Improving Gas-Lift Performance in a Large North African Oil Field

A significant increase in oil production has resulted from improved gas-lift efficiency in the Nasser field in Libya. This paper discusses the studies involved and the methods used to bring about this improvement. Such techniques can be adapted readily to other producing areas through the use of engineering procedures and computer programs generally available throughout the industry.

Nasser (Zelten) Field

Field and Reservoir Characteristics

Nasser field is in the Socialist People's Libyan Arab Jamahiriya (Libya), approximately 110 miles (178 km) south of the Mediterranean coast on the edge of the Libyan desert (Fig. 1). Oil is found in a highly porous limestone reservoir 180 to 320 ft (55 to 98 m) thick at a measured depth of 5,400 to 5,800 ft (1646 to 1768 m). Commercial oil production from the field began in Aug. 1961, and today there are 130 active producing wells, 119 of which are on gas lift.

Most of the wells have been completed with 9 5/8-in. (245-mm) or 7-in. (178-mm) casing set at the top of the pay and then completed in an open hole drilled to within 50 to 100 ft (15 to 30 m) of the oil/water contact. The wells generally were acid-washed upon completion and produced up the annulus between 2 7/8-in. (73-mm) tubing and the casing. The field has an extremely strong water drive, which has replaced in excess of 95 0,10 of the reservoir withdrawals.

Water production began in 1964 and by 1969 amounted to 250,000 B/D (39 700 m³/d) compared with an oil production rate of 565,000 B/D (89 800 m³/d). Currently, water production amounts to about 79% of a total fluid production that exceeds 500,000 B/D (79 500 m³/d). Initial reservoir pressure was 2,457 psig (16 940 kPa) at 5,200 ft (1585 m) subsea. In 1969, after the production of 1,040 MMSTB (165×10⁶ stock-tank m³) of oil from the reservoir, the pressure had dropped only 186 psig (1280 kPa) to 2,271 psig (15 660 kPa). The current reservoir pressure ranges from a low of 2,254 psig (15 540 kPa) in north Nasser field to as high as 2,306 psig (15 900 kPa) in southeast Nasser field.

The reservoir produces 39° API (0.83-g/cm³) undersaturated crude with GOR's ranging from 345 scf/STB (61.5 std m³/stock-tank m³) in southeast Nasser field to 683 scf/STB (121.7 std m³/stock-tank m³) in north Nasser field. Table 1 summarizes information on reservoir conditions and fluid properties.

Total liquid productivity of the wells in the field ranges from 1 to more than 500 B/D/psig (0.02 to 11.5 m³/d/kPa) of pressure drawdown at the reservoir. Table 2 gives the distribution of productivity among the active wells. From the table, it can be seen that more than half of the wells have total liquid productivities in the range of 5 to 100 B/D/psi (0.115 to 2.31 m³/d/kpa). Very little change in well productivities has been noted over the life of the field, and straight-line extension of drawdown has been quite accurate. The current status of wells in the field is shown in Table 3.

Field and Reservoir Characteristics

Nasser field is in the Socialist People's Libyan Arab Jamahiriya (Libya), approximately 110 miles (178 km) south of the Mediterranean coast on the edge of the Libyan desert (Fig. 1). Oil is found in a highly porous limestone reservoir 180 to 320 ft (55 to 98 m) thick at a measured depth of 5,400 to 5,800 ft (1646 to 1768 m). Commercial oil production from the field began in Aug. 1961, and today there are 130 active producing wells, 119 of which are on gas lift.

Most of the wells have been completed with 9 5/8-in. (245-mm) or 7-in. (178-mm) casing set at the top of the pay and then completed in an open hole drilled to within 50 to 100 ft (15 to 30 m) of the oil/water contact. The wells generally were acid-washed upon completion and produced up the annulus between 2 7/8-in. (73-mm) tubing and the casing. The field has an extremely strong water drive, which has replaced in excess of 95 0,10 of the reservoir withdrawals.

Water production began in 1964 and by 1969 amounted to 250,000 B/D (39 700 m³/d) compared with an oil production rate of 565,000 B/D (89 800 m³/d). Currently, water production amounts to about 79% of a total fluid production that exceeds 500,000 B/D (79 500 m³/d). Initial reservoir pressure was 2,457 psig (16 940 kPa) at 5,200 ft (1585 m) subsea. In 1969, after the production of 1,040 MMSTB (165×10⁶ stock-tank m³) of oil from the reservoir, the pressure had dropped only 186 psig (1280 kPa) to 2,271 psig (15 660 kPa). The current reservoir pressure ranges from a low of 2,254 psig (15 540 kPa) in north Nasser field to as high as 2,306 psig (15 900 kPa) in southeast Nasser field.

The reservoir produces 39° API (0.83-g/cm³) undersaturated crude with GOR's ranging from 345 scf/STB (61.5 std m³/stock-tank m³) in southeast Nasser field to 683 scf/STB (121.7 std m³/stock-tank m³) in north Nasser field. Table 1 summarizes information on reservoir conditions and fluid properties.

Total liquid productivity of the wells in the field ranges from 1 to more than 500 B/D/psig (0.02 to 11.5 m³/d/kPa) of pressure drawdown at the reservoir. Table 2 gives the distribution of productivity among the active wells. From the table, it can be seen that more than half of the wells have total liquid productivities in the range of 5 to 100 B/D/psi (0.115 to 2.31 m³/d/kpa). Very little change in well productivities has been noted over the life of the field, and straight-line extension of drawdown has been quite accurate. The current status of wells in the field is shown in Table 3.