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Module B: In Situ Stresses Earth 437 Maurice B. Dusseault University of Waterloo 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 s1,s2,s3: 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 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 Δs’v = +75 MPa. The stress change is what is important; it is defined as Δs’ = s’ final - s’ initial This Δs’ value is used to compute subsidence, rock behavior (shearing, collapse), and so on In hard rocks (mining), [s]ij can be calculated from direct strain measurements – [De]ij 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) Stress Definitions s1 > s2 > s3 s3 Principal s2 Stresses s1 s1 s2 max planes sa = s1 sr sr = s3 s3 slip planes Triaxial Test Stresses sa y We usually assume sv is a principal stress x sv sHMAX > shmin sr sq sHMAX shmin In Situ Stresses q r ri z Borehole Stresses 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 ~4 km 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 ~400 m Borehole region: 1-5 m Drawdown zone (well scale) 100-1000 m Small Scale Stresses (less than 10-20 cm) 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 is s3 (least stress) Strike-slip regime: sv is s2 (intermediate stress) Listric (growth, down-to-sea or GoM) fault regime: sv changes from s1 to s3 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. Faults and Plate Tectonics The Big Picture! Compression region Regions of crustal extension 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 Basins: Major Examples in USA Rockies Foreland Basins, compressive stresses controlled by mountain thrust Thrust basins WILLISTON BASIN Powder River B. San Joachim, a rift valley Paradox B. Southern CA basin complex, strike-slip and normal faulting MICHIGAN BASIN MIDCONTINENT BASINS Atlantic coastal plain and offshore basin complexes, passive margins 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 Stresses and Basin Shape USA Cross-section New Orleans shoreline Houston Regional s3 directions Gulf of Mexico listric faults Edge of continental shelf 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 Normal Fault Regime The San Joachim Valley in California, the Rhine Valley between France and Germany, the Gulf of Thailand are all normal fault grabens graben horst extension sv = s1 sHMAX = s2 Horst-graben structure shmin = s3 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. Some Classic Normal Fault Areas Red Sea Around UK, Ireland 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. 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 = s3 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” … Strike-Slip or Wrench Fault Block diagram shmin = s3 acute angle sv = s2 sHMAX = s1 ~vertical fault plane shmin sHMAX Associated normal faults Surface view Strike-Slip Stress Regime Very high angle faults (>80° usually) Indicates sv = s2 (sHMAX = s1, shmin = s3) 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 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 Southern CA basin complex Small basin 15 km 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 strong lateral thrust B A high-p shale static basal sheet compression highly fractured zone C largely unfractured shale 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° Sometimes, thrust faults can take on very complex, stacked structures Thrust Faults and Mountains NWT Canadian Shield Nunavut Athabasca Basin (Precambrian) Alberta Syncline BC Basin edge Tectonic stress Edmonton ALTA Canadian Shield Breakouts to sHMAX SASK Williston Basin MAN USA The Western Canadian Sedimentary Basin Alberta is the “classic” compressional (thrust fault) regime Compressional Basin Section not to scale Thrust faults SW Rockies Alberta Syncline Massive heavy oil deposits Salt solution and Edmonton collapse features NE + + Cretaceous sands, shales Regional Cretaceous unconformity Jurassic and older carbonates, sandstones, shales + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + ++ Precambrian rocks Schematic cross-section through Edmonton, Alberta + 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) Cross Section: Stress & Structure forebasin Mountains Golden Colorado Banff Alberta Distant plains Eastern Colorado Calgary Alberta sHMAX Near mountains: •Very high sHMAX •For great depth, sv = s3 •Thrusts, folds… •Fractured strata •Low to modest po sHMAX Distant from mountains: •Moderate to high sHMAX •For some depth, sv = s3 •Flat-lying, no faults •Strata are relatively intact •Low pressures Sing07.024 Generally very high pore pressures are not found in thrust regimes and their