Examples illustrate sour gas and oil facility design

WELL TIE-IN-Conclusion

Ken J. Vargas
Falcon EDF Ltd.
Calgary

Two examples illustrate the design required to tie-in sour gas and oil wells to production facilities. These designs are typical for the western Canadian environment and economics.

This article is the conclusion of a two-part series on techniques and methods to quickly and effectively connect wells to the production facilities and pipelines. The first was in OGJ, Sept. 21, p. 78.

The detailed design includes:

Equipment layout
Process flow diagram (PFD)
Environmental/regulatory approvals
Pile/civil design layout
Piping and instrument drawings (P&IDs)
Process control strategy (PLC)
Electrical single-line drawings
Electrical load balance
Shutdown key
Isometric drawings
Bill of materials.

Sour gas well

The first example is for a sour gas well that requires a dehydration unit and pipeline to sales.

The first step in the well tie-in design is to recombine the hydrocarbon well effluents and saturate them with water. The recombination helps simulate the reservoir effluent.

The gas and liquid hydrocarbon components were derived from the well test analysis. By recombining the gaseous and liquid phases and saturating them with water, one can simulate the energy consumed or given off in the process.

Fig. 1 [77,000 bytes] shows the two-phase envelope for the gas and the hydrate line. Note the hydrate temperature is 73° F. at the operating pressure of 1,000 psia.

Process flow diagram

For the sour gas well, the recombined hydrocarbons were input to the Hysim simulator. The process flow diagram (Fig. 2 [137,826 bytes]) shows the stream and unit operations. Fig. 3 [110,225 bytes] shows the site plot. The facilities required to handle the sour gas include:

Separator-This unit removes the water and condensate. The condensate is pumped to a nearby oil line and the water is trucked out of a buried tank. All vents go to the flare.
Dehydration unit-The dehydration unit consists of the absorber tower, the regenerator (which includes the regenerator still, reboiler and overhead condenser), inlet/outlet exchangers and circulation pump.
Incinerator-The incinerator takes the water evaporated from the regenerator and noncondensable gases (H₂S), and transforms them to SO₂. This eliminates the toxic H₂S vapors from being released to the environment.
Flare-All blowdown and emergency discharges are vented through the flare line to an auto-ignited flare.

Fig. 2 shows the streams required for the detailed design of the project. The numbers in the diamonds indicate points at which a material balance for the streams was determined.

The triethylene glycol (TEG) exiting the absorber not only picks up water but also picks up 0.0761 mole fraction of H₂S. This H₂S is given off in the regenerator still when the water is boiled off.

With a sweet-gas well, the incinerator is not required because H₂S is not present. In this case the vapors off the regenerator still could be fed to a flare; however, a flare would have to be located a considerable distance away.

The close proximity of the incinerator minimizes the backpressure on the top of the still, allowing the water to boil off easily. This permits good regeneration of TEG, making the TEG very lean (very little water). Lean TEG maximizes the water pickup in the contactor, allowing the dehydrated gas to be on specification (4 lb H₂S/MMscfd).

To aid TEG regeneration, stripping gas can be added to the regenerator tower; thus, more water is stripped from the TEG. In this case, sweet fuel gas is available and is used for incineration.

Sour fuel gas could have been used in the incinerator to achieve the desired temperature for complete conversion of H₂S to SO₂. Because of the large amounts of equipment required to eliminate H₂S, a line heater could have been used with an insulated line to keep the gas temperature above hydrate temperature until it reaches the transfer point. But for distances over 10 km, this may be too expensive.

In this example the gas was being delivered into a dehydrated-gas gathering system, and dehydrated gas was mandatory.

It is important to run project economics when facilities are selected, to ensure the most cost-effective alternative is selected.

Design

Based on the PFD material balance, all equipment was sized and specified.

The separator was sized for 10 MMscfd and 23 b/d condensate. The dehydration unit was designed for these flows and the reboiler duty of 122,200 BTU/hr/0.6 or about 250,000 BTU/hr (48.22 hp).

The incinerator was sized by means of stream "OVHD" in the material balance. The 2,400-m pipeline is designed for 10 MMscfd.

All long-delivery equipment was ordered based on the PFD information and associated equipment sizing.

Table 1 [205,567 bytes] and Fig. 4 [264,695 bytes]show the cost and schedule for this project.

