Critical Borehole Orientations – Rock Mechanics Aspects

DRILLING

By R. BRAUN*

Abstract again and again during the development of 3 Significant “Rock Mechanics” This article discusses rock mechanics as- the inclination, and these can often only be Factors pects of the relationship between borehole controlled to a limited extent by classical 3.1 In situ stresses stability and borehole orientation. Two countermeasures (variation of mud pressure The magnitudes and orientations of in situ kinds of instability are considered. One is and its composition). It is therefore worth- stresses have a decisive influence on the failures resulting from normal stresses in while in every case to take a more careful borehole loadings. In order to make a rea- planes perpendicular to the borehole, or look at the rock which is to be penetrated and sonable assessment of these in situ loadings along its axis. These may be tension or shear at the possible interactions of this with the it is essential to distinguish between the vari- failures in the direction of the minimum borehole. These interactions may be geolog- ous different components. and/or the maximum normal stress compo- ical/mineralogical or geochemical, but the First the external loading (also called the to- nent related to the borehole, or peeling/loos- ones discussed here concern rock mechan- tal loading) is to be considered. The vertical ening around the complete borehole perime- ics. The significant rock mechanics factors component of this is the total overburden ter. These can be avoided, or at least mini- and the way in which they can influence pressure and it can therefore be determined mized, by the selection of an appropriate borehole stability are discussed in the directly (for example from the average den- borehole pressure. following sections. sity of the overburden). The minimum total In a borehole in an anisotropic in situ stress stress component corresponds to the closure field, which deviates from a principal in situ pressure of a hydraulic frac. Against these effective stress direction, there occur in ad- 2 Presentation of 3D Parameters loadings acts the pore pressure as an inner dition shear stresses parallel to the borehole Rock mechanics analyses considering three loading. This is however often (above all in axis. These can result in shear failures which dimensions are facilitated by having a suit- low porosity and low permeability rock) cannot be influenced directly by the bore- able method for visualizing the 3D parame- only partially effective and directional in its hole inner pressure. They can therefore only ters (see, top right in Fig. 1, an example of a effect [1]. This behaviour is described by the be avoided by the selection of an appropri- stress state). For this the Schmidt plot is gen- pore pressure effectiveness (3D Biot coeffi- ate borehole alignment. erally used (see the centre of Fig. 1). The pa- cient). The magnitude of this depends on the It is shown that the intensity of borehole in- rameters are plotted unambiguously on a 2D rock structure and the stress situation and stability increases significantly with in- disc using their direction (azimuth) in rela- may have a value between 0 (no pore pres- creasing anisotropy of the effective in situ tion to geographical stresses. The borehole orientations most at north and their dip in risk are approximately the same in compact relation to horizon- and in disturbed rock masses, however the tal. Around the out- presence of weak surfaces results in an ex- side of the disc is tension of the instability. shown the azimuth, beginning at the up- per edge with 0° 1 Introduction (north) and then in- The planning of a borehole’spath usually de- creasing clockwise pends on economic factors. These are ini- (as in a compass). tially defined by the drilling position and the The values of dip are target zone. Then, especially in cases of the shown as circles of development of resources in low permeabil- different radii, with a ity strata and in disturbed rock masses, co- vertical orientation mes a further consideration, that the part of (dip angle 90°) cor- the borehole relevant to production (often a responding to the horizontal end zone) should have an opti- mid-point of the mum orientation in relation to the preferred disc. Parameters flow direction and the propagation direction with a horizontal ori- of hydraulic fracs. Thus not only the end entation (dip angle point but also the way to it comes into the 0°) are plotted planning. However, on understandable cost around the outer reasons, a path which is as short as possible edge of the disc. In will always be sought. addition, the param- With deviated boreholes instabilities occur eter magnitude in * Dr. Roland Braun, Consultancy in Rock Mechanics, Ca- each orientation is puth/Germany (E-mail: [email protected]). indicated by the 0179-3187/10/II colour (see the lower © 2010 URBAN-VERLAG Hamburg/Wien GmbH part of Fig. 1). Fig. 1 An example of a Schmidt plot showing a 3D stress state

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faulting, distinct strength anisotropies can principal stresses and on the deviation of the occur (see the example in Fig. 2). These are borehole path from the principal stress di- not necessarily identical with the stress rections. As deviated boreholes always have anisotropies. (at least in part) an orientation differing from In zones of looser material (bedding planes, a principle effective in situ stress direction, faults, fracs etc.) the strength may be lower there are (in an anisotropic in situ stress than in compact rock. Such features gener- field) always sections with shear stresses ally have a planar