A Brief Guide to Geothermal Wells

Transcription

1 A Brief Guide to Geothermal Wells Key Features, Fundamental Differences Ralph Winmill Senior Drilling Engineer, Contact Energy 21 June 2011

2 WHO ARE CONTACT ENERGY? Publically-Listed New Zealand Energy Company Electricity Generation (Geothermal, Hydro, Gas-Fired) Natural Gas Retail Electricity Retail 51% Owned by Origin Operator of 2 nd oldest geothermal field in the world (Wairakei) Major project developer in NZ Energy Sector 21 June 2011 Geothermal Drilling and Wells 2

3 The Ring of Fire Geothermal Drilling and Wells 3

4 Taupo volcanic zone, NZ 21 June 2011 Engineering Science Careers 4

5 Active Volcanoes Recently Active Contact Energy Fields Wairakei (prod/dev) Ohaaki (prod) Tauhara (prod/dev) Taheke (expl) Mighty River Power Fields Mokai (prod) Rotakawa (prod) Kawerau (prod) Ngatamariki (dev) 21 June 2011 Geothermal Drilling and Wells 5

6 Heat Permeability Groundwater Geothermal Reservoir 21 June 2011 Geothermal Drilling and Wells 6

7 Typical Reservoir 21 June 2011 Geothermal Drilling and Wells 7

8 GENERATING POWER AND REVENUE Production ($$$) is Generated by; H. Energy per Unit Mass (kj/kg) Reservoir Fluids Enthalpy Q. Mass Flowrate (kg/sec) Formation permeability Wellbore diameter POWER = H x Q High flowrates are required for economic production 6/21/2011 Geothermal Drilling and Wells 8

9 Vertical Discharge - Movie 21 June 2011 Geothermal Drilling and Wells 9

10 Typical Temp and Pressure Cross-Sections Heating, Static Discharging 21 June 2011 Geothermal Drilling and Wells 10

11 Thermal Loads Feature Casing wants to expand upon heating Example (ΔT = 200degC) 1000m of free casing will expand 2.4m F Severe stresses if casing goes into buckling so it is fully cemented into place. - Casing cannot expand or buckle due to cement Heating converts to stress increase (independent of cross-sectional area) - Increase in stress of 480 MPa (69,000psi) K55 casing goes into yield ΔT σ High compressional loads on connections 13-3/8 x 68ppf = 609 MT (1342 k-lb), or limited by body yield Couplings can be damaged by thread jump Over time, stresses relax due to creep High tensile loads upon well cooling Stress approaches zero at high temperatures over a period of weeks/months Similar high tensile stresses into yield. Damaged connections can pull apart Cooling must be carefully managed F 21 June 2011 Geothermal Drilling and Wells 11

12 Casing Connections First wells used STC, LTC Connections weaker than pipe body, high prevalence of connection failures BTC Now Commonly Used Connections stronger than pipe body, >100% strength in compression Little or no issues CE now using GeoConn for Production Casings Fully BTC-compatible Little increase in price over BTC (say 5-15% more) Designed for high compression Internally flush High torque capacity for RwC / DwC applications 21 June 2011 Geothermal Drilling and Wells 12

13 Trapped Water Hazard of water trapped between two casings, with cement above and below On heating, trapped water expands (will not boil) and can collapse the casing Inner surface of 13-3/8 csg no cement in annulus Milled surface of bulge Bulge in 9-5/8 production casing Depressed water level for imaging Casing Cementing is Critical 21 June 2011 Geothermal Drilling and Wells 13

14 Chemical Environment Geothermal fields have prevalence of CO 2 and H 2 S (typically 95%:5% ratio) Casing: Drillpipe: L80 or below Wellhead: G105 or below Cement: NACE MR01-75 <15% Silica Gas can build up inside shut-in wells, creating a cold, gasladen environment in tension potential for problems Operationally, it is recommended to keep shut-in wells on bleed (bleeding off pressure through an orifice plate and slim tubing) to eliminate the gas column, and keep the well and wellhead hot and not in tension 21 June 2011 Geothermal Drilling and Wells 14

15 Nature of Formations PERMEABLE SIGNIFICANT LOSSES Severe Lost Circulation, from Porous formations Fractured formations Faulting BAD BEFORE RESERVOIR GOOD WITHIN RERSERVOIR Difficulty in clearing cuttings Hole instability due to loss of column pressure Cementing methods must be modified Mud is lost to formations Final well production is from permeable formations Linked to well productivity Option: Heal with LCM / cement plugs Drill sections blind (no returns) Drill sections with aerated fluids Notes: Time consuming Unreliable (they can break down over time) Sometimes nearly impossible Not an option for production section Cuttings packoff is a real risk Riskier for larger hole diameters No cuttings (geology) obtained Not suitable for surface hole Additional equipment and expense reqd Higher BOP stack reqd (rig clearance) Additional operational complexity High Capacity Water Supply is Essential 21 June 2011 Geothermal Drilling and Wells 15

