The Alteration of Rock Properties by Percussion Sidewall Coring

Abstract

Introduction The use of percussion-type sidewall cores as an aid in detection and evaluation of hydrocarbon shows and in examination of rock properties has become increasingly popular in recent years. Sidewall coring has an advantage over normal bottom-hole coring methods in that the zones may be surveyed with the electric log prior to selection of sample depths. This allows an efficient sampling of all zones of interest, a procedure not possible with bottom-hole coring techniques unless the costly procedure of continuous coring is utilized. Percussion sidewall coring presently provides the most rapid and lowest cost method of obtaining sidewall cores. It has been recognized for some time that certain discrepancies exist between core analysis data obtained from percussion cores and data obtained from other types of core samples. This paper presents existing field and laboratory data that were available to the authors. Several examples of comparative core analysis data obtained from various fields are presented along with photomicrographs of thin sections of conventional and percussion cores. The results of this investigation are presented to throw more light on the possible disturbed condition of percussion sidewall cores. Sample Disturbance and Fracturing The principal cause of discrepancy between conventional core analysis data and percussion sidewall core analysis data is believed to be the shattering and re-adjustment of the grains due to the large impact shearing forces produced by mechanics of percussion coring. Figs. 1 through 5 are photomicrographs of thin section slides of formation samples obtained by means of diamond cores, conventional cores, and percussion sidewall cores.

A New Wireline Rotary Coring Tool: Development Overview and Experience from the Middle East

“…The applications and limitations of percussion coring were recognized and reported in the 1950s and 1960s (Webster and Dawsongrove 1959;Koepf and Granberry 1961). The applications and limitations of percussion coring were recognized and reported in the 1950s and 1960s (Webster and Dawsongrove 1959;Koepf and Granberry 1961).…”

“…Cannot be used to accurately determine porosity, permeability, or other special core analysis because of rock property alteration from the aggressive process of the percussion coring method (Webster and Dawsongrove 1959). Limited to soft and medium formations; in hard or tight rocks, the bullet barrels tend to break.…”

Comparison of Conventional Core Data, Well Logging Analyses, and Sidewall Samples

“…Unfortunately, published comparisons of rock properties obtained from core and sidewall samples are scarce.1.z Included in the many generalized conclusions in those papers are that (1) percussion sample porosities in softer, looser sands are only slightly higher than those of conventional cores; (2) sidewall sample permeabilities are decreased in higher permeability formations; and (3) water saturations of the sidewall cores are higher and oil saturations slightly lower than those of conventional cores. Although similar conclusions can be drawn from this study, our investigation uncovered additional complications.…”