Abstract

Field data from the Prudhoe Bay oil field in Alaska was used to develop a neural network model of the cross-country gas transit pipeline network between the production separation facilities and central gas compression plant. The trained model was extensively tested and verified using 30% of the data that was not used during the training process. The results show good accuracy in reproducing the actual rates and pressures at the separation facilities and at the gas compression plant. The correlation coefficient for rate and pressure were 0.997 and 0.998 respectively.

This is the first phase in the development of a tool to maximize total field oil production by optimizing the gas discharge rates and pressures at the separation facilities. The second phase, development of a state-of-the-art genetic algorithm to perform the optimization, is currently being tested and will be the subject of a future paper.

Background

Prudhoe Bay has approximately 800 producing wells flowing to eight remote, three-phase separation facilities (flow stations and gathering centers). High-pressure gas is discharged from these facilities into a cross-country pipeline system flowing to a central compression plant. Figure 1 illustrates the gas transit network between the separation facilities and the inlet to the compression plant.

Fuel gas supply (at the flowstations and gathering centers) and artificial lift gas supply for the lift gas compressors at GC1 are taken off the gas transit line upstream of the compression plant. This reduces the feed gas rate and pressure at the inlet to the compression plant.

Gas feeding the central compression plant is processed to produce natural gas liquids and miscible injectant. Residue gas from the process is compressed further for reinjection into the reservoir to provide pressure support.

Business Motivation

Ambient temperature has a dominant effect on compressor efficiency and hence total gas handling capacity and subsequent oil production. Figure 2 illustrates the range of daily average temperatures from 1990–2000, and the actual daily average for 2001 and 2002. Observed temperature variations during a 24-hour period can be as high as 40 degrees Fahrenheit (°F).

Figure 3 is a curve fit to total shipped gas rate to the compression plant versus ambient temperature for 2001. A significant reduction in gas handling capacity is observed at ambient temperatures above 0°F. Individual well GOR ranges between 800 scf/stb and 35,000 scf/stb, with the lower GOR wells in the water-flood area of the field and higher GORs in the gravity drainage area. Gas compression capacity is the major bottleneck to production at Prudhoe Bay and typically field oil rate will be maximized by preferentially producing the lowest GOR wells.

As the ambient temperature increases from 0 and 40°F, the maximum (or "marginal") GOR in the field decreases from approximately 35,000 to 28,000 scf/stb. A temperature swing from 0 to 40°F in one day equates to an approximate oil volume reduction of 40,000 bbls, or 1000 bopd per °F rise in temperature.

The reduction in achievable oil rate, per degree Fahrenheit increase in temperature, increases with ambient temperature. This is due in part to the increase in slope of the curve of shipped gas versus temperature, and also to the reduction in limiting or "marginal" GOR as gas capacity decreases. At higher temperatures, larger gas cuts are required at lower GORs to stay within compression limits.

Business Motivation

Ambient temperature has a dominant effect on compressor efficiency and hence total gas handling capacity and subsequent oil production. Figure 2 illustrates the range of daily average temperatures from 1990–2000, and the actual daily average for 2001 and 2002. Observed temperature variations during a 24-hour period can be as high as 40 degrees Fahrenheit (°F).

Figure 3 is a curve fit to total shipped gas rate to the compression plant versus ambient temperature for 2001. A significant reduction in gas handling capacity is observed at ambient temperatures above 0°F. Individual well GOR ranges between 800 scf/stb and 35,000 scf/stb, with the lower GOR wells in the water-flood area of the field and higher GORs in the gravity drainage area. Gas compression capacity is the major bottleneck to production at Prudhoe Bay and typically field oil rate will be maximized by preferentially producing the lowest GOR wells.

As the ambient temperature increases from 0 and 40°F, the maximum (or "marginal") GOR in the field decreases from approximately 35,000 to 28,000 scf/stb. A temperature swing from 0 to 40°F in one day equates to an approximate oil volume reduction of 40,000 bbls, or 1000 bopd per °F rise in temperature.

The reduction in achievable oil rate, per degree Fahrenheit increase in temperature, increases with ambient temperature. This is due in part to the increase in slope of the curve of shipped gas versus temperature, and also to the reduction in limiting or "marginal" GOR as gas capacity decreases. At higher temperatures, larger gas cuts are required at lower GORs to stay within compression limits.

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