Wellbore Stability Self Learning Package
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SUGAR LAND LEARNING CENTER Wellbore Stability SELF-LEARNING COURSE
USEFUL PRE-REQUISITES Basic understanding of drilling terms and procedures Stuck Pipe Self Learning Package
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Table of Contents OBJECTIVES………………………………………………………………………….. 3 THE STRESS IN THE EARTH BEFORE WE DRILL A BOREHOLE……………...…………….……4 THE STRESS IN THE EARTH AFTER WE DRILL A BOREHOLE….…………………….…...……..8 ROCK FAILURE……………………………..………………………………………………..……...….10 REVIEW QUESTIONS I…………………………………………………………………………..……..14 WELLBORE STABILITY PLANNING AND PREVENTATION…………………………….………..15 REVIEW QUESTIONS II…………………………………………………………………………….…..29 ANSWERS TO REVIEW QUESTIONS …………………………………………………………….…..30
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Objectives Upon completion of this training module you should be able to:
Describe the stresses in the earth before we drill a borehole Describe the stresses in the earth after we drill a borehole Describe the different types of rock failure Describe the characteristics of a mini-frac Describe the 2 main outputs of wellbore stability planning Understand the differences between Tabular, Angular and Splintered Cavings Describe the common wellbore monitoring techniques and the 4 most common wellbore instability mechanisms Describe remedial actions that are taken to fix a failed / failing wellbore
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The Stress in the Earth before we Drill a Borehole
Before we drill a borehole the rock in the earth is in a state of equilibrium. This state is called the “Initial State”.
In the earth, there are 3 stresses that are perpendicular to each other:
σv Principal Stress in vertical axis σh Principal Stress in horizontal axis σH Principal Stress in horizontal axis
σV σh
σH
σH is the maximum of the 2 horizontal stresses and σh is the minimum. (ie σH > σh )
In Rock Mechanics we also describe earth stresses in order of magnitude:
σ1 Maximum Earth Stress σ2 Intermediate Earth Stress σ3 Minimum Earth Stress
These can be ordered in any way: for example σ1 could be the vertical stress or one of the horizontal stresses, depending on the sedimentary basin in which we are drilling.
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Normal Fault Regime
σv = σ1 Steep sloping
σh = σ3
σH = σ2
Figure 1: Tectonic dependence on earth stresses
Thrust (Reverse) Fault Regime
σv = σ3 Gentle sloping
σH = σ1 σh = σ2
Slip Fault Regime
σv = σ2
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σh = σ3
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The earth’s stresses are related to a number of different variables including: Tectonic Setting, Pore pressure, Depth, Lithology, Temperature, Structure The tectonic setting can affect the relationship of the earth’s stresses. Consider figure 1.1 a) In a Normal Fault Regime, the vertical stress (σv) is the maximum principal stress (σ1):
σv > σH > σh
b) In a tectonically stressed regime, horizontal stress (σH) is the maximum principal stress (σ1):
σH > σh > σv
c) Slip fault regime, the horizontal stress (σH) is the maximum principal stress (σ1):
σH > σv > σh
Pore Pressure supports a portion of the total applied stress in a rock. In general: Total stress (in given direction) = Effective Stress of Rock Grains (given direction) + Pore Pressure
If a formation is “normally pressured” the pore pressure mechanism can be described as following: Sediment burial → full pore fluid escape → porosity decreases → effective rock stress increases → pore pressures are hydrostatic (normal) If a formation is “over-pressured” the pressure in the formation is greater than the pressure exerted by a column of water at that same depth. There are 2 main mechanisms causing overpressure: a) Loading mechanisms: Sediment burial → pore fluid escape fully restricted → porosity & effective stress are both constant → pore pressures increases at the same rate as the overburden (ie overpressure) b) Unloading mechanisms7: (i) Aquathermal expansion or hydrocarbon generation or mineral dehydration (smectite→illite) or osmosis → sealed formation → fluid-volume increase can result in rapid pore pressure increases that unload the rock grain matrix. (ii) Uplift / Erosion → unloading rock grain matrix → sealed formation → formation has same pore pressure as before but due to closed system is abnormally pressured compared with neighbor formations at same depth.
