Module L:More Rock Mechanics Issues in Drilling Argentina SPE 2005 Course on Earth Stresses and Drilling Rock Mechanics Maurice B. Dusseault University of Waterloo and Geomec a.s.
“Predicting” Onset of Instability • Now, we have methods of estimating in situ stress conditions • Also, we have methods of measuring or estimating strength • Furthermore, we have methods of calculating stresses around a circular opening, subject to several assumptions… • Putting this together allows prediction of shearing initiation on the borehole wall • …An estimate of “breakouts initiation”
Linear Poroelastic Borehole Model… • Eqn: • Where: • pw]cr critical wellbore pressure, shear initiation • pi pressure just inside the borehole wall • σ1, σ3 largest, smallest ppl σ in borehole plane • A = α(1-2)/(1-) ( = Poisson’s ratio) • α Biot’s coefficient (1.0 for soft rocks) • N friction coefficient = (1 + sin’)/(1 - sin’) • UCS, Unconfined Compressive Strength, friction angle (MC yield criterion) • Δp “drawdown” = pi - po
Discussion of Parameters pw pi po • pw – pi is support pressure • Usually, we ignore effects of “α”, except in low porosity, stiff shales (E > 30-40 GPa) • UCS and N are equivalent to the c’, ’ of the linear MC yield criterion for shear • Poisson’s ratio for shales, 0.25 to 0.35 • σ1, σ3 are computed using equations converting 3-D stress to stresses in the plane of the borehole (90° to hole axis) radius - r
Control Parameters in Drilling • Mud weight, mud rheological properties, the geochemistry of the filtrate, cake quality, mud type (WBM, OBM, foam, etc.) • LCM content, type and gradation • Tripping and connection practices: • Surging (run-in), swabbing (pull-out) pressures • Drilling parameters: • ROP, bit type… • Hydraulics and hole cleaning • ECD (BHA characteristics, mud properties) • Well trajectory, and maybe a few others
Defining Limits in Our Well Plan Gradient Pressure or stress Predicted MW for onset of unmanageable sloughing shmin, danger of LC sv Onset of ballooning in shale zones sv po, onset of blowout if in a sand zone Depth Depth
How are the Limits Defined? • Lower MW limit • Pressure control • Rock Mechanics stability, experience, use of correlations to predict stability line, etc. • How much sloughing can we live with? • Underbalanced Drilling is a good example of RM • Upper MW limit • Avoiding massive lost circulation • Fracture gradient, earth stresses analysis • Effects on ROP • The new concept of overbalanced drilling is an example of RM extending this envelope
Are All Limits Absolute? • No, and here are examples: • Drilling underbalanced? OK as long as it is shales or lower permeability sands, and if the shales are strong (little sloughing) • Drilling overbalanced? OK for up to ~1000 psi with properly designed LCM in mud! • Drilling below sloughing line? OK if good hole cleaning, use increased MW for trips… • Pushing the envelope is typical in offshore drilling, HPHT wells… (e.g. mud cooling…) • Vigilance and RM understanding needed…
Example: Drilling Underbalanced • It is a Rock Mechanics issue, a pore pressure issue, and a fluids type issue • If the shale is strong enough to be self supporting in a bore hole with a negative r • If the pore pressure is not so high that it “blows” sand and shale into the borehole • If the fluids that enter the hole are “safe”, i.e., not oil and gas in large quantities • Excellent for drilling through depleted zones, fast drilling through good shale, entering water sensitive gas-bearing strata, reservoirs that are easy to damage
Underbalanced Stress Conditions s – stress sq High shear stress at the borehole wall shmin = sHMAX sr po pw < po pw radius Some tensile stress exists near the hole wall in underbalanced drilling because po > pw