forebasins, as rocks are somewhat fractured, pressures dissipate Real Thrust Faulting Structures Rocks are permeable because of fractures and folds, po is rarely overpressured High stresses near the tectonic compression front Lower stresses far from the tectonic compression front Folding Thrust sheets Thrust fault planes Folded belt in front of last thrust Sing07.024 Undeformed sediments Moderate deep overpressure may remain in the deepest part of the foreland basin (po ~ 1.3-1.4 gw·z) Thrusting Aided by High po Axis of geosyncline Zone of abnormal fluid pressure Normal faults Undeformed Eroded Thrust fault Undeformed strata 30 20 10 0 10 20 MILES Sing07.025 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 30 Listric Faulting and Stresses grabens steep at top “down-to-the-sea” faults Listric faults on continental margins lead to unusual stress regimes where the major stress changes from vertical to near-horizontal at depth sea stress sv sh slip planes zone where faults coalesce (detachment or décollement zone) depth Stresses change with z! 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 India and Tibet Examples Active basins Passive basins 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. Tectonic Structure Map of the Region 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! 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… North America (World Stress Map) (available online at www.world-stress-map.org) 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 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 Thrust condition 20 – 100 km Folds and closed structures 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 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…) Stresses at Depth σv from density logs, σhmin, σHMAX from various methods (geological estimation, HF…) We often use the “K” coefficient. sh min p o sh min K s v po sv Ratio of least horizontal effective stress to the vertical effective stress (in situ) <1 – vertical fracturing >1 – “horizontal” fracturing 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… Burial Stresses, Friction Control These values are the limits, not actual values in situ E = stiffness UC sand 0.5E shale 0.75E Ka = 0.70 salt is viscoplastic salt sandstone Ka = 0.33 Ka = 1.0 Ka = 0.33 E Note: s = s - po (Terzaghi’s law) po 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 shmin 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 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… Deep Salt Diapir Example Gas Pull Down Mid-Miocene regional pressure boundary Top Balder Top Chalk Intra Hod/Salt Sands and Shales, sh vs. Depth Ko - effective stress ratio stress, units of density (s/z) 1.0 1.5 2.0 mud 2.5 0.5 0 n ~ 0.45 sands & sandstones n ~ 0.25 1.0 clays & shales (n = Poisson’s ratio) clay mudstone hydrostat = gwz sv, vertical stress sh sand sh shale This model applies only to upper 2000 m of soft sediments and no tectonics! sh/sv sh/sv shale depth Stress plot (density) 2.0 = 16.7 ppg depth Stress plot (ratio of sh / sv) Porosity-Depth Relationships 0 0.25 0.50 0.75 1.0 porosity sands & sandstones mud clay & shale, “normal” line clay mudstone effect of overpressures on porosity 4-8 km depth shale The specific details of these relationships are a function of basin age, diagenesis, heat flow ... 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, s(ie: depth), t, chemistry…] Stress and Diagenesis diagenesis sv burial Sand burial in a non-tectonic environment results in sh < sv because of friction. Diagenesis seems to reduce this stress difference slowly over long periods of time. f is the friction angle for sand sh Stress & Diagenesis, no Tectonics If there has been no tectonic activity, sh is less than sv In sands, the ratio Ko (defined as the ratio of horizontal to vertical stress, sh/sv), can be as low as 0.3, usually 0.4 – 0.6 Shales have a low angle of friction, usually Ko is 0.6 – 0.8, even as high as 0.95 in muds Thus, the fracture gradient is higher in mud or shale in non-tectonic areas (GoM) Deep burial and diagenesis tend to reduce the stress differences Diagenesis and Strength max planes shear stress - chemical cementation diagenesis effects on the strength sa diagenetic strength increase s3 s1 strength increase Principal stresses – see inset slip planes sr sr = s3 densification (more interlock) cohesion sa = s1 Triaxial Test Stresses original sediment normal stress - sn Diagenesis, Strength, Stiffness Mechanical compaction in the early burial stages increases the friction angle, f Chemical effects increase cohesion, c, often cementing particles together