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Figure 2: The 3 Wellbore Stresses
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The Stress in the Earth after we Drill a Borehole Before a wellbore is drilled the rock is in a state of equilibrium. This state is called the “Initial State”. The stresses in the earth under this condition are known as the Far Field Stresses (σh , σH , σv ) or in-situ stresses. When a well is drilled it introduces a perturbation in the initial stress field. The perturbation causes a ‘new’ set of stresses known as wellbore stresses that act on the formation at the wellbore wall. There are 3 wellbore stresses. These are:
• • •
Radial Stress Tangential Stress Axial Stress
Figure 2 shows these 3 wellbore stresses. The wellbore stresses depend on 2 different things: a) The mud weight used b) The magnitude of the far field stresses (σv , σH and σh)
If we know what these wellbore stresses are then we will have a better idea of whether a borehole will fail when we drill it.
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Figure 3: The 2 different ways a rock can fail
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Rock Failure Generally a rock can fail in 2 different ways:
a) Shear Failure: This is caused by 2 perpendicular stresses that are different in magnitude.
b) Tensile Failure: This is caused by one stress exceeding the tensile strength of the rock.
Figure 3 shows schematically a shear failure and a tensile failure.
Both of these failures can cause wellbore instability. When a rock fails by either shear or tensile failure, 2 things can happen depending on the type of shear/tensile failure: a) Loss circulation can occur (due to mud losses in the cracks of the rock) b) Stuck pipe can occur (pack off due to the borehole collapsing)
We need to prevent these failures from occurring (if we can) to minimize the amount of Non Productive Time (NPT)
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Formation Breakdown Pressure pbd Pumping stops Tensile Strength To Leak off Pressure
Fracture opening pressure
Fracture Closure Pressure =
σh
Time
Figure 4: A Mini-Frac Test
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Figure 4 shows an example of a mini-frac. The y-axis shows the wellbore pressure (ie the mud weight). The formation is basically broken down and the pressure trace is examined – from this we can determine certain properties of the rock and this will give us geomechanical information that will ultimately help us manage wellbore stability. It can be seen that there is a linear trend (the elastic region) until The Leak Off Pressure. At this point (the Leak off Pressure) the plot deviates from the straight line; the formation grains start to move apart and take mud. The formation is on the threshold of moving from an elastic state to a plastic state. The Formation Breakdown Pressure pbd represents the “maximum strength” of the rock before it breaks. This will be equivalent to the pressure exerted by the mud in the borehole. The tensile strength To of the rock is the corresponding Tangential Stress at this mud weight. (For simplicity of this SLP we will neglect Axial and Radial Stress). Therefore, the condition for tensile failure is when the tangential stress is equal to the tensile strength of the rock.
Figure 5 shows some examples of borehole failure from RAB images. swbo, ssko and shae are examples of shear failures tver is an example of a tensile failure (a vertical fracture in this case)
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Figure 5: Borehole Failure in RAB images 13
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Review Questions I 1)What is the relationship between the earth stresses while drilling in a tectonically active region? 2) What are the 2 main mechanisms that cause a formation to be overpressured ? 3) What are Wellbore Stresses and what do they depend on ? 4) Describe the 2 ways that a rock can fail 5) What is the difference between the Leak off Pressure and Formation Breakdown Pressure ?
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Wellbore Stability Planning and Preventation Wellbore instability / Rock Failure is undesirable because it can lead to Non Productive Time (NPT) such as:
• • • • • •
Pack offs (formation failure leading to excess of cuttings) Excessive trip and reaming time Mud losses Stuck Pipe and BHAs → Loss of equipment / Fishing / Sidetracks Inability to land casing, casing collapse Poor logging and cementing conditions
These can be caused by the following: breakouts, sloughing, natural fractures/weak planes, drilling induced fractures, faulting, undergauge hole, interbedded sequence, overpressured formation, unconsolidated formation, mobile formation, permeable formation, chemical activity. Even relatively minor wellbore stability problems in tectonically passive settings can be extremely expensive ($100,000 to $250,000 per day offshore). The key to effective reduction of NPT is planning for wellbore stability. One process used to reduce the NPT is the Mechanical Earth Model. This integrates all geomechanical data available from a field/basin into one “database” which is then used to predict wellbore stability problems that are likely to occur in an upcoming well.
