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Module F: Drilling in Unusual Stress Regimes Part I – Overpressured Cases

Module F: Drilling in Unusual Stress Regimes Part I – Overpressured Cases. Maurice B. Dusseault University of Waterloo. Drilling in Overpressured Zones. For practical purposes ($), reducing the number of casings or liners is desirable

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Module F: Drilling in Unusual Stress Regimes Part I – Overpressured Cases

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  1. Module F:Drilling in Unusual Stress Regimes Part I – Overpressured Cases Maurice B. Dusseault University of Waterloo

  2. Drilling in Overpressured Zones • For practical purposes ($), reducing the number of casings or liners is desirable • However, drilling in OP zones carries simultaneous risks of blowouts and lost circulation that are difficult to manage. • There now exist new options that help us: • Drilling slightly above shmin with LCM in the mud • Bicentre bits and expandable casings • Understanding overpressure and also the deep zone of stress reversion will help

  3. Pressures at Depth Fresh water: ~10 MPa/km 8.33 ppg 0.43 psi/frt Sat. NaCl brine: ~12 MPa/km 10 ppg 0.516 psi/ft pressure (MPa) ~10 MPa Hydrostatic pressure distribution: p(z) = rwgz 1 km Underpressured case: underpressure ratio = p/(rwgz), a value less than 0.95 Overpressured case: overpressure ratio = p/(rwgz), a value greater than 1.2 underpressure overpressure Normally pressured range: 0.95 < p(norm) < 1.2 depth

  4. Some Definitions • For consistency, some definitions: • Hydrostatic: po = “weight” column of water above the point, r = 8.33 ppg to 10 ppg in exceptional cases of saturated NaCl brine • Underpressure is defined as po less than 95% of the hydrostatic po, usually found only at relatively shallow depths (<2 km) or in regions of very high relief (canyons…) • Mild overpressure: po of 10 ppg to 60% sv • Medium overpressure: po of 60 to 80% sv • Strong overpressure: po > 80% of sv

  5. Abnormal Pressure, Gradient Plot 1.0 2.0 • Typically, po is close to hydrostatic in the upper region • shmin is close to sv in shallow muds, soft shale, but lower in stiff competent deeper shale • A sharp transition zone is common (200-600 m) • The OP zone may be 2-3 km thick • A stress reversion zone may exist below OP 0 1 16.7 ppg po hmin 2 v thick shale sequence 3 po 4 Target A 5 Target B 6 Target C depth - kilometres

  6. GoM –The Classic OP Regime

  7. Other Well-Known Strong OP Areas • Iran, Tarim Basin (China), North Sea, Offshore Eastern Canada, Caspian • In many thick basins, OP is found only at depth, without a sharp transition zone • Most common in young basins that filled rapidly with thick shale sequences • Good ductile shale seals, undercompaction • Watch out for OP related to salt tectonics! • These are most common offshore: • Land basins have often undergone uplift • Tectonics have allowed pressures to dissipate

  8. Eastern Canada Overpressured Areas Nova Scotia Gas Belt Importance of Geomechanics Exports

  9. Porosity vs Depth & Overpressure 0 0.25 0.50 0.75 1.0 porosity sands & sandstones mud clay Anomalously high f, low vP, vS, and other properties may indicate OP clay & shale, “normal” line mud- stone In some cases, 28% f at depths of 6 km! shale effect of OP on porosity 4-8 km +T depth slate (deep)

  10. Permeability and Depth 5 10 15 20 25 Permeability – k – Darcies • Muds and shales have low k, < 0.001 D, and as low as 10-10 D • Exception: in zones of deep fractured shale, k can approach 0.1-1 D • Sands decrease in k with z • Exception, high f sands in OP zones can have high k • Anhydrite, salt k = 0! • Carbonates, it depends 0 1 2 3 4 5 Muds and Shales Sands and Sandstones Intact muds and shales have negligible k Depth – z – 1000’s ft High porosity OP sands have anomalously high porosity & permeability Fractured shales at depth may have high fracture permeability