The sediments also become stiffer (higher Young’s modulus) In general, rocks become stronger, ƒ(f) However: deep, intense shale diagenesis often generates shrinkage fractures; they become weaker, shmin and sHMAX , k, even though they are lower porosity Erosion and Stresses s’v diagenesis sv sh Elastic behavior governs unloading because the rock is stiff and strong; lateral stresses increase naturally s’h Effect of Erosion Once a sediment is buried and diagenetically indurated, it behaves elastically Direct erosion without tectonic loading leads to the so-called “Poisson effect”: n Ds'h Ds' v 1 n Thus, erosion naturally leads toward the shallow condition K’o > 1.0 (except for salt, which behaves as a viscous fluid) Rocks, Stresses in an Eroded Basin s, po Erosion has created a thrust “skin” near the surface stress where sHMAX = s1, and sv state sv = s3 sh sv = s3 Deeper, a strike-slip regime condition sHMAX = s3, sv = s2 is found sv = s2 I.e. fracture gradient increases with depth strike-slip stress Rocks are stronger, state stiffer High pore pressures Z (po > 1.3 sv) are rare in Assuming that both horizontal stresses are equal eroded basins Eroded Basin The “Poisson effect” during unloading generates a region at shallow depth where horizontal stresses are larger than vertical Also, the rocks are strong Drilling underbalanced is becoming common in such regions because of rock strength Pore pressures in such regions are rarely in the overpressure domain For large overpressures, there is a special module Conclusions on Erosion Generation of high Ko values requires plastic deformation (incl. diagenesis) This happens naturally in sedimentary basins, even without tectonics, during the burial phase, however, after induration… The unloading phase is largely elastic It is unlikely that glaciation has a large effect on previously eroded basins Any eroded basin will have a skin that is “overstressed”, with sv = s3 to some depth Changes in Stresses Opening a hole (drilling) changes stresses massively sr is now s3 at a local scale In drilling, heating or cooling the wall can change the stresses, affecting stability High mud pressures can cause joints to slip Stresses changed by injection or production E.g.: Depletion of the reservoir decreases the fracture gradient (loss of horizontal stress) Hot or cold water injection changes stresses Pressurization or depletion can result in shearing of casing, faulting, minor seismicity In Situ Stresses: Summary Stresses are needed for casing programs, borehole stability analysis, etc. etc. Pore pressures are a vital aspect as well Often stress directions and relative magnitudes can be estimated from the tectonic and burial history of the sedimentary basin, or its structure The most recent fault patterns often reflect the stress regime Features such as salt domes, etc., invariably indicate locally altered stresses Lessons Learned Stresses in the earth arise from: Gravity effects (rock bulk density – sv) Tectonic effects (compression, salt tectonics…) Recent faults indicate stress patterns Basin shape is a stress pattern indicator Geology and history allow us to estimate relative stress magnitudes and orientations Stress is involved in many basin processes Basin fabric, diagenesis, overpressure, oil migration, gas and oil valving… Recommendations Offshore or inshore, it pays to have some stress information for drilling, hydraulic fracturing, reservoir modeling… The first step is to use geological history to build a regional stress model The pore pressure conditions should be inferred as well, (also offset well data…) Then, examine reservoir & local scale factors Faults, salt features, reefs and drapes, hydrothermalism, and other features These may “perturb” regional stresses Extra Materials Breakouts and Natural Stresses Vertical borehole sHMAX principal stresses, s1 > s3 shmin breakouts damage, ravelling high sq Breakouts are evidence of stress anisotropy and are caused by shear rupture of the borehole wall However, care must be taken in assessing breakouts, as other factors can “interfere” Use only vertical wells (10) to get good stress orientations Some “Confusing” Effects sHMAX material anisotropy shmin jointed laminated shale slabbing, ravelling Material anisotropy means the mechanical properties are different in different directions, as in a fissile shale bedding planes s3 ravelling s1 s3 ravelling sHMAX Borehole Features: Wall Scan 0 90 180 270 360 axial fractures Stress directions sHMAX “en-echelon” axial fractures if hole is slightly inclined shmin breakouts large washout Sinusoidal fracture traces higher