An Overview Of Computer-Enhanced Analyses Of Core Data

Abstract

Introduction Using modern digital computers, a core analysis laboratory can now measure and display petrophysical parameters so as to maximize information content and ease of interpretation. Computer graphic presentations of core analyses in log format, x-y crossplots and parametric frequency distributions permit direct visual comparison and thus cut down on time spent by geological and reservoir technologists. Graphical and statistical analyses may also reveal significant trends in reservoir quality which are not readily extracted from numerical core data. This is particularly true in multi-well studies. Core Analysis in Log Format Routine core analysis of consolidated carbonate and clastic sequences includes natural gamma ray activity, porosity, permeability, fluid saturation, and grain density(1,2,3). By using standard API format log grids and 1:240 vertical scales, these parameters can be readily compared to geophysical well logs. Figure 1 illustrates core analyses of both Marly and Vuggy Members of the Midale Formation from a single well in southeast Saskatchewan. The components of track 1 Highlight changes in lithology. Total gamma ray activity is presented in standard 0 – 150 API scale together with grain density (Dg) in kilograms per cubic metre. Track 2 contains maximum horizontal permeability (Kh) on a five-cycle logarithmic grid, scaled between .01 and 1000 mD, similar to resistivity logs. Track 3 plots porosity (Φ) as the percentage of bulk volume, measured by Boyle's Law helium porosimetry. Oil and water saturations are represented by horizontal shading and horizontal stripes respectively. Cursory inspection of the three tracks reveals the essential features of the core, a fairly homogeneous dolomite with good permeability and excellent porosity underlain by a more heterogeneous limestone with good permeability and fair to good porosity. To facilitate comparison with wireline data, a transparency of the graph is laid over the open hole logs and depth shifted until the symmetry of log and core gamma ray is matched, thus compensating for the usual discrepancy between core and log depth. Vertical scales may be expanded to provide a direct overlay for digital open hole logs played back on the 1:48 "dip-meter" format. Analyses of unconsolidated sandstone cores are more complex due to sample preparation and cost of analysis(4,s.6l. After coring and prior to analysis, core temperature is reduced to and maintained at -30 °C to preserve fluid saturations and grain orientation. Figure 2 shows the analysis of a plastic sleeved heavy oil core from the Lloydminster area. Track 1 contains gamma ray and grain density. Track 2 presents particle size distribution from sieve analysis. Weight percentages for silt and grades of sand are expressed relative to total solids. The graph indicates a general coarsening from top to bottom. The pattern of the core gamma ray is mirrored in the distribution of very fine sand. Particle size analysis of unconsolidated core yields information regarding both permeability and production of sand. In track 3 mass fractions of bitumen and water appear as horizontal shading and horizontal stripes respectively on a 0 – 25 weight percentage scale.