Mud Rheology • High gel strength can cause mud losses on connections, trips • Increases surge and swab effects when BHA is in a small dia. Hole • Also affects ECD • Mud rheology & density can be changed for trips to sustain hole integrity • Hydraulics is a vital part of borehole stability! Mud Rheology Diagram Static condition m – mud viscosity Shearing resistance Yield point YP Dynamic conditions Shearing rate
Effect of Mud Weight Increase , shear stress MC failure line yield Mohr’s circle of stresses no yield c n, normal stress r a Increasing MW (with good cake) reduces the stresses on the wall
Effect of Loss of Good Filter Cake , shear stress MC failure line failure Mohr’s circle of stresses c n, normal stress r a With loss of mudcake effect, radial support disappears, shear stress increases
Stresses and Drilling sv sHMAX ~ sv >> shmin To increase hole stability, the best orientation is that which minimizes the principal stress difference normal to the axis 60-80° cone sHMAX shmin sv Favored hole orientation sv Drill within a 60°cone (±30°) from the most favored direction sHMAX sHMAX shmin shmin sHMAX >> sv > shmin sv >> sHMAX > shmin
Uncontrollable Parameters • Constrained trajectory (no choice as to the wellbore path) • Sequence of rock types (stratigraphy) • Rock strength and other natural properties • Fractured shales • Clay type in shales (swelling, coaly, fissile) • Salt, etc. • Formation temperatures and pressures, plus other properties such as geochemistry • Natural earth stresses and orientations
Can You Live with Breakouts? • Yes, in most cases the breakouts are a natural consequence of high stress differences, and can be controlled • In exceptional cases, the breakouts are so bad that massive enlargement takes place • If hole advance is necessary, there are special things that can be done: • Some new products, silicates, polymers that set in the hole and can even be set and then drilled • Increase MW, even to the point of overbalance • Gilsonite and graded LCM can help somewhat • In desperation, set casing!
Some Diagnostic Hole Geometries d. a. General sloughing and washout Swelling, squeeze b. sHMAX drill pipe Keyseating e. shmin Breakouts Fissility sloughing Induced by high stress differences c. c. f. Only breakouts are symmetric in one direction with an enlarged major axis
Equivalent Circulating Density • Viscous resistance increases the apparent mud weight at the bottom of the hole • This is a kinematic (viscosity) effect, and takes place only as the mud is circulating • ECD can lead to fracture at the bit though static pressure of mud column is below PF • As high as 2.0#/gal recorded in 4¾” hole! • Real-time BHP pressure data allow it to be measured and managed (offshore drilling) • This leads to early warnings of high ECD • This leads to better control and mitigation
ECD Pressure gradient plot 15 16 17 18 19 ppg PF (shmin) MW = 16.7 ppg (static value) reamers and stabilizers mud rings also increase ECD Dynamic pressure (ECD) because of friction, hole restrictions, high mud m A hydraulic fracture is induced at the base of the hole where the ECD exceeds PF (shmin). When the pumps stop, much of the mud comes back into the hole! BHA and collars High ECD! Depth
ECD • pBH = mud weight plus friction Dp loss • High ECD values (>0.5 ppg) are related to: • High mud viscosities and gel strengths (evident on connections and trips as “breathing” of hole) • Rapid slim hole drilling leading to large cuttings loads in the drilling fluids near the bit • Limited clearance with BHA (MWD system), reamer system, extra large collars… • Sloughing of shales leading to partial mud rings or high cavings loads in the mud • Reducing ECD is the same as expanding your safe MW window for drilling!