angle of intersection (joint plane) low intersection angle (bedding?) Geometry of joint plane intersection breakouts no breakouts axial fractures Reconstructed breakout data from Schlumberger borehole scanner logs Axial fractures and breakouts are stress direction indicators. If the stress difference is large, breakouts are also larger (deeper and wider). s Directions: Breakouts, Fractures This is a LWD log trace taken during a trip, so resolution is poor Natural fracture plane Borehole wall tensile fractures Small breakouts (90° to tensile fractures) shmin is at 40°Az in this example Natural fracture plane Modest breakouts, no tensile fractures Use of Breakouts, Axial Fractures For s orientations, use only wells that are vertical +/- 10° (rarely more inclined) Establish quality control on your data (length, symmetry across hole, quality…) Grade your data (“A” “B”, “C” quality…) Breakouts: sHMAX is at 90° to breakout axis Borehole wall axial fractures: sHMAX is parallel to the fractures axis Combine with geology, sv calculations from density log data, LOT data, HF data… Build a stress map for your region & use it More about Breakouts, Fractures Don’t confuse breakouts with hole enlargement (breakouts are symmetrical, and the minor axis ~ hole gauge size) Don’t confuse breakouts with sloughing in a fissile shale when the hole dip is close to the dip of the shale fissility Joints and planar features trace sinusoidal patterns on the borehole wall; induced axial fractures do not 4-arm dipmeter data must show ~symmetry, consistency, reasonable length, etc. (QC) Full wall scans are easier to interpret Log Data as Stress Indicators 4-arm dipmeter response Some other log properties may give stress orientation information Formation, po Conductivity response Pad region b borehole c borehole, a pw d sHMAX Anisotropically invaded zone, & stress microfissured region, giving anisotropic permeability shmin Mud Weight Window Prognosis 1.1 1.3 1.5 1.7 1.9 2.1 prognosis for shmin prognosis for po Convert to density units by dividing pressures and stresses by vertical depth density, g/cm3 MW =1.92 sv XLOT shmin value overpressure transition zone 2.3 Previous casing string shoe shoe location for next casing string area indicates possible MW depth strong overpressure zone This is a conservative approach: often, we can stretch the lower limit a bit, carefully Mud Weight Prognoses Data garnered from many sources, offset wells, geology, XLOT, MWD… The interval defined is the secure window: MW below pfrac (= shmin) at the shoe MW above pmax, usually at or near the bit However, the window can be pushed a bit! Drilling MW + ECD can be a bit above pfrac If shales are strong and no high k sands, drilling may take place slightly below the pore pressure Tricks such as high weight pills on trips Underbalanced drilling in strong shales Pushing the Envelope in Drilling! (2.0 = 16.7 ppg) 1.1 1.3 1.5 1.7 1.9 2.1 2.3 density, g/cm3 prognosis for shmin prognosis for po Convert to density units by dividing pressures and stresses by vertical depth MW =1.92 sv XLOT shmin value overpressure transition zone area indicates possible MW depth Previous casing string shoe deeper shoe for casing string! strong overpressure zone Using high weight trip pills and careful monitoring, the lower limit can be extended Pore Pressure Generation sv Poisson effect Dsh = Dsv(n/1-n) faulting n/(1-n) Pore pressure generation reduces effective stresses, but not in a path parallel to hydrostatic because of Poisson effect. This can lead to rock shear if po is high sh Diagenesis, Burial, Erosion 25 sv - MPa diagenesis -2000 m- 20 This stress path explains the presence of 15 high horizontal stresses near the surface 10 Burial to 2000 m, erosion to 500 m erosion, n ~ 0.2 -500 m5 sh =17 MPa sv = 7 MPa Stress path sh - MPa 0 0 5 10 15 20 A simple calculation of the probable effect of erosion of 1500 m of rocks on the stresses. We assumed initial stress state (red star), took a reasonable Poisson’s ratio for erosion (0.2), and made the calculation. (Assume that po is always 8.33 ppg) 25 Fluid Flow in Shales Shales are almost always the source rocks; sands, limestones the reservoirs Most shales are water-wet, except for the rich kerogenous shales Gas or oil as the non-wetting phase flows only with gradients of > 500 Two-phase permeability of shales = zero Thus, HC flow must be through fissures, fractures, or other macrodiscontinuities Shales are originally intact, so O&G expulsion is likely through induced fractures Stress and Petroleum Migration How are microfissures generated? When s3 normal to fissures is < po