Effect of Carbonate Heterogeneity on Core - Log Integration
Authors: MacDonald Robin Michael , Kersey David G. , Zhang Tianhua , Akbar Mahmood , Mousa Wail 
Published: December 2009

Carbonate heterogeneity exists in different scales and affects different petrophysical estimations. Heterogeneity must be considered for effective core-log integration in order to ensure both core and logs measure rock volumes large enough to sample all rock properties. The conventional integration method is to compare data from the analysis of 1 in. or 1.5 in. diameter plugs (1.5 in. in length) or whole core samples, and compare the results with the same properties derived from logs. Because of heterogeneity, the results do not always compare well unless the rock volumes measured by both core and logs embody the variations and heterogeneity of the rock properties.

Validation of petrophysical models through core-log integration requires an understanding of the scale of reservoir heterogeneity. This paper develops a workflow from a case study evaluating the effects of porosity and mineralogical heterogeneity on core-log integration. Log and core data were available from carbonate intervals in three wells. In each well, porosity and mineralogy were determined from open hole density, neutron, and elemental capture spectroscopy (ECS) logs. The log values were compared to porosity, X-ray diffraction (XRD), and X-ray fluorescence (XRF) data from 86 core plugs (1 foot spacing) and XRD and XRF data from 656 ft of homogenized core slabs. In addition, a special signal processing technique, including homomorphic filtering, reversing intensity values and automatic thresholding, was developed to estimate anhydrite volume percentage from core plug and core slab photographs.

The degree of mineralogical heterogeneity was quantified with spatial variograms. In the case study, the petrophysical evaluation of carbonates was optimized after determining that, among the open hole logs and capture spectroscopy logs, the heterogeneity lengths of neutron porosity and density are similar, and also similar to that of calcite and dolomite. Although, the anhydrite mineralogy is very complex and heterogeneity exists in all scales.

Heterogeneity length was used to identify zones where core mineralogy from core plugs was sufficient for core-log integration and zones where mineralogy from core slabs was required. Heterogeneity is affected by the bed thickness and size and abundance of nodules, and the decision to use core plug or slab data must satisfy the parameter with the most complex heterogeneity. If heterogeneity length is less than 8 in., core plugs can be used. If the heterogeneity length is larger than 8 in., homogenized core slabs are required.

In the case study, the accuracy of the petrophysical models was improved using the scale of heterogeneity to select the proper type of core sample. In addition, this heterogeneity quantification methodology has the potential to improve the selection of samples for lab measurement of permeability, density, etc.

In summary, the workflow provides a robust method to quantify heterogeneity to improve petrophysical evaluation through effective core-log integration.