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Increasing Mud Weight
slae ssko tcyl
snbo sdko
Safe Mud Weight Window
Pore Pressure
tver
σh
swbo
shae
Figure 6: Designing a Mud Weight Window
Figure 7: A typical Mud Weight Window (North Sea)
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Horizontal well
Increase the mud weight or increase the risk of shear failure
Mud Weight (g/cc)
Vertical well
S
Sh
Figure 8: Trajectory Analysis for Anisotropic Stress Field, Relaxed Basin (σv is max)
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Two of the most important outputs that emerge from wellbore stability planning are the determination of a safe mud weight window and the safest direction to drill, especially for highly deviated wells. Figure 6 shows that it is often desirable to drill with a mud weight between swbo (a shear failure condition) and σh (the minimum horizontal stress). Figure 7 shows an example from the North Sea where the safe mud weight window should between the black dashed line (Minimum Borehole Stability or shear failure) and the formation propagation pressure (or the minimum horizontal stress). Figure 8 shows that in a relaxed basin it is often safer to drill the well in the direction of the minimum horizontal stress (σh). Also it can be seen that the safe mud weight window narrows as well deviation increases (ie you need to increase the mud weight to keep the wellbore stable but be careful because the maximum mud weight before borehole instability occurs will now be lower). The open hole section of a wellbore must be maintained in a condition that is good enough to allow drilling and casing to be run. This does not mean that it is necessary to eliminate all formation failure. Indeed the wellbore can remain stable even after a period of prolonged formation failure. An example of this is the Cuisiana field, Colombia where the wellbore has remained stable because the cavings from borehole failures can be cleaned out of the hole. In this example the wellbore instability was managed (or contained) rather than prevented. In these cases it becomes difficult to find a solution that will completely prevent the instability from occurring in the first place and wellbore stability management is required: for example, loss circulation might be avoided at all costs, and techniques to manage the shear failure are implemented such as good hole cleaning practices.
Real time Wellbore stability management (control) is a twofold process involving: a) Continuous monitoring – ie downhole/surface signatures to diagnose onset of a problem. b) Remedial actions – ie drilling parameters to fix a failed or failing wellbore.
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a) Continuous monitoring Real Time Wellbore Stability Control relies on an integration of all data available at the Wellsite: Surface signatures: Cavings analysis – Wellbore Failure, Cuttings volume – Hole Cleaning, Pit volumes – Gains (overpressured zone), losses, Surface Drilling Parameters MWD data: Downhole Drilling Parameters DWOB, DTORQ – Friction / Drag ECD behaviour – Hole Cleaning, pack off LWD data: Gamma Ray, Resistivity – Identify zones of potential instability from MEM Sonic – Pore pressure prediction while drilling Caliper measurements – if pattern is forming in some intervals, can identify unstable formations
A reliable diagnosis of the instability mechanism requires use of all available data. If tabular cavings due to natural fracturing are observed then the resistivity log should be checked for evidence of mud invasion into fractures and the mud records require examining for losses. Similarly, if splintered cavings due to over-pressured formations are seen then high gas levels, kicks or mud gains may also be present. The observation of angular cavings due to breakouts requires the debris levels in the hole to be discerned. In all cases, the cavings volume should be compared to the ECD and the degrees of tight hole and restricted circulation to discern the effectiveness of the hole cleaning and the severity of instability. (see cavings analysis on the following pages)
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Figure 9: Tabular Cavings
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Cavings Analysis: An analysis of cavings can provide a signal that the borehole is failing and indicates both the nature of the instability and the troublesome formations. Cavings dimensions range from a few millimetres to 10 cm or more, with larger examples rising to the surface while lodged in the BHA. There are four main types of caving: Tabular, Angular, Splintered Those which cannot be characterized.
Tabular cavings are the result of natural fractures or weak planes. In the case of natural fractures, the fluid pressure in the annulus exceeds the minimum horizontal stress, resulting in mud invasion of fracture networks surrounding the wellbore. This can result in severe destabilization of the near wellbore region, due to the movement of blocks of rock, leading rapidly to high cavings rates, lost returns and stuck pipe. The blocks of rock are bounded by natural fractures planes and, therefore, have flat, parallel, faces. Figure 9 shows examples of tabular cavings due to natural fractures. The other characteristic is that bedding, if any, will not be parallel to the faces of the caving. In the case of weak planes, the combination of low mud weight and a borehole axis that is within approximately 15 degrees of the bedding direction can induce massive failure along the planes of weakness, leading to the symptoms described above. Cavings that are the result of weak planes are characterized by having flat, parallel, faces. The bedding direction is also parallel to the faces.
