Intermittent Production from Liquid Loaded Gas Wells by P. Verbeek (Shell, Rijswijk), H. Frydenlund (NTNU, Trondheim), T. Clemens (RAG, Vienna), R. Eylander (NAM, Assen)
Intermittent Production from Liquid Loaded Gas Wells Topics 1. The hydraulic model 2. Field test versus model prediction 3. Intermittent production 4. Downhole deliquification & injection
Typical production response in liquid loaded gas well 1,000 Production Response (example) Unloading Region Loading Region Unstable (cycling) Region Intermittent production Gas Rate (MCFPD) 100 10 Gas Rate Liquid Rate Forecasted Production Recharge (reservoir) Drainage (well) 1 J-98 J-99 J-00 J-01 J-02 J-03 J-04 J-05 J-06 J-07 J-08 J-09 J-10 Time
Liquid loading problem definition Water Gas Water collects down hole and drains off P wf DD P res Depth OB Pressure
Well configuration with separate production and drainage intervals PWH Hydraulic model Production interval Drainage interval Z=325m PBH A3B20: 1691-1695.5m P res1, k 1, h 1 Upper pressure gauge 1708m SSD 1885m Water column in well Lower pressure gauge 2076m CF40: 2081-2083m upper zone: gravity gas/water separation lower zone: water drainage under gravity P resd, k D, h D
Hydraulic Model (cont.) Producing (upper) & injection (lower) zones vertically separated to build up hydrostatic head lower injection zone pressure-depleted favourable permeability Applications Layered reservoir: High pressure in upper zone and low pressure in lower zone can be used to perform downhole gas/water separation & injection driven purely by gravity Thick reservoir: liquid column builds sufficient head over pay zone for liquid drainage in lower part
Producing zone - liquid fall-out Production interval P res1, k 1, h 1 Gas velocity profile along hole VG Production interval Velocity profile : liquid fall-out in lower part due to low upward velocity Water column in well VT Velocity In deviated well: Turner critical velocity for liquid loading depends on tube inclination (Shell experiments)
Back-up slide: Turner criterion for liquid loading v t σ 1.59 ( ρ 1/ 4 L = 1 2 ρg ρ ) g 1/ 4 There are many factors affecting the onset of liquid loading, e.g. large liner diameter and tubing inclination (Rijswijk experiments):
Pressure Pres Pwell Injection zone - Liquid build-up Pressure profile with water column Pressure profile without water column h Liq block PWH Production interval P res1, k 1, h f1 Gas inflow rate: hindered by liquid column providing back pressure Water column in well Well Reservoir Water drainage: h Liq PBH ID Water column in the well creating hydrostatic head Drainage interval P res, k D, h D Hydrostatic head and pressure depletion governs rate
Figure demonstrates that increased permeability thickness (product k*h) increases water drainage rate back into reservoir and decreases equilibrium liquid column height Liquid column height H as a function of kh 100 90 80 70 Height (m) 60 50 40 30 20 10 0 1 m3/day 3/d 2 m3/day 4 m3/day 0 50 100 150 200 250 300 350 400 450 500 permeability-thickness (mdarcy-meter)
Back-up slide: Gas rate hindered by liquid column providing back pressure in layered reservoir q g = πk ( h h )( p p ) 2πk ( h )( p p ρ gh ) 2 f 1 Liqblock Re s1 WH Liqblock Re s1 µ g ln r r e w + S + µ g ln r r e w WH + S w Liqblock Injection rate modeled with Darcy: q inj = 2πkh p r e µ ln + S rw Hydrostatic head P = ρ gh L Liq + P WH P Re sd
Test downhole separation/injection in Austrian field producing and injection zones connect by open sliding side door 55 50 SSD open Compression on Compression out SSD closed 140000 120000 BHP ~55bar lower gauge, 2076 m depth Downhole pressure in bar 45 40 35 30 25 gas rate upper gauge, 1708 m depth 100000 80000 60000 40000 Gas rate in m3/d Production interval Gas production ~50.000 m³/d WHP A3B20 P res1, k 1, h 1 20 15 15.04 08:24 15.04 12:00 15.04 15:36 15.04 19:12 15.04 22:48 16.04 02:24 16.04 06:00 16.04 09:36 16.04 13:12 20000 0 SSD open Liquid to drainage zone
Downhole gas/water separation & injection Response simulated by hydraulic model @ test conditions 70 120 BHP ~65bar PBH [bar] 60 50 40 30 20 10 100 80 60 40 20 Production rate m3/d Injection rate ~100m³/d Gas prod.~50.000 m³/d PBH Lower gauge, bar Drainage rate, m3/d 0 0 0 20 40 60 80 100 120 140 Time [h] Gasproduction, 1000x m3/d
Intermittent production with downhole separation & gravity drainage controlled by automatic shut-in/startup valve 60000 Response simulated by hydraulic model 1200 50000 1000 Gas prod. [ m3/ d ] 40000 30000 20000 10000 800 600 400 200 Liquid height [ m ] Drainage rate [m3/d] Gas Prod. Zone 1 Qdrain 0 0 0 5 10 15 20 25 30 Time [ h ] Liquid height
Back-up slide: Definitions for next slide Normalised Gas rate Actual gas rate / Minimum gas rate at onset of liquid loading Normalised liquid rate Actual liquid rate / Liquid Drainage rate at equilibrium hydrostatic liquid column
Solutions for liquid unloading classified on basis of critical rates for gas production and liquid drainage Normalized Gas Rate 8 Qgas/QTurner 1 Cyclone Sep. Cyclone Sep. + + Intermittent prod. Gravity Inj. or Injection pum p Gravity Sep. Gravity Sep. + + Intermittent prod. Gravity Inj. or Injection pum p 0 1 Normalized Liquid Rate 8 Cyclonic gas/liquid separation Q prod/q inj
Liquid loading & unloading - discussion Flow Characteristics dynamic multi-phase flows in long, thin deviated tubing dynamic interaction between far-field, near-wellbore reservoir & well backflow of condensate / water and back into reservoir Dynamic Modelling time-scale of drainage process pressure-recharge and liquid drop-out around well transient state and PVT properties Fit-for-purpose solution presented here Controlled intermittent operation to stabilise production
Concluding remarks A hydraulic model for depleting gas fields may predict capacity and characteristic time for intermittent production beyond onset of liquid loading. Predictions compare favourably to rates observed from test of downhole gas/water separation and injection (producing and injection intervals have significant vertical distance to build up hydrostatic head for liquid to drain into pressure-depleted lower zone). A liquid loaded well may be produced intermittently in controlled manner using automated shut-in valve; during shut-in time liquid is allowed to drain away. The application may be extended above critical gas rate by installation of cyclonic separation device, and above critical drainage rate by installation of downhole injection pump. Expected benefits from model are better predictability and control of gas capacity at tail-end production, and extended life of liquid loaded wells.