Basic Properties obtained from Core Analyses
A solid understanding of geology, combined with core and log analyses form the basis of petrophysical interpretation A good discussion of integrating data is found in the Petroleum Engineering Handbook, Vol V(A), beginning on page 421. (see reference 1). Since the log data covers much more of the formation than the core, the core data may be used to improve or extend the log data.
Core analyses may be conducted on plug samples or the whole core. In carbonate reservoirs with a high degree of heterogeneity, whole core samples are preferred. Plug samples will provide only horizontal permeabilities.
Routine core analyses includes:
At the completion of the routine core analyses, the core is normally slabbed and photographed. Some core samples may be retained for additional tests. All other analyses done on cores are considered special core analyses or SCAL. The most common SCAL are capillary pressure and relative permeability tests. Other SCAL tests include water susceptibility tests, wettability studies, rock compressibility, interfacial tention, cation ion exchange, mineral descriptions, formation resistivity factors, acoustic velocity and many thermal flood tests. The more common SCAL tests will be discussed in a separate section.
The core is normally cut with a diamond bit core barrel and the core is retrieved by pulling the barrel to the surface with the drilling string. Slips inside of the core barrel keep the core from falling out as it is pulled to the surface. Typical core barrels are 30 ft long, so selected intervals to core are a multiple of 30 ft. (see discussion at end on long core barrels) Cores cut are used in routine core analyses, and SCAL analyses are performed on selected samples. Generally-available core barrels outside diameter range from 5.75" to 7" and cut cores outside diameters range from 2" to 2.75."
The residual dead oil is normally reported in the core analyses. It can be a very low saturation and not representative of the residual oil from waterflooding. Extreme flushing from a core can take place as a result of jetting of drilling mud as the core is taken. Variables that influence the degree of flushing include: a) overbalance of pressure at the point where core is being cut, b) interfacial tension c) wettibility d) permeability e) core penetration rate f) core diameter g) type of drill bit h) drilling mud compositon. Further reduction in residual oil will occur as oil is expelled from core due to gas expansion. However, the residual oil saturations may help identify an oil-water contact. (Reference 1) Connate water saturation identification is possible if cored with an oil based mud, although the cores must be specially cut and preserved for this purpose.
Core barrels sometimes jam and can not take samples. This can occur in fractured carbonate formations, and can be taken of a good indicator of fractures. Often, all mobile oil is flushed from the cores by the drilling mud. Cores when taken to the surface may continue to bleed oil. This may be a qualitative sign of a tight oil bearing formation.
Coring in friable, uncemented and unconsolidated sands demands special coring, handling and analysis techniques so that the grain structure in not altered. (Reference 1) Core liners of fiberglass or aluminum help reduce friction. Cores should be cut as rapidly as possible and then slowly tripped to the surface. The inner core barrel may be cut into short sections and the ends plugged. Freezing the core before shipping is sometimes done.
Cores typically require cleaning prior to testing for air permeability to remove mud contamination. . In some cases, cleaning is insufficient and the permeability will still be affected. In whole core analyses, horizontal permeability is affected more, and unusually high K-vert/K-horiz ratios can result. In some cases, an outer layer of the core must be removed (trepanning the core), to obtain realistic values of permeability.
Core values of porosity are frequently compared with log values. (see Reference 1) The reported gamma ray from cores is used to aligning the core depths with log depths. The grain density may be used in the log analysis, particularly with dolomites which may have a grain density less than the textbook value of 2.87 gm/cc (this is based on personal experience). The porosity in cores may be higher than logs because they are not being subjected to overburden stresses. SCAL can identify the reduction of porosity and permeability in cores at varying levels of stress.
Coring in drilled wells
Cores are very desirable in new discoveries or initial delineation wells. However it is difficult to pick the best interval to core in the undrilled sections. Two options exists for taking cores in previously drilled but uncased wells- sidewall and sidetrack coring.