po is the pressure in the shale pores s3 is the lateral confining stress at the scale of the microfissure This can occur by various geological and tectonic processes: Processes which increase po Processes which decrease s3 These are linked to shrinkage of shale, loading… Reduction in s3 Tectonic unloading reduces s3 (shmin) Any shrinkage in the material reduces s3 -Clay compaction, thermal shrinkage (uplift) -Loading of anisotropic shales -Smectite to mixed-layer to illite changes When s3 drops below po, fissures can open stably, and remain open Fissures can be dominantly vertical (usual case), or also horizontal Now, fluids can easily flow through the open cracks, migrate to traps & accumulate Increase in po HC generation increases pore pressures Retarded compaction (overpressure) Smectite diagenesis releases H2O Thermal pressuring increase po Accumulation of a thick gas zone in a reservoir with good vertical closure Tectonic loading (?) may increase po Other processes (e.g. gypsum dewatering) These processes give the driving forces for fluid flow and HC migration Can Oil or Gas Flow in Shales? clay particle Typical pore size in shale 0.1-0.5mm gas or oil bubble clay particle water pw po r oil or gas Dp = g/2r g = interfacial tension r = radius of curvature Dp = po - pw Darcy flow requires huge Dp Conclusion: oil or gas cannot flow through intact shale! Can Water Flow Easily in Shale? Cations Hydrated cations Bound water Na+ H2O molecules Much of the water in shales is not free to move easily: Adsorbed on the clay fragments Hydrated onto cations Pore throats are “blocked” by adsorbed water Bound water Free water, partly on ions Conclusion: oil or gas cannot flow through intact shale! Fluid Generation & Fracturing Flow in shales must be through fractures! 3-20 mm sv shale T, p, s increase kerogen microfissure fluid flow sv high T, p, s oil and gas generation of hydrocarbon fluids semi-solid organics, po < sh < sv po = sh < sv, fractures develop and grow fluids are expelled through the fracture network, po declines Effects of Increasing po This is called “valving” of gas A stresses along A-A stress gas cap effect oil, density = 0.75-0.85 A po depth sh hydraulic fracture when po > sh gas cap, low density sn = sn - poil o fault slips when > (sn - po)·tanf, i.e. when > sn·tanf Fluid migration is a function of stress as well as pressure! sn Valving of Reservoirs & Stresses Gas is generated and accumulated, increasing the gas cap height Because gas is light, it has a different gradient of pressure (0.1 instead of 0.8 for oil and ~1.04 to 1.10 for brine) This causes po to increase at the crest When po exceeds sh, hydraulic fracture! This is like a valve that opens episodically over geological time, releasing pressure Responsible for shallow gas in some areas Tectonic Stressing lithotype stiffness UC sand 0.5E shale 0.75E stress mud salt is viscoplastic salt sandstone E limestone 1.5E stresses are isotropic loading unloading depth assumed initial sh Tectonic Stressing From its “virgin state”, unloading (rifting) or loading (compression) can occur This has a relatively larger effect in the stiffer rocks (small strains = large Dsh) Stress contrasts can be generated among the beds (except salt) This was proven by measurements in the Appalachian Basin thrust zones in 1980s This is easily simulated by numerical models, but real data need measurements Typical Stress Conditions stress (or pressure) stress (or pressure) vertical stress. sv vertical stress, sv horizontal stress. sh horizontal stress, sh pore pressure, po pore pressure, po strongly overpressured region at depth 4 km depth 4 km depth mild overpressure a. Gulf Coast of USA b. Western Alberta, 100 km from Rockies Relaxed continental margin Tectonically stressed rocks A Well Plan, North Sea • classical mud weight window is too narrow; cannot avoid instability • low mud weight breakouts • high mud weight destabilized fractured zones & losses • breakout problems are controllable by good hole cleaning; fracture zones are uncontrollable Strategy: • keep mud weight low • manage breakouts with good hole cleaning before increasing mud weight • monitor cavings and mud losses for warning of fractured zones Stresses and Drilling To increase hole stability, the best orientation is that which minimizes the principal stress difference normal to the axis Favored hole orientation sv 60-90° cone sHMAX shmin sv Drill within a 60°cone (±30°) from the most favored direction sHMAX shmin sv >> sHMAX > shmin sHMAX ~ sv >> shmin sv sHMAX shmin sHMAX >> sv > shmin
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