Module B: In Situ Stresses

1. Module B: In Situ Stresses Earth 437 Maurice B. Dusseault University of Waterloo

2. Common Symbols in Earth Stresses • sv,shmin,sHMAX: Vertical, minor and major horizontal stresses (usually sv to surface) • Sv,Sh,SH: Same as above, different symbols • s1,s2,s3: Major, intermediate, minor stress • s1,s2,s3: Effective or matrix (solid) stress • E, n: Young’s modulus, Poisson’s ratio • f: Porosity (e.g. 0.25, or 25%) • r, g, po: Density, unit weight, pore pressure • k: Permeability (kv, kh…) • These are the most common symbols used in discussing stresses in the earth

3. Stresses in the Earth: Intro I • In situ stresses: a vital initial condition for all geomechanics issues, not just drilling! • To carry out any quantitative analysis, it is necessary to start from the initial stress state • For example, deep reservoir depletion can lead to a Δp of perhaps -75 MPa, so that Δ‛v = +75 MPa. • The stress change is what is important; it is defined as Δ‛ = ‛final - ‛initial • This Δ‛ value is used to compute subsidence, rock behavior (shearing, collapse), and so on • In hard rocks (mining), []ijcan be calculated from direct strain measurements – [De]ij

4. Stresses in the Earth: Intro II • In sedimentary rocks (oil and gas applications), it is far more difficult • The locations are deep, hard to get to • And, the strains are small, hard to measure • The rocks are porous, poor strain response • So… hydraulic fracture-based methods are widely used - Minifrac™, LOT, XLOT • +Core-based methods (DSCA, vP(q), …) • +Geophysical logging based methods • +Geological inference (burial and tectonic histories of the basin give excellent clues)

5. Stress Definitions a = 1 slip planes max planes r r = 3 Triaxial Test Stresses a HMAX > hmin 3 Principal Stresses 1 > 2 > 3 2 1 1 2 3 y We usually assume sv is a principal stress q x v r q r HMAX ri hmin In Situ Stresses Borehole Stresses z

6. Local, Reservoir and Regional Scales ~200 km • Regional Scale Stresses • Basin scale: 50 km to 1000 km • Often called “far-field stresses” • Reservoir Scale Stresses • A reservoir, or part of a reservoir • Scale from 500 m to several km • Salt dome region: 5-20 km affected zone • Local Scale Stresses • Borehole region: 1-5 m • Drawdown zone (well scale) 100-1000 m • Small Scale Stresses (less than 10-20 cm) ~4 km ~400 m

7. Common Stress Regimes • The most common stress regimes are: • Relaxed, or non-tectonic (no faulting, flat-lying): vertical stress, sv, is= s1 (major stress) • Normal fault regime: sv is s1 • Thrust fault regime: sv iss3 (least stress) • Strike-slip regime: sv iss2 (intermediate stress) • Listric (growth, down-to-sea or GoM) fault regime: sv changes from s1 tos3 at depth, then back to s1 • Most sedimentary basins with O&G have relatively simple stress regimes • But, there are local complications, such as multiple faults, salt domes, uplift, etc.

8. Faults and Plate Tectonics The Big Picture! Compression region Regions of crustal extension

9. Where Are Tectonics Important? • Near active plates (eg: California, Sumatra, Colombia), tectonics governs stresses • Near mountains, tectonic forces dominate • Away from plate margins and mountains (eg: Williston Basin, Kalimantan, GoM), other factors are important • In continental margin basins salt tectonics (domes and tongues) can be very important • In non-tectonic intracontinental basins (Michigan, Williston, Permian…), the shape and burial/erosion history are more important than tectonics

10. Basins: Major Examples in USA Rockies Foreland Basins, compressive stresses controlled by mountain thrust Thrust basins WILLISTON BASIN MICHIGAN BASIN Powder River B. APPALACHIAN BASIN San Joachim, a rift valley MIDCONTINENT BASINS Paradox B. Atlantic coastal plain and offshore basin complexes, passive margins PERMIAN BASINS COMPLEX Southern CA basin complex, strike-slip and normal faulting Gulf Coast Basin – GoM, passive or relaxed basin

