The First Eocene is a multi-billion-barrel heavy oil carbonate reservoir in the Wafra field, located in the Partitioned Zone (PZ) between Kingdom of Saudi Arabia and Kuwait. After more than 60 years of primary production, the expected recovery is low and provides a good target for enhanced oil recovery (EOR) processes. A phased piloting approach has been used to reduce the uncertainties (subsurface and surface) related to application of thermal EOR processes in this field. This paper outlines the motivation for and benefits of the phased approach leading to steamflood development in the First Eocene reservoir. The key learnings from each phase, with focus on the progress made in identifying and mitigating key uncertainties for carbonate steamflooding, are discussed in detail.
The first step of the phased approach was an extensive EOR screening study in 1996 that identified steam flooding as a viable EOR process for this reservoir. In 1999, the Eocene Steam Stimulation Pilot, a multi-well cyclic steam stimulation (CSS) test, was conducted to provide information regarding the applicability of steam injection to Eocene reservoirs. The pilot evaluated reservoir, production and operational factors associated with injecting steam into the First Eocene reservoir. This was followed in 2005 by single 5-spot pattern pilot known as the Small-Scale Test (SST). The primary objectives of the SST were to address uncertainties regarding steam injectivity and the ability to generate steam from Eocene produced water. The SST successfully proved sustained injectivity in the dolomite reservoir using steam generated from produced water. The SST was followed in 2008 by a 16-pattern pilot referred to as the Large-Scale Pilot (LSP). The LSP was designed to evaluate the technical and operational feasibility of continuous steam flooding and further reduce 1) subsurface uncertainties such as recovery, vertical (potential barriers and baffles)/horizontal (high permeability layers or streaks) heterogeneity, steam-rock/steam-water interactions, and, 2) operational uncertainties associated with scaling and corrosion.
Key objectives of each pilot were defined and phase-appropriate subsurface/engineering design, reservoir surveillance and subsurface response plans (linked to the uncertainty management plan) were formulated to meet pilot objectives. Detailed data analysis (logs, cores, production, pressure, temperature, fluid properties, geochemical), reservoir characterization (stratigraphy, petrophysics, earth-modeling) and dynamic modeling (predictive and prognostic/validation) helped to understand production behavior, quantify impact of uncertainty parameters on reservoir response and estimate oil recovery. Finally, poststeam cores were acquired in the SST and the LSP pilots, adjacent to existing presteam cored observation wells, and detailed analysis (routine and SCAL) helped to quantify the residual-oil-to-steam and steam-rock interactions.
The learnings from the phased piloting approach to this carbonate field are vital to mitigate key uncertainties for full field steamflood development (FFSFD). The lessons learned and the best practices for successful steamflooding of carbonate reservoirs will be highly leveraged for optimum development design.