Core Plug Analysis
Consolidated Formations
Core plug analysis is most useful in those fairly homogeneous formations that can be effectively characterized by plug-size samples. Typical plug sizes are 1.5 inches (3.8 cm) to 2.5 inches (6.35 cm) in diameter and 1 inch (2.5 cm) to 1.5 inches (3.8 cm) long. Successful rotary sidewall cores are of similar dimensions.
By nature of the hardware design, rotary sidewall core plugs are always cut perpendicular to the side of the wellbore, irrespective of the bedding planes’ dip angle and direction. The interval to be cored is selected from either wireline or LWD log responses subject to their associated vertical resolution and depth correlation limitations.
By contrast, a plug sample from a full diameter core can usually be cut in the optimal orientations (both parallel and orthogonal to the bedding planes) with respect to the main bedding and over an interval determined after careful visual inspection of the whole core, allowing even small vertical changes in rock characteristics to be accommodated by fine-tuning the core plug sample location, as shown in Figure 1.
Plug samples are usually cut from a full diameter core with a diamond core bit both parallel and orthogonal to the main bedding planes, and trimmed so as to create a cylindrical plug from the center of the core where the minimum amount of mud filtrate flushing and invasion of drilling mud chemicals is likely to have occurred. The objective of coring is to recover representative samples of the formation under study. To be fully representative, the samples should not be severely damaged or invaded by non-reservoir fluids. Therefore, sample plugs taken from the center of the core have the highest chance of being the most representative of the reservoir rock.
Vertical permeability samples are drilled at right angles to the bedding planes, while horizontal permeability samples are cut parallel to the bedding planes. Although primarily used for bedded sandstone reservoirs, this technique is also satisfactory for the more homogenous, non-fractured and non-vuggy carbonates.
Unconsolidated Formations
Unconsolidated sand and rubble recovered within a plastic inner barrel liner, a foam-lined core barrel, or a fiberglass barrel is often stabilized by freezing prior to shipping to the laboratory. Figure 2 shows a fairly extreme example of a crumbled core resulting from a combination of poor coring tool selection, sub-optimal coring practice, and careless core handling.
Freezing is one method of stabilizing weakly cemented to uncemented cores, usually with dry ice. This is most commonly done with the core in the liner before sectioning it into 3 ft. lengths. If dry ice is not available, annular-fill (the annulus of the liner) methods are used. Frozen interstitial water present within grains immobilizes the rock particles. Core plugs are generally drilled using liquid nitrogen as the bit lubricant.
In an alternative technique, an unconsolidated core is first immobilized by surrounding it with wax, plaster of Paris, foam or other stabilizing materials, after which the sample may be frozen and drilled. When a core is completely unconsolidated, plug samples can be removed by the insertion of a hollow punch into a non-frozen core. Friable cores, however, should not be punched because additional porosity and permeability will be artificially created in the core. Instead, such plug samples should be confined in a metal or plastic sleeve and subjected to simulated overburden pressure during analysis. Failure to treat unconsolidated cores in this fashion will generate much higher porosity and permeability values from the core analysis than those actually present within the reservoir.
A plastic inner liner can be a successful solution to the recovery of unconsolidated, shallow tar sands. These formations are often subsequently mined, and it is essential that the tar content be accurately defined. A modified evaluation technique does not rely on plugs extracted at selected intervals but uses a small representative portion of the full diameter core.
This process requires the plastic sleeve to be cut into 1 ft (30 cm) lengths, which are then cut in half vertically. Two 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 samples approximately 1 ft (30 cm) long and 1 square inch (6.45 sq. cm) in area (Figure 3).
The center portion of the half is used for determination of the saturation in tar sands 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 analysis techniques.
