Introduction to Core Analysis

Rotary and Percussion Sidewall Core Analysis

Rotary sidewall cores (Figure 1) are frequently of comparable size to the plug samples taken from full diameter cores. Although the orientation of the rotary sidewall cores is sub-optimal unless the bedding is at right angles to the wellbore, they can be analyzed in much the same manner as standard plug-sized cores.

Rotary sidewall core samples and the core bit, Core Analysis
Figure 1: Rotary sidewall core samples and the core bit

Percussion sidewall cores are often much smaller and more variable in length. At the same time, they are usually being damaged by the percussion process such that permeability measurements are invalidated, and porosity measurements need to be incorporated into reservoir descriptions with caution.

Sidewall core analysis is made on all samples sent to the laboratory. The sampling is at the option of the well operator selecting the depth sampling points, normally from the interpretation of well logs, at which to recover each sidewall core. Results of these laboratory analyses are often used to support the log interpretation, define the lithological content and the rock’s grain size, sorting, roundness and texture. Consequently, samples are selected so as to be spaced at reasonably regular intervals throughout the full section to be evaluated.

It is important that the analyst receives these samples in their correct measured depth sequence, as this is crucial for the interpretation. In areas where sidewall core-to-conventional core correlations are not known, it is beneficial to take a conventional, full diameter core in a reservoir and then to follow this with samples of sidewall cores. From this, a sidewall-to-conventional core data relationship can be developed for use in subsequent wells in the same field.

Sidewall core samples are used more widely in softer sandstone formations. Percussion sidewall cores are often small and need additional care when being analyzed. 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. All sidewall cores are almost always coated with some drilling mud, which should be removed before the analysis.

Permeability measured on percussion sidewall core samples very rarely represents the true in-situ reservoir values. For hard formations, permeability values are too high due to impact fracturing as the core is taken, while unconsolidated sands can be compacted by the percussion process and yield erroneously low values.

Data show that porosities measured on percussion sidewall samples are closest to the conventional core analysis values in formations having true porosities ranging from 32% to 34%. In hard formations, percussion sidewall porosity values are normally higher than conventional plug values. The porosity within hard, well-cemented rock is increased by grain shattering during bullet impact, and these alterations in properties severely limit the usefulness of percussion sidewall cores in reservoir engineering evaluations.

All sidewall cores are good indicators of the reservoir lithology, provide supporting evidence to well logs and wireline formation tester data on the presence or absence of oil and gas, and are normally suitable for a wide range of sedimentological and petrographic work.

Percussion sidewall core samples are often small, with the consequence that the entire sidewall sample is often used just for the porosity and saturation determinations. In this circumstance, a visual assessment of the rock’s grain size and sorting, the shaliness of the sample, the measured porosity and the natural density of the fresh core is combined with correlation charts appropriate to the area to arrive at an empirical, and very approximate, estimation of the permeability.

Technological improvements in the assessment of grain size and sorting have been developed and are now used in many laboratories. One type of instrument is a laser diffraction particle size analyzer (Figure 2). Other procedures use Stokes’ Law to provide the distribution of grain sizes for each sidewall core sample. Applying Stokes’ Law, 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 generates 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 generate approximate values of permeability.

Particle size analyzer, Core Analysis
Figure 2: Particle size analyzer

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

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