Abstract

Extra-heavy oil (XHO) reservoirs in South America represent some of the largest hydrocarbon accumulations (> 500 billion barrels) in the world. Primary production using long horizontal wells is a commercially proven technology for XHO reservoirs. The expected ultimate recovery with primary production is generally limited to less than 12% of original-oil-in-place (OOIP) and thermal EOR is critical for increasing recovery to 30–60% OOIP. Economic and environmentally viable thermal development of these reservoirs will require the use of horizontal steam injectors. Our results reveal that a continuous steam injection with a horizontal injector placed vertically above a horizontal producer (CSI-HIHP) is a very effective method for XHO reservoirs, with high peak oil rate and significantly high recovery. This study, first of its kind for an XHO reservoir, outlines an integrated workflow to evaluate production potential of a large XHO green-field using primary production followed by thermal exploitation. The workflow, based on a probabilistic framework (involving designs of experiments, proxy methods and Monte Carlo simulations), evaluates reservoir performance for the whole lifecycle of the field under a range of uncertainties and quantifies the impact of key parameters affecting the reservoir performance.

XHO reservoirs usually have significantly higher pressures than those of typical conventional heavy oil reservoirs, where continuous steam injection (CSI) has been applied commercially. Therefore, pressure in these reservoirs must be reduced before CSI can begin. Cyclic-steam-stimulation (CSS) after initial stage of primary production can be used to accelerate pressure reduction in the reservoir, while providing additional recovery. Our results demonstrate that geological features such as shale baffles have a significant impact on delaying pressure reduction during primary and CSS. In all the cases investigated, primary production for one year followed by CSS for four years has been found to be successful in reducing pressure to the target pressure for CSI.

High pressure-drop in the horizontal steam injector can cause pressure near the toe region of the injector to be lower than the producer pressure. This results in poor steam injection and poor steam-chest development in that region, thus, greatly reducing efficiency of the thermal recovery process. We quantify pressure drop in a horizontal steam injector and its impact on the thermal performance and suggest a novel well configuration that uses two injectors for every long producer during CSI. The proposed configuration with a sequential development plan can significantly improve economics of the projects.

A novel probabilistic workflow for a full-field development plan (primary, CSS and CSI) of XHO reservoirs provides robust production forecast over the entire lifecycle. The workflow developed and the insights obtained would be very valuable in preparing effective exploitation plans and optimal facility design, a key economic variable in large projects of developing giant XHO reservoirs.

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