The detailed design involves a equipment layout drawing, environmental approval, process and instrumentation drawings, civil drawings, piping layout/material lists, line list, shutdown key for PLC programming, and area electrical classification.

Construction

The construction can proceed once the detailed design and procurement for the piping, foundations, electrical instrumentation, equipment, and well completion are finished.

Because the project was fast-tracked, construction was started when the first critical item was finished, in this case the well completion. Installation/commissioning was complete in 3-4 weeks.

Sour oil battery

For designing a sour-oil battery for 1,600 b/d, a similar process to the sour gas well example was followed.

The well effluents were recombined in the given ratios. In this case the gas/oil ratio was used to calculate the gas flows. The expected maximum water cut was used.

The production was as follows:

Oil = 1,600 b/d
Water = 14,400 b/d (this is the highest expected water cut)
Gas = 0.7 MMscfd.

Fig. 5 [214,821 bytes]shows the PFD for the battery. Material balance calculations on the streams were done at the points designated with numbered diamonds.

The model was created in such a way that the duties of the test heaters and treater fire-tubes were conservative with more water than would be normally expected. The PFD shows the following equipment:

Inlet manifold with fluid going to either a test heater separator or to the group or inlet separator.
Test and group/inlet separators, with a heater ahead of the test separator to provide heat to obtain good well tests
Two-phase (oil/water) free-water knockout (FWKO)
Treater consisting of both a heater and separator in the model
Gas recompressor for all gas from of the pressurized equipment (treater, and separators)
Crude and water storage tanks, at atmospheric pressure
Vapor recovery unit (VRU) blower, to collect the remaining gas coming out of solution in the storage tanks and to prevent the tanks thief hatch from opening and releasing sour gas to the environment
Crude recirculation pump
Water booster and reinjection pump
PLC (programmable logic controller) based emergency shutdown system (ESD).

Equipment sizing

Sizing of the equipment was done from the Hysim stream unit operations. The test separators operate at 90° F. and 45 psia and have a capacity of 100 bo/d and 900 bw/d. The test line heaters are designed for an emulsion in at 40° F. and out at 90° F.

The free-water knockout flow is for 15,000 b/d of water/oil emulsion. The treater is designed to operate at 170° F., from 40° F. in at 40 psia. Total fluid is 2,960 b/d with 44% water.

The compressor is designed for 64° F. and 40 psia suction with a discharge of 300 psia and 700 Mscfd. The vapor recovery unit (VRU) takes gas at atmospheric pressure and 80° F. and has an outlet pressure of 50 psia.

Material balance flow (Stream 29) times a safety factor accounts for upsets. Filling and blanket gas is 10-15 times and equals 2,900 scf times 15 or about 50,000 scf.

The oil storage is designed for 1,600 b/d and with a slop of two 1,000 bbl tanks. Water storage initially is in two 1,000 bbl tanks, which will be expanded as water cut increases

Water pumping has an initial capacity of 6,000 b/d from atmospheric suction to 2,000 psia discharge. This will also be expanded as water cut increases

The fluid flow lines are designed for full multiphase flow with 300 psia at the well head screw pumps to the battery. The distance is 5,000 m

The water flow line design will handle 14,400 b/d at 2,000 psia.

The flare is designed to handle all of the gas depressurizing from all vessels with a 5-7 min emergency shutdown (ESD).

The ESD system includes full block and bleed of all gas. All the sources are blocked and the system allows full bleed of all vessels.

Table 2 [195,051 bytes] and Fig. 6 [304,345 bytes]detail the cost and schedule for this battery.

The major equipment for the project was specified and bid or located. Because of the fast-track nature of the project, some existing used equipment was located and modified.

When purchasing used equipment, the cost of suitably modifying and installing such equipment must be carefully considered. Our company's experience has been that it is quicker and cheaper to custom build equipment rather than modify used equipment. A prime example was the treater. A new treater could have been built in 12 weeks vs. 10 for modifying a used one. The cost of the modified treater was very similar to a new one.

Design, construction

The detailed design steps are similar to those of the previous sour-gas dehydration example. The only major difference was more equipment and different construction techniques.

Fig. 7 [131,493 bytes]shows the plot of the battery. Drawings of the line lists were also needed as well as complete PIDs. The full battery required 23 PIDs.

Because of delivery problems with equipment, battery construction was completed in 11 weeks instead of the planned 7 weeks. The main reason was the overall high demand for oil and gas equipment and services in Alberta in 1997.