Coring and Core Analysis (Major Types of Core Analysis)

Major Types of Core Analysis

Plug (Conventional) Analysis

Consolidated Formations

This technique is normally restricted to homogeneous formations that can be characterized with plug-size samples. Typical plug size is 1 inch (2.5 m) in diameter, and 1 inch (2.5 cm) long. Cylindrical samples ( Figure 1 , Core sample for conventional analysis with properly and improperly selected horizontal and vertical plugs) are normally cut with a diamond core bit parallel to bedding planes and trimmed to yield a plug from the center of the core where minimal filtrate flushing and invasion of mud solids is to be expected. Vertical permeability samples are drilled at right angles to bedding planes. Although generally used for sandstones, this technique is also satisfactory for the more homogenous, nonfractured, and nonvuggy carbonates.

 

Unconsolidated Formations

Unconsolidated sand recovered within a rubber sleeve core barrel, a plastic inner barrel liner, or a fiberglass barrel is often stabilized by freezing prior to sampling. Frozen interstitial water present at the grain contacts immobilizes the rock particles. Plugs are drilled using liquid nitrogen as the bit lubricant.

In other cases a rubber sleeve core is first immobilized by surrounding it with wax, plaster of Paris, foam or other suitable materials, following which the sample may be frozen and drilled. When a core is completely unconsolidated, plug samples can be removed by insertion of a hollow punch into nonfrozen core. Friable cores, however, should not be punched, as porosity and permeability will be created in the core. Instead, such plug samples should be confined in a metal, plastic, or rubber sleeve, and be subjected to simulated overburden pressure during analysis. Failure to treat unconsolidated cores in this fashion will yield much higher porosity and permeability values than those actually present in the reservoir.

The plastic inner liner has been a successful solution to recovery of unconsolidated Canadian tar sands. These formations are subsequently mined, and it is essential that tar content be accurately defined. A modified evaluation technique is used that does not rely on plugs cut at selected intervals, but uses a small representative portion of the full diameter core. This process requires that the plastic sleeve be cut into 1 ft (30 cm) lengths, which are then cut in half vertically. Three additional cuts down the full length of the sample are made on one of these core halves. This results in three continuous wedge sections of rock approximately 1 ft (30 cm) long and 1 square inch (6.75 sq cm) in area. The center portion of the half is used for determination of tar saturation and can be related to a given volume or weight of reservoir rock. Plugs are taken from the one-half full diameter slice resulting from the original cut. These are confined in jackets, and are then analyzed for porosity and permeability, using standard techniques.

 

Full Diameter Analysis

Routine Analysis

Full diameter analysis was introduced to allow testing of rocks with complex lithology, such as heterogeneous carbonates ( Figure 1 , Heterogeneous carbonate requiring full diameter analysis) and fissured, vugular formations unsuitable for plug analysis. Analysis of these rocks requires samples that are as large as can be obtained, so that pore spaces are small compared to the bulk volume of the samples. Lithology and pore space in carbonates may be highly variable, and the porosity can exist as micro-porosity, intergranular, vuggy, fracture, or a combination of all four. The full diameter technique does not differentiate between the contribution made by each of the various types of porosity, but yields a single porosity value that includes all pore type combinations.

Samples in the form of a right cylinder up to 10 inches (25 cm) long and approximately 5 inches (12.5 cm) in diameter are often used. Data generated include Boyle's law porosities, utilizing helium as the saturating medium. Two horizontal permeability values are determined. When fractures or vugs are present, one of the permeability measurements is visually oriented through the more permeable section, and the second permeability is at right angles to this measurement. In this manner, the effect of vugs or fractures on horizontal permeability is indicated. Vertical permeabilities are also frequently determined.

Property	Plug data	Whole core data
Air permeability, md.	0.1	69
Porosity, %	10.3	11.3
Residual oil,% pore space	14.7	17.1
Total water, % pore space	24.6	37.7

Table 1.
Comparison of plug and whole-core data on micro fractured oil-productive sandstone samples

A method for differentiating between matrix properties and full diameter data affected by fracture or vug porosity is to drill and test plugs selected from the more uniform matrix. A comparison of such data using this type of test is shown in Table 1. The difference is significant. Matrix properties are important because they control initial water content and, hence, matrix hydrocarbon saturation.

Pressure Core Analysis

The analysis of full diameter pressure cores follows, in a modified form, the procedures normally employed in more routine analysis. Full diameter samples are cut in the form of a right cylinder and then placed in specialized, airtight containers where they thaw, so that fluids expulsed from the core can be collected and measured. The cores are subsequently moved through a Dean-Stark device (Section 6.4.2) for measurement of water saturation in each sample. Pressure core samples should be further cleaned in the toluene-CO2 pressure fluxer after removal from the Dean-Stark device. This requires that the samples be placed in a surgical stocking so that any rock fragments that come loose from the core during cleaning are retained. This is necessary because the residual oil saturation value that is obtained from the analysis is at least partially dependent upon weights taken during the analytical process.

