Adjusting the Basis of Design

1. Adjusting the Basis of Design Why Facilities Engineers Need to Think Ken Arnold, Senior Technical Advisor WorleyParsons Offshore Technology Conference, Houston │ 6-9 May 2013

2. Outline • What Makes Basis of Design (BOD) Susceptible to Interpretation and Change? • Fluid Properties • Water Production • Flowrate • Interaction of Topsides and Production Rate • Inlet Separator Pressure and Compressor Configuration • Sizing Equipment • Conclusions

3. What Makes BOD Susceptible to Interpretation and Change? • Reservoir studies are educated guesses at a point of time of future properties effecting design: • Flowrates • Pressure • Expected water production • Number of wells, flowrates and reserves often changes during design and construction due to more data and reservoir studies • Often another reservoir with different properties of fluids is discovered prior to or after startup

4. What Makes BOD Susceptible to Interpretation and Change? • All input variables change with time • Rapid changes due to well slugging • Long term changes due to reservoir conditions • Hard to change out equipment as needs change • Energy is bought at wholesale prices • Processes are mostly gravity separation of immiscible phases • Not particularly sensitive to small changes in BOD • Changes in flow, pressure and temperature may have little impact on equipment’s ability to function

5. What Does This Mean? Be Flexible!! Optimizing the process for givens in BOD is probably the wrong thing to do Design for easy modification Consider modifications to BOD based on practical affects on facility equipment selection and sizing

6. Fluid Properties • BOD based on fluid sample • Small and perhaps not representative of all reservoir zones • Contaminated with drilling mud • Weathered in field or laboratory • Probably no sample of reservoir water • Laboratory emulsion tests based on above sample and synthetic water • Laboratory GORs based on separator flashes which are probably different than what will be used in the design • Fluid gravities, GORs and required retention times may be significantly different than BOD

7. Water Production • Dependent on drive mechanism, reservoir uniformity and continuity • Need extended production data to accurately predict • Often oil reservoirs show earlier water breakthroughs than anticipated while there is still significant reserves of oil to recover • Higher water treating needs than BOD • Longer life to produce oil

8. FTP PC SEPARATOR Gas Lift Gas Casing LC Q = f (PR – BHFP) FTP = P +Pflowline BHFP = FTP + H +PTubing Tubing Gas Lift Valves Q = flowrate into wellbore PR = reservoir pressure BHFP = bottom hole flowing pressure FTP = Flowing tubing pressure  = average density of fluid in tubing P = separator pressure H = depth of reservoir Packer Reservoir BHFP Flow From the Reservoir

9. Flow From the Reservoir • Flow rate into the bottom of the well is a function of the difference between Reservoir Pressure (RP) and Bottom Hole Flowing Pressure (BHFP) • The RP is determined by pressure depletion due to fluids produced less any influx from the edges of the reservoir to fill the void left behind by the production • RP may stay constant or decline with time • At any point in time there is a curve of flowrate from the reservoir versus BHFP • RP depends on the time history of fluid production

10. Determining Flow • Flow into the bottom of the well (and thus out the top) is controlled at any point in time by adjusting the BHFP. This can be done by: • Adjusting the pressure drop across the wellhead choke • Adjusting the Inlet Separator operating pressure • Artificial lift • Some combination of the above • Thus the minimum Inlet Separator operating pressure can affect flowrate from the reservoir once the wellhead choke is wide open

11. Inlet Separator Pressure - High • Benefits • Conserve reservoir energy • Reduce compression power requirements • Stabilize the crude with multiple flashes (increase sales) • Smaller size if dependent on gas capacity • Detriments • Higher cost for separator, PV&F, instruments and controls • ANSI 600 (100 bar), 900 (150 bar), 1500 (250 bar) • Backpressure on wells (decreases flowrate)

12. Inlet Separator Pressure - Low • Benefits • Reduces backpressure on wells (increases flowrate) • Minimizes need for artificial lift • Lower cost for separator, PV&F, instruments and controls • Detriments • Compressor power requirements greatly effected by suction pressure • Larger size to handle the same amount of gas • Limits ability to use flash stabilization of crude • Need to pump liquid through system

13. Compressor Suction Pressure • Power is a function of flowrate • Power is a function of compressor ratios (Pd/Ps) • Once a compressor configuration is chosen there is a curve of throughput vs. suction pressure • Suction Pressure determines Separator Pressure which determines Wellhead Pressure which determines BHFP • Thus compressor configuration defines a curve of BHFP versus topsides flow capacity • At any point in time there is a curve of flowrate from the reservoir versus BHFP • The flowrate will be the intersection of these two curves and independent of the BOD

14. Compressor Selection • In high GOR crude reservoirs and gas fields trying to meet the desired BOD of flowrate and wellhead pressure late in life may reduce project economics • Initial capital plus maintenance over life • Late in life deferred production • In oil reservoirs requiring gas lift, lower wellhead pressures require less gas lift gas for the same BHFP, and can thus result in lower compressor power requirements • Consider changing the BOD

15. Sizing Equipment - Separation Actual gas capacity of a separator Liquid capacity of a two phase separator Oil and water capacities of a three phase separator Capacity of oil treating equipment Capacity of water treating equipment Bottom line: Changes in BOD may not affect separation equipment sizing but could affect size of nozzles, relief valves and controls

16. How About Compressors • Changes in flow can affect suction pressure • Low suction pressure limit controlled by high discharge temperature - recycle • High suction pressure limit controlled by driver power requirements or rod load reversal – line pack or flare • Do we really need a spare compressor for all potential load conditions?

17. How About Glycol Dehydration Low flow depends on design of internals – structured packing High flow limited by carry-over, pump capacity and/or heat capacity of reboiler Judicial use of lagniappe

18. Conclusions • The Basis of Design is always a good starting point for configuring a facility design • The BOD does not normally consider in detail the affects on facility selections • The facilities engineer must use judgment in selecting equipment design • Understand the accuracy of the data used to develop the BOD • Understand the affects on equipment selection and sizing • Modify the BOD based on practical limitations of the facility equipment and related costs