11. Non-Tectonic Regimes • On stable continental plates, far from active plate boundaries. Some examples • Mid-continent basins: Williston, Michigan, Permian age basins, East Texas, Songliao Basin (richest Chinese basin), interior Russia… • On passive continental plate oceanic margins such as GoM, Kalimantan, Nova Scotia, NW Norway coast, Angola, etc. • Basin geometry, history of sedimentation, compaction, burial, erosion, diagenesis, salt dissolution features…, salt tectonics affect stress states locally & substantially

12. Stresses and Basin Shape Cross-section USA New Orleans shoreline Houston Florida Regional s3 directions listric faults Gulf of Mexico Edge of continental shelf Mexico Yucutan GoM example: regional stress directions are dominated by the continental slope, except locally near salt domes and a few structures such as the Mississippi canyon

13. Normal Fault Regime sv = s1 shmin = s3 sHMAX = s2 Horst-graben structure horst graben extension The normal fault regime is also called the extensional regime. It is characteristic of shallow rocks in all non-tectonic sedimentary basins without large erosion. The San Joachim Valley in California, the Rhine Valley between France and Germany, the Gulf of Thailand are all normal fault grabens

14. Some Classic Normal Fault Areas Red Sea Around UK, Ireland

15. Normal Fault Zones (Pull-Apart) • Mid-ocean rifts • East African Rift • Upper zones, GoM • Gulf of Thailand • Upper Cook Inlet • On flanks of thrust faults, etc.

16. Normal Faulting Regimes • High angle faults at surface (60°-70° dip) • This indicates that sv = s1 when faulting occurred. (But, is the fault old or active?) • Also, sHMAX = s2 and shmin = 3 • Characteristic of extensional strain • Also, typical of non-tectonic basins • Hydraulic fractures are vertical,  to shmin • However, high angle surface faults may “flatten” at greater depth (as in the GoM) • Many continental margins, passive basins, regions of crustal “pull-apart” …

17. Strike-Slip or Wrench Fault sv = s2 Block diagram shmin = s3 sHMAX = s1 acute angle ~vertical fault plane shmin sHMAX Surface view Associated normal faults

18. Strike-Slip Stress Regime • Very high angle faults (>80° usually) • Indicates sv = s2 (sHMAX = s1, shmin = 3) when the fault formed • Characteristic of plate margins • Common at depth in eroded basins • Common some distance from compression • Usually, normal faults are found nearby at the surface, away from the main fault trace, to accommodate strata movements • Hydraulic fractures vertical,  to shmin

19. Small Window Basins • Small “window” basins between strike-slip faults have complex stress conditions Small basin opened up between parallel strike-slip faults. Locally, stresses can vary from normal to thrust regimes, very complex Graben basin San Joachim Valley Basin Small basin 15 km Southern CA basin complex

20. Thrust Fault Regime and Structures The shale bed in zone A has gone through one hinge point, through two in zone B, and through three hinge bends in zone C. sv = s3 sHMAX = s1 hinge points overthrust sheet highly fractured zone strong lateral thrust overthrust sheet B C brittle quartz-illite shale A largely unfractured shale RAMP static basal sheet high-p shale compression

21. Thrust and Reverse Faults • Less than 45° angle on fault plane • If less than 20-25°, it is almost always called a thrust fault rather than a reverse fault This angle is always less than 45°, usually less than 30°

22. Sometimes, thrust faults can take on very complex, stacked structures

23. Thrust Faults and Mountains NWT Nunavut Canadian Shield Athabasca Basin (Precambrian) Alberta Syncline BC Basin edge Canadian Shield Breakouts  to sHMAX Tectonic stress Edmonton Rockies MAN ALTA SASK Williston Basin USA The Western Canadian Sedimentary Basin Alberta is the “classic” compressional (thrust fault) regime

24. Compressional Basin Section + + + + + + + + + + + + + + + + + not to scale Thrust faults SW NE Massive heavy oil deposits Salt solution and collapse features Rockies Alberta Syncline Edmonton + + + Cretaceous sands, shales + + + + + Regional Cretaceous unconformity Devonian reefs Prairie Evaporites (halite) Precambrian rocks + Jurassic and older carbonates, sandstones, shales + + + + + + + + + + + + + + + + + + + + Schematic cross-section through Edmonton, Alberta + + + + + +

