Colleen Barton, PhD Senior Technical Advisor Baker Hughes RDS GMI. HADES - Hotter And Deeper Exploration Science Workshop

Transcription

1 Geothermal Reservoir Geomechanics: The Application of High Temperature / High Pressure Borehole Logging Technologies to Reservoir Characterization and Production Prediction HADES - Hotter And Deeper Exploration Science Workshop Taupo, New Zealand 25-26th May 2011 Colleen Barton, PhD Senior Technical Advisor Baker Hughes RDS GMI

2 The Geomechanical Model S v The geomechanical model of a reservoir involves detailed knowledge of: In situ stress orientations In situ stress magnitudes UCS Pp Pore pressure Effective rock strength Fracture patterns Structure S hmin S Hmax

3 Geomechanical Data Parameter Stress Orientation & Orientation Anisotropy Vertical Stress Pore Pressure Horizontal Stress Magnitudes Stress Path - z 0 S v ( z 0 ) = r g dz 0 Data Source S Hmax azimuth Image data interpretation Density logs S hmin minifrac (sand), XLOT (shale) S Hmax magnitude Constraint based model using observations of wellbore failure P p RFT, MDT, sonic, seismic, DST 3 Rock strength Fracture and Fault Strike and Dip UCS Lab measurements, geophysical logs, constraint based model using observations of wellbore failure Wellbore image data interpretation, seismic interpretation

4 Characterizing Geothermal Prospects: The Problem

5 Open Hole HP/HT Logging Instruments Nautilus Ultra Formation Evaluation Tool O.D. (in.) Temperature Rating ( o F) Pressure Rating (psi) Gamma Ray GR-HP/HT 4 1/ ,000 Digital Spectralog DSL-HP/HT 4 1/ ,000 High Definition Induction Log HDIL-HP/HT 4 1/ ,000 Compensated Neutron CN-HP/HT 4 1/ ,000 Compensated Density CDL-HP/HT 4 1/ ,000 Cross-Multipole Array Acoustilog XMAC-HP/HT 4 1/ ,000 AR01 SP C Nautilus Ultra TM 500F and 30,000 psi SP Current: 30 Cross-Dipole ( XMAC-F1) LI C Formation Evaluation Induction Resistivity (HDIL) Porosity (CN) Density (CDL) Natural GR (GR/SL) Borehole Geometry (XY-Cal) Future: Acoustic Borehole Imaging

6 Geothermal Ultrasonic Fracture Imager Geothermal Imager Goal is the development of wireline borehole televiewer which can operate at 300 o C with DOE in U.S. Demonstrated 300 o C transducer Tool, Pressure & Temperature test by the end of Q1, 2012 Efforts will directly feed into Nautilus-Ultra Borehole Imager for the Oil & Gas industry Baker Hughes Incorporated. All Rights Reserved.

7 Geothermal Ultrasonic Fracture Imager Beam Field Spot and Test at 300 o C

8 Case Histories Successful HP/HT operations Maximum temperature run to date: 474 F (246 C) Maximum pressure to date: ~28,000 PSI Conventional Gas Alice, Texas: HT Gas well logged using Nautilus Ultra SM tools. Maximum BHT recorded was 450 F (232 C) at a depth of 21,053 ft (6,417 m). Geothermal Central Oregon: Nautilus Ultra tools were deployed in a very difficult geothermal, where BHT was expected to exceed 450 F (232 C). Data was recorded successfully and max. temp. recorded was 474 F (246 C). Moomba, Australia: Baker Hughes successfully logged this HT land well that reached a depth of 16,109 ft (4,910 m) with 39 of deviation and a bottom hole temperature of 470 F (243 C). Ultra Deep Gas US Gulf of Mexico: Nautilus Ultra deployed at the McMoRan Exploration Co. Davy Jones ultra-deep gas discovery on the shelf in the Gulf of Mexico. Nautilus Ultra was deployed to a depth exceeding 28,000 feet, using our HP/HT Pipe Conveyed Logging system. Maximum pressure exceeded 25,000 PSI. 8

9 Geothermal Exploration Cooper Basin Estimated crustal temperature at 5 km depth (Holdgate and Chopra, 2004) The Cooper Basin is characterized by high heat flow arising from the presence of Paleozoic granites. Images courtesy of Geodynamics Ltd.

