Abstract
Recent exploration efforts in the Campos Basin, offshore Brazil, have resulted into important discoveries of what is known as "offshore heavy oil" (API gravities below 18 and reservoir viscosities above 20 cp). Actually, these accumulations have been known since the beginning of the development of the Basin, in the 80s, but they were never given a chance of development due to the technology challenges involved. The maturation of lighter oil fields, associated with recent technology developments (1,2,3), has boosted the interest for these heavy oil resources set aside for many years.
This paper describes a case history concerning the strategy for the appraisal of a large offshore shallow carbonate 12 API reservoir, at 100m water depth. To justify the investments in obtaining static and dynamic information of the reservoir behavior, a preliminary development plan based on long horizontal wells was established. The value of the new information (VOI) was estimated through the assessment of the impact that uncertain reservoir and production attributes could have on the development plan. Uncertainty analysis based on reservoir simulation allowed the identification of the main variables affecting the project: reservoir oil viscosity, permeability and effective footage of the horizontal well. This analysis revealed the best strategy for the appraisal of the well planning and led to the approval of a 2000m horizontal well.
On a second stage of the study, after the successful drilling and testing of the well, another VOI study was carried out, aiming at the evaluation of the benefits of a one-year extended well test, which is planned to start in 2007. At this stage, the main reservoir uncertainty was the aquifer support, which could relief the need for sea water injection (besides this there are other production uncertainties as flow assurance, performance of the pumping system and oil water separation).
This paper describes the simulation model building process, the difficult task of assigning scarcely sampled rock and fluid properties, the uncertainty study and the use of quality maps for placing the wells. Simulations results have shown the importance of the well footage and its effectiveness in contacting the reservoir rock.
