Fractured Reservoir Characterisation: A Fully Integrated Study, From Borehole Imagery, Cores and Seismic Data to Production Logs - Example of an Algerian Gas Field (Sbaa Basin, SW Algeria)

Abstract

GDF-SUEZ and Sonatrach will develop in partnership the main fields of the prolific Sbaa basin, SW Algeria. In this basin, the main gas levels comprise the Cambrian and Upper-Ordovician reservoirs, sealed and sourced by the Silurian "Hot Shales" Formation. During the exploration and appraisal phases of the project, an intensive drilling program, including coring, standard logging and imaging (FMI), following a 3D seismic campaign was performed. Integrated study was done to investigate the suspected impact of natural fractures on the reservoir performances, to suggest potential geological drivers on fracture type (such as diffuse fractures and fracture swarms, if any) and on fracture distribution. The hydraulic properties of the fractures, based on available dynamic data were also evaluated. Indeed, accurate information on fault network and on fracture properties is essential to improve strategies for maximising hydrocarbon recovery, by optimising well location or directional drilling.Various seismic anisotropy "fracture relevant" attributes (RMS, Coherence and Dip) were selected and jointly used in a multivariable statistical process called Seismic Facies Analysis (SFA). This gave a preliminary interpretation of the seismic facies in terms of fracture density that was completed and validated with well data and curvature analysis. The curvature pointed out various scale lineaments fully consistent with previous work. It concludes on predominance of fault-related fractures. According to dynamic data, well productivity may be enhanced by fracturation by a factor up to 12. The resulting fracture probability map was used to build a 3D fracture model with IFP and FracaFlow® software dedicated for fracture analysis and fracture modelling. For each well, a local discrete fracturation model was built. The length and intrinsic permeability of fractures were then calibrated using KH determined by well test interpretation and production log analysis. The final result consisted in computing the equivalent properties of the fracture network in each cell of the reservoir grid in order to perform the reservoir simulation. In this study, we present the results of production log (PLT) matched with success. The same fracture parameters were also used to match consistently the production log of the other wells . The integration of a wide type of data to analyse the natural fractures of this Algerian gas field leads to a fully consistent and well-constrained fracture model.