On-Line Process Optimization in a North Sea Field
Authors: Jalali Y. , Van Wassenhove W. , Romano C. , Pickles R. 
Published: December 2007

The goal of this paper is to underscore the advantages that can be drawn from the utilization of process simulation during the operational phase of an oil and gas separation process. Life-cycle simulation of a six-month extended well test (EWT), for instance, indicates the potential for improving liquid hydrocarbon yield by 0.64% and reducing flare gas by 1.32%. For a typical North Sea EWT, this represents 35,000 bbl. more oil and 55 million scf less gas. The potential for improvement may be higher for volatile crudes and more complex processes.

Introduction

In 1996, there were at least five EWTs in the North Sea. One of these, which can be considered representative, had the following objective — evaluate reservoir uncertainties to better understand reservoir potential and reduce development risk by producing 5.5 million STB of commercial quality crude in six months without exceeding the regulatory-dictated flare limit of 4.25 billion scf. To achieve this objective. the operating company opted for an early production facility (EPF) by subcontracting a semi submersible drilling rig equipped with custom-built separation facilities.

The reserve estimates for this discovery ranged from 20 to 120 million STB, and hence it was essential to test the reservoir for an extended period of time to narrow the reserve estimate and assess the sustainability of production. The formation consists of a fractured clastic-carbonate rock sequence that partially drapes a salt dome.

Two wells intersect this structure, at angles conforming to the natural deviation of the formation. The true vertical depth (TVD) of these wells is about 7500 ft. The wellheads are completed at the subsea floor. The wellhead lines are joined at a subsea template, which feeds the platform through a flexible riser. Water depth is about 280 ft.

The separation process is single-train, two-stage and three-phase. Export capacity is 40,000 STB/D. The process has a modular configuration, so if additional processing capacity is required, a second train can be laid out in parallel, thereby increasing the throughput capacity. For the EWT under consideration, there was no water production, so the basic process was oil and gas separation. The first-stage separator is where the bulk of the gas is removed from the feed stream. This is the high pressure (HP) separator. The HP flare emerges from this separator. The second-stage separator is where the remaining gas is removed through the low pressure (LP) flare. The total flare capacity is 35 million scf per day. The stabilized crude is directed to export pumps, which deliver the oil to the tanker through a 1-mile export line. The tanker has a storage capacity of 750,000 bbl. As such, it takes eight cargo shipments to deliver the total produced oil to the market.

Figure 1 shows the overall schematic.

RVP constraint

A commonly used index of crude stability is Reid vapor pressure (RVP). RVP is a commercial requirement specified by tanker operators with typical values in the range of 8-12 psia. For RVP measurement, a cell is filled to one-fifth level with the crude oil at atmospheric pressure. The container is heated to 100 F. Hydrocarbon gas evolves, exerting a pressure that is the RVP. The more stable the crude, the lower the RVP. This is because stable crudes vaporize less easily.

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