10 The HFR Reservoir Concept Challenges: Granitic basement (>3,500m) Deep wells (~ 5,000 m) Temperatures > 240 C Overpressures ( >1.7 SG) Thick sedimentary cover (>3,000m) Weak shale and coal intervals Image courtesy of Geodynamics Ltd. An accurate geomechanical model will help to manage wellbore stability in this hostile environment

11 Aplite Images are the key Observations Breakout Continuous breakouts develop in the granitic basement. DITFs Drilling induced tensile fractures (DITF) develop in quartz-rich intervals. Breakouts and DITFs occur within the same depth intervals. Natural fractures DITFs develop as vertical features under a reverse faulting stress regime. Acoustic image data (CBIL). (after Fernández-Ibáñez, et al., 2009, IPA)

12 Observations of Breakout Rotations ROTATED BREAKOUT 13 UNPERTURBED BREAKOUT Drilling experiences and stress rotations suggest that the high P p in the basement is mostly confined to natural fractures.

13 Drilling time scale Thermoelastic Effects on Wellbore Stress 11/11/ /11/ /11/2007 The effect at the wellbore wall of a temperature difference DT between the wellbore fluid and the wall rock is given by the equation: s qq DT = (a E DT)/(1-n) Cooling increases the tensile stresses (and decreases the compressive stresses) at the wellbore wall. trips ΔT min = -60 C ΔT max = -100 C DITFs BOs 18/12/2007 1/1/2008 DITFs form while drilling due to the temperature contrast between the drilling fluids (cool) and the formation (hot). Breakout development will be also affected by the temperature contrast. As the hole warms up, compressive stresses will be increased and breakouts will increase in severity. (after Fernández-Ibáñez, et al., 2009, IPA)

14 Stress Model Summary P p ramp FITs S Hmax modeling points Transitional Strike-Slip to Thrust Faulting Stress Regime Overburden (S V ) determined from integrated pseudo density log data. Minimum horizontal stress (S hmin ) is presumed to be comparable to S v based on wellbore failure observations and available FITs. Pore pressure (P p ) determined from drilling experiences (gas readings), wireline logging data, and mud weights used to drill wells in the area. Overpressure confined by natural fractures Maximum horizontal stress (S Hmax ) magnitudes based on modeling of observed wellbore failure. It is found to be >> S v. S Hmax azimuth ~E-W. (after Fernández-Ibáñez, et al., 2009, IPA)

15 Temperature-dependent Borehole Collapse Pressure Collapse pressure can be thought of as the required bottom hole pressure to prevent breakouts exceeding 90 width. Collapse pressure at thermal equilibrium During drilling ΔT~100 C borehole collapse pressure was generally lower than the pore pressure. As the hole warms up after drilling the compressional stresses increase and so does the collapse pressure. Over time the hole falls into thermal equilibrium (ΔT~ 0 C) and the collapse pressure increases meaning a higher mud weight is required to control excessive breakout development (>90 ). Top granite Collapse pressure while drilling Collapse pressure increases more rapidly within the first 5-10 days after drilling the well. After this period, the increase in collapse pressure is less pronounced. (after Fernández-Ibáñez, et al., 2009, IPA)

16 Temperature-dependent Breakout Width Breakout width at thermal equilibrium Breakout width while drilling Coal (change in temp. contrast / hole warming) Top granite An increase in the collapse pressure will directly impact breakout width. Breakouts form at the time of drilling and get wider with time (i.e. warming). There is an important temporal and thermal component to breakout development in this hostile environment. A higher DT and less well exposure time could inhibit breakout development or at least decelerate their growth. (after Fernández-Ibáñez, et al., 2009)

