CONVENTIONAL CORING METHODS
Porosity, permeability, grain density, and mineralogy of reservoir rocks are important elements in a reservoir description. We can estimate these properties from well logs or measure them from rock samples in the laboratory. The rock samples are obtained by cutting a piece of rock from the well bore; the process is called coring.

Conventional cores are cut using a specialized subassembly at the bottom of the drill string. This consists of a coring drill bit (usually a diamond bit), a core barrel to hold the recovered core, and fingers in the core barrel to hold the core in place while the coring assembly is pulled out of the hole. At the surface, the core is retrieved from the core barrel and placed in transport boxes, which are transported to a laboratory for further study.

         Coring assembly on bottom of drill string and coring bit

SIDEWALL CORING
If coring while drilling is impractical, small cores can be taken on wireline using a sidewall core gun or a sidewall rotary coring tool. The sidewall core gun uses black powder explosives to fire a steel bullet into the rock adjacent to the toole. The hollow bullet captures a small piece of rock that is pulled to the surface by the tool. Such guns can recover up to 48 samples in one trip in the hole. Depth control is monitored using a gamma ray log to correlate to previous logs run in the well.

The rotary coring tool uses an electrically driven  diamond bit to drill a small core from the formation adjacent to the tool. Several cores can be taken at different depths before the tool is brought to the surface.

     
Rotary core drill on wireline                  Sidewall core gun with steel bullets     

SELECTING CORE PLUG LOCATIONS
Most routine core analysis is oerformed on core plugs drilled from slabbed or whole core. Plugs are 1.0 to 1.5 inches in diameter and 2 to 3 inches long. Locations for each plug are selected based on visual observation of changes in rock texture, porosity, or lithology / mineralogy, giving rise to irregular intervals between core plugs. Some operators prefer equal intervals b etween samples.

The orientation of the core plug relative to the core axis and bedding dip angle is important. Horizintal rock properties are determined from plugs drilled parallel to bedding, not at right angles to the core's verical axis. Likewise, plugs for vertical properties are taken perpendicular to bedding planes.

       
Core slab, core plug, full diameter, and whole core definition               Core photo of slabbed core                 

Taking core plugs for horizontal and vertical rock properties requires care and attention to dipping beds, fractures, lithology variations, and porosity heterogeneity.  Do NOT high-grade the selection of core plugs by choosing only good porosity points - this will not provide useful information to control petrophysical evaluations, reserves, productivity or other performance calculations.

CORE CLEANING METHODS
Before measuring porosity and permeability, the core samples must be cleaned of residual fluids using solvents, then thoroughly dried. There are numerous lab techniques available to do this. Most methods also provide the oil and water saturation of the core samples on an “as-received” basis.

The number of cycles or amount of solvent which must be used depends on the nature of the hydrocarbons being removed and the solvent used. Often, more than one solvent must be used to clean a sample. The solvents selected must not react with the minerals in the core. Toluene and benzene are most frequently used to remove oil and methanol and water is used to remove salt from interstitial or filtrate water. The cleaning procedures used are specifically important in special core analysis tests, as the cleaning itself may change wettability.

The core sample is dried to remove connate water from the pores, or to remove solvents used in cleaning the cores. When hydratable minerals are present, the drying procedure is critical since interstitial water must be removed without mineral alteration. Drying is commonly performed in a regular oven or a vacuum oven at temperatures between 500C to 1050C. If problems with clay are expected, drying the samples at 600C and 40 % relative humidity will not damage the samples.

Direct Injection of Solvent
The solvent is injected into the sample in a continuous process. The sample is held in a rubber sleeve thus forcing the flow to be uniaxial.

Centrifuge Flushing

A centrifuge which has been fitted with a special head sprays warm solvent onto the sample. The centrifugal force then moves the solvent through the sample. The used solvent can be collected and recycled.
 

Gas Driven Extraction

The sample is placed in a pressurized atmosphere of solvent containing dissolved gas. The solvent fills the pores of sample. When the pressure is decreased, the gas comes out of solution, expands, and drives fluids out of the rock pore space. This process can be repeated as many times as necessary.

Soxhlet Extraction
A Soxhlet extraction apparatus is the most common method for cleaning samples, and is routinely used by most laboratories.
 

Soxhlet (left) and Dean-Stark (right) extraction apparatus

As shown in the illustration, toluene is brought to a slow boil in a Pyrex flask; its vapors move upwards and the core becomes engulfed in the toluene vapors at approximately 1100C. Eventually, water within the core sample in the thimble will be vaporized. The toluene and water vapors enter the inner chamber of the condenser, the cold water circulating about the inner chamber condenses both vapors to immiscible liquids. Recondensed toluene together with liquid water fall from the base of the condenser onto the core sample in the thimble; the toluene soaks the core sample and dissolves any oil with which it come into contact.

When the liquid level within the Soxhlet tube reaches the top of the siphon tube arrangement, the liquids within the Soxhlet tube are automatically emptied by a siphon effect and flow into the boiling flask. The toluene is then ready to start another cycle.

A complete extraction may take several days to several weeks in the case of low API gravity crude or presence of heavy residual hydrocarbon deposit within the core. Low permeability rock may also require a long extraction time.

 

Dean-Stark Distillation-Extraction

The Dean-Stark distillation provides a direct determination of water content. The oil and water area extracted by dripping a solvent, usually toluene or a mixture of acetone and chloroform, over the plug samples. In this method, the water and solvent are vaporized, recondensed in a cooled tube in the top of the apparatus and the water is collected in a calibrated chamber (Figure 2.1b).

The solvent overflows and drips back over the samples. The oil removed from the samples remains in solution in the solvent. Oil content is calculated by the difference between the weight of water recovered and the total weight loss after extraction and drying.

Vacuum Distillation

The oil and water content of cores may be determined by this method. As shown in Figure 2.2, a sample is placed within a leakproof vacuum system and heated to a maximum temperature of 2300C. Liquids within the sample are vaporized and passed through a condensing column that is cooled by liquid nitrogen.

Summary

The direct-injection method is effective, but slow. The method of flushing by using centrifuge is limited to plug-sized samples. The samples also must have sufficient mechanical strength to withstand the stress imposed by centrifuging. However, the procedure is fast. The gas driven-extraction method is slow. The disadvantage here is that it is not suitable for poorly consolidated samples or chalky limestones.

Distillation in a Soxhlet apparatus is slow, but is gentle on the samples. The procedure is simple and very accurate water content determination can be made. Dean-Stark method crushes the sample, so it cannot be used for other purposes, but accurate oil, water, and pore volumes can be determined.  However, if clay bound water is present some or all of it may be driven off, adding to the water recovered from the pore space.

Vacuum distillation is often used for full diameter cores because the process is relatively rapid. Vacuum distillation is also frequently used for poorly consolidated cores since the process does not damage the sample. The oil and water values are measured directly and independently of each other.