Measuring Rock Properties

1. Measuring Rock Properties Maurice Dusseault

2. Process Control The Optimization Loop In situ state -p,σ,T Science studies Behavioral laws Simulations Experience Better physics Better models Predictions Other applications New processes DESIGN This ongoing process requires measuring material parameters RISK MANAGEMENT & OPTIMIZATION MONITOR PRODUCE

3. Common Symbols in RM • E, n: Young’s modulus, Poisson’s ratio • f: Porosity (e.g. 0.25, or 25%) • c′, f′,To: Cohesion, friction , tensile strength • T, p, po: Temperature, pressure, initial pres. • sv, sh: Vertical and horizontal stress • shmin,sHMAX: Smallest, largest horizontal σ • s1,s2,s3: Major, intermediate, minor stress • r, g: Density, unit weight ( =  × g) • K, C: Bulk modulus, compressibility These are the most common symbols we use

4. Stress and Pressure • Petroleum geomechanics deals with stress & pressure • Effective stress: “solid stress” • Pressure is in the fluid phase • To assess the effects of Δσ', Δp, ΔT, ΔC… • Rock properties are needed • Deformation properties… • Fluid transport properties… • Thermal properties… sa– axial stress pore pressure A po sr– radial stress

5. REG. TIPO SVS-337 Obtaining Rock Properties… Properties data bank Depth Fric. Coh. XXX YYY ZZZ Rock Properties (E, ν, , c′, C, k,…) 3-D Seismic Reflected and direct paths Borehole seismic

6. The Geology… (Lithostratigraphy) How many rock types must I define and test for a reasonable, useful Geomechanics Analysis? Source: University of Texas - Bureau of Economic Geology

7. This is a Challenging Problem… • How can I determine field rock behavior from limited quantity, questionable quality core? • How do I cope with massive heterogeneity? • What about anisotropy (e.g.: shales)? • Can I test shale realistically in the laboratory? • Are laboratory results representative? • How many tests do I need? • I have no core (or bad core)! What do I do? • How many rock types to test (see diagram)? • And so on and so forth…

8. REG. TIPO FLANCO OESTE FLANCO ESTE REG. TIPO ER-EO ER-EO ER-EO C-4 B-SUP ER-EO C-5 B-SUP B-SUP B-6/9 C-3 C-6 B-6/9 C-1 C-2 C-4 C-1 C-3 FALLA VLE-400 B-6/9 B-6/9 C-2 C-7 C-3 C-5 C-4 C-2 C-5 C-3 C-2 C-1 C-1 C-4 C-3 C-5 C-2 C-6 GUAS C-6 C-4 C-2 C-3 C-5 C-7 C-4 C-6 C-3 C-5 C-4 C-7 C-7 C-6 C-5 C-6 C-7 C-7 GUASARE GUASARE GUASARE FALLA ICOTEA SVS-30 SVS-337 Geological Models: Logs vs. Rocks SMI Fault Structure, Center of Lago de Maracaibo (Venezuela)

9. What is a GMU? • Geo-Mechanics Unit • Nature is too complex to “fully” model • Simplification needed • A GMU is a “single unit” for design and modelling purposes • 1 GMU = 1 set of mechanical properties • GMU selected from logs, cores, judgment Log data Core data GMU 1 GMU 2 GMU 3 GMU 4 GMU 5 GMU 6 GMU 7 GMU 8

10. GMU’s and Rock Mechanics • Rocks are heterogeneous, anisotropic, etc… • For analysis, we divide systems into GMU’s… • Includes critical strata, overburden, underburden… • Too many subdivisions are pointless • Can’t afford to test all of them • Too few subdivisions is risky TOO FEW? TOO MANY?

11. Correlations for Properties • An adequate data base must exist • The GMU* is properly matched to the data base, for example, using the following: • Similar lithology • Similar depth of burial and geological age • Similar granulometry and porosity • Estimate of anisotropy (eg: shales and laminates) • Correlations based on geophysical properties • Use of a matched analog is advised in cases where core cannot be obtained economically *GMU = geomechanical unit

12. …UNCERTAINTY… Reservoirs are heterogeneous & aniso-tropic at all scales (microns to kilometers) 70 m of Athabasca Oilsands,  = 30%, So = 0.8,  > 1,000,000 cP North of Fort McMurray, Alta Even sandstone reservoirs show a great variability, especially vertically, and properties can change over distances as small as a few millimeters. Clearly, simplifications are needed for analysis.

