Comparison of Conventional Core Data, Well Logging Analyses, and Sidewall Samples
Authors: Fertl W.H. , Cavanaugh R.J. , Hammack G.W. 
Published: December 2007

Conventional core data and analyses of sidewall samples have been compared with data derived from well logs in reservoir rocks of varying lithologies. Correlation of porosity, hydrocarbon and water saturation, and permeability variations makes it possible to use one data set-taking special precautions when only sidewall samples are available-in the absence of other measurements.

Introduction

The comparison of core data from both conventional and sidewall samples with log-derived data makes possible a more realistic and reliable evaluation of possible a more realistic and reliable evaluation of the reservoir. Unfortunately, published comparisons of rock properties obtained from core and sidewall samples are scarce. Included in the many generalized conclusions in those papers are that

percussion sample porosities in softer, looser sands are only slightly higher than those of conventional cores; sidewall sample permeabilities are decreased in higher permeability formations; and 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.

Comparison of Conventional and Sidewall Samples

An example from Offshore Louisiana shows the comparison between conventional cores using a rubber-sleeved core barrel and sidewall samples (Figs. 1 and 2) in a Pliocene sand. Handling of this Pliocene reservoir rock was complicated by its semiconsolidated nature and by the presence of hydratable clays, which gave up water even at room temperatures if exposed for long periods of time. Since the core-gamma ray log showed multiple sand and shale partings corresponding to lithology variations over short distances, it was difficult in many cases to find comparable porosities within a 6-in. interval of the original core sample. Windows were cut in the rubber sleeve, test plugs were removed and the rubber sleeve was plugs were removed and the rubber sleeve was rescaled to preserve the remaining core. As the study progressed it became apparent that the Dean-Stark extraction method for determining saturations was releasing clay-bound water along with pore water, resulting in too high a measurement of water saturation; using the summation-of-fluids method with varying water plateaus gave more reasonable results. Finally, the plateaus gave more reasonable results. Finally, the remaining rubber sleeve was slit lengthwise and a continuous colored photographic log was taken of the core. Such photographs show clearly the varying degrees of both lenticularity and consolidation. The effect of these two core analyses on the porosity and permeability data in this shaly pay sand as reported permeability data in this shaly pay sand as reported by a commercial core laboratory are shown in Tables 1 and 2. We do not mean to imply by the tables that fluid saturations by "summation of fluid" (SOF) is generally or usually better than the Dean-Stark method. In fact, we prefer the latter method over the SOF method almost exclusively. However, in this particular case, the SOF method under closely controlled particular case, the SOF method under closely controlled conditions (not routine procedures) resulted in data that were reasonable and acceptable. A Miocene Gulf Coast formation was also cored using rubber-sleeve coring equipment.

JPT

P. 1409