Reservoir Paraffin Precipitation: The Oil Recovery Challenge in Dom Jo?o Field?

Abstract

Abstract Dom Joao Field, located at Reconcavo Basin in northeastern Brazil, has been under production for more than 50 years. From the very beginning in 1947, exploitation has been plagued with problems that standard geology and reservoir engineering practices were not able to cope with. With an OOIP of 113 MMm3 of 38 °CDATA[API paraffinic crude, the current recovery factor stands at only 15%, in spite of the early implementation of waterflooding. Three major reservoir studies by different organizations, Petrobras included, were conducted over the years with significant impact on the geological model and appraisal of the original volumes, together with recommendations as to the variation in the injection and production patterns. They were implemented accordingly but with limited success and, other than the heterogeneity of the reservoirs in the main Sergi Formation, no suitable explanation was found for the poor recovery performance, not to mention a solution to the problem. The size of the target oil and the constant threat of field abandonment, led to the approval of a research project in late 1993 as part of the portfolio of PRAVAP, Petrobras Strategic Advanced Oil Recovery Program. Its goal was twofold: verify the assumption of paraffin precipitation in the reservoir as the cause of the low recovery factor, and eventually put to the test recovery strategies that would ensure the economical exploitation of the shalloweposits of paraffinic crude in Reconcavo Basin. This paper summarizes the major accomplishments of this project throughout the 3 1/2 years of research and development and the corresponding IOR field implementations. Introduction As reported by Leite(1), Dom Joao Field, the second oldest commercial accumulation in Brazil, was discovered in March 1947. Situated in the southwestern part of Reconcavo Basin in Bahia State, Brazil, it is 23.5 km long and 0.6 - 3.5 km wide for an area of 47.3 km(2) (Figure1). Structurally, it is a slightly dipping (1 - 5 °CDATA[) horst, limited by a major fault zone with 400 to 500 m of displacement in the West, and smaller ones in the East. Two thirds of the field are located offshore in Todos os Santos Bay, at an average water depth of 3 m. It produces from the 8 zone (C-J) Sergi formation at depths between 146 m and 376 m. C zone, with an average net pay of 10 m, is the major producer. For management purposes only, the field is divided into Dom Joao Onshore and Dom Joao Offshore. Also, due to a series of normal faults across the structure, the offshore part of the field is further divided into Northern Dom Joao Offshore and Southern Dom Joao Offshore. General data on Dom Joao Field is given in Table 1. Over the years, a total of 828 wells were drilled in Dom Joao Field to tap an original volume of oil in place of 113 MMm3. Yet, after more than 50 years of exploitation, the recovery factor stands at barely 15﹪ OOIP.