16 Nature of Formations ABRASIVE 5 x 19.5ppf G105 NC50 pipe Good premium with Arnco 100XT hardbanding Came out like this after a single run 21 June 2011 Geothermal Drilling and Wells 16

17 Nature of Formations STRUCTURE Complex mix of lava flows, ash-falls, breccia (and siltstone/mudstone) Old volcanic features, intrusions, faulting Typically hard formations - (1-8 m/hr) Significant variability even within geological units Often significant permeability in many units 21 June 2011 Geothermal Drilling and Wells 17

18 Nature of Formations HOT, PRESSURE CHARGED Well pressure is derived from; Formation fluids boiling (steam condition) Gas pressure in formation (gas condition) Much of the well has the potential to come under pressure during drilling, not just in the reservoir Well kicks are related to communication via permeability fairly variable and unpredictable Formations can be quite weak Safe well construction calls for multiple casing strings to maintain well control More casings Larger casings to maintain final diameter More flatspots 21 June 2011 Geothermal Drilling and Wells 18

19 Multiple Casings Lesson 204 Do NOT skimp on casings 21 June 2011 Geothermal Drilling and Wells 19

20 Typical Geothermal Well Structure CASINGS Typically 3-6 casing strings Casings fully cemented back to surface Typical Sizes /8 13-3/8 9-5/8 PERFORATED LINER Sitting on hole bottom sometimes hung 10-3/4 9-5/ /2 Simple, robust design No tubing, packers, or elastomers Well diameter maximised 21 June 2011 Geothermal Drilling and Wells 20

21 Wellheads Summary of modern wellhead setup API 6D Expanding Gate MV rated for geothermal conditions 10 or 12 Class 900 API 6A Screw on Casing Head Flange 3-1/8 x 3M side valves rated for geothermal conditions Casing stump - API 5CT Cut off casings and annuli open to atmosphere Protected from rainwater ingress to prevent corrosion 21 June 2011 Geothermal Drilling and Wells 21

22 Well Control Well pressure can come from either GAS (CO 2 /H 2 S) ingress, or BOILING within the wellbore (or combination) Gas Action: Bleed off gas Action: Bullhead back into formation Gas can instigate a steam kick Steam Action: Kill kick with cold water (cooling, and repressurising) Prevention: Keep formations cool /below boiling with cold water at all times Kicks occur and escalate rapidly due to cascade boiling throughout the wellbore as the pressure drops (5 seconds from first sign to washing the crown) Well response the time to for the well to come under pressure after water off - runs from seconds to days, depending on the well WELLS ARE INHERENTLY UNSTABLE NEED CONSTANT COOLING FOR WELL CONTROL 21 June 2011 Geothermal Drilling and Wells 22

23 Depth (m) Downhole Temperature Profile During Drilling Temp During Circulation (degc) T-In T-Out KEY POINTS TO NOTE BHCT is less than BHST BHCT is greater than T_out Mud cooling is required to knock out the heat Aerated drilling produces higher T_out, BHCT BHCT BHST 21 June 2011 Geothermal Drilling and Wells 23

24 Bottom Hole Temperature (degc) Downhole Temperature Management Water Off Stopping circulation or cooling - the well heats up. How fast? Conductive - Long Cooling Time Conductive - Short Cooling Time Convective - Internal Flow Time (minutes) from stop of circulation 21 June 2011 Geothermal Drilling and Wells 24

25 Limitations to Tools Jars, Accelerators, PDM motors and MWD can be used, but must be managed carefully Turbodrills are suitable for high temperature operations, but lack suitable MWD equipment Elastomers (bit seals, packers) have a limited life and temperature limitations PDC bits give little increase in ROP, but can stay in the hole longer Runs are typically terminated by i) bit hours for tricones, or ii) jar hours for aerated with PDC Limitations to electronic tools that can be used 21 June 2011 Geothermal Drilling and Wells 25

26 Summary - KEY DIFFERENCES OF GEOTHERMAL DRILLING LARGER HOLE AND CASING SIZES Bigger rigs and handling gear required for similar depths More pump power reqd LOST CIRCULATION More pump horsepower reqd More drilling problems related to this HIGH TEMPERATURES Limitation on tools that can be used Cooling tower required Negative effect on mud condition WELL CONTROL VIA PREVENTION OF BOILING Reliable high-flow water supply required to site Wells require active control and are naturally unstable LACK OF EXPERIENCE FROM MAJOR SERVICE COMPANIES Often unable to call on external expertise because there is little in the oil and gas industry 21 June 2011 Geothermal Drilling and Wells 26

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