Abstract
A group of mature oil fields in Argentina, that share the same sandstone reservoirs, were declining rapidly when applying established methodologies. New wells drilled away from the exploited areas, generally encountered a water column, while infill wells saw either low pressures or injected water. To revitalize these fields, a new approach to both field management and identification of new opportunities was required. This paper discusses the methodologies applied as part of the revitalizing approach, along with the newly identified opportunities.
The first of these field studied comprised multiple stacked reservoirs all producing oil through comingled wells; oil production and allocation to date was uncertain. An integrated modelling effort showed unexpectedly that reservoirs containing lighter oil were exhausted; however, the shallower, viscous oil-bearing reservoir (Rayoso Formation) was virtually untouched.
To improve the efficiency of the project, one dedicated multidisciplinary team was assigned to identify opportunities over the entire extent of the formation containing viscous oil (~ 860 km2). The key question was where to find the remaining oil within and across the fields.
This work enabled reservoir understanding, developed over a long period of time, during the first study, to be rapidly applied to subsequent ones. Frequent iterations between the static and dynamic models were used to test hypothetical scenarios, to speed up the modelling process and to reduced uncertainty.
Use of analogues across the sandstone reservoirs reduced the uncertainty associated with commingled production by comparison with fields where injection and production was reservoir specific. Through analogues we also determined that water was not injected in secondary sands and that this reservoir was being bypassed due to the presence of similar petrophysical properties but lower viscosity oil in underlying reservoirs.
This combined fields study resulted in significant new volumes being identified with less effort and time required in respect to studying each field separately, this reducing history matching times (from two years to a number of weeks).
As a result of this study, gravity forces and different PVT properties between reservoirs were found to adversely affect production in Rayoso, in addition to the high mobility ratio. A polymer pilot is currently being implemented, with another one currently being planned as the future outlook for these previously rapidly declining fields has now significantly improved.
