Improved Fractured Reservoir Models Using Borehole Images and Cores from Horizontal Wells: Monterey Formation, Elk Hills Field, California

Campbell, Tania C.¹, Michael R. Gross², Jon R. Schwalbach³ (1) Occidental Oil and Gas, Tupman, CA (2) Florida International University, Miami, FL (3) Aera Energy LLC, Bakersfield, CA

A prerequisite for accurately modeling fractured reservoirs is high quality data that captures the nature and distribution of fractures and faults. Because fractures are discrete features, often with irregular spacing and dimensions, it is difficult to capture valid statistical data describing the fracture network from vertically-oriented data sets. Unfortunately, most reservoir models rely on limited or extensive vertical coring, borehole image logs, and analog comparisons from allegedly similar outcrops. The data base for the Miocene Monterey Formation siliceous shale reservoirs at Elk Hills is unique because it contains an extensive number of electrical image logs from horizontal wells, and a core-image log pair from a horizontal well that provides valuable calibration.

The core-image log data set reveals at least two scales of fracture features. The bed-bounded fractures are typically restricted to one or a small number of cm-thick stratigraphic layers (beds or bed-sets). Bed-bounded fractures are most common in the brittle rocks, and clearly reflect the fine-scale mechanical stratigraphy in the reservoir. Image logs from horizontal wells provide excellent measurements of the spacing of these features, and comparison with the core reveals lower limits of detection that can be extrapolated when building reservoir models. The greatest benefit from image logs coupled with cores from horizontal wells, however, is derived from characterizing larger-scale features of the fracture network, sometimes referred to as backbone- or mega-fractures. The image-core comparison reveals the nature of these features. The cores provide information about apparent aperture and mode of deformation. The image logs complement the core data with information delineating the spacing and orientation of the feature. Calibrating the core and images from horizontal wells significantly improves our confidence for populating fractures in the reservoir model.