The airtight vessel in which each frozen core is placed is evacuated for a short time interval to remove air surrounding the core. As the rock thaws, the gas that evolves from the residual oil saturation escapes from the core and is retained in the vessel surrounding the core. The volume of this gas is measured and its composition determined by chromatograph. The latter is helpful if exotic gases have been injected into the formation and you must know what portions of the reservoir have been swept by this injected gas. The surface volume equivalent of the residual oil saturation present in the core at reservoir conditions is determined by summing the oil that is expulsed during the thawing process with the oil that is subsequently removed during the Dean-Stark and toluene-CO2 cleaning.

To summarize the handling process for pressure cores:

· The metal barrel is milled down its length and the core is removed.

The drilling mud is chipped from the core surface.

The core is cut into full diameter right cylinders.

The core is weighed and then thawed in evacuated glass chambers.

The oil, water, and gas expulsed are collected and the gas volume is determined. The composition of the collected gas can also be measured.

The core samples are cleaned in a Dean-Stark apparatus. This furnishes water saturation data and partial data for the determination of residual oil.

The core is cleaned in a toluene-CO2 extractor.

A Boyle's law porosity value is determined, as well as horizontal and vertical permeability. The water and residual oil saturations are calculated and a correction for oil shrinkage is applied.

At selected intervals, plugs from sections of the frozen rock not used in full-diameter analysis are drilled vertically down the center line of the core. The water that is present in the centermost plug and in the surrounding doughnut is analyzed for the presence of tracers previously added to the filtrate. This yields insight into core flushing.

Sponge Core Analysis

Full diameter analysis of samples recovered within the sponge barrel proceeds along the usual lines once the core has been removed from the barrel. The sponge itself is cut from the core barrel and the fluids it contains are extracted using a vacuum retort technique. Both oil and water volumes within the sponge are measured. Table 2., below shows residual oil saturation data for the core alone and for the core plus sponge for a specific field example. Note that the contribution of the sponge is variable and may be significant.

DepthFeet	Core residual oil % pore space	Sponge residual oil % pore space	Total (sponge pluscore) residual oil% pore space
4636	23.1	0.8	23.9
4637	23.1	0.8	23.9
4638	21.7	8.3	30.0
4639	20.4	10.0	30.4
4640	28.7	7.0	35.7
4641	22.1	6.2	28.3

Table 2.
Core, sponge, and core-plus-sponge residual oil saturation data

Sidewall Cores

Sidewall core analysis is made on all non-shale samples sent to the laboratory. The sampling, therefore, is at the option of the operator selecting depth points at which to recover a core. Results of these analyses are often used to define the gas, oil, and water zones; hence, samples should be spaced at regular intervals throughout the vertical section to be evaluated. It is important that the analyst receive these samples in correct vertical depth sequence, as this assists in the interpretation of the probable production. In areas where sidewall core-conventional core correlations are not available, it is important to take a conventional core in a reservoir and then to follow this with samples of sidewall cores. From this, a sidewall-conventional core data relationship can be developed for use in subsequent wells.

Sidewall Core Analysis

Sidewall samples are used extensively in softer sand areas. (Note, however, that a sidewall-drilled plug from a new sidewall coring device can be used for harder formations and can be analyzed in the same manner as a standard plug-sized core.) Percussion sidewalls are often smaller and demand additional attention. Equipment for them is miniaturized to reduce dead volume in the testing apparatus, although techniques used for analysis are similar to those utilized in conventional plug analysis. The sidewall cores are normally coated with drilling mud, which is removed prior to analysis. In areas of high API-gravity (light) oils, sidewall cores are often smaller than 1 inch (2.5 cm) in diameter. In areas of low API-gravity (heavy) oils, samples are often larger.

Permeabilities measured on percussion samples rarely yield true in situ values. For hard, low-permeability formations, permeability values are too high due to impact fracturing, while unconsolidated sands are usually compacted and yield erroneously low values. This conclusion was documented by Reudelhuber and Furen (1957), as well as Koepf and Granberry (1960).

Data show that porosities measured on sidewall samples approach conventional analysis values in formations having true porosities ranging from 32% to 34%. In hard formations, sidewall porosity values are normally higher than conventional plug values, as shown by Webster and Dawsongrove (1959).

Porosity in hard, well-cemented rock is increased by grain shattering during bullet impact, and these alterations in properties limit sidewall sample usefulness in reservoir engineering evaluations. However, sidewall cores are excellent indicators of lithology, furnish data on the presence or absence of oil and gas, and are valuable for interpretation of probable production. They also furnish samples suitable for petrographic work.

Because of the small sample size, techniques employed in some areas require that all the sample be used for porosity and saturation determinations. In this circumstance, a visual assessment of the grain size, the shaliness of the sample, a measured porosity, and the natural density of the fresh core is used with correlation charts appropriate to the area to arrive at an empirical value of permeability. In the hands of an experienced and competent analyst, such estimated values of permeability are suitable for formation evaluation.