17 Fault and Fracture Dominated Basin and Range Geothermal Reservoir An accurate geomechanical model will help to determine the optimal well placement to enhance production in this hostile environment (after Moos and Ronne et al., 2010, GRC)

18 Selecting the optimal logging suite for geothermal reservoir evaluation (after Moos and Ronne et al., 2010), GRC

19 Probability Probability General Properties of Fracture Zones A fracture zone is usually a zone of intensely deformed rock (cataclastic rocks), with a long history of deformation and fluids circulation, high permeability, and associated mineral deposits. Logging response in fracture zones: Reduction in V p Reduction in resistivity Increase in porosity Reduction in density Increase in GR Shear waves anisotropy Reduction in V p /V s ratio Fracture zone signature while drilling: Losses Well inflow Drilling breaks Gas peaks Changes in cuttings mineralogy Reduction drillstring vibrations These are general indicators, but it does not necessarily imply in all cases.

20 Crossed Dipole versus Image Analysis UPPER SEDIMENTS LOWER GRANITES Bedding (blue x) Breakouts (black) Anisotropy (green) S Hmax = bedding strike Circulation losses where anisotropy aligned in strike direction steeply dipping fracture set Fractures are more compliant because they are hydraulically conductive (i.e., losses) In hard, stiff, fractured rock, anisotropy is useful to identify zones of aligned fractures that can provide a directionally consistent permeable pathway for fluid migration (after Moos and Ronne et al., 2010, GRC)

21 Propagation of Critically Stressed Shear Fractures Coulomb Criterion Frictional Sliding Sliding occurs when: t - ms n - S o > 0 S o = Cohesion m = Coefficient of Friction (sliding friction) s n = Effective normal stress = S n - P p 26

22 Why does slip enhance permeability? Shear (Coulomb failure model) crack: Slips, creating opening, if t - ms n - S o > 0 Wall offset stiffens the crack t Shear fractures self-prop Mohr Diagram s n t Mode 1 (extensile) crack: opens only if s n < 0 Does not self-prop s n >0 Final stress state s n = S n - P p P p P p >S 3 S 3 P p <S 3 s n = S n - P p P p <<S 3 P p <S 3 28

23 Fractured Reservoirs Normal Different Stress Regimes Activate Different Fractures S v S hmin S Hmax S hmin b a. S Hmax S v > S Hmax > S hmin Normal S v S hmin Strike-Slip S v S hmin S Hmax S Hmax X S hmin b. S Hmax > S v > S hmin Strike-slip Reverse S v S hmin S Hmax S Hmax S Hmax c. S hmin S Hmax > S hmin > S v Reverse S v Map V iew Cross-section Stereonet 29

24 Predicting the Best Well Orientation Perpendicular to the most fractures Best well Fracture distribution is clustered Stresses are not the same as when the fractures formed S Hmax Therefore The best well is not always oriented perpendicular to the most fractures The best well can be oblique to the stress field Perpendicular to S Hmax The best well intersects the maximum number of stress sensitive fractures. 30

25 State of Stress Along Natural Fractures While Drilling (1) Pre-drill: fractures has a very low critical injection pressure. Effect: High pore pressure is confined within the fractures. (2) While drilling: MW+ECD effect increases BHP and fractures become critically-stressed. Effect: fractures release pressure to the borehole; well flows. (3) Circulating kill mud: Extremely high BHP; more fractures become critically-stressed. Effect: BHP>>Pp; losses.

