Robert M. Moss and John W. Russo
Texaco Inc., Houston, Texas
ABSTRACT
A new technique has been developed to quantify secondary porosity using x-ray computed tomography (CT) of whole cores. These cores were from a heterogeneous limestone-dolomite formation with water production problems. Identification of secondary porosity heterogeneity by. CT agreed well with core descriptions and well log results.
A detailed analysis of single-scan x-ray CT data from well “A” has shown that the permeability is strongly influenced by the amount of secondary (vugular) porosity present in the formation. Well “A” is a good producer with water coning problems. Good agreement was found when the amount of secondary porosity was compared to the horizontal plug permeability as a function of depth. The single-scan results were confirmed by directly measuring the porosity distributions of eight core plugs from well “A” using a dual-scan CT method, This correlation to horizontal permeability indicates a distribution of secondary porosity that is uniform in three dimensions.
Well “B” is a poor producer with early water breakthrough. Selected areas were scanned and analyzed for the amount of secondary porosity for comparison to the original data from well “A”, The porosity distributions seen in well “B” have quite a bit less secondary porosity on average, and much of the material scanned shows evidence of very high vertical permeability. A poor correlation of horizontal permeability with CT data in well “if’ indicates a non-uniform distribution of secondary porosity.
Differences between density-neutron porosity and sonic porosity were correlated with the CT data to establish a method of identifying porosity heterogeneity in intervals where no core exists. The log-derived secondary porosity shows good agreement with the quantitative, whole core CT data and core descriptions. Previous attempts to correlate log-derived secondary porosity with core data rely on thin section measurements.
By greatly increasing the volume of core analyzed, this method provides confidence that logs can be used to identify heterogeneities in this reservoir. Such information can help decrease water production by indicating the extremely high permeability zones prior to perforating.