  11. Abnormal po Causes • Delayed compaction of thick shale zones • Water is under high pressure • Leak off to sands is very slow (low k) • Thermal effects (H2O expansion) • Nearby topographic highs (artesian effect) • Hydrocarbon generation (shales expel HCs, they accumulate in traps at higher po) • Gypsum dewatering ( anhydrite + H2O) • Clay mineral changes (Smectite  Illite + H2O + SiO2) • Isolated sand diagenesis (Df, no drainage)

  12. Mechanisms for OP Generation Compaction = H2O expelled to sand bodies, especially from swelling clays Mud, clays 0-2000 m Sand H20 H20 H20 Shale 2000-4000 m Montmorillonite = much H2O Sandstone Diagenesis 4000-6000 m Illite Kaolinite Chlorite Compaction and Clay Diagenesis + Free H2O + SiO2

  13. Mechanisms for OP Generation • Artesian effect (high elevation recharge) • Thrust tectonics (small effect) • Deep thermal expansion rain clays and silts Artesian charging 3-10 km Artesian charging is usually shallow only Thrusting can lead to some OP +DT = +DV of H2O: thermal expansion at depth 20-100 km

  14. Offshore: Trapping of OP Listric faults on continental margins lead to isolated fault blocks, good seals, high OP in the isolated sand bodies from shale compaction “down-to-the-sea” or “listric” faults sea stress v h po shale slip planes shale Sand bodies that have no drainage because of fault seals, OP is trapped indefinitely depth Stress reversion zone

  15. HC Generation and OP micro- fissure HCs generated in organic shales sv T, p, s increase shale Semi-solid organics, kerogen, po < sh < sv kerogen sv po = sh < sv, Fractures develop and grow high T, p, s sands Pressured fluids are expelled through the fracture network, po “stored” in OP sands oil and gas fluid flow generation of hydrocarbon fluids

  16. OP From Gas Cap Development Thick gas cap development, perhaps charged from below, can generate high OP A pressures along A-A stress gas cap, low density gas cap effect oil, density = 0.75-0.85 A Gas migration along fractured zones, faults, etc. Fractured rock around fault sh po Deep gas source depth Gas rises: gravitational segregation

  17. Abnormal Pressure – Sand-Shales • Overpressure is often generated due to shale compaction and clay diagenesis • Montmorillonite (smectite) changes to lllite/Chlorite at depth. H20 is generated and is a source of OP. • Pressure is generated in shales, sands accumulate pressure • PF commonly higher in shales than sands • Sand-shale osmotic effects (salinity differences) can also contribute to OP

  18. PF in GoM Sand-Shale Sequences Absolute stress values Stress gradient plot stress shmin sv PF in sand line shmin sv z z shale sandstone shale sandstone limestone shale depth depth Pore pressure distribution, top of OP zone

  19. Some Additional Comments • Casing shoes are set in shales (98%) • The LOT value reflects the higher shmin in the shales, therefore a higher PF • As we drill deeper, through sands, the actual shmin value is less! By as much as 1 ppg in some regions • Can be unsafe, particularly when we increase MW rapidly at the top of the OP zone • You should test this using FIT while drilling

  20. Examination of a “Typical” Synthetic OP Case

  21. Particularly Difficult OP Case 2.0 (16.7 ppg) 1.0 (8.33 ppg) • Deep water drilling, mud heavier than H2O • Thick soft sediments section, PF ~ sh ~ sv • Thin, shallow, gas-charged sand • Zone where sh is roughly unchanged • Sharp transition zone • High OP, 90% of sv • Deep zone of stress and pressure reversion 0 Sea water depth 800 m 1 800 m soft sediments 2000 m medium stiff shales and silts 2 po sh sv 3 seal sharp transition 4 1400 m OP zone 5 Reversion zone 6 Z – kilometers (3279 ft/km)