26 Injection or Depletion is a Dynamic Process!! Initial Conditions Injection Test (lower pressure) Injection Test (higher pressure) Fall off Best well Best well Best well Best well 35 Permeability is a tensor Stimulation / depletion change magnitudes and principal component orientations of the permeability tensor

27 Characterizing the Dynamic Process How do we verify the initial conditions of statistically based discrete fracture network models, how do we capture changes in the permeability tensor? How can we determine how a naturally fractured reservoirs will perform with stimulation? What are the required stimulation pressures? How much production improvement can be attained with stimulation? What will be the reservoir performance with cooling? 36

28 Shear slip Relative permeability Permeability Enhancement with Stimulation a o Aperture = f ( a o, s closure n ) Q = Aperture 12 3 P ao closure s n closure s n Effective normal stress Pre-slip: Small open aperture (a o ) s closure Soft (small ) n Post-slip: Larger open aperture (a o ) closure Stiffer (larger s ) n 37

29 Injectivity Test and Calibration Four injection stages were carried out using increasing total injection rates Injection rates were converted to an effective injectivity (red dots) Fracture properties are determined by fitting a computed injectivity to this data (blue curve) Key model parameters: S o = Cohesion s n = Effective normal stress m s = Sliding Friction a 0 = Fracture aperture 38 Barton and Moos, 2009, AAPG

30 Injectivity Predictions Initial injection reduces the stress holding fractures closed but does not open anything Pressure fall-off or flow-back due to permanent enhancement induced by slip, the productivity after stimulation is higher than before stimulation permanent productivity enhancement A rapid increase in potential productivity only occurs after fractures begin to slip 39 Onset of microseismicity

31 Best Well Analysis Calibrated with Injection 1000 PSI injection S 0 = 522 psi Best Well = 78 /356 Slipped fractures = 116 WELL PRODUCTIVITY 3000 PSI injection S 0 = 522 psi Best Well = 63 /356 Slipped fractures = 501 WELL PRODUCTIVITY Using analyses from injectivity tests the fracture flow parameters are established. The best match to the increased production is cohesion, S 0 = 522 psi and coefficient of sliding friction, m=0.6. The increase in permeability using all modeled fracture flow parameters is a factor of five. 40 Barton and Moos, 2009, AAPG

32 Uncertainty - Well Productivity Ratio ± 3% To reduce uncertainty, measure productivity of an existing well S hmax magnitude Fracture strength Fracture flow properties 41 Barton and Moos, 2009, AAPG

33 What is the best wellbore orientation? Orthogonal to hydrofracs S Hmax Most productive wells Easiest wells to drill S Hmax S Hmax S Hmax 42 Productivity vs. orientation S Hmax Open hole breakdown vs. orientation Mud weight vs. orientation Easiest wells to break down

34 Lessons Learned Drilling in HT environments may benefit wellbore stability if a proper drilling strategy is planned. The high temperature contrast temporarily reduce the compressional stresses around the wellbore. If a higher ΔT and a shorter well exposure time can be achieved it is possible to control breakout development, drill with lower mud weights, and therefore, minimize the risk of formation damage. Cooling of the rock will induce thermal cracking that may eventually work as a pre-stimulation test of the reservoir.

35 Lessons Learned Optimizing production in geothermal fractured reservoirs requires characterization and verification of the fracture network geometry, the fracture flow properties, the response of fractures to the pre-drilled reservoir stress state and the response of fractures to the changes in the reservoir state of stress with production The most effective way to reduce prediction uncertainties in planning Best Well trajectories and in predicting stimulation pressures to enhance natural fractures is to calibrate fracture flow properties, cohesion (S o ), sliding friction (m s ), effective normal stress (s n ), and fracture aperture (a 0 ), against the productivity of a pre-existing well Applying geomechanics and the reservoir fracture distributions to model permeability at both ambient pressure conditions and with shear-enhanced permeability under injection pressure conditions appears to be a promising improvement to existing fractured reservoir flow models