Angular cavings are a consequence of breakouts. These cavings are characterized by curved faces with a rough surface structure. The surfaces intersect at acute angles (much less than 90 degrees). Figure 10 shows Angular Cavings.
Splintered cavings have two nearly-parallel faces with plume structures. This type of caving is due to tensile failure occurring parallel to the borehole wall and commonly occurs in overpressured zones drilled with a small overbalance. Figure 11 shows Splintered Cavings.
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Figure 10: Angular Cavings
Figure 11: Splintered Cavings
The higher the cavings rate the more severe the failure for a given hole cleaning efficiency. The dominant caving should be noted not the proportion of different cavings. The cavings rate is measured by the time required to fill a bucket placed underneath the shakers. The cavings volume is then proportional to the amount of cavings in the bucket. CARE MUST BE TAKEN – IF HOLE CLEANING IS POOR THERE WILL BE FEWER CUTTINGS
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Chemical Wellbore Instability: Wellbore instability can be classified as either mechanical (for example, failure of the rock around the hole because of high stresses, low rock strength, or inappropriate drilling practice) or chemical. Chemical Wellbore Instability arises from damaging interactions between the rock, generally shale, and the drilling fluid. The integration of understandings of chemical and mechanical damage remains problematical. In wellbore stability monitoring, it is important to determine whether a particular drilling problem is mechanical or chemical in origin. Figure 12 describes how to diagnose the 4 most important wellbore stability mechanisms. 3 of these are mechanical and 1 of these is chemical in origin. The 3 tables that follow show examples of wellbore stability from surface, downhole and miscellaneous signatures.
Figure 12: Diagnosing the 4 most common wellbore instability mechanisms
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Mechanism
Lost Time
Permeable formation
Stuck pipe
Interbedded soft/strong rocks Fault slip/ activation Sloughing
Stuck pipe Stuck pipe, excessive reaming
Overpressured formation Undergauge hole Unconsolidated formation Mobile formation
Chemical activity
Wellbore Trajectory
In-situ stresses
Formation Stength
Pore Pressure
Geology
Low compared to mud pressure Frequent changes
Tortuous
Thick sections collapse more
High stress deviation
Faults present Weak
Proximity to salt dome or faults, tectonically active
Hole fill after trips Hole fill after trips Excessive slack off while tripping Restricted pipe movement Hole fill after trips Problems worsen with time, slight flow Stuck Pipe
Breakouts Drilling induced fractures Closely spaced natural fracs / weak planes
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High High mean stress
Recently crossed fault
Low yield strength Large sand or fractured section
High overburden
Proximity to salt dome, evaporate sequence Low
High stress/stength ratio
Stuck Pipe, hole fill after trips
Planes of weakness
Mud pressure>pore pressure
Figure 13: Wellbore instability – Miscellaneous signatures 24
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Figure 14: Wellbore instability – MWD, LWD & Wireline
Mechanism Permeable formation Interbedded soft/strong rocks
ROP
DWOB Decreases
Frequent & rapid changes
Fault slip/ activation
γ-ray
Thick filter cake
GAPI ≤ 60 GAPI>60, & GAPI ≤ 60 often
Decreases
Low
Overpressured formation Undergauge hole
High, given rock strength
Low
Resistivity
UBI