13. Scale of Specimens to Test… Is a 35 mm core representative of a conglomerate with 20 mm pebbles?

14. Plugging a Larger Diameter Core… 25 mm specimens plugged from a 125 mm core Issues of scale and representativeness always arise in Petroleum Geo-mechanics testing

15. Scale and Heterogenity

16. How Do We “Test” This Rock Mass? • Joints and fractures can be at scales of mm to several meters • Large  core: 115 mm • Core plugs: 20-35 mm • If joints dominate, small-scale core tests are “indicators” only • This issue of “scale” enters into all Petroleum Geomechanics analyses A large core specimen A core “plug” 1 m Machu Picchu, Peru, Inca Stonecraft

17. Laboratory specimen (“intact”) 70-200 mm Scale of Discontinuities A tunnel in a rock mass Rock vs Rock mass --Intact rock --Single discontinuities --Two discontinuities --Several disc. --Rockmass 20-30 m

18. Discontinuities & Rocks • Rocks are heterogeneous at all scales (microns to kilometers) • In granular media, macroscopic stresses are transmitted through grain contact forces (fn, fs) fs = shear force fn = normal force

19. Difficult Materials to Get and Test • Very high porosity materials (e.g.: diatomite) • Materials containing viscous oil with gas in solution (expansion – e.g.: oil sands) • Highly fractured materials such as fractured quartz-illite shales • Highly heterogeneous layered material from great depth (core breaks apart at each layer, referred to as “disking”…)

20. Unusual Materials (Diatomite)… Increasing stress on diatomite (through pressure depletion) causes material compaction and eventually pore collapse Δσ′v Source: Bruno and Bovberg, 1992 cylindrical specimen εh = 0 εh = 0 SPE75230, Barenblatt et al, 2002

21. 95 mm Oil-rich sample expands to completely fill the liner Oil Sand Core Expansion… Radially Axially Core has expanded from 120.7mm to 127mm diameter and is now acting like a piston in a cylinder Schematic Diagram of Expansion of an 89 mm Core 89 mm 90-91 mm Oil sand PVC liner 127 mm Corrugated surface characteristic of thinly-bedded and laminated fine-grained sands of variable oil saturation Oil-poor to oil-free silty sands, expansion much less than other material Ironstone band, no expansion Cores separate readily along cracks which form between zones of differing expansion potential Gas pressure inside liner Observed Expansions of 89mm Core: • Ironstone 89 mm • Basal clays, clayey silts 89-91 mm • Oil-poor to oil-free silty sands 90-93 mm • Fine-grained oil-rich sand 91-95 mm • Coarse-grained oil-rich sand 94-95 mm ref. Dusseault (1980) Fig. 5 & 6

22. Quality Control – Oil Sand Cores CT-Scan Evidence of Damage in Heavy Oil Cores Courtesy of Glen Brook, Nexen and Apostolos Kantzas, U of Calgary

23. Venezuelan Core Damage Oil sands core from the Faja del Orinoco, depth of about 900 m. Massive core expansion from gas exsolution.

24. Core – General Statement Is this core useful for Geomechanics tests? ANY CORE is better than no core. However, with poor core condition, all we can realistically expect is a qualitative assessment, grain size, clay mineralogy, fluids…, perhaps some rough index tests of strength and deformability. However, dry shale core – no strength tests Best is high-quality intact core collected just for geomechanics tests. Obtain, preserve and transport the core carefully. Test it soon, test it appropriately, but be aware that there is always some damage…

25. Use of Time-Lapse Seismics… Seismic Attributes Relative Change Matrix …as T goes up, Qp drops… T σ po Sg D …as σ goes up, Qp increases… etc. Modified from Doug Schmitt, UofA, 2004

26. MS & Integrated Monitoring… Microseismic data can be collected and used to update a Whole Earth Rock Properties Model (Mechani-cal Earth Model - MEM) based on combined lab, log, geological and seismic data. This is an example of microseismic sources located in a cyclic steam stimulation process in Peace River, Alberta (Shell Oil). In geomechanics, because of massive uncertainly and scale issues, we exploit whatever data sources we can. We try to regularly update our MEM’s with new logs, new core, new seismic data, better geological models, and other information. Also, remember that the properties can change, especially with large Δσ′, Δp, or ΔT. Shell Oil, Peace River

27. Testing Heterogeneous Materials? These materials respond radically different to stress: one flows, the other fractures. How might we incorporate such behavior in our testing and modeling for a natural gas storage cavern? Original specimen - Post-test appearance

28. Inherent Anisotropy • Different directional stiffness is common! • Bedding planes • Oriented minerals (clays usually) • Oriented microcracks, joints, fissures… • Close alternation of thin beds of different inherent stiffness (laminated or schistose) • Imbricated grains • Different stresses = anisotropic response • Anisotropic grain contact fabric, etc. stiffer less stiff

29. 0° 30° 60° 90° Stiffness Anisotropy Δσ′a Apparent axial stiffness - M Vertical core L    Bedding inclination 0° 30° 60° 90° e.g.: shales, laminated strata

30. Cracks and Grain Contacts Microflaws can close, open, or slip as s changes E2 E1 E1 E3 • Flaws govern rock stiffness The nature of the grain-to-grain contacts and the overall porosity govern the stiffness of porous SS

31. Issues to Remember… • Natural lithological heterogeneity • Wide range of properties (e.g.: compressibility or Chalk vs. low- limestone) • Scatter of experimental data • Log data – lab test correlations (variance) • Core damage and quality control • Issues of scale (especially in fractured rocks) • Representativeness and GMU delineation • We must cope with all of these sources of uncertainty…