Use of Pressure Transient Analysis to Evaluate the Impact of Salt Deposition in Gas Wells. E.H. Smith

Transcription

1 Use of Pressure Transient Analysis to Evaluate the Impact of Salt Deposition in Gas Wells E.H. Smith

2 Key Points Causes and risks of down-hole salt deposition Difficult to identify early changes in well performance Pressure transient analysis is a useful tool for Detecting subtle early changes in well performance Identifying location of salt deposition How to Minimize Loss of Production from Salt Damage

3 Down-hole Salt Deposition Caused by evaporation of formation water Southern North Sea Gas Well Requires sustained low water rates ~1bbl/mmscf Restricts perforations Creates salt bridges Causes reduced and erratic flow Can totally block wells Bailer Sample Central North Sea Gas Cond Well East Irish Sea Gas Well SPE Salt Across Perforated Intervals

4 Formation Water Evaporation Evaporation occurs when the ability of gas to contain water vapour increases Reducing pressure at constant temperature increases the water vapour in saturated gas (bbls/mmscf) Reducing temperature however decreases the water vapour in gas and leads to condensation Reducing pressure with no change in enthalpy causes Joule-Thomson cooling Thermal conduction through the rock warms cooled gas Geo-thermal gradient causes gas to cool as it flows to the surface

5 Saturated Gas Water Vapour Content Gas Water Content (BBLs/MMscf) (BBLs/MMscf) For a constant temperature (240 o F) Water content increases as pressure reduces Formation water evaporates (Salt may be deposited) 4.0 Pressure is reduced by flow through restrictions Choke, Blockage, Skin 3.0 J-T cooling will therefore occur at pressure drop The temperature is therefore reduced This reduces the ability of gas to hold water vapour Water may condense (salt may even be dissolved) No salt deposition until gas warmed by thermal conduction However below about 1500 psi pressure Pressure decline dominates J-T cooling Salt deposition enhanced at a restriction Pressure Psia

6 Potential for Salt Deposition Gas cools as it enters the well Limit of salt deposition Cooling Thermal conduction warms gas Potential for salt snow when cool gas mixes with hot dryer gas Potential drifts of loose salt that could move and create blockages Salt bridges may form but are limited by J-T cooling Water seeping out of silts

7 Estimate Risk Using a Salting Envelope Drawdown (psi) Salting tendency increases with drawdown 3500 Halite deposition Well Drawdown Salting tendency worse at low pressures 3000 WGR 0.5 bbl/mmscf WGR 1 bbl/mmscf 2500 WGR 2 bbl/mmscf Worse in low perm depleted reservoirs 2000 WGR 5 bbl/mmscf WGR 10 bbl/mmscf Worse at lower formation water rates 1500 Salting worse for high salinity reservoirs 1000 However at sustained low rates salting is possible for moderate salinity reservoirs 500 No Halite Deposition Reservoir pressure (psia)

8 Case Study of Salt Impairment Well features Horizontal well Fluvial reservoir with alternating intervals of channel sands and overbank Water seeping out of very low permeability wet overbank is potential source for formation water Completed using sand screens Down-hole gauge Well history Initial high rates 100+ mmscf/d Rapid decline in rate and pressure Compression upgraded, suction pressure reduced, well flow declines rapidly and becomes erratic Water washes from surface gave little benefit Coiled tubing water wash removed salt plugs and well productivity was restored

9 Using Back-Pressure Plots To Check Well Down-hole Wellhead with Static Depth Correction IMPAIRED IMPAIRED OK OK 4hr Transient 4hr Transient

10 Well Bottom Hole Productivity Coiled Tubing Clean-out A OK B OK? Surface Water Washes D OK? C IMPAIRED N=0.7

11 Well Log-Log Pressure Transient Plots A Early build-ups show no change B Possible slight loss in deliverability Significant changes in PTA response Increased skin Shift in late time response Indicative of loss of productive interval (toe?) C Significant loss of deliverability Much higher skin Wellbore Storage & Skin Reservoir Geometry & KH (Changes may indicate loss of productive intervals) D Deliverability restored by Coil Tubing Well appears to be restored to same state as B Coiled tubing job aborted 1,000ft short of TD Productive interval at toe still lost

12 Evaporation Potential at Downhole Gauge Temperature Loss of production from toe Pressure Significant J-T cooling Suction pressure reduced Tubing becoming blocked predominantly above gauge below 1500 psi Coiled Tubing Intervention Significant evaporation potential Water Vapour Initial Temperature & Shut-in Pressure High risk of evaporation Water Vapour Capacity at Gauge Water Vapour in Gas at Initial Conditions

13 Use of Wellhead Pressure Transient Plots Many wells do not have BHP gauges Good quality high frequency WH data can be used Baseline initial data needed prior to deterioration This example well has salt restrictions in the tubing Late time reservoir response is stable Water washing from surface effective Washing needs to be repeated regularly Infrequent washes can allow salt to build-up and totally block the well Coiled tubing would then be needed to deliver water Initial Rig Test Early Performance Impaired Post Wash

14 How to Minimize Loss of Production from Salt Damage Be alert to signs of salt deposition, especially wells that appear to be dry Surface water washing can be effective Water needs to be able to reach salt (will not remove salt in horizontal wells) Needs to be repeated regularly Once well is totally blocked water cannot reach the salt plugs Coiled Tubing Very effective removing salt Need to have coiled tubing access to the salt plug Very expensive if treatment needs to be regularly repeated Permanent down-hole water injection Choke well back to reduce drawdown Avoid suction pressure reductions until salt is managed Case off tight wet zones

15 Acknowledgements The author thanks the following for their permission to present this paper GDF Suez E&P UK Ltd Tullow Oil Ltd And acknowledges the assistance of ConocoPhillips colleagues 15