Guidelines for Geomechanics Rev 00

Guidelines and manuals

GM-FPL-GEO-01

Guidelines for geomechanics Exploration & Production

Rev: 00

Date: 01/12/03

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TABLE OF CONTENTS

1. PURPOSE...................................................................................................................3 2. GLOSSARY ABOUT THE MECHANISMS OF WELLBORE INSTABILITY...............3 3. GENERALITIES ON BOREHOLE STABILITY...........................................................4 3.1 Mud Weight Window............................................................................................................ 4 3.2 Types of rock failure and methods of detection.................................................................... 4 3.3 Effect of the mud ................................................................................................................. 5 3.4 Summary............................................................................................................................. 5

4. DRILLING DIFFICULTIES RELATED TO WELLBORE INSTABILITY ......................9 5. TYPICAL STRUCTURE OF A WELLBORE STABILITY STUDY ............................11

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GM-FPL-GEO-01 guidelines for geomechanics Rev 00.doc


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1. PURPOSE When drilling ERD or highly inclined wells, it is important to avoid large instability of the drilled rock. The instability leads either to hole closure (tight hole), enlargement in all directions (washouts) or enlargement in one preferential direction (breakouts) and might be the cause of severe difficulties: reaming, back reaming, greater risk of pack off due to bad hole cleaning, bad cementing, string hang up on ledges. Under some circumstances, it might not be possible to prevent rock instability by just controlling the mud weight. Other possible options could be to adjust the mud system, to modify the well trajectory (azimuth and inclination) or to change the casing points. When instability problems can not be avoided at all, it becomes important to set up a strategy to manage these problems and reduce the risk of stuck pipe. The experience shows that time and money can be saved if rock mechanics studies are carried out at the preproject stage before deciding about the well architecture.

2. GLOSSARY ABOUT THE MECHANISMS OF WELLBORE INSTABILITY Shear failure of the intact rock : The rock breaks at the well bore due to high compressive stress and insufficient mud weight Destabilisation of rock blocks in fractured formations : if the fractures are naturally open, the destabilisation occurs due to mud invasion even at low mud weighs. If the fractures are naturally closed, the might open up if a high mud weight is applied. Bedding shear : the rock breaks due to high shear stresses on bedding plans acting as weakness surfaces. The failure along bedding is very sensitive to the angle of intersection between the well and the bedding dip. Over-pressured formations : overpressure is a major source of instability in shale rocks. The instability caused by overpressure can only be solved by increasing the mud weight. Hydraulic fracturing : heavy losses can be the result of hydraulic fracturing of the formation due to mud weight higher than the fracture propagation pressure. The risk of hydraulic fracturing increases in depleted reservoirs. Losses in natural fractures : this kind of losses can only be solved by using LCM products. It might occur even at mud pressures in balance with the formation pressure. It has nothing than to do with in-situ stresses.




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3. GENERALITIES ON BOREHOLE STABILITY 3.1 Mud Weight Window In general, the mud weight window required to drill a well safely is controlled by the hole stability. The minimum mud weight will be that to control the pore/collapse pressure of the drilled formation(s) while the fracturing pressure determines the maximum mud weight. The fracturing pressure is the pressure that leads to uncontrolled propagation of a hydraulic fracture, resulting in heavy mud losses. Fracturing risk increases considerably in depleted reservoirs. Losses by hydraulic fracturing must be treated differently from losses through natural fractures. The collapse pressure is defined as the mud weight necessary to prevent significant diminution of the hole diameter or to avoid large breakouts/washouts. Washouts are usually attributed to formation pressures higher than the mud pressure. Such a pressure under balance situation might happen in the following circumstances: n

When stop circulating: the pore pressure is close to the ECD as a consequence of mud pressure penetration while the well pressure is equal to the static mud pressure.

n

When the circulating temperature becomes higher than the static temperature

3.2 Types of rock failure and methods of detection Breakouts (ovalization of the well in one preferential direction) are the result of formation shear failure under anisotropic stress and/or rock strength. Breakouts or a mix of washouts and breakouts can also occur when drilling fractured formations. It might be possible to fix the origin of instability, permanent or transient under-balance, shear failure or fractured rock, from the shape of caving that can be recovered on the shakers on one side and the type of ovalisation it produces and that can be inferred from calipers and images on the other side : •

Permanent Under-balance : very large caving, very large washouts

•

Transient under-balance : thin caving, time increasing washouts

•

Shear failure of intact rock: quite large caving with round irregular faces. Breakouts oriented perpendicular to the maximum in situ stress

•

Fractured rocks : thick caving with regular shape and parallel faces. Breakouts or washouts/breakouts not oriented according to the direction of maximum in situ stress.

•

Shear failure along bedding: caving show laminated structure. Instability and breakouts orientation depend on the angle between formations and bedding dip.