  22. Upper Part of Hole 2.0 (16.7 ppg) 1.0 (8.33 ppg) • The vertical lines are several MW choices • Riser and first csg. MW • 9.16 ppg does not control gas, but only fractures above 950 m • 10.0 ppg controls gas, but losses above 1200 m will be a problem. It does allow deeper drlg. • Solution, riser seat at ~1000 m • Casing shoe at ~1400 m 0 Sea water - 800 m 9.16 ppg 10.0 ppg 800 m soft sediments 1 2 Medium stiff shales and silts Z – kilometers (3279 ft/km)

  23. Riser Issues in this Example • Sea water is ~ 1.03 ~8.6 ppg • At great depth, MW may be as high as 2.02 (17 ppg) if the riser is exposed fully • The D-pressure at the riser bottom is very large: 800m  9.81  (2.02 – 1.03) = 7.8 MPa • The riser must be designed to take this • Or, special sea-floor level equipment must be installed • Special mud lift systems from the sea floor, etc.

  24. Approaching the Transition Zone 2.0 (16.7 ppg) 1.0 (8.33 ppg) 0 • LOT of 1.3, 10.83 ppg • This limits us to 3.6 km for the next casing • However, this will require a liner to go through transition zone • Liner from 3600 m to 3750 – 3800 m • If it is possible to drill 100 m deeper initially, to 3700 m, we may save the liner ($1,000,000) Sea water - 800 m 800 m soft sediments 1 2000 m shales and silts 2 po sh sv 3 sharp transition 4 OP zone Z – kilometers (3279 ft/km)

  25. Solution A: Casing or Liners 2.0 (16.7 ppg) 1.0 (8.33 ppg) 0 • This is the most conservative, safest, and the most costly • Black line is MWmax • If shale problems occur in the 1.6-3.6 km shale zone, requiring an extra casing… (i.e., little margin for error) Sea water 1 2000 m shales and silts 2 po sh sv 3 4 OP zone Z – kilometers (3279 ft/km)

  26. Sol’n B: Drill OB With LCM? 2.0 (16.7 ppg) 1.0 (8.33 ppg) 0 • Dashed line is from the previous slide • Drilling with the purple line, saves a liner! • This is ~1.2 ppg OB at the shoe (quite a bit!) • Place upper casings deeper if possible • Drill with LCM in mud (see analysis approach in Additional Materials) • Place a denser pill at final casing trip • (Approach with caution) Sea water 1 2000 m shales and silts 2 po sh sv 3 4 OP zone Z – kilometers (3279 ft/km)

  27. Solution C: Deeper Upper Casings 2.0 (16.7 ppg) 1.0 (8.33 ppg) 0 • 300 m subsea primary casing depth • Casing at 1850 m depth • Drill long shale section with MW shown as dashed black line • Increase MW only in last 100 m (LCM to plug ballooning at the shoe) • Slight OB of 0.2-0.3 ppg needed • Casing may be saved (?) Sea water 1 2 Slight OB needed 3 sh sv po OP zone 4 Z – kilometers (3279 ft/km)

  28. Deeper Upper Casing Shoes • Depending on the profile of OP stresses and pressures, this approach can be effective, but in some cases it is not • Of course, the best approach is always to place the shoes as deeply as possible • This may give us a one-string advantage deeper in the well if problems encountered • At shallow depths (mudline to ~4000 ft), use published correlations with caution because there are few good LOT data

  29. Comments on the Approaches • There is risk associated with saving a casing string: risks must be well-managed … • The stress/pressure distribution sketched is a particularly difficult case: • Shallow pressured gas seam at 1500 m subsea • PF (sh) is quite low around 3000 m subsea • Transition zone is very sharp (~250 m) • OP is high (88-90% of sv) • However, it could even be worse! • More gas zones, depleted reservoirs at 3.6 km • Etc…

  30. Drilling Through a Reversion Zone • Below OP, usually a zone where po, sh (PF) gradually revert to “normal” values. This is rarely a sharp transition as at top of OP • This is related to fractured shales that “bleed off” OP (i.e. lower OP seal is gone) • Also, when shales change and shrink, the sh value (PF) drops as well • “Reverse” internal blowout possibility • Blowout higher in hole • Fracturing lower in hole

