Construction of Geological Model Scenarios in a Highly Faulted Reservoir: Finding the Appropriate Model That Best-Fit Reservoir Simulation Objectives

Abstract

An integrated subsurface modelling workflow (Geology, geophysics, petrophysics, reservoir and production engineering) was utilised to rebuild a static and dynamic model of PetroSA’s F-A gas field to optimise reservoir management and recovery as well as identify remaining potential for infill well drilling. The last model built was 6 years after production, whereas now in 2018, after 26 years of production, considerably more data is available to guide the model building process. Subsequent to the first well drilled in 1970 in the F-A field area (F-A1), which encountered hydrocarbon shows in a shallow marine syn-rift sandstone, it was only in 1980 that the true hydrocarbon potential of the so-called F-A gas/condensate field was ascertained. Fifteen follow-up appraisal wells were drilled from 1981 to 1985 to delineate the field, all on the basis of seismic 2D lines. Six of these wells encountered commercially viable gas-bearing reservoirs and provided the basis of the F-A field development with the start of gas and condensate production at the end of 1992. Over the past 26 years since commencement of production, major technical milestones like seismic 3D surveys in 1986 and 1997/8, history matching studies in 1994 and 1999, a geochemical study in 1998 as well as past and current production and reservoir pressure history have contributed to changes in hydrocarbon estimates and the view regarding reservoir connectivity. A greatly revised static and dynamic model was built in 2017/18 to replace an old 1999 dynamic model. Static Modelling of the reservoir depends greatly on the accuracy of the structural model, which forms the container into which the reservoir properties can be populated. Structural models were created using different modelling algorithms. The inputs to the structural model were horizon and faults interpretations based on seismic character, fault seal analysis, material balance and pressure analysis. Updated petrophysical modelling which includes a facies dependant permeability model and saturation height model was integrated with Seismic attributes like Acoustic Impedance as a secondary variable, to model properties such as porosity away from the wells. This paper aims to demonstrate the importance of integrating different disciplines to build a robust static model; how different methods of building the structural model impacts on the reservoir simulation; and how the geological understanding has changed since the field was discovered.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018