We present a case study of locating and estimating preferential flow and permeability in a fractured carbonate oil/water reservoir about 40 km north of Lisbon, Portugal. The limestones in the study area show very poor reservoir properties with negligible matrix porosity and permeability. Fractures and faults are the main conduits to flow and also constitute the main porosity. An extreme heterogeneity in fault permeability is demonstrated by core descriptions and geophysical logs in combination with hydraulic testing.

Three wells were drilled to depth of up to 328m with full core coverage, within a square of 200 m. Two of the wells are inclined about 60 degrees from horizontal. A suite of geophysical logs including optical and acoustic image logs were run. The log interpretation is correlated with observations in the core and a unique characterization where fracture properties are related to all log data is established. An increased intensity of conductive bed-parallel slip-surfaces, which are of critical importance for the connectivity of the fracture network, is interpreted from the geophysical log data to occur in one of the reservoir units. Higher porosity and permeability is interpreted in younger faults compared to older ones and individual faults show large heterogeneity in fault rock structure between the wells.

A number of hydraulic well tests were performed at discrete intervals by use of a straddle packer. The well tests include analysis of intervals e.g. covering a major fault intersecting all three wells. The results quantify spatial variation in fault permeability of several orders of magnitude within a 100m distance. The performed tests reveal a highly complicated behaviour of the hydraulic system controlled by the distribution and hydraulic parameters of fractures and faults. This pattern is supported by biological marker analysis of liquid oil and oil stain samples from the wells that point towards the presence of more than one hydraulic system within the reservoir.