Observed Fracturing in a Middle East Oil Well – A Combined Subsurface Study

Abstract

Abstract The paper presents the results of an integrated study comprising different disciplines and scales (core, log and seismic). The study followed the drilling of a well through a Jurassic carbonate reservoir in a NE-SW direction, which was cored near the tip of a NW-SE striking fault. Core and FMI data show a number of open and closed fractures, all striking in a NW-SE direction, contrary to what was expected from the regional structural model. The structure grew through the Mesozoic with NE-SW extension producing a set of NW-SE striking normal faults and related fractures. A pulse of growth occurred during the Late Cretaceous-Tertiary, followed by continued Tertiary growth. 3D seismic data clearly shows the Lower Cretaceous fault pattern. The faults decrease in throw with depth, and only a limited number cut the Jurassic. Interference tests show a preferential NE-SW communication pathway between wells. In addition, borehole and core data from other wells show open fractures, which tend to have a NE-SW strike, and a second set of closed fractures orthogonal to these. The following were concluded from a review of the data: Correlation between core goniometry and FMI image analysis is excellent.The well is located near the tip of a NW-SE striking fault, and is likely to be in the transfer zone between two faults.Two sets of fractures occur in the Jurassic. A fault related NW-SE striking set and a NNE-SSW striking set occurring further away from faults.Fractures occur as near vertical, broken non-mineralized, and completely healed; and tend to occur in swarms.Fractures in the reservoir are likely to provide additional permeability.To intersect the maximum number of open fractures in the Jurassic, wells should be horizontal and oriented WNW-ESE away from faults and NE-SW close to faults.