High ECD Effects 15 16 17 18 19 ppg Gradient plot PF = shmin po Top of restrictive BHA MW = 16.7 ppg (static value) reamers mud rings Dynamic pressure (ECD) because of friction, hole restrictions, high mud m Cannot reduce the MW much because of borehole instability uphole or blowout danger on trips, connections, gas cutting… BHA and collars stabilizers Large mud losses at hole bottom because of fracturing High ECD! Depth
Reducing High ECD Values • High ECD: excessive ballooning, high losses, increased risk, reducing the drilling window • The high ECD values can be reduced in several ways, here are a few examples: • Reduce the mud weight (careful about gas cuts!) • Reduce the viscosity and gel strength • Avoid sloughing above bit (increases ECD) • Circulate out cavings and cuttings as needed • Use less restrictive BHA, reduce ROP • Use an off-center bit (lower friction losses) • Redesign well plan (one less casing, larger hole) • OBM probably somewhat better than WBM
North Sea ECD Example • Serious ECD problems, but extra depth needed • Very long & restrictive BHA was being used • Drill (mud motor) to Z with 8.5” hole size • Trip out, replace bit with eccentric 9¾” bit • Ream to bottom & trip • Drill to TD with the 8.5” drill bit size • Set 7” casing to TD 10¼” casing High ECD Underream Drill to TD
Some Other Comments on ECD • If high drill chip loads from rapid ROP are contributing to ECD, reduce ROP • Lower viscosity and gel strength during drilling, but increase it a bit for trips • Break the gel strength of the mud during trips by pumping, rotating pipe as you are breaking circulation • Be careful in inclined and horizontal holes where pipe is not being rotated much, better to rotate more aggressively • Use LCM in mud to plug fractures
ECD Services • Example of output from BHI service • MWD gauges used • Gives ECD, MW, annular pressure, connection effects… • This data can be used in a diagnostic manner during drilling to manage ECD and aid well performance • This website gives many useful formulae http://www.tsapts.com.au/formulae_sheets.htm
Drilling and Shale Fissility • If a hole is within 20° of strong fissility… • Sloughing is more likely • Shale breaks like small brittle beams • Breakouts can develop deep into strata • In this GoM case, in the “tangent” section, the hole angle was 61° • Vertical offset hole, no problems bedding direction Courtesy Stephen Willson, BP
Coping with Fissile Shale Sloughing • If possible, stay at least 30° away from the fissility dip direction (see sketch) • Otherwise, keep your mud properties excellent, keep circulation rate & ECD low, gilsonite and fn-gr LCM in mud may help… Keep the drillhole within this cone to avoid severe fissility sloughing problems 100-120° cone Normal to bedding planes
DENSITY NEUTRON IMAGE OF 12500’ MD SHALE BREAK OUT From: Bruce Matsutsuyu SECTION OF SHALE BREAKOUT Note that the majority of the shale sloughing appears to be from the top of the borehole. PHOTOELECTRIC FACTOR CURVES DENSITY CURVES BOTTOM OF BOREHOLE GR Density Neutron Image
Drilling Through Faults • The fault plane region is often: • Broken, sheared, weak shales and rocks • It may have a high permeability • It can be charged with somewhat higher po • First, expect the faults from your data: • Seismic data analysis • Near salt diapirs, especially shoulders • Accurate mud DV(t) measurements can be of great value to good drilling • Cavings monitoring • MWD (ECD, resistivity, bit torque…)
Borehole Shear Displacement • High angle faults, fractures can slip and cause pipe pinching • Near-slip earth stresses condition • High MW causes pw charging • Reduction in sn leads to slip • BHA gets stuck on trip out • Can be identified from borehole wall sonic scanner logs (profile logs) • Raising MW makes it worse! Lowering MW is better… • Also, LCM materials to plug the fault or joint plane are effective pw sn
Slip of a High-Angle Fault Plane borehole sv = s1 casing bending and pinching in completed holes sh = s3 high pressure transmission pipe stuck on trips slip of joint surface slip of joint (after Maury, 1994)
Slip Affected by Hole Orientation! OFFSET ALONG PRE-EXISTING DISCONTINUITIES FILTRATE TYPICAL MUD OVER-PRESSURE Courtesy Geomec a.s.
