Seals and CO 2 : Techniques and Issues. Dave Dewhurst CSIRO Petroleum

Transcription

1 Seals and CO 2 : Techniques and Issues Dave Dewhurst CSIRO Petroleum

2 Why do Seals Fail? Capillary Failure Buoyancy Pressure > Capillary Entry Pressure Mechanical Failure Fracturing (tensile/shear) Faulting Reactivation Presence/Efficiency at time of Charge Alteration, post charge.

3 SEAL CLASSIFICATION and CRITICAL RISKS

4 Capillary Failure GWC 2 P b = P e GWC 1 P b < P e GWC 3 Pb > Pth Courtesy John Kaldi, CO2CRC

5 SEAL CAPACITY HEIGHT OF TRAPPED HYDROCARBON/CO 2 COLUMN THRESHOLD PRESSURE BETWEEN SEAL AND RESERVOIR DENSITY DIFFERENCE BETWEEN FORMATION WATER AND HYDROCARBON PHASE PORE GEOMETRY INTERFACIAL TENSION CONTACT ANGLE CHEMISTRY OF FLUID PHASES TEMPERATURE PRESSURE Courtesy John Kaldi, CO2CRC

6 Mercury Porosimetry. Evaluation of Seal Capacity Allows estimation of P th from: P = 2σ Cos r θ σ = Interfacial tension θ = Contact angle Sneider et al., 1997, AAPG Memoir, 67

7 Calculating Seal Capacity ( P th ) = P b / CO ( th ) 2 a / m σ b / σ a CO 2 / m Cosθ Cosθ b / CO a / m 2 Air-mercury contact angle = 140 Air-mercury interfacial tension = 480 dyne/cm CO 2 -brine contact angle =?? CO2-brine IFT = Reasonably constrained Need CO 2 wettability at reservoir conditions

8 Calculating Seal Capacity H max = P th ( seal) 0.433( ρ w P th ρ ( res) CO 2 )

9 Seal Capacity Issues Sample size, representative? Use regional geology. Geometry, lateral extent lithology changes, how to predict away from well sequence strat relation to shale properties embryonic seismic resolution Wettability of CO 2 not well constrained how physically interacts with rocks

10 FAULT SEAL RISK BUOYANCY PRESSURE CAPACITY vs DISPLACEMENT PRESSURE COLUMN HEIGHT JUXTAPOSITION FAULT ZONE PROPERTIES XFAULT LITHOLOGY Diagenesis Clay smear Cataclasis Grain sliding REACTIVATION (regional stress / fault orientation)

11 FAULT SEAL APPROACHES Fault Plane Mapping ALLEN DIAGRAMS Fault Rock Processes SHALE GOUGE RATIO (SGR) MICROSTRUCTURAL CHARACTERIZATION Pressure Data PRESSURE COMPARTMENTS Stress Analysis TRIAXIAL TESTING RISK OF REACTIVATION GEOMECHANICAL MODELLING

12 FAULT SEAL RISKS BUOYANCY PRESSURE CAPACITY vs DISPLACEMENT PRESSURE COLUMN HEIGHT JUXTAPOSITION FAULT ZONE PROPERTIES XFAULT LITHOLOGY Diagenesis Clay smear Cataclasis Grain sliding REACTIVATION (regional stress / fault orientation)

13 Juxtaposition Situations Shale Gas Oil Water Sand Water Gas Oil Water 1) Sand juxtaposed opposite shale. 2) Sand juxtaposed opposite sand. Water Oil Gas Gas Oil Water Water Oil Oil Water 3) Common GOC and HWC across the fault. 4) Different OWC across the fault. After Smith (1980) Slide courtesy of John Kaldi, CO2CRC

14 FAULT SEAL RISKS BUOYANCY PRESSURE CAPACITY vs DISPLACEMENT PRESSURE COLUMN HEIGHT JUXTAPOSITION FAULT ZONE PROPERTIES XFAULT LITHOLOGY Diagenesis Clay smear Cataclasis Grain sliding REACTIVATION (regional stress / fault orientation)

15 Fault Rock Types After Fisher and Knipe, 1998

16 Juxtaposition Situations Shale Gas Oil Water Sand Water Gas Oil Water 1) Sand juxtaposed opposite shale. 2) Sand juxtaposed opposite sand. Water Oil Gas Gas Oil Water Water Oil Oil Water 3) Common GOC and HWC across the fault. 4) Different OWC across the fault. After Smith (1980) Slide courtesy of John Kaldi, CO2CRC

17 Clay Smears Lehner and Pilaar, 1997, NPF 7

18 Clay Smears Form in sediments with clay content > 40% Smearing of adjacent host lithologies into fault zone In ductile clays/shales shearing or injection In brittle shales abrasion smears. Perm often < 500 nd, P th = 2000 to >10,000 psi. Common in clastic sedimentary sequences Can be predicted through various methods.

