Polymer Injection in Deep Offshore Field: The Dalia Angola Case

Abstract

Whereas on-shore polymer injection may be qualified as a mature EOR technique, considering the hundreds of operations that have been conducted all over the world, only one polymer pilot has been implemented offshore, and none in deep offshore conditions. A very thorough feasibility study of polymer injection has been made on the Dalia field in Angola, a typical deep-offshore high permeability (>1D as an average) sandstone reservoir containing medium viscosity oil (3 to 7cP under reservoir conditions). The study has demonstrated that high molecular weight hydrolyzed polyacrylamides could be used under a wide range of salinities covering sea water and a mixture of sea water and produced water. Additional recoveries in the range of 3 to 7 % can be expected in this particular context of large well-spacing development of a medium viscosity field. Powder polymer supply is achievable for deep offshore fields either with a specific bulk carrier or using standard international containers to transport big bags (750 kg). Although the on-deck option is simpler, even in the case where no room is left on the existing FPSO, marine options can be found to safely process the polymer on a barge connected to the FPSO (depending on the sea conditions), but the on-deck option is more simple. The need for an injectivity pilot is compulsory to demonstrate the operability of the facilities, and the injectivity of the polymer solution. A single well test has been designed and is planned fall 2008 in Luanda on one of the well of the Camelia reservoir. A skid dedicated to the injectivity test has been designed, assembled and constructed to prepare a mother solution of polymer from powder. Tested in France (no injection), the skid has been shipped to Luanda for installation on the FPSO during summer 2008. Future of the project will depend on the injectivity tests results and on-going studies on a phased approach. Introduction Deep offshore reservoirs may be good candidates for EOR by polymer injection :they are generally shallow (below sea floor), which means rather low temperature;waterflooding is the preferred basic recovery mechanism, for pressure maintenance and sweeping;very often, the reservoir oil is rather viscous, as a result of biodegradation which has occurred due to temperature conditions; recovery by water injection is then adversely affected by an unfavourable mobility ratio, and can be improved by injecting viscosified water;the reservoirs are often turbidites of very good characteristics, allowing an efficient propagation of high molecular weight polymers;the wells are quite prolific, for both production and injection; injecting a viscous solution should not be an issue. Despite this long list of positive criteria there is still no implementation of polymer injection in deep offshore conditions and only two small off shore pilot operation [1–2] whereas on-shore polymer injection may be qualified as a mature EOR technique, when considering the hundreds of operations that have been conducted all over the world. Several key issues have to be faced in deep offshore, that are a major step out versus existing commercial onshore projects: the range of salinities to be met over field life, particularly if sea water is to be injected (most existing - on shore- projects are injecting almost fresh water)the larger well spacing: typically 500m to 1500m instead of 150m to 400mthe facilities: multi-well injection through common subsea lines, FPSO with no/reduced room for additional facilitiesthe logistics to ship the polymerthe incremental oil evaluation in fields with little to no water injection history

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