallet should be laid down upon which the filled boxes can be stacked.

Recovery from the Barrel

When the core reaches surface, the outer core barrel is suspended in the rotary table. The inner barrel is lifted out and suspended with the traveling block over the core retrieval area. The core catcher is removed, and replaced with a barrel clamp and core tongs, which allow the core to be slipped from the barrel at a safe and controlled rate ( Figure 2 , Conventional retrieval of the core from the inner barrel).

The first two or three feet of the core should be slipped from the barrel, briefly inspected, wiped dry of drilling fluid, and placed in box #1. The next section of core should not be "slipped" until this is completed. If loose debris falls from the barrel, it should be swept clear with the broom and placed into the appropriate position in the core box. It may be possible later to piece together this broken material.

At no time should anyone reach under the core barrel or block the view of the driller operating the drawworks brake. Loggers should remove core pieces from beneath the barrel with a broom or hammer and stand clear when not doing so.

If the core ceases to slide from the barrel, it may be jammed (the barrel is not empty until the rabbit slides out). Light tapping with a light metal hammer or mallet should free the core. Do not use the geological hammer, which can damage the barrel. If hammering cannot free the core, it must be pumped out ( Figure 3 , Retrieval by pumping in the case where the core is jammed). As core pieces are pumped free of the barrel, they will fall to the ground and may be pushed away with the broom to be dried and boxed. Do not walk in front or handle the end of the core barrel, or any core that is protruding from it.

 

Boxing the Core

On arrival at the work area, the core must be transferred to clean core boxes, fitted together where possible, and measured. Most oil companies require core boxes to be numbered in the manner mentioned, i.e., from the bottom upward, but some European companies prefer a reverse order, so that the core is reboxed with the top of the core in box #1. Whatever method is to be used should be decided on beforehand.

Each piece of core should be taken from the catching box and wiped clean with a dry rag; its obvious features (such as fractures, bedding planes, oil or gas bleeding from porosity) should then be noted on a worksheet. The piece is then transferred to its resting place in a clean, new core box. Loose rubble should be put in a clean sample sack and labeled with well and core identification information, as well as an estimation of the number of feet it represents ( Figure 4 , Labeling of the core for shipping).

When each box is about full, the core pieces should be rotated and pushed together to obtain the best possible fit between broken ends. Ends should then be marked with appropriate symbols to indicate the quality of fit: double chevrons for no fit, single chevrons for a poor fit, and no symbol for a perfect fit. The core is then oriented by marking two lines on it in felt tip pen from top to bottom (red to the left, black to the right), with the pieces pushed together for best measured fit. A tally sheet should be kept with the measured length of core and estimated rubble content of each box. When the total length of core is known, these length measurements are translated into depths ( Figure 5 , Example core tally sheet ) --missing core is always assumed to have been lost from the bottom of the barrel.

Finally, the pieces are moved slightly apart and separated by rags, which are also stuffed into the space between pieces and at the ends of the box. This prevents damage to the core during transport. The process is repeated for each box until the whole core is reboxed. The catching set of boxes may now be removed, and the geologist can proceed with sampling and geological evaluation.

Other Coring Devices

RUBBER, PLASTIC, AND FIBERGLASS CORES: Rubber sleeve, plastic sleeve, and fiberglass cores should be marked with waterproof materials for top, bottom, and depth. and then sealed with caps (when available) furnished by the coring company. Tape should be wrapped around the sleeve-cap joint. This in turn can be dipped in strippable plastic to ensure air tightness.

The sleeves are sometimes transported frozen in full-length wooden boxes. Often they have first been cut into 3 to 5 ft (1 to 1.5 m) lengths. The shipping boxes should also have top, bottom, and depth identified. Care should be exercised to prevent the sleeves from bending, since this would destroy the grain-to-grain integrity of the core.

PRESSURE CORE: Core handling of the pressure core barrel is managed by the coring company. The samples remain in the inner barrel for transport to the laboratory. Typically, the inner barrel is frozen at the wellsite, cut into 3 to 5 ft (1 to 1.5 m) lengths while in a frozen state, and then shipped in a freeze box. Samples must remain frozen until analysis in the laboratory.