Specialized core barrels
Unconsolidated Formation coring
One of the most difficult type of formations to core is unconsolidated formations. As a result of the damage during coring and transport, the excessive grain movement may sufficiently to damage cores to the point that laboratory restoration is impossible. Properties most sensitive to these properties are velocity, compressibility, and rock mechanics.
Success in this area relies on a coordinated plan from wellsite to laboratory, with the understanding that any error at any point can result in loss of obtain valuable information. In this case, the rubber sleeve core barrel is an option.
:Some of the keys to success as discussed in Reference 3 are: a) Trip out at a calculated rate that allows for gas expansion, particularly important for the last few stands of pipe b) Freezing all cores using dry ice and maintaining them in cryogenic freezers (at temperatures well below electric freezer temperatures) c) In the lab, cutting cores with a high-quality milling machine that minimizes vibrations and d) Cutting plugs using a drill bit cooled with liquid nitrogen. Plugs should be tested for porosity using a stress cell, designed for quick loading so samples due not thaw until under stress.
For water saturated shales where freezing could cause mechanical damage, even more damage occurred to cores that were not preserved with dry ice. (Reference 3) Alternatively, cores can be preserved with gypsum, however this results in massive damage throughout cores, making plugs unusable for analyses.
Other options/ advances
a) Low Invasion core heads: Standard coring can be done with three low invasion core heads: (1) PDC (polycrystalline diamond compact) core heads, suitable for softer formation, (2) thermally stable polycrystalline core heads, suitable for medium to hard formations, and (3) full diamond impregnated core heads, suitable for hard formations.
b) Specialized tools for core handling, logging and cutting: Once the core is brought to the rig floor, it must be handled with care. Hydraulic shear clamps are designed to prevent damage while breaking connections. On site, a portable gamma logger provides GR logs, which allows for rapid decisions on further coring and testing. An on site plug taker cuts sample plugs to ship to the laboratory.
c) Wireline conveyed core barrels: Coring is expensive in terms of rig cost as the drill string must be run in the hole and out again. . Wire line coring provides significant rig time savings by recovering the cores without tripping the whole drill string.
d) Specialized core barrels for oriented Cores: Oriented cores provide additional information on the anisotropic permeabilities trends, useful in reservoir management and water flood pattern programs.
e) Slim hole coring: Slim holes are drilled to perform evaluation of formation at a minimal cost. Specialized core barrels are available for these wells.
f) High pressure/ temperature and long core barrels: Long core barrels and/or high pressure formations result in gas release as the core barrel is tripped out of the hole. The standardization of 30 ft core barrels was due to concerns of gas release. However, venting of gas is possible with the "pressure relief check valve" which can safely vent the gas as the core is tripped out of the hole and also prevent the drilling fluid from entering the inner tubing. The use of the relief valve permits long cores to be taken, up to 600 ft. This has obvious application in drilling high inclination or horizontal wells.
g) Hard rock micro-coring: It has been recently reported that a new drill bit, the micro-coring bit (MCB), for hard rock drilling (>40,000 psi compressive strength) which retreives rock fragments, suitable for mineralogy examination. The micro-core is generated in the center of the bit. It has been reported that the "no center bit" improves the rate of penetration in addition to providing rock fragments, instead of pulverized rock ("rock powder"). See reference 5.
1. Holstein, E.D, Reservoir Engineering and Petrophysics, Volume V, See discussion beginning on page V-83 (Chapt 3A, Petrophysics) for fluid saturations in cores, by E.C. Thomas, and the discussion beginning on page V-421 (Chapt 3H, Petrophysical Applications, by H. R. Warner and R. Woodhouse.
2. Swift, T. E., Kumar, R., Goodrich, J., and McCoy, R.L., "Pressure Coring Provides Innovative Approach", Petroleum Engineering International, August 1981.
3. Rosen, R., "Proper Core Analysis Yields Value", Petroleum Engineering International, October 1, 2008.
4. Baker-Hughes website, www.bakerhughesdirect.com, Overview- High Temperature High Pressure Coring
5. Deschamps, B., Desmette, S. and Birch, R., "Generate Micro-cores of Formations While Drilling", February 19, 2008.