Simulation Model Robustness Check- Indicators of Pessimistic Models Pre and Post History Match Case Study Fotis Reservoir

Abstract

The essence of reservoir simulation is to forecast field performance, ultimate recovery and to evaluate the effects of different operational conditions on recovery while keeping the economics of each scenario in view. It is important to note that in simulation one of the characteristics of a history match is that the outcome is a non-unique solution and it is influenced by many factors including porosity, permeability, thickness, saturation, PVT, relative permeability etc. A combination of these parameters result in a match and is not unique to that reservoir, thus this may not perfectly represent the condition of that reservoir. A common area of struggle is the transition of a model from pre-production vintage to a history matched solution where the only additional data available is production without having to build the model from scratch. It is therefore important to have a rule of thumb to validate the robustness of a simulation pre-production forecast prior to the onset of production. The effect of this may result in a pessimistic or overly optimistic model which translates to a non-representative reserves addition. The Fotis reservoir has been used as a case study to highlight the importance of a robust pre-production forecast prior to the onset of production and history match. Fotis is a NAG reservoir with a fault assisted dip closure trap. The reservoir sand is interpreted to be deposited in a shallow marine setting (consisting of shoreface and channels deposits). Reservoir properties are good (Avg Por. 17%; perm ~700 mD). Base case In-Place volume is about half a Tscf. Some 27% of the inplace volumes have so far been produced. The reservoir simulation model went live in 2010 without a history match because there was no production and pressure data. Production commenced in 2011. Well and Reservoir Management Philosophy for the reservoir was to produce the well at 100MMscf/d thus this was captured in the model though model forecast plateau seemed conservative for the low case model. The model was history matched in 2015 post the acquisition of pressure data. Inability to achieve history match within acceptable limits on the low case, even as the current well production rate of 100MMscf/d was sustained confirm the conservative nature of the low case model. This paper analyzes and highlights pitfalls to be avoided in the pre-production models and how multi-disciplinary solutions could drive a more representative output that supports robust life cycle predictions from the model.