25. Thrust Faults • Low angle faults (dip of 0° to 30° usually) • Indicates sv = s3 (sHMAX = s1) when the fault formed (or if it is still active) • Characteristic of compression regions, associated with thrust mountain ranges • Same stress condition can often be found at shallow depths in eroded basins • Usually, thrust fault “sheets” are bounded by systems of normal and strike-slip faults • Hydraulic fractures will be horizontal (in fact, usually they propagate gently upward)

26. Cross Section: Stress & Structure Sing07.024 forebasin Mountains      Distant plains Golden Colorado      Eastern Colorado Banff Alberta      Calgary Alberta sHMAX sHMAX • Near mountains: • Very high sHMAX • For great depth, sv = s3 • Thrusts, folds… • Fractured strata • Low to modest po • Distant from mountains: • Moderate to high sHMAX • For some depth, sv = s3 • Flat-lying, no faults • Strata are relatively intact • Low pressures Generally very high pore pressures are not found in thrust regimes and their forebasins, as rocks are somewhat fractured, pressures dissipate

27. Real Thrust Faulting Structures Sing07.024 Rocks are permeable because of fractures and folds, po is rarely overpressured Lower stresses far from the tectonic compression front High stresses near the tectonic compression front Folding Thrust sheets Thrust fault planes Undeformed sediments Moderate deep overpressure may remain in the deepest part of the foreland basin (po ~ 1.3-1.4 gw·z) Folded belt in front of last thrust

28. Thrusting Aided by High po Axis of geosyncline Zone of abnormal fluid pressure Thrust fault Eroded Normal faults Undeformed Undeformed strata 30 20 10 0 10 20 30 MILES This is a massive gravitational “landslide” (Wyoming), similar to listric faults. This could not be possible without high local pore pressures in shales, which allowed the fault block to virtually “float” along the fault plane Sing07.025

29. Listric Faulting and Stresses stress depth Listric faults on continental margins lead to unusual stress regimes where the major stress changes from vertical to near-horizontal at depth grabens “down-to-the-sea” faults steep at top sea v h slip planes Stresses change with z! zone where faults coalesce (detachment or décollement zone)

30. Listric Faults • Characteristic of passive continental margin basins that are “open-to-the-sea” (GoM) • Look like normal faults at the surface • At depth, the faults flatten to become thrust faults • Stress regimes change with depth! • Often associated with overpressured zones • These faults are like massive landslides

31. India and Tibet Examples

32. Active basins Passive basins

33. Bay of Bengal Region and North • A continental margin basin exists offshore south of Dacca and Calcutta, we will expect a relaxed stress condition, GoM features • A strong thrust basin to the north, along the Himalaya front, fractures, no oil • Strike-slip to the east (Sagaing zone) • Shan-Thai Plateau is partly a zone of extension, some N-S faults are normal • Sichuan Basin, relatively undeformed, but under strong compression • Etc.

34. Tectonic Structure Map of the Region

35. Tectonics Give Stress State Clues This NASA image of Ganymede shows complex tectonism, giving clues about the stresses and dynamics which caused the structures Normal faulting Shear zones Extraterrestrial Geomechanics!

36. North Sea Stress Trajectories • From the World Stress Map Project • In the central part, complex of grabens and wrench faults • Many “blocks”, each with a stress pattern • Farther north, the Continental Shelf is “open to the sea” • Breakouts, LOT, HF tests…

37. North America (World Stress Map) (available online at www.world-stress-map.org)

38. Stress Map of Europe • Many solutions for earthquake focal mechanisms in southern Europe give the dense stress coverage • In the hard-rock areas – strain relief methods • In quiescent basins, data from breakouts, hydraulic fracturing, LOT

39. Geological History!! • This basin opened, filled, was compressed (thrusts and folds), uplifted and eroded • Later, it subsided with new sediment fill • The different lithologies compacted differently, leading to normal faults clays and silts gravels Normal faults Relaxed stresses 3-10 km Folds and closed structures Thrust condition 20 – 100 km