Around vertical wells, stress anisotropy is linked to tectonic Figure 1-a). Around inclined wells, stress anisotropy is the result of differences between the vertical stress (usually equal to the weight of the overburden) and the horizontal stresses (Figure 1-b).




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Rock strength anisotropy is most usually observed in shale due to bedding and/or laminations. Figure 2 shows how the strength of shale cores can vary with bedding orientation, measured with respect to the direction of coring. A minimum strength is observed for cores drilled with an angle of 50° relative to the bedding. Anisotropy of strength and of stresses make the well bore stability strongly dependent on the well trajectory. As shown Figure 3, the most problematic wells will be those intersecting the bedding at critical angles. Figure 4 shows the types of failures that occur in laminated rocks versus isotropic rocks. It s worth noting that it is not always possible to drill according to the stress or bedding orientations that would minimise the risks associated with well bore instability. Also, the orientation of stress and bedding might vary considerably within the same field, especially in the vicinity of faults. Faults are also associated with the presence of highly fractured zones called rubble zones. Rubble zones might be quite large (up to 500 ft). Rubble zones might lead to severe washouts if the fractures are sufficiently open to mud invasion. In some cases, the fractures open up due to too high mud weights. In such cases, the increase of mud weight does not necessarily improve the hole stability, it might even render the problems worse.

3.3 Effect of the mud Pressure penetration is possible in intact shale when the mud is water based. In fact, shale are most usually water wet and do not allow filter cake building up because of their low permeability. The solution is either oil based mud (OBM), which can not invade the shale thanks to capillary barrier, or specifically designed WBM such as silicate mud which after precipitating plugs the shale porosity. Pressure penetration phenomenon can be studied in the laboratory. The experimental set up and results of experienced with sea water and silicate mud are portrayed in Figure 5. With sea water, the mud pressure fully penetrates and balances the pore pressure. In the case of silicates, the mud can not penetrate and the mud pressure always overbalances the formation pressure. It is worth noting that pressure and heat transfers from the mud into the rock are time dependent phenomena and make the size of washouts increasing with time. Reducing the open hole time is a way to avoid severe well bore instability troubles. This can be achieved by optimising the number of trips, controlling the deviation to reduce the frequency of correction runs, etc..

3.4 Summary In summary, Well bore instability is a complex phenomenon which might involve : •

Shear failure when the mud weight is too low to relax the high compressive stresses around the well (large breakouts, rapid and important instability)

•

Shear failure along weakness planes (breakouts, stability depends on the angle between well axis and bedding dip vector)

•

Shear failure due to pore pressure penetration in either the rock porosity or into microfissures (breakouts, time dependent, controlled by rock permeability)

•

Tensile failure due to permanent or transient under balanced mud pressure (washouts, time dependent, function of rock permeability)




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•

Mix of breakouts and washouts/breakouts in highly fractured or completely unconsolidated rocks (time dependent according to fractures permeability)

•

Hole restriction due to high stresses and/or low rock stiffness

σH

σV

σθ=(3σH-σh)

σθ=(3σV -σh)

σh

σh

h

Figure 1 Figure 1. Stress anisotropy around a well a) due to tectonic compression. b) due to difference between vertical and horizontal stresses.

4000 3800 3600

(σ1-σ3)/2 (psi)

50o

Weak Plane Parameters Friction Angle = 150 Linear Cohesion = 1700 psi

3400 3200 3000 2800 2600 2400 2200 2000 0

10

20

30

40

50

60

70

80

90

Orientation (deg)

Figure 2. Ex. of strength anisotropy (i.e., max shear strength vs orientation to bedding) for shale samples. Results from triaxial compression tests were fitted to a model representing a rock containing a plane of weakness (Jaeger and Cook, 1979). The orientation to bedding is measured in relation to the direction of o loading. The orientation of minimum strength was calculated to be 50




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o 40 50°

Figure 3. Drilling intersecting bedding.The most problematic directions during drilling will be those intersecting the bedding. For those directions, wellbore breakouts will occur along the direction of o weakness (at 50 from the direction of bedding).

A

B

Figure 4. Figure 4 Extreme example of wellbore failure on laminated rock (A), drilled parallel to the laminations (i.e., horizontal well) and isotropic rock (B) drilled perpendicular to bedding. The magnitude of the failure depends on the degree of strength anisotropy and the orientation of the stress field in relation to bedding. (Photographs after Okland and Cook ,1998(A) and Ewy, 1989 (B)).




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Vertical load

P1

Pore fluid

Sample P2

Mud filtrate

Pressure (bar) 160

Pressure (bar) 160

Mud Pressure

140

Mud Pressure

140 Pore Pressure

120

Sea water

Virgin pore pressure

100

Silicate NaCl

120

Virgin pore pressure

100

80

Pore Pressure

80 0

10 20 Time (hour)

30

0

20 Time (hour)

Figure 5 : Pore pressure transmission test



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4. DRILLING DIFFICULTIES RELATED TO WELLBORE INSTABILITY •

Hole restriction : leads to tight hole and associated reaming and back reaming difficulties.