19 Predicting Clay Smear : Shale Gouge Ratio e.g. Yielding et al., 1997, AAPG SGR can be calibrated to column height by using either 1.field pressure data or 2. laboratory-based analyses of immiscible fluid breakthrough

20 SGR 3D Projection Courtesy Julian Strand, CSIRO Petroleum

21 FAULT SEAL ANALYSIS WORKFLOW Seismic interpretation Fault surface map Across fault juxtaposition maps m 100m Calibration Fault seal potential maps SGR Threshold pressure (bar) 10 1 Line of weakest fault seal IF FRAMEWORK MODEL INADEQUATE SUBSEQUENT FSA MEANINGLESS % Phyllosilicates (SGR) 70 Courtesy of Wayne Bailey, CSIRO Petroleum (ex)

22 Issues with Fault Capillary Seal Methods Mechanical incorporation of material only No account of diagenetic changes Assumes all sealing is due to clays Assumes V clay from gamma-ray is accurate CO 2 wettability not well understood yet Microstructural/lab-based approaches Scale, representative, continuity etc

23 FAULT SEAL RISKS BUOYANCY PRESSURE CAPACITY vs DISPLACEMENT PRESSURE COLUMN HEIGHT JUXTAPOSITION FAULT ZONE PROPERTIES XFAULT LITHOLOGY Diagenesis Clay smear Cataclasis Grain sliding REACTIVATION (regional stress / fault orientation)

24 Reactivation Risking Different fracture types form at various angles to principal stress planes Forms basis for assessing reactivation risk for pre-existing faults/fractures Courtesy of Richard Hillis, ASP.

25 Reactivation Risking

26 Reactivation Risking

27 Reactivation Risking Courtesy of Scott Mildren, ASP.

28 Issues with geomechanical approaches Treats faults as discrete entities Stress field rotation near faults Generally 2D analyses stress is 3D, intermediate stress can influence failure How to account for P p changes with injection poroelasticity issues not well understood in practice. Fault size issues Geomechanical modelling may aid understanding.

29 Geomechanical Modelling Head Gradient

30 Geomechanical Modelling DEPTH (m) Cap Injection Pressure 33 MPa Upper Aquifer Injection Zone Base CO P(MPa) Coupled mechanical-fluid flow modelling (TOUGH- FLAC) of CO 2 injection P P Distribution DISTANCE FROM INJECTION POINT (m) Stress change From Rutqvist and Tsang, 2002, LBNL. Issues input properties, complexity of mechanical layering and faulting for meshing. DEPTH (m) Increased Horizontal Stress at the Injection Point Caused by Poro-elastic Effects σ XX (MPa) DISTANCE FROM INJECTION POINT (m)

31 Courtesy Mike Kendall, University of Bristol Microseismics Passive seismic monitoring during production Resolved location of clusters onto steeply dipping faults Data can ground truth geomechanical models

32 4D Monitoring Issue: Often considerable overburden response rather than reservoir Shale behaviour not well constrained From Rodney Calvert, Shell, SEG Distinguished Lecturer

33 4D Monitoring Estimated saturation and pressure changes at top Cook interface OWC Saturation changes Pressure changes - 27% of remaining reserves in this segment has been produced in Notice that pressure anomaly crosses the OWC and terminates close to faults - Observed pore pressure increase in the segment is bar Courtesy of Martin Landrø, NTNU, & Statoil

34 Summary Many techniques - lots of issues/uncertainties Critical knowledge gaps: - Wettability - Reactivation & Poroelasticity - Diffusivity Critical methods aiding in understanding subsurface processes and monitoring: Geomechanical modelling, microseismics and 4D and their integration. Currently embryonic in oil industry but growing rapidly.