| Popis: |
Deepwater fields, such as those encountered in the Gulf of Mexico (GoM), often exhibit reservoir tar deposits and asphaltene instability (Dumont et al, 2012, Mullins et al, 2016). This instability may manifest itself within the reservoir in the form of out of equilibrium asphaltene gradients and tar deposits. These tar deposits can present drilling hazards which are a major nuisance for drilling operations, and can force operators to drill multiple sidetracks or bypass wells. This of course leads to increase well spend and rig time usage. For instance, the last two years showed that approximately 5% of the wells drilled in the GoM encountered severe drilling issues related to asphaltenes/tar deposits. Even more critical and concerning, is that asphaltene instability can have major implications for field developments, stretching expenditure risks beyond a single well to that of an entire field. From a flow assurance point of view, comingling incompatible oil produced from different zones or reservoirs can precipitate, and worse, deposit asphaltenes in the production path even when the unmixed oils are individually stable. Thus for reservoir management and field development plan, tar deposits can seal off aquifer support and have a detrimental impact on the productivity index of the field. During field development planning, it is essential to know the Asphaltene Onset Pressure (AOP) of the produced oils at reservoir conditions and its change with pressure and temperature along the wellpath. In addition, for unstable oils, AOP gradients variations across the field should be evaluated as AOP can vary largely as a result of Reservoir Fluid Geodynamics (RFG) processes occurring within the reservoir. For instance, fluid mixing due to a secondary charge into a reservoir after trap filling increases the AOP of the resulting oil. In addition, the non-homogeneity of AOP can then be mislabeled as AOP laboratory uncertainty. As AOP field variability can impact all aspects of the journey of the oil to surface, (well placement, completion, recovery factor, injection for pressure support, surface facility, and subsea equipment) and because no model exist to predict AOP gradients, the measurement and the evaluation of these AOP gradients, coupled with simple chemistry understanding are key inputs to a successful field development plan. This paper discuss AOP gradients within a reservoir, their impact on Field Development Plan (FDP), their origin, and proposes a methodology to characterize them. |