•

Caving problems : the hole becomes larger than the bit size. The main risks are bad hole cleaning and pack off and stuck pipe due to the so called phenomenon of hang up on ledges at the passage from over gauge to in gauge intervals during tripping out (see Figure 6). The problem of hang up on ledges is exacerbated by hole inclination when the caving develop on the bottom and the top of the hole. This is for instance the case in tectonic environments with wells drilled parallel to the minimum stress (or perpendicular to the maximum compression).

Hole restriction occurs mainly in soft formations. It might take place in relatively hard formations in the very beginning of the destabilising process. In fact, the passage from fully stable to completely unstable situations occurs progressively in several stages : Stage 1 : mud weight high enough, well bore fully stable (Figure 7-a). Stage 2 : lower mud weight, rock starting to fissure (Figure 7-b). Stage 3 : invasion of the fissures by the mud and destabilization of the disintegrated rock. Caving start moving toward the hole, hence the hole restriction (Figure 7-c). Stage 4 : Caving washed out by the mud (Figure 7-d). In tectonic environment, the hole enlargement takes place in the direction perpendicular to the maximum compression while there might be restriction in the direction parallel to the compression. Apart from the problem of ledges illustrated in Figure 6, hole enlargement (beyond stage 4) might generally be lived with, unless the amount of additional cuttings is huge and cannot be managed. Stage 3 might be the most difficult to manage because it generally leads to severe tight spots requiring hard back reaming with a high risk of getting stuck. To mitigate such kind of risks, it might be useful to add sealing agent to the mud to avoid the invasion of fissures. It is also important to avoid vibrations and hard manoeuvring to prevent the collapse of the destabilised rock, especially at the passage of the BHA in front of the unstable zone. Frequent circulation stops should be avoided too because it creates transient under-balance situations, with the pressure equal to the ECD inside the rock and to the static mud weight in the well, which tends to destabilise the well bore. It is also worth mentioning that increasing the mud weight once stage 3 has been crossed is not suitable. This means that the right mud weight must be applied from the beginning, not after the hole has become unstable.




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Top hole Hard

soft hard soft

Side hole Bottom hole

Figure 6 : Schematic of the phenomenon of hang up on ledges. Breakouts oriented according to top and bottom hole in inclined wells exacerbate the risk of hang up.

(a)

(b)

(c)

(d)

Figure 7 : different stages from stable hole to completely unstable hole : (a) hole stable, (b) well bore starts fissuring, (c) invasion by the mud, disintegrated rock starts moving inwards, (d) broken rock washed by the mud resulting in larger hole (caves) and the beginning of a new cycle of hole destabilisation.




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5. TYPICAL STRUCTURE OF A WELLBORE STABILITY STUDY The mud weight window available to drill wells safely is function of several factors : 1. Fixed, non modifiable factors • Formation(s) pressure(s) • In situ stresses, magnitudes and orientations • Rock strength, including the effects of bedding or lamination • Geological accidents (faults, rubble zones) 2. Controllable factors • Mud weight • Well trajectory, i.e. inclination and azimuth • Mud chemistry (WBM versus OBM) • Circulating temperatures The methodology used to calculate the collapse pressure can be summarised as follows : 1. Prediction along the trajectory of the well(s) of : •

Formations pressures -

RFT, DST in permeable formations

-

Indirect methods in shale (e.g. Eaton, using sonic, density and GR logs) calibrated against washouts/kicks/reservoir pressures

•

Orientation of stresses -

Regional stresses from the dominant breakout direction non perturbed by faults

-

Possible re-orientation of stresses in the vicinity of faults : from bore hole images and structural geology tools using geological cross sections and faulting maps.

•

Magnitudes of stresses : -

Vertical stress by integration of a density log

-

Minimum horizontal stress from LOTs and minifracs

-

Maximum horizontal stress from the interpretation of bore hole breakouts and drilling induced fractures observed on bore hole images. This supposes that we have strength measurements in front of few breakout intervals.

•

•

Constraints on stress magnitudes -

Stress magnitudes should respect fault(s) equilibrium

-

In compressive environments there might be a link between stress and lithology through elastic properties Rock strength with possible effect of bedding/laminations :




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-

Laboratory tests on core samples (if any)

-

Shape of caving if any of them were recovered in the previous wells

-

Correlation with sonic, lithology and bed dip based on previous experience. The maximum stress and strength data should be consistent with breakout data in different lithologies.

2. Calculation of the collapse and fracturing pressure using standard rock mechanics software 3. Issue best practices for managing well bore instability problems if the recommended mud weights cannot cope with the operational constraints.



Guidelines for Geomechanics Rev 00

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