Frequent & rapid changes
Frequent well diameter changes
Local borehole elongation
Decrease
Low Low High
Caliper
Frequent & rapid changes
Sloughing
Unconsolidated formation
DTOR
Decreases
Borehole enlargement Borehole enlargement Diameter less than gauge Borehole enlargement
GAPI > 60
Hole tightens with time, or dissolves Borehole enlargement
GAPI > 60
High dip (> 60°)
GAPI < 60
Detected. Rotation of breakouts Borehole enlargement Borehole enlargement Diameter less than gauge Borehole enlargement
Decreases
Mobile formation Chemical activity
Decreases
Decreases
Breakouts Drilling induced fractures Close spaced fracs/weak planes
Decreases
Low
Low Low
Borehole enlargement
Decreases
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GAPI > 60 GAPI > 60
Swelling detected
Diametrically opposed &long Fracture & bedding plane orientation
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Orientation & span detected Possible detection Borehole geometry
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Figure 15: Wellbore instability – surface signatures
Mechanism
Pump pressure
Circulation
Mud
Permeable formation Interbedded soft/strong rocks
Gradual decrease
Flow decreases
Water loss, high solids
Spikes
Flow erratic
Spikes Increase
Flow erratic Flow decreases Pit level increase
Fault slip/ activation Sloughing Overpressured formation Undergauge hole Unconsolidated formation
Increase
Volume rate changes frequently Loss Large & flat Background gas high
Flow erratic Spikes Increase Increase
Mobile formation
Flow decreases Flow decreases
Chemical activity
Increase
Flow decreases
Breakouts Drilling induced fractures
Spikes
Flow erratic
Decrease
Flow decreases Flow decreases
Close spaced fracs/weak planes
Cuttings and cavings
Decrease
Salt present, rise in Cl MW & solids increase Apparent loss
Large, brittle, fissile, concave Abrasive & hard Unconsolidated & uncemented Salt grains Soft,water soluble.Gumbo High volume
Hookload
Surface Torque
Increases
Higher
High
Erratic
Packed off
High High when pumps off Large overpull at connections
Increase High
Diametrical wear Packed off
High Large overpull at connections Large overpull at connections Large overpull at connections High
Drillstring
Increase High & erratic Erratic
Undergauge BHA
Erratic
Packed off
Increases High Packed off
Loss Loss at similar weights across field
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Squarish, high volume
High
High
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Frequent Wiper Trips
Infrequent Wiper Trips
Decrease ROP
Increase Mud Gel Strength
Increase Mud Circulation Rate
Limit OD size / Drill Collars
Increase Mud Weight
Decrease Mud Weight
Use Minimum Overbalance (200 psi)
Ensure Overbalance exceeds 200 psi
Add Fluid Loss Agents
Use Inhibitive Mud
Minimize swab and surge affects
Breakouts
1
0
1
1
1
0
1
0
0
0
0
0
1
Sloughing
0
1
0
1
0
0
0
0
0
0
0
0
0
Natural Fractures / Weak Planes Drilling Induced Fractures Fault Activation
0
1
1
1
0
0
0
0
1
0
1
0
1
0
0
0
1
0
0
0
1
0
0
1
0
1
0
0
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0
0
0
0
1
0
1
0
1
Undergauge Hole
1
0
0
0
0
1
1
0
0
0
0
0
1
Interbedded Sequence
1
0
0
1
1
0
1
0
0
0
0
0
1
Overpressured Formation
0
0
0
0
0
0
0
0
0
1
0
0
1
Unconsolidated Formation
0
1
0
1
1
0
1
0
0
0
0
0
1
Mobile Formation
0
1
0
0
1
0
1
0
0
0
0
0
1
Permeable Formation
0
1
0
0
0
0
0
0
1
0
1
0
1
Chemical Activity
0
1
0
0
0
0
0
0
0
0
0
1
0
Figure 16: Actions inhibiting the instability mechanisms. A "1" indicates that the action suppresses the instability. A "0" indicates that the action has no influence on instability or makes it worse.
