Arun Field High-Pressure Gas Reinjection Facilities
Authors: Giles A.J. 
Published: December 2007

Summary
This paper discusses the high-pressure natural gas reinjection facilities at the Mobil Oil Arun natural gas field in north Sumatra. Each of the three compression trains at Arun comprises a four-stage centrifugal compressor with a 28,000-bhp [20 888-kW] gas turbine driver plus associated cooling, scrubbing, and control facilities. The present throughput of each train is around 250 × 10 6 scf/D [7.08 × 10 6 std m3/d] at 6,100 psig [42 MPa] at 6,500-psig [44-MPa] discharge pressure. The paper first discusses the overall Arun field facilities and operation. It then describes in detail the compression train facilities including the causes of, and solutions to, the many and various problems associated with the commissioning and operation of these prototype compression facilities. The paper also describes performance monitoring of the trains and the efforts paper also describes performance monitoring of the trains and the efforts made to optimize train capacity. The paper concludes with 12 specific practical recommendations based on the problems and experiences encountered practical recommendations based on the problems and experiences encountered at Arun during the last 7 years' operation.

Arun Field Development
Field Statistics. Arun field is a giant natural gas field situated onshore north Sumatra in the province of Aceh. The natural gas reservoir is a carbonate reef at a depth of 10,000 ft [3048 m]. Initial reservoir pressure was 7,100 psig [49 MPa]. The field, discovered in 1971, became pressure was 7,100 psig [49 MPa]. The field, discovered in 1971, became operational in 1977. The current field production is 2,300 × 10 6 scf/D [65.13 × 10 6 std m3/d] natural gas with an associated liquid condensate production of 140,000 B/D [22 258 m3/d]. Of the gas, 1,550x 10 6 scf/D production of 140,000 B/D [22 258 m3/d]. Of the gas, 1,550×10 6 scf/D [43.89 × 10 6 std m3/d] goes to the P.T. Arun liquefied natural gas (LNG) plant, which is 19 miles [30 km] away on the northeast coast of Sumatra. plant, which is 19 miles [30 km] away on the northeast coast of Sumatra. The remaining gas production, around 750 × 10 6 scf/D [21.24 std m3/d], is reinjected into the Arun reservoir by using three gas-turbine-driven centrifugal compressors. The reinjection facilities act primarily as a backup to ensure a continuous gas flow to the LNG plant in the event of a production upset. Moreover, the additional gas production has accelerated production upset. Moreover, the additional gas production has accelerated the production of liquid condensate and, hence, field revenue.

The Cluster Concept
The Arun field production facility is designed around a "cluster" concept. At present there are four clusters sited along the length of the field, with 16 production wells per cluster. Each cluster is an independent, self-contained production area except for electrical power, which is generated at a central common facilities area. Each cluster has two identical production trains (Fig. 1) designed to handle 375 × 10 6 scf/D [10.62 × 10 6 std m3/d] dry gas. Each train has independent pressure-reduction, cooling, liquid separation, and condensate-pumping facilities.

Reinjection Facilities
The four clusters are essentially identical to each other except for Clusters 2 and 3, which also site the three Arun field gas reinjection trains. There are two compressor trains at Cluster 3 (K301 and K351), and one compressor train at Cluster 2 (K201). The suction to the compressors is taken from the cluster gas production pipeline and the discharge goes to the cluster gas injection header (Fig. 2). From the header, separate lines transport the injection gas to each of nine gas injection wells (GIW's). Six GIW's tie in to the Cluster 3 injection header and three GIW's into the Cluster 2 injection header. There is also an injection header interlink line between Clusters 2 and 3 for balancing of GIW flows if required.

Gas Reinjection Trains

Design Parameters
The three identical compressor trains at Arun originally were designed to the conditions listed in Table 1.
Description of Main Compressors. The original injection compressors were skid-mounted, four-stage, centrifugal compressors, consisting of a 272B four to four low-pressure barrel (Fig. 3) and a 181B three to three (Fig. 4) high-pressure barrel.
The compressors comprise the following main components.

Compressor Case
Both the 272B and 181B compressor sections comprise a one-piece, forged steel barrel containing a bundle and rotor assembly. The case is the main pressure-containing device of the compressors. The 272B case is pressure tested to 7,500 psig [52 MPa] and the 181B case to 12,750 psig [88 MPa]. Bundle Assembly. The bundles include the intake walls, inlet guide vanes, diaphragms, and division walls. They are horizontally split with O-ring cordage for sealing between halves and taper pins for assembly alignment. O-rings also are used for interstage sealing between the bundle and the compressor case. Rotor Assemblies. These consist of a shaft, impellers, (four per stage on the 272B shaft and three per stage on the 181B shaft), impeller spacers, and thrust-bearing disks.
JPT
p. 701