An improvement in the visual assessment of grain size and sorting was recently developed and is now used in selected laboratories. The instrument is referred to as a particle size analyzer. This procedure utilizes Stokes' law and rapidly furnishes the distribution of grain sizes for each sidewall core sample, using a Stokes' law device. A small portion of the sample is disaggregated and allowed to settle in a water bath. Material settling to the bottom of the tube is retained on a balance pan and the increasing weight is transmitted electronically to a computer. The settling time within a tube of known height is related to the grain diameter. Interpretation is made by a computer, which yields both tabular and graphical histogram reports. The grain size distribution and the median grain diameter are then used to assess the quality of the rock and, with correlations, to furnish estimated values of permeability.

Sidewall samples from heavy oil formations are sometimes encapsulated in metal or plastic jackets prior to analysis. This maintains the integrity of the core as the heavy oil is extracted during analysis. A common method of analyzing encapsulated samples utilizes a Dean-Stark cleaning process, followed by a Boyle's law porosity test.

 

B.6. Core Sample Preparation

Cleaning

The measurement of permeability and porosity using Boyle's law and resaturation techniques requires that residual fluids be removed and the cores be cleaned and dried. The solvents used to remove oil must not react with the rock; they include toluene and xylene. Typically, water is removed by heating the rock, with subsequent vaporization, and trapping of the resultant water vapor. Although not part of routine core analysis, samples may be leached with water or methanol to remove salt left from the vaporized interstitial or filtrate water.

Cores may be cleaned by:

refluxing in a Dean-Stark or Soxhlet apparatus, which is a slow but gentle process;

flushing in a centrifuge, which is limited to plug-size cores and requires that the mechanical strength of the core be sufficient to withstand the centrifugal forces;

pressure flushing of solvent through the core, which is a slow process;

repeated pressure cycling of the core with a carbon dioxide and toluene mixture (a relatively fast process not suitable for poorly consolidated sand or chalky limestone, and the best technique for full-diameter cores);

vapor soaking, with condensed toluene dripping on the core. This technique is suitable for nonclay and non-gypsum-bearing formations.

Certain clays (primarily montmorillonite) will dehydrate at temperatures lower than 180° F (82° C) if the relative humidity is reduced to zero during the cleaning process. It is important to avoid this because the water on the clay surface is chemically bound to the clays in the reservoir and reduces pore space available for hydrocarbons. Data indicate that reservoir clays at moderate depths retain two molecular layers of water on their surfaces. Removal of this water in the laboratory results in an increase of approximately 3.3 porosity points for each 10% of montmorillonite present. Removal of this water will also result in an increase in the measured grain density.

The Dean-Stark and Soxhlet cleaning techniques may not be suitable for clay-bearing rock, since samples have been known to crack during the cleaning process when these techniques have been used. The Soxhlet technique can be used in some circumstances by cooling the toluene prior to the time it contacts the samples. Low-temperature solvent flushing by centrifuging is recommended, as it is relatively fast and has proven to be a reliable technique when clays are troublesome. Bush and Jenkins (1970) discussed these phenomena and techniques for handling clay-bearing samples.

Gypsum poses similar problems and also requires special analytical techniques that expose the samples to temperatures no greater than 147°F (64ºC). Hurd and Fitch (1959) addressed this problem in detail and found that the presence of 10% gypsum in a rock sample will increase the porosity 4.7 points if all water of hydration is removed from this material.

Drying

Drying poses no problems in stable rocks, and temperatures of 240ºF (115ºC) can be maintained with no damage to the cores. Formations containing hydratable clays can be dried in a humidity-drying oven set at 45% relative humidity and 145ºF (64ºC). These conditions leave two molecular layers of water on the clay surfaces, an amount that clay chemists believe to be reasonable. Observation of samples taken from depths of 1000 ft (305 m) or less shows that some cores crack while in the humidity oven even under these conditions. This would indicate that more than two molecular layers are actually present on the clay surfaces at these lesser depths.

Property	Oven dried @ 240° F	Humidity Dried*
Bulk volume: cm3	10.0	10.0
Pore volume: cm3	2.6	2.27
Porosity: %	26.0	22.7
Grain volume: cm3	7.4	7.73
Dry weight: g	19.61	19.94
Grain density: g/cm3	2.65	2.58

*145° F and 45% relative humidity

Table 1. Example of porosity and grain density variations between humidity-dried and oven-dried clay-bearing rock samples

The humidity-drying technique is sometimes utilized in core analysis studies. Water bound to the sand grains reduces both porosity and grain density. Clay-bearing samples subsequently dried in a regular oven will show both an increase in porosity and an increase in grain density (an example is given in Table 1). The user of the core analysis data should know the core analysis drying technique that was employed.

High permeability samples can sometimes be dried within several hours, and this time framework can be reduced by application of a vacuum to the samples as heat is applied. As permeability decreases, drying time increases. In some cores, with permeabilities of less than one millidarcy, 48 hours have been required in nonvacuum ovens. As the sample size increases so does drying time. Unconsolidated rock samples enclosed within metal or plastic jackets will require longer drying times than samples exposed on all surfaces.