Multiphase Production Through Hilly Terrain Pipelines in Cusiana Oilfield, Colombia T.J. Hill, SPE, C.P. Fairhurst, SPE, and C.J. Nelson, BP Exploration Operating Company, London, H. Becerra, BP Exploration Company (Colombia) and R.S. Bailey, BP International
The Cusiana oilfield in Colombia is currently producing about 180,000 bpd through a complex multiphase flowline network. The terrain of the area is very hilly, with substantial elevation changes along the length of the lines. Prediction of pressure drop using industry standard correlations has been very variable in its accuracy. A revised pressure drop method, including the effect of slug formation and decay, has been produced, with appreciably better performance.
Field data on flow regime characteristics from several of the lines are presented to show a transition from surging/slugging to a steady 'homogeneous' flow at relatively low mixture velocity. The effect of slug flow on slugcatcher performance has also been assessed, both by direct measurement, and by use of a dynamic simulator. The simulator is used to test new control schemes prior to implementation.
At low flowing velocities one line has been seen to undergo large pressure swings and to exhibit slug production due to liquid accumulation and sweepout. This effect is described, and re-produced using a transient simulator.
The Cusiana oilfield containing 1.6 billion barrels of 350 API oil, is located in the Eastern Cordillera of the Andes mountains, in Colombia. Current production rates are of the order of 180,000 bpd, with a GOR of 1400-2000 scf/bbl and little water, from thirteen wells drilled to about 13,000 ft.
The wells are connected to a central processing facility (CPF) via a complex network of multiphase flowlines. These range in diameter from 8 to 20", and in length from 0.4 to 12.5 miles. Figure 1 shows a plan view of the layout of the network. The topography of the line from wellpad T, subsequently co-mingled with fluids from wellpad Q, to the CPF is given as Figure 2, illustrating the large variations in elevation over the length of the line.
During the design phase of the field a long term test (LTT) was established from well BA-1 through a flowline to a 13,000 bpd stand-alone process facility located 4.3 miles away. The detailed topography of this line, for which early pressure drop data was made available, is given in Figure 3. Both the T-Q and LTT lines pass under the Cusiana river (albeit flowing in opposite directions), as well as over large hills. The CPF is located on a mesa, so the majority of lines arriving at it have a final section of upwards inclination. The LTT process centre is at a different location to the CPF. The line arriving at this process plant has a final section that is inclined downwards, as shown in Figure 3.
The key multiphase hydraulic issues arising in the Cusiana field are the prediction of pressure drop, and the characterisation of slug flow, through hilly terrain multiphase flowlines - do we have enough energy to get the required production rates from reservoir to process, and can the process accommodate satisfactorily any slugging flow arriving from the flowline?
At the end of the flowline network is a manifold system connecting to two finger type slugcatchers (each with four 42" epoxy lined pipe 'fingers', 90 feet long), as shown in Figure 4. The slugcatchers have two main functions:
to 'catch' slugs of liquid, and hence smooth the flow to downstream facilities
to buffer, to some extent, the arrival of gas to avoid rapid rates of change of gas flow through the process.
The slugs are formed in one of three ways - normal steady-state hydraulic slugs; transient slugs due to changes (increases) in throughput; terrain slugging in low flowrate operation. Finger-type slugcatchers were installed primarily because of transportation limitations on vessel size, and the relative ease of on-site fabrication. The two slugcatchers feed four parallel, nominally 40,000 bpd, processing trains.