  31. Stress Reversion at Depth vertical stress, v horizontal stress, h pore pressure, po 4 km stress (or pressure) Note that hmin can become > v Region of strong overpressure depth Stresses “revert” to more ordinary state Higher k rocks (fractured shales) Z

  32. Same Example… 2.0 (16.7 ppg) 1.0 (8.33 ppg) • OP casing was set at 3800 m depth • Drill with 16.7 ppg MW • At 5.5 km, large losses • If we reduce MW, high po at 4.6 km can blow out, flow to bottom hole at 5.5 km (reverse internal BO) • Set casing at 5450 m • Drill ahead with reduced MW 4 1400 m OP zone 5 Reversion zone po sh sv 6 Z – kilometers (3279 ft/km)

  33. Real Deep Overpressure Drilling • Watch out for shallow gas sands • Dark black line: MWmax for the interval • Dashed black line is the actual drilling MW • Red stars: excessive shale caving, blowouts • Green stars: ballooning and losses • Surface casing string not drawn on figure This is a deep North Sea case, west of Shetlands

  34. Detecting OP Before Drilling • Seismic stratigraphy and velocity analysis • Anomalously low velocities, high attenuations • Can often detect shallow gas-charged sands (unless they are really thin, < 3-5 m) • Geological expectations (right conditions, right type of basin and geological history…) • Offset well data, good “earth” model, so that lateral data extension is reliable

  35. Detecting OP While Drilling • Changes in the “Dr” exponent, penetration rate may increase rapidly in OP zone • Changes in seismic velocity (tP increases) • Changes in porosity of the cuttings (surface measurements or from MWD) • Changes in the resistivity of shales from the basin “trend lines” • Changes in the SP log • Changes in drill chip and cavings shapes, also volumes if MW < po • Mud system parameters, etc

  36. Comments on LWD • Methods of data transmission… • Mud pulse – 2 bits/s @ 30,000’, 12-25 b/s is good at any depth • Issues in data transmission: • Long wells, extended reach • OBM, electrical noise, drilling noise • ID changes in the drill string • Pump harmonics, stick/slip sources • “Wire” pipe – extremely expensive • High rate on out-trip, then download on rig • New technologies will likely emerge soon…

  37. Reasons for Pore Press. Prediction Drilling Problems Due to Pressure Imbalance: • Overbalance: Slow drilling, Differential Sticking, Lost circulation, Masked shows, Formation damage. • Underbalance: Imprudently fast drilling, Pack- offs, Sloughing shales, Kicks, Blowouts.

  38. Pore Pressure Prediction Basics I • Data from offset wells • Logs, Dr data, sonics, neutron porosity, resistivity, etc. • Transfer data to new well stratigraphy, z • Plot sv gradient, sonic transit time, Dr, resistivity, porosity, etc. with depth • Use trend analyses and published methods, to determine the “normal compaction line” • Use an Eaton correlation chart if you have it for this area (use offset and other data) • This is the prognosis profile for new well

  39. Pore Pressure Prediction Basics II • With seismic data and geological model of the new well region, assess: • Existence of OB conditions (seals, sources…) • Existence of faults, salt tectonic features… • Plot depth corrected velocities on profile: • Carefully compare the two: • Lower velocities = greater OP risk… • Explain existence of any undercompacted zones and anomalies you have identified • You now have as good a prognosis as you can develop with existing data

  40. Sonic Transit Time Differences 2.0 (16.7 ppg) 1.0 (8.33 ppg) Log of sonic transit time 0 Sea water depth 800 m 650 ms/m 1 Normal trend from the basin, offset data Soft seds. Seismic velocity model Stiff shales and silts 2 po sv 3 Sonic transit time from offset wells seal Expected OP transition PROGNOSES FROM OFFSET WELL DATA, CORRECTED FOR Z, ETC… 4 Critical region OP zone 5 Reversion zone 6 Z – kilometers (3279 ft/km)

