An Integrated Geological-Reservoir Engineering Technique for Increasing Confidence in Field Development Planning for a Reservoir Under Primary Depletion
Authors: Biswas Aakash , Jumah Ali Al , Lalji Farhad , Doush Mohamad 
Published: November 2017

One of the principle inputs to project economics and all business decisions is a realistic production forecast. This becomes particularly challenging in supergiant oil fields with medium to low lateral connectivity. In this case, three distinct production forecasts were generated: Short Term (ST) – 3 monthsMedium Term (MT) – 2 yearsLong Term (LT) – 5+ years

The main objectives of the Medium Term Production Forecast (MTPF) are: Provide an overview of the total expected production profile, expected wells potential/ spare capacityHighlight the requirements to maintain production targetsProvide an anchor point for the ST and LT production forecast generation

The main tool used for MTPF was Integrated Production System model or IPSM (GAP © PETEX) since it can predict reservoir behavior, honor physical constraints and capture bottlenecks and back-pressure effects within the production system. The different components of the IPSM are the reservoir component, well component and surface network.

This paper covers the methodology for building the reservoir component of the IPSM to evaluate and accurately predict reservoir performance in the MT period. The IPSM model is not only used for the MTPF generation, but also for real time Production System Optimization (PSO), key decisions regarding well hookups, etc. by the Well, Reservoir and Facility Management (WRFM) team. There was a business need to build a pragmatic IPSM model with an optimal run time.

Thus, tank models were built to replicate the 3D reservoir models which had been built in MoRes™ (dynamic reservoir simulator developed by Shell). The MoRes™ model was divided into multiple sectors (tanks) based on pressure sinks predicted by the model and supported by geological studies.

From the MoRes™ model, a Grid cell Pressure histogram (Pressure versus frequency plot) was used to divide the reservoir into certain sectors. A close-to-normal distribution per sector was obtained to divide the reservoir into an optimal number of representative sectors. This was done because a single pressure value per sector is used by the well models connected to the sector to calculate the inflow performance in IPSM. Acquired Static Bottom Hole Pressures (SBHPs) in wells were used as anchor points for the calculated average pressure of the sectors and to test the validity of the divided sectors.

This methodology has been tested successfully in the stated super giant oil field, which has both sandstone and carbonate reservoirs. An example this is covered in the paper. It was concluded that utilizing multiple sectors (tanks) results in reservoir pressure decline predicted being closer to what is actually seen by the wells. The IPSM model built represents reality sufficiently well in the MT period, thus increasing confidence in the generated production forecast. Another technical advantage of the described method is the higher sustainability of the model.

The suggested histogram method, in combination with geological information available, can be applied to majority of the reservoirs. This combination is paramount to ensure the divided sectors concur with reality.