32 nd Gas-Lift Workshop The Hague, The Netherlands February 2-6, 2009 Continuous Injection of Hydrate Inhibitor in Gas Lift Gas to Mitigate Downtime Due to Downhole Annular Plugging John R. Cliver and Chris M. Robinson* ExxonMobil Production Company** * C. Robinson currently with Chevron Texaco ** ExxonMobil Production Company is a division of Exxon Mobil Corporation (ExxonMobil). As used in this material, the term ExxonMobil may refer to Exxon Mobil Corporation, one of its division, or a subsidiary or affiliated company. The shorter term is used here only for convenience and simplicity. This presentation is the property of the author(s) and his/her/their company(ies). It may not be used for any purpose other than viewing by Workshop attendees without the expressed written permission of the author(s).
Outline Downhole Annular Plugging Downtime Defining the Problem Modeling Approach Solution Evaluation Implementation and Results 2009 Gas-Lift Workshop 2
Downhole Annular Plugging Downtime Gas Lift Gas Well #1 3000 6 Casing Pressure 1 IPO for unloading SCSSV GLM Mud Line Vacuum Insulated Tubing Casing Pressure (PSIG) 2500 2000 1500 1000 500 5 4 3 2 1 Gas Lift Injection Rate (MMCFD) 1 Orifice Valve GLM BHP CIM X XN 0 0 2 4 6 8 10 12 14 16 18 20 Casing Pressures Time (Days) Gas Lift Rates Pressure and Gas Lift Rates for a Problem Well 0 Reservoir / Perfs Typical Well Configuration Similar trends seen in 30% of the platform s producing wells 2009 Gas-Lift Workshop 3 3
Defining the Problem Sweet Produced Gas Sour Associated Gas Glycol Contactor & Depropanizer Gas from Production System (1100psi) Injection Gas Compressors Injection gas Coolers Problem Assumed to Stem from 1 of 2 Causes: To Gas Injection headers (2600 psi) 1. Hydrates forming due to lack of sweet gas dehydration 2. Asphaltenes asphaltene precipitation from sour gas 2009 Gas-Lift Workshop 4
Two Potential Mechanisms Theory Support for Theory Asphaltenes Solid organic material plugs inside of gas lift orifice Bleeding down pressure causes material to fall out of orifice Observed black hydrocarbon material on exterior of production tubing when pulled for workovers Problem did not go away when methanol pumped during gas lift delivery event Hydrates Pressure drop across valves causes temperature to drop into hydrate formation region Bleeding down casing pressure melts the hydrate plug Sweet gas lift gas not dehydrated before going to injection Problem wells contain vacuum insulated tubing (VIT), nonproblem wells generally did not Potential Solutions Replace gas lift valves Wash backside of tubing using xylene Heat gas before going downhole Pump hydrate inhibitor chemical into annulus with gas lift gas 2009 Gas-Lift Workshop 5
Vacuum Insulated Tubing 3.5 OD Inner Tubing 4.5 OD Outer Tubing Welded on ends, at connection Significantly reduces heat loss by limiting conduction and radiation Intended to prevent heat loss in the oceansubmersed portion of tubing Isolates heat transfer between hot, produced fluids and gas lift gas 2009 Gas-Lift Workshop 6
Hydrate Prediction Modeling Approach q in q out R 1 R 2 R 3 R 4 R 5 R 6 Gas Lift Mud Line T production T gas lift T ambient Casing Pressure Gas Lift Valves SCSSV GLM Vacuum Insulated Tubing 1. Heat Balance in Gas Lift Gas Stream to predict temperature profile GLM BHP CIM X XN 2. Thermodynamic Modeling of fluids to predict hydrate temperatures and pressures Reservoir / Perfs 2009 Gas-Lift Workshop 7
Modeling Output waterline Elevation [ft] seafloor 0 25 50 75 100 125 150 175 T-gas lift T-ocean/ soil T-hydrates Temperature [ o F] 2009 Gas-Lift Workshop 8
Modeling Results Gas lift gas is more likely to form hydrates at lower injection rates waterline waterline Elevation [ft] seafloor 0 25 50 75 100 125 150 175 T-gas lift T-ocean/ soil T-hydrates Elevation [ft] seafloor 0 25 50 75 100 125 150 175 T-gas lift T-ocean/ soil T-hydrates Temperature [ o F] Gas lift temperature profile: 1.6 MMcfd injection Temperature [ o F] Gas lift temperature profile: 0.5 MMcfd injection 2009 Gas-Lift Workshop 9
Modeling Results Summary minimum gas lift gas temperature critical hydrate zone Temperature [ o F] Increasing 2600 psig, HYD 2400 psig, HYD 2200 psig, HYD 2000 psig, HYD 1800 psig, HYD 0.5 1.0 1.5 2.0 Gas Lift Rate [MMcfd] Modeling indicates that hydrate formation is likely below 1.1 MMcfd 2009 Gas-Lift Workshop 10
Potential Solutions Evaluated Maintain gas lift rates above 1.1 MMcfd Install dehydration facilities for sweet produced gas Eliminate vacuum insulated tubing Eliminate the gas lift cooler upstream of gas lift injection headers Inject hydrate inhibitor when a plug develops Continuously inject hydrate inhibitor chemical into gas lift 2009 Gas-Lift Workshop 11
Potential Solution: Remove Gas Lift Gas Coolers waterline waterline Elevation [ft] seafloor 0 25 50 75 100 125 150 175 200 225 250 275 300 325 T-gas lift T-ocean/ soil T-hydrates Elevation [ft] seafloor 0 25 50 75 100 125 150 175 200 225 250 275 300 325 T-gas lift T-ocean/ soil T-hydrates Temperature [ o F] Temperature [ o F] With Coolers (current) Without Coolers Removal of injection gas coolers upstream of injection headers will not prevent hydrate formation 2009 Gas-Lift Workshop 12
Potential Solutions Evaluated Maintain gas lift rates above 1.1 MMcfd Install dehydration facilities for sweet produced gas Eliminate vacuum insulated tubing Eliminate the gas lift cooler upstream of gas lift injection headers Inject hydrate inhibitor when a plug develops Continuously inject hydrate inhibitor chemical into gas lift 2009 Gas-Lift Workshop 13
Implementation and Results Further thermodynamic modeling and evaluation showed Ethylene Glycol injection to be the most cost-effective hydrate inhibitor Currently injecting chemical continuously at low dosages (0.5 gal / day / well) 3000 Well #1 6 3000 Well #1 6 Casing Pressure (PSIG) 2500 2000 1500 1000 500 5 4 3 2 1 Gas Lift Injection Rate (MMCFD) Casing Pressure (PSIG) 2500 2000 1500 1000 500 5 4 3 2 1 Gas Lift Injection Rate (MMCFD) 0 0 0 2 4 6 8 10 12 14 16 18 20 0 0 2 4 6 8 10 12 14 0 Casing Pressures Time (Days) Gas Lift Rates Casing Pressures Time (Days) Gas Lift Rates 2009 Gas-Lift Workshop 14
Questions? Continuous Injection of Hydrate Inhibitor in Gas Lift Gas Mitigates Downtime due to Downhole Annular Plugging Acknowledgements Phil Carrigee Cliff Corbell James Cunningham Mike Johnson John Martin Joseph Pergler Bryan Pickett Bryan Wesley Lester Matthews 2009 Gas-Lift Workshop 15
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