  41. Prognoses Based on Seismics Normal compaction line for the basin General seismic profile data, depth corrected for new well Corrected sonic transit time, calibrated with the general seismic velocity data Regions of substantial deviation are highlighted as “critical”, experience used to choose likely top of OP OP magnitude estimated, based on correlations OP beginning Large OP expected

  42. Seismic Cross-Sections • Depth Converted • 1:1 Horizontal / Vertical Ratio • Offset Well Ties (Regional) • Planned Wellbore (Local) • Full Structural Picture • Fully Annotated • Radial Animation

  43. North Sea Seismic Section - Diapir Well A 1b Gas Pull Down Mid-Miocene regional pressure boundary Top Balder Top Chalk Intra Hod/Salt Courtesy Geomec a.s.

  44. Other “Trend Line” Approaches • Methods exist for using trend analysis for many different measures, including: • Drilling exponent data • Resistivity trends lines (salinity of strata) • Deviations from expected porosity (less sensitive) • SP log characteristics • Perhaps some others… • Shale data are used because sand porosity is less “predictable” in general

  45. Gas Cutting of the Drilling Mud • Shale behaves plastically at elevated pressure and temperature gradients. • Significance (and insignificance) of gas cut mud (GCM). Gas from CH4 in shales? • Very large gas units: 2,000 to 4,000 units ? • Connection gas (CG) - better indicator. Use it for well to talk. Ineffective when too much overbalance. • CG increase from 20, 40, 60 to 80 points. Yes, you are underbalanced.

  46. Is MW a Pressure Indicator? • No. The lower limits of MW in most OP regimes are related to shale stability, rather than to pore pressure • Usually, in difficult shales, 1 to 2 ppg above po is needed to control excessive shale problems • HOWEVER! MW limits from offset well drilling logs are useful to estimate MWmin • Of course, this can change as well: • More inhibited WBM, using OBM instead, etc… • Faster drilling, less exposure, etc…

  47. MWmin Prognosis • Offset well pressure, stress, drilling data… • Estimate target MWmin for new well prognosis • If this generates too narrow a MW window, assess approaches • Will OBM allow a lower MWmin? (on the plot, the dashed blue line is the estimated OBM MW for shale stability) • Other factors?

  48. MWmin, MWmax Well Prognosis 2.0 (16.7 ppg) 1.0 (8.33 ppg) • Use a rock mechanics borehole stability model, calibrated, to estimate MWmin from geophysical logs and lab data • Use offset well losses, ballooning, LOT, etc. to estimate MWmin • This defines the local “safe” MW window • Now, combine with casing program prognosis to plan the MW for the well 0 Sea water depth 800 m 1 Soft seds. Weak rocks Stiff shales and silts 2 sv 3 po Expected OP transition PROGNOSES FROM OFFSET WELL DATA, CORRECTED FOR Z, ETC… 4 OP zone 5 Reversion zone Strong rocks 6 Z – kilometers (3279 ft/km)

  49. During Drilling… • Remember, in OP drilling we are trying to “push the envelope” to reduce casings • Update the well prognosis regularly with actual LOT, MWD, ECD data • Monitor, measure, observe… • Kick tolerances, ballooning behavior, gas cuts • Chip morphology and volumes • Flow rate gauges on flowline, pumps • Mud temperature monitoring MWD temperature • Sticky pipe, torque, ECD, mud pressure fluctuations • Cuttings analyses: vP, Brinnell hardness are used

  50. Increasing Depth of Casing Shoe 1.1 1.3 1.5 1.7 1.9 2.1 2.3 (2.0 = 16.7 ppg) density, g/cm3 prognosis for shmin MW =1.92 Previous casing string prognosis for po sv XLOT shmin value shoe overpressure transition zone deeper shoe for casing string! area indicates possible MW strong overpressure zone depth Using high weight trip pills and careful monitoring, the lower limit can be extended

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