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b) Remedial Actions If wellbore instability becomes severe as detected from a) continuous monitoring, and hole cleaning cannot remove cavings from the wellbore then the wellbore would be unstable. The ability to deal effectively with the consequences of the unstable wellbore depend on the instability mechanism and its severity. Remedial action generally involves the control of surface parameters (e.g. ROP, RPM, flow rate, mud weight/rheology). For example, if mud losses are currently occurring, but a mud weight decrease is not possible due to conditions that will be encountered while drilling through formations below the current hole bottom (cavings generation), then decreasing the ROP will reduce cuttings loading and therefore the ECD. This may be sufficient to eliminate mud losses and also reduce cuttings loading in deeper intervals The emphasis when considering remedial actions, which either suppress instabilities or minimize their consequences, should be the entire open hole interval, rather than focusing on problem fixing at the bit. The ROP and hole cleaning efficiency form the key links between wellbore instability and operations. Rock debris in the annulus, resulting from drilling and/or wall failure, will increase if hole cleaning is inadequate, raising the risk of pack-offs and stuck pipe. The ability to clean the hole is also related to the ROP. Figure 16 outlines the various actions that are recommended for various given wellbore stability mechanisms. It can be seen that minimizing swabbing and surging affects helps to suppress more instability mechanisms than any other drilling practice. Also it can be seen that drilling practices such as wiper trips that are often considered as routine are sometimes detrimental to wellbore stability. Minimising wiper trips can help suppress actions that are sensitive to mechanical agitation of the formation such as mobile formations / sloughing shales, weak planes. Increasing mud weight is not necessarily the answer to wellbore stability problems. Whilst this practice can help suppress breakouts, it can cause drilling induced fractures or activate natural fracture networks by drilling above the minimum horizontal stress. However, where overpressure occurs, it is desirable to drill with an overbalance that exceeds 200 psi. In all cases, calculations are required prior to drilling to determine optimal parameters. This problem becomes amplified in deviated and especially horizontal wells where the mud window between shear and tensile failure becomes so small that sometimes there is no stable mud weight window. Good drilling practices such as Circulation, Rotation, Reciprocation, of the drillstring to remove excess cuttings in highly deviated wells, and close examination of shale shakers to examine volume of cuttings and their geometry is desirable to manage (suppress) unstable wellbores.
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Review Questions II 6) When planning a mud program, how is the mud weight often determined ? 7) In a relaxed basin, as well inclination changes from vertical to horizontal, what happens to the “Mud Weight Window” ? 8) Describe the 3 main types of cavings found on the shale shakers ? 9) For the following wellbore instability problems, what drilling practices would you use to surpress or control the problem ? a) Borehole Breakouts b) Natural Fractures / Weak Planes c) Unconsolidated formations
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Answers to Review Questions 1)
σH > σh > σv
2)
Loading mechanisms – where the pore fluid cannot escape as quickly as the rock compaction rate, and the pore fluid gets squeezed and pressured because it can’t escape. Unloading mechanisms – where a formation rises to a shallower depth, and the pore fluids cannot escape, then the formation is overpressured compared to surrounding (shallower) formations (because the pore fluids still have the same pressure as before the formation rose). Hydrocarbon generation where the pore fluids are trapped is another example.
3)
When we drill a hole in the rock, we replace the rock with a cylinder of mud and a set of stresses are created in the region of the wellbore wall. These stresses are known as “Wellbore Stresses”. They depend on the mud weight used, and the far field stresses σH , σh and σv
4)
Tensile failure – occurs when the rock grains are held in tension and are pulled apart. Shear failure – occurs when the rock grains are under a state of compression by 2 stresses that are acting perpendicular to each other and their magnitudes are very different.
5)
Leak off Pressure – the wellbore pressure at which the rock begins to yield and the formation grains begin to move apart and take mud. Formation Breakdown Pressure – the wellbore pressure at which the rock physically breaks down.
6)
Often (but not always) between the condition for shear failure and the minimum horizontal stress σh
7)
It generally becomes more narrow (ie you have to less of a margin in which to drill safely )
8)
Tabular – from natural fractures (where the cavings will have flat, parallel faces with bedding not parallel to the parallel faces of the caving). or from weak planes (the same as natural fractures but the bedding is parallel to the faces of the cavings). Angular – from borehole breakouts (they have curved faces with rough surface structure) Splintered – from overpressured zones (concave flat, thin, planar structures)
9 a) Perform frequent wiper trips, ensure hole is kept clean by: increasing mud gel strength, increasing mud circulation rate, increase mud weight. Also minimize swabbing / surging to stop borehole breakouts from getting worse, circulate / rotate / reciprocate (in extended reach or highly deviated wells). b) Minimise wiper trips otherwise might make situation worse (especially if problem is due to weak planes), increase mud gel strength to help decrease fluid mobility, add fluid loss agents to help control loss circulation, ensure drilling with minimum overbalance, minimize swab/surge. c) Minimise wiper trips, increase mud weight, minimise swab/surge, ensure hole cleaning. 30
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