The Amplitude Versus Azimuthal AVAZ analysis has proved to be an important tool for characterizing fracture distributions and orientations of hydrocarbon reservoirs. This paper is aiming at the application of this tool for characterizing the fractures in Bahrain field reservoirs. Better understanding of faults and fractures distribution is essential to optimize EOR strategy and reservoir management. Regional analysis is possible by looking at faults distribution characterized by structural attributes analysis validated by regional stress and geological information. The investigation at local scale is more cumbersome but an Amplitude Variation with Azimuth (AVAz) method based on azimuthal Fourier Coefficients (FCs) proves to be a simple and powerful tool to characterize fractures distribution validated by FMI data. The anisotropy information was then used to update and improve the reservoir model long production history matching.
Ahmadi is a shallow carbonate reservoir consisting of two units AA and AB. These units are thin, highly faulted and irregularly-fractured. The matrix permeability of Ahmadi is very low; it ranges between 0.01 and 10 md. However, the secondary permeability is playing a big part on achieving some remarkable production figures. The reservoir is believed to have some fractures which have been proven from image logs. These make the simulation of the reservoir quite challenging. This paper describes how fractures were incorporated in the simulation of Ahmadi where three different methodologies have been considered. These are: single porosity with seismic attributes enhancement, dual porosity dual permeability (DPDP) and virtual fracture network modeling. In the single porosity model, the fracture properties were represented along with the matrix properties in one cell. It was done by considering some seismic attributes such as Anttrack and Anisotropy and then using the Attributes distributions to generate permeability multipliers. For the Dual Porosity Dual Permeability model where the matrix and fracture cells are different, the seismic attributes were used again but to generate a stochastic fracture network. Alternatively, the virtual fracture network model was created by having fractures intersecting each well based on some assumptions related to well spacing, fracture aperture and fracture direction. The listed methodologies are discussed in details in this paper. It was found that the virtual fracture network approach led to fast and robust history matching results. It was also observed that Anttrack and Anisotropy seismic attributes helped to represent the fault and non-fault related fractures in both single porosity and dual porosity dual permeability models.
The Late Cretaceous Mishrif limestone is known locally in Bahrain as the "Rubble" due to extensive tectonic fracturing and karst brecciation which may have tens to hundreds of times the permeability of the tight matrix where most oil resides. If these secondary features are not properly managed, steam utilization during thermal heavy oil recovery is compromised, so heat transfer to viscous crude in the matrix is less effective. To address this challenge, field pilots are conducted across Awali field to apply customized thermal processes to varying geology. Designing the thermal pilots uses 3D static and dynamic models representing the heterogenous reservoir to test various thermal processes (cyclic steam stimulation, forced imbibition, and steamflood). Initial geocellular models are multi-well secular models utilizing simplified erosional surface truncations and fault-fracture networks to optimize run times for multi-component thermal simulation. Reservoir properties are guided by well log data and geostatistics, whereas an effective permeability hierarchy accounts for historic well performance and observations. Additional inputs for simulation include thermal properties for rock and fluids, temperature-dependent relative permeability curves, temperature-viscosity relationships and vapor-liquid equilibrium ratios for each crude component, and injected steam quality. Piloting in a sparsely fractured portion of Awali field gave unexpected results that required revisiting conceptual geologic models and testing new ideas with dynamic simulation. This effort was accelerated by multiple scenario playing with "pseudo-fractures" strategically relocated to honor dynamic data while remaining consistent with the known fault, fracture and karst trends. This paper describes the results of this particular pilot and the versatile modeling approach taken to understand the outcomes. It recommends early quick multiple scenario playing to help guide subsequent time-intensive detailed fault-fracture-karst modeling.
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