Diagnostics for Fault Slip Problems • In tectonic areas, near salt diapirs… • On trips, BHA gets stuck at one point • Easy to drop pipe, hard to raise it • Borehole scanner shows strange shapes: not the same as keyseating or breakouts drill pipe Start of keyseat Serious keyseat Evidence of fault plane slip
Curing Fault Plane Slip Problems • Usually occurs up-hole in normal faulting regimes that are highly faulted, jointed, as MW is increased to control po downhole • May occur suddenly near the bit when a fault is encountered • Back-ream through the tight zone • High pw contributes to the slip of the plane, thus reduce your MW if possible • Condition the mud to block or retard the flow of mud pressure into the slip plane: • Gilsonite, designed LCM in the mud • Use an avoidance trajectory for the well
Mud Volume Measurements • Extremely useful, but, accurate DV/Dt needed • Case A: fracture/fault encountered, quickly blocked, now analyze data for k and aperture! • Case B: fractured rock not healed by LCM • Other cases have their own typical response curves (ballooning, slow kick…) • Diagnostics! Losses - gpm 20 A 15 10 5 Hole deepening rate Filtration fluid loss Time - min 0 5 6 7 8 9 B Losses - gpm 20 15 10 5 Hole deepening rate Filtration fluid loss Time - min 0 5 6 7 8 9
A Precise Mud Volume Installation (taken from SPE 38177 - Agip well ) Outlet mud line Precision flow meter
Actual Field Example of Analysis Hydraulic Aperture (mm) Average permeability (D/m) 0 20 40 60 0 0.5 1 2890 2890 2910 2910 2930 2930 2950 2950 Depth (m) 2970 2970 2990 2990 3010 3010 3030 3030 3050 3050 Courtesy Geomec a.s This information proved extremely valuable for reservoir engineers in this case, as a gas reservoir was found
Losses Identify Fractured Zones Mud Loss Rate – litres/min Likely, each event involved filling a single fracture Depth - m
Problems in Coal Drilling • OBM are worse than WBM in Coal • Filtrate penetrates easily (oil wettability) • Coal fractures open easily if pw > po • Coal is extremely compressible • Difficult to build a filter cake on the wall • Fissure apertures open with surges • Sloughing on trips, connections, large washouts, … • Packing off of cuttings and sloughed Coal around the pipe, even during trips
Drilling in Coal sq sr stresses around wellbore Mud rings and pack-off caused by slugs of cavings and cuttings Deep pore pressure penetration because of coal fractures Massive sloughing fracture-dominated coal
Drilling Fractured Coal Safely • Keep jetting velocities low while drilling through the coal (avoid washouts) • Keep MW modest to avoid fractures opening and coal pressuring, low ECDs while the BHA is opposite the coal seams • Drill with graded LCM in the mud to plug the fractures and build a cake zone • Avoid swabbing and surging on trips • See Appendix to Module H for some results on drilling overbalanced with LCM
North Sea Case, Shallow Depth Well A 1a Shallow Gas 2000 m Gas Pull Down Courtesy Geomec a.s.
Above a Deep Diapir, North Sea • Normal faulting observed well above the top of the diapir, these will likely be zones of substantial mud losses (low shmin) • Beds are distorted, likely shearing has occurred along the bedding planes (weaker) • Seismic data show strong “gas pull-down effect”, lower seismic velocities because of free gas in the overlying shales and high po • Free gas zones are noted in the strata, and these will increase gas cuts • (Gas “pull-down” refers to the effect of free gas on seismic stratigraphy)
Deeper, Around the Diapir This region avoided Well A 1b Gas Pull Down 2000 m Mid-Miocene regional pressure boundary Top Balder Top Chalk Intra Hod/Salt 3000 m Courtesy Geomec a.s.
MWD RESISTIVITY LOG SIGNATURE (OBM) Invaded Zone Courtesy Geomec a.s. Time-lapse and different spacing resistivity logging data identified fractured zone clearly
INVADED ZONE Symmetry(O-B) Courtesy Geomec a.s.
What Was Done to Improve Drlg? • A trajectory was chosen to avoid the worst of the crestal faulting and gas pressures • Shales also intersected at ~ 90 to fissility • Mud losses were carefully monitored with depth in the critical zones, then analyzed • Designed LCM in the mud allowed a bit of overbalance in a critical region • Of course, gas cuts, shale chip geometry, total cutting volumes, etc., and many other things were monitored in “real-time”
Statfjord Case: North Sea STATFJORD 6 5 Mud Pressure minus stress in MegaPascals 4 3 2 1 0 B-06B B-23AT2 B-39A B-39BT2 Well OVERBALANCED! -800 psi These wells were drilled with overbalance: a MW above the lowest estimated shmin in the zone Courtesy Geomec a.s.
Conclusions • Fracturing pressure can be increased by several 100 psi by graded LCM, analysis • Young’s modulus (E) is the control parameter • Induced fractures or even natural fractures encountered open up almost immediately to their final width: • This aperture controls LCM design • The plugging happens rapidly with right LCM • The effect is enhanced with high viscosity mud and slower ROP • Design tools are available for this