36 Summary of Geothermal Logging Tools for Geomechanics Application Log Information Supplied Comments Density Bulk density computed from electron density Estimate of content of Fe, Ca, and Mg relative to Si, from PEF Required to compute overburden stress by integration Important for porosity Important constraint on gravity modeling PEF can help differentiate limestone, dolomite, and mafic-rich rocks from those with high quartz content, and help with clay mineralogy Spectral natural gamma Neutron porosity Resistivity Separately measures the contributions of K, Th, and U to the total GR Porosity from hydrogen content Volume of clays or hydrated alteration products A measure of the volume of conductive fluids (i.e., porosity) In low porosity rock, a measure of the volume of conductive minerals Total volume of clay and K-rich minerals U is mobile; high values could indicate paleo-flow zones Th and K help with clay mineralogy Recommended over standard GR Important for porosity Useful for clay volume if non-clay minerals are radioactive Not sensitive to porosity variations for porosities below a few percent Important constraint on magnetotelluric modeling Low resistivity indicates higher porosity, or the presence of electrically conductive minerals e.g. clays, oxides, or pyrite May provide estimates of total dissolved solids of fluids 45 (after Moos and Ronne et al., 2010, GRC)

37 Summary of Geothermal Logging Tools for Geomechanics Application Acoustic Crossed dipole acoustic Electrical images Compressional and shear elastic-wave velocities With density, dynamic elastic moduli Stoneley-wave reflections and attenuation Azimuthal shear-wave anisotropy Combined with Stoneley modeling, TI elastic moduli Hole ellipticity and its orientation, from centralizer calipers Centimeter-scale image of wall rock resistivity Fine-scale fractures, resistive vs. conductive, NOT open vs. closed Subtle stratigraphy Calibration for seismic or vertical seismic profile Measure of degree of consolidation (stiffness) Can be used to compute rock strength to help constrain stress from observations of wellbore failure Detecting compliant / conductive fractures Estimate of matrix permeability by Stoneley-wave inversion Useful for better seismic ties using transversely anisotropic velocities Sensitive to stress to determine orientations of maximum and minimum horizontal stresses Sensitive to intrinsic anisotropy (steep open fractures; dipping bedding) Independent information is required to differentiate stressinduced from intrinsic anisotropy Can substitute for 4-arm dipmeter to detect breakouts Processing is carried out off-site Important for structural analysis Can be so sensitive to fine scale features it obscures useful information Identifying drilling-induced tensile fractures for stress In contrast to an acoustic image, does not provide complete wellbore coverage Cannot be used alone to detect permeable fractures Pads can be damaged by high temperature 46 (after Moos and Ronne et al., 2010, GRC)

38 Summary of Geothermal Logging Tools for Geomechanics Application Acoustic images Azimuthal resistivity Nuclear magnetic resonance Wireline straddle packers Pulsed neutron Caliper Several cm-scale image of wellbore wall reflectivity Several cm-scale image of wellbore radius Dip of electrically-anisotropic materials Resistivity perpendicular and parallel to bedding Porosity Estimate of permeability Pore pressure Stress from micro-fracturing tests Mass fraction of individual elements Hole size, using one or more independently articulated arms Excellent to identify mechanically weak fractures Less resolution than electrical image logs Provides 100% wellbore coverage Excellent for breakout tensile fracture analyses Additional structural constraint May help separate structural from stress-induced elastic anisotropy Structural information available post-acquisition Pad-type tools are sensitive to wellbore roughness Permeability estimate requires calibration May have temperature limitations The stress and pore pressure data can be very important to supplement or replace an extended leakoff test May not be available from all service providers Have severe temperature constraints and pressure limitations Detailed mineralogy Carbon content Precise depth delineation of lithologic contacts Detects weak fractures and faults that cause wellbore enlargements Single-arm caliper provides information to correct other logs for hole size Multi-arm caliper allows determination of hole shape; if oriented can be used to detect and orient wellbore breakouts for stress determination 47 (after Moos and Ronne et al., 2010, GRC)