Abstract

The paper describes a method for modeling reservoir true effective k incorporating k from core, fractures &well test. A 3D permeability model is developed based on detailed lithotype mapping within which horizontal matrix permeability is simulated. Comparison of modelled permeability with actual test permeability values suggests that there is a genuine permeability contribution from fractures in the reservoir. The resultant model is thus combined geostatistically with fracture k so that the final permeability matches well test permeability. We used strain field as an indicator of fracture locations and hence to flow from them. Strain field over the reservoir, which correlates with test permeability, is obtained from curvature analysis and calibrated to strain calculated from core fractures. High strain areas are set to be associated with highly fractured zones that are commonly marked by increased conductivity. Computer geostattistical module computes fracture permeability from a power-law relationship and conditions it to match interpreted test permeability in 3D space. The geostatistical method offers means of predicting fracture permeability distribution field-wide and better accounts for water encroachment trends. The results helped achieve better history match results more easily than would have been possible without the combined model.

Introduction

This work evaluates effects of open natural fracture networks on total permeability of Zone-I reservoir system. Zone-I is a shallowing upward carbonate reservoir composed of predominantly grainstone and packstone with subordinate wackestone/mudstone rocks. The structural development history of fractures in the zone is interpreted from a collage of information. The information includes well cores, FMI/FMS images, dipmeter logs, wireline logs and lateral thickness variations. Collectively, this data suggests that there are at least five elements contributed to the development of fractures in Zone-I reservoir. 1) Rejuvenation of Permo-Triassic N-Strending basement normal faults, which generated intense fracturing whose trend spans from NNW to NNE. Fractures produced are mostly closed, mineralized and associated with regional lineaments crossing the field. 2) Upper-Middle Cretaceous rotation of crestal blocks resulted in parasitic fracture system with left lateral shears trending NE-SW. 3) Synthetic fractures emanating from and orthogonal to the rotated basement faults are observed crosscutting the reservoir. 4) Curvature and fracture trend analyses indicate that the prevailing tectonic stress orientation over the field is about N10°E. It is considered responsible for generating conjugate pair of fractures trending NW and NE. Most of these fractures are fairly recent and open. Their trends superimposed and, probably reopened, closed fractures created by (2) and (3) processes above. 5) Finally, salt diapiric movement in the region has assisted in the development of fold resulting in concentric and radial fracture domains.

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