40. Conclusions on Tectonics and Faults • The tectonic condition and the nature and orientation of faults give important clues: • The principal stress directions • The relative magnitude of the stresses • Whether stresses are intense or relaxed • To be confident of the stress conditions, the faults must be shown to be “active” • Geological history can be complex, giving different stress fields at different depth • The first task in a new area is to study the stresses and tectonic features

41. Burial and Diagenetic History • What controls stresses during burial? • How do stresses change with diagenesis? • What happens during uplift and erosion? • Do all rocks behave the same? • What happens if pore pressures change? • When there is tectonic loading or unloading, how are stress changes partitioned in strata? • Hydrocarbon generation effect? • Etc, etc… (it gets complicated…)

42. Stresses at Depth • σv from density logs, σhmin, σHMAX from various methods (geological estimation, HF…) • We often use the “K” coefficient. • Ratio of least horizontal effective stress to the vertical effective stress (in situ) • <1 – vertical fracturing • >1 – “horizontal” fracturing

43. Friction Angle Control of Stresses • If soft sediment is in a state of plastic yield during sedimentation and burial: K’]min = (1 - sin f)/(1 + sin f) (soft seds) • f is the Mohr-Coulomb friction angle • For loose sand: f = 30°, thus K’]min = 0.33 • For shales, much lower friction angles • f = 10°: this gives K’ = 0.70 • We observe that horizontal stresses in soft shale are much higher than in sands during burial, until sediments are indurated • Upper GoM, Gulf of Thailand…

44. Burial Stresses, Friction Control These values are the limits, not actual values in situ E = stiffness Ka = 0.33 UC sand 0.5E shale Ka = 0.70 0.75E salt is viscoplastic Ka = 1.0 salt Ka = 0.33 sandstone E Note: s = s - po (Terzaghi’s law) po

45. Frictional Control of Stresses • In fact, the strata we encounter are rarely purely frictional materials • They also have cohesion • The frictional stress control “model” is only intended to give the theoretical lower bound of shmin for high porosity strata • If rocks are strongly cemented, it is possible to have stresses lower than this • In exceptional cases, open fractures! • Shallow, above flanks of salt domes • In mountainous areas

46. Stresses In and Around Salt • Salt is a very special material: • Highly soluble • Low density (2.16 g/cm3 or 18 ppg equivalent) • Viscoplastic, so all stresses are the same • Drilling long sections of salt is a challenge • Drilling near salt structures such as diapirs and sand tongues is challenging • Therefore, a special Module on salt drilling is included, and not treated here… • See Module G for a full discussion…

47. Deep Salt Diapir Example Gas Pull Down Mid-Miocene regional pressure boundary Top Balder Top Chalk Intra Hod/Salt

48. Sands and Shales, h vs. Depth 0.5 1.0 0 clays & shales sands & sandstones sh/svsh/sv depth Ko - effective stress ratio stress, units of density (s/z) 1.0 1.5 2.0 2.5 mud n ~ 0.45 n ~ 0.25 (n = Poisson’s ratio) clay sv, vertical stress mud- stone This model applies only to upper 2000 m of soft sediments and no tectonics! sh sand hydrostat = gwz sh shale shale depth Stress plot (density) Stress plot (ratio of sh / sv) 2.0 = 16.7 ppg

49. Porosity-Depth Relationships 0 0.25 0.50 0.75 1.0 porosity sands & sandstones mud clay clay & shale, “normal” line mud- stone shale effect of overpressures on porosity The specific details of these relationships are a function of basin age, diagenesis, heat flow ... 4-8 km depth

50. Diagenesis and Rocks • Mechanical compaction, most important in shales, drives the particles closer together • Pressure solution, important in sands but not shales, lowers porosity substantially • Cementation, usually SiO2 or CaCO3, bonds grains together, reducing porosity as well, most important in sands (sandstones) • Very deep, clay minerals change, leading to fracturing & stress changes (shales only) • Diagenesis rate and intensity is ƒ[T, (ie: depth), t, chemistry…]