Optimization of Riser Design and Drill Centers with a Coupled Reservoir and Facility Network Model for Deepwater Agbami

Abstract

The utilization of multiple wells connected at a subsea manifold providesthe opportunity to reduce the number of risers and to reduce capital expense.The problem is the proper modeling of the reservoir well flow coupled with thecombined flow into the facility network (manifolds, flowlines and risers), which is necessary to prevent under design resulting in flow rate bottlenecksor over design resulting in extra expenditures. This paper presents a tool andmethodology for better modeling of the well to riser flow and the optimizationof riser count and configuration. Although reservoir models coupled with facility networks is not new, software enhancements provided the capability of including operation logic thatcould duplicate operations in the field. The reservoir model coupled with thefacility network provided more reasonable and accurate modeling of multiphaserates and pressures as wells were combined into a single riser. To optimize the riser count, operation logic was applied to maximize rate inthe riser at all times against the erosional velocity limit. This in turnprevented over designing by placing more risers than needed to obtain the fieldfacility capacities. The utilization of the coupled model and operation logic allowed theoptimization of the riser count for any particular reservoir model. The optimumriser count was then determined for 3 earth (geologic) models. Introduction The Agbami structure is a northwest/southeast trending four-way closureanticline, and is located on the Niger delta front approximately 65 milesoff-shore Nigeria in the Gulf of Guinea (see map in Fig. 1). Thestructure spans an area of 45,000 acres at spill point and is located in 4800ft of water. The Agbami No. 1 discovery well was drilled in late 1998. Theappraisal program was completed in 2001 and included five wells and onesidetrack drilled on the structure with each encountering oil pay. These fivewells and a sidetrack penetrated an average of approximately 350 ft of oil. One of the key outcomes of the previous phase (Phase 2) work was theselection of the facilities design. The Agbami team adopted an all-subsea, field development scheme. This scheme involved modeling all field productionand injection fluid flow through numerous subsea manifolds. One of the important objectives in the current phase (Phase 3) ofdevelopment was to determine the optimum number and locations of the productionrisers. The problem was determining how to properly model the reservoir andfacility design such that the impact of riser count can be properly quantified.The determination of this impact was necessary in order to make a qualitydecision on the riser count and configuration. Several key development decisions were determined in the previous phase(Phase 2) of the development process. These decisions were taken as givens inthis study and are listed as follows:Recommended pressure maintenance scheme and gas disposition strategy for thelower, 17 million-year (MY) reservoir is a combination of crestal gas injectionwith peripheral water injection.Recommended pressure maintenance scheme and gas disposition strategy for theupper, 14MY–16MY reservoirs is crestal gas injection only.Facility design capacity recommendations are:250,000 stock-tank bbl per day (STB/D) oil450,000 thousand cubic ft per day (Mcf/D) gas production250,000 STB/D water production450,000 STB/D liquid production450,000 STB/D water injection Reservoir Description and Uncertainty Geological Model Framework. Although the seismic quality is less than perfect, the data indicates that ahigh degree of complexity is prevalent at Agbami. The structural andstratigraphic complexity needs to be discussed briefly to set the backgroundfor the earth (geologic) and reservoir modeling.

Flow assurance analysis in Front End Engineering Design (FEED) of Subsea Flowline and Riser

“…This is not however without its shortfalls, because facility design to handle such a wide range (14%) in production-rate, necessitates ensuring that all wellbore equipment, surface processing facilities, and transportation infrastructure are adequately sized and designed to accommodate this flexibility. Narahara et al [16] in their optimization study on Agbami No.1 Discovery well in the Gulf of Guinea, suggest that reasonable care should be taken when performing a coupled reservoir-facility network model design, to avoid over-designing facility capabilities as it can lead to significant upfront capital expenditure (CAPEX)…”

New Methods to Predict Inflow Performance of Multiply Fractured Horizontal Wells under Two-Phase Condition and Optimize Number of Fracture Stages

“…The deliverable PI is then used as an input to the surface network model which takes into account all the engineering constraints from facilities and wellbores. This requires the use of network simulation methods (Narahara et al 2004). The network simulation results might predict that a horizontal well will not produce at the maximum PI.…”

An Offshore Field Development Plan to Optimize the Production System from the Reservoir to the Tank

“…Restrictions in compression power or pump capacity, for example, could impose significant limitations over the well and surface network performance. According to Narahara et al (2004), although reservoir models coupled with facility networks are not new, software enhancements provided the capability of including operation logic that could duplicate operations in the field. Morales et al (2007), state that efficient optimization of large and complex production facilities (involving wells, pipeline networks, process facilities, etc.)…”