SPONGE CORE: Thirty ft (9 m) of formation is normally cored with the sponge barrel, after which the core is hoisted to the surface. The inner core barrel is removed from the outer barrel and laid down. This inner barrel contains the sponge liner, which in turn contains the core. Hydraulic pressure is used to force the 30 ft (9 m) of sponge liner from the inner barrel. As 5 ft (1.5 m) sections are exposed, the sponge-liner-enclosed core is broken off and placed in PVC handling tubes for transport to a core analysis laboratory. If desired, the handling tubes can be filled with drilling mud or formation salt water to eliminate exposure of the cores to air.

Core Preservation Techniques

Cores are packaged to prevent fluid loss between the time of recovery and analysis. Heavy-duty plastic bags are commonly used for short-term storage. For long-term storage, samples are often sealed by first wrapping the core in layers of SaranTM wrap followed by several layers of aluminum foil, and then dipping the wrapped core in strippable plastic or wax. Freezing, canning, and submerging a core under fluid have all been successfully employed as means of preserving cores. Cores cut to evaluate interstitial water, measure fluid levels, or to interpret gas, oil, or water production can be packaged by any technique other than submersion, since exposure of the core to liquid would result in imbibition of that liquid and alteration of residual saturations.

Exposure of a core to the atmosphere should be minimized when preservation of core wettability is desired. Immediate submersion under deaerated water is suitable for cores taken with water-base mud. Cores cut with oil filtrate muds can be stored under nonoxidized oil. The SaranTM wrap-foil-strippable plastic preservation technique is suitable for all coring fluids and is generally accepted as the best.

Some coring techniques have a built-in preservation ability that can be used to good advantage, at least for short-term storage. Rubber sleeve, pressure, plastic-sleeve, and fiberglass core barrels need only be cut into suitable lengths and then capped to properly preserve the core.

Sealing in Plastic Bags

Sealing a core in plastic bags is an excellent short-term preservation technique that should be restricted to no longer than several days. The sample should be placed within the plastic bag and air space squeezed from between the core and the bag wall. The top can then be twisted to seal the sample and taped against the plastic or sealed with a rubber band. If the samples are to be shipped, the core should be double-bagged with insulating material placed between the cores. Each plastic bag should be labeled for depth interval, top, and bottom. The plastic bags normally used are suitable for cores of up to approximately 1 ft (30 cm) in length. A newer, 3 ft (1 m) bag with a zip-lock liner exists for fast preservation within cardboard boxes. This long bag can be opened and folded over the side of the box, and then samples may be put in depth sequence within it. This also allows the top to be opened so that the geologist may secure small chips of the rock for description or view the core without the cover of a plastic film.

Sealing in Strippable Plastic or Wax

This technique is the best for long-term storage. Individual pieces of core should be wrapped with several layers of SaranTM wrap or Dow HandiwrapTM (other plastic products affect sample wettability). The plastic should then be wrapped with several layers of aluminum foil, making sure the foil is pressed to the sample to eliminate sharp corners where subsequent sealing materials may run off and not coat. Each sample should be marked clearly for well name, depth, and top or bottom with a permanent marker or an attached label.

Heavy wire or twine should then be tied to each piece of wrapped core to allow immersion of the sample into molten wax or plastic, or the samples can be dipped from opposite ends with a sealant overlap. The samples should be immersed and removed rapidly so as not to melt the inner plastic wrap. Samples should be dipped at least twice, and then hung on a rack to cool. The wire or twine should be clipped near the core. The clipped end must be sealed to prevent a wick effect that will allow moisture to escape. If paraffin wax is used as the preservative material, the wax should be heated only slightly above the melting point; otherwise, the temperatures may be too high, and damage may occur to the inner layer of SaranTM

Plastic materials used for coating cores must be stable over long periods of time, must not react with water or oil, and should not exude materials when set. Recent research has resulted in the development of a product with superior sealing properties known as CoreSealTM The CoreSealTM product is new; history on its performance is available only for a two-year period.

When tests are to be made on cores that have been stored for long periods, selected samples should be analyzed for water saturation. The results should be compared to saturations present in the core at the time the rock was preserved in order to assess the amount of fluids that might have been lost.

Sampling for Core Analysis

Once the core is cleaned, boxed, and measured, the core analysis samples should be selected and preserved. Normally, samples are taken at regular intervals in massive formations considered to have reservoir potential (porosity and permeability), but must be more carefully selected in thin, inhomogeneous beds.

Thick, impermeable formations, meanwhile, need to be sampled only at the top, bottom, and once in the middle (or every 50 feet, whichever is less). All permeable zones more than two inches thick within such formation should also be sampled. Thicker permeable intervals should be sampled every foot, or every two feet if lithologically uniform. In less homogeneous formations, the sampling interval should be reduced to obtain reasonably representative samples for the whole core.

For analysis, a coherent piece, about 3 inches long and showing no splits or cracks, should be taken. At each sample point, the minimum amount of core that will ensure 3 inches of homogeneous, unbroken rock should be selected. Core samples must be sealed in order to preserve contained fluids until analysis. There are a number of methods used for this. Canning is a common method, but one that is not recommended. Although it provides a tight seal, the air space in the can allows evaporation and expansion of core fluids. Improvement is gained by wrapping the sample in a large quantity of nonabsorbent material prior to canning.

Dipping the core sample in wax or a thermoplastic gel is an excellent method of sealing and is very long-lasting. To prevent surface porosity damage to the core, it should be wrapped in Saran Wrap™ or aluminum foil before dipping.

A modern method, widely used to preserve pressurized cores, involves freezing in dry ice. It requires special facilities at the wellsite, however, and either rapid or refrigerated transportation to a core laboratory.

Where core analysis is to be performed at the wellsite, and preservation of the core sample is required for only a few hours, double wrapping is quite successful. The sample is first wrapped in Saran Wrap™ (Saran Wrap™ only! Other forms of food wrap are gas permeable), and then in aluminum foil. If longer preservation is required, it is recommended that the wrapped sample be further enclosed in a heat-sealed polyethylene bag or sleeve.

If core analysis is to be performed at the wellsite, the sample need only be labeled by depth and returned to its appropriate location in the core box. If the sample is to be shipped to a core laboratory, then it must be labeled in more detail:

oil company name and division;

well name and location;

core number;

sample number;

sample depth interval.

Samples should be packed in a wooden or metal container with rag, straw, or newspaper stuffing between the samples. A sample inventory should be prepared, and copies of it made. One copy should be enclosed in the sample box. The other should be retained at the wellsite until receipt of the samples has been acknowledged. The sample inventory should contain the following:

name of the laboratory and responsible person;

oil company name and division;

well name and location;

core number and depth interval;

total number of samples;

total number of boxes;

list of samples by number, depth, and box;

type of analysis required;

drilling fluid type and special peculiarities;

drilling fluid filtrate loss, salinity, nitrate concentration, and any other special tests that were run;

name and location of person to whom results are to be reported.

Gaps in the core box from removal of a core analysis sample should be stuffed with rags and the notation "CORE ANALYSIS SAMPLE: INCHES" made on the inside of the box at that point.

Geological Evaluation: Hand Samples

Following the brief examination made while boxing the core, a more thorough evaluation should be made to describe in detail all significant macroscopic features of the core. While doing this, the geologist should also be extracting small chips of core that will be used both for microscopic examination and to take the place of washed and dried cuttings samples in the well sample set.

Sample description should include:

lithology and thickness of major formation units;

dip (apparent or true-if known) and strike of boundaries, beddings, fractures, and other structures;

nature of lithological boundaries; nature of bedding planes, and of sedimentary and diagenetic structures;

gradational features in grain size, sorting, etc.;

spacing and surface texture of fractures and joints.

Microscopic Examination

Collected core chips should be subjected to the full lithological and hydrocarbon examination that is standard for cuttings examination. Samples may be broken with a hammer and crushed in the blender to obtain fine-grained material for examination of grain texture and to liberate gas for analysis by the mudlogger. Sieving or point counting of this disaggregated material can give a quantitative estimate of grain size and sorting.

A sample log, consisting of a graphical representation of lithology and structure, and a written description should be prepared from the notes made during all stages of sample examination.

Core Slabbing

The core slabber is a table mounted, diamond circular saw ( Figure 1 , Diamond saw for cutting core slabs). It may be used to cut flat horizontal or flat vertical slabs of core. The faces on these slabs give very fresh exposure of geologic features and improve visual inspection at the microscope ( Figure 2 , Orientation of etched core slab for examination). After slabbing, the core may be polished with carborundum and etched with dilute hydrochloric acid.

 

Thin Sections

Inspection of porosity, texture, mineralogy, and microfossils can be much improved by the production of thin sections from slabbed samples of core. These can be prepared by most core analysis service companies.

Some mud logging contractors will also be able to provide thin sections at the wellsite. This can be especially useful for cores cut in carbonate sections, where determining the specific genetic origin and distribution of porosity can be more important than measuring its quantitative value.

Acetate Peels

An alternative to thin sections is the acetate peel, which can also be prepared at the wellsite. Softened plastic is applied to the slabbed and etched surface of the core and allowed to set. The plastic is then peeled off the surface and, when mounted between glass sheets, used as a photographic negative and enlarged.

The traditional method requires the mixing of unpleasant chemicals in order to prepare a solution of cellulose acetate. An improved method uses precut sheets of acetate film that may be softened with acetone and applied to the core surface.

Laboratory Sample

Core Sampling

In some cases wellsite sampling is performed, but it is preferable to preserve and transport the entire core to the laboratory for sampling under more controlled conditions. When the entire core reaches the laboratory, it should be placed in depth order on a layout table. After the core is refitted, the natural gamma activity of the core can be logged and photos of the native rock can be taken. A detailed core description should then be made.

Two basic approaches exist in sample selection. One is a statistical approach, in which cores are sampled from the top or middle of each foot of rock, independent of the lithology. The second approach requires that the analyst secure a representative sample, regardless of its location, from each foot. The actual approach will depend upon company philosophy and the formation characteristics.

Conventional (Plug) Analysis

In homogeneous formations a core segment of approximately 4 inches (10 cm) in length or less taken from every foot of core is sufficient for core plug analysis. If the core has great lithological variations, however, samples should be obtained more frequently. It is important that the samples be representative — core data have been skewed by improper sample selection.

Full Diameter Analysis

If a full-diameter analysis is to be performed, samples 6 inches (15 cm) or longer are prepared in the form of right cylinders from each foot of core, using a diamond saw. The core ends are sometimes used for saturation determinations. It is important that the lubricant selected for the saw blade corresponds to the filtrate of the coring fluid, so that additional extraneous fluids are not added.

Rubber, Plastic, and Fiberglass Cores

Rubber-sleeve cores arrive in the laboratory within the rubber sleeve, and a core gamma log is run on the core to assist in selecting sampling points. In thinly laminated sands, the core is sometimes X-rayed to locate bedding planes and sand stringers suitable for sampling. In some cases the cores are frozen and then slabbed. In other cases the rubber sleeve is placed in a wooden or cardboard box and a stabilizing foam or plaster of Paris is used to surround the core. The uppermost part of the rubber sleeve is removed, the mud cake is cut off, and the core is then available for sampling. This technique exposes the full length of the sample, and the core can be photographed prior to or after the samples have been taken.

Another technique is to cut a window in the sleeve, leaving one side hinged so that it may be folded back while the sample is taken. The window may subsequently be closed and taped. The rock is sometimes frozen and plugs are drilled with liquid nitrogen to assure that grain-to-grain contacts are maintained until tests commence.

Plastic sleeve and fiberglass core barrels containing unconsolidated reservoir rock are often frozen and then slabbed. Plugs can be drilled from the frozen core at desired intervals. In other cases a hole is drilled into the sleeve and samples are either drilled from the encased rock or removed by a punch when the rock is very unconsolidated. In other applications twin cuts are made down the sides of the barrel parallel to the long axis of the core. The sleeve material is lifted away, exposing the core for examination and sample selection.

Pressure Cores

Cores recovered in a pressure core barrel arrive in the laboratory frozen within the inner metal barrel, having been transported in insulated chests filled with dry ice. Selected lengths are placed on a milling machine and grooves are cut along the opposite sides of the metal barrel almost to the core. A tissue-thin layer of metal is allowed to remain to preserve the integrity of the barrel. At the time the analysis is to start, a screwdriver is wedged into the groove and turned, which removes the upper half of the metal barrel and exposes the core for subsequent sampling. While a plug analysis can be performed, it is common to use full diameter samples of up to 8 inches (20 cm) in length.

Sponge Cores

The sponge core barrel arrives at the laboratory in a PVC handling tube. The aluminum shell and the sponge are split open to expose the core. Full diameter analysis techniques are employed. Samples are selected and marked on both the core and the sponge before the core is removed from the sponge. Sponge samples for analysis are taken directly adjacent to the corresponding core samples. This is to insure that the oil recovered from the sponge is attributed to the pore space from which it was expulsed. The sponge samples are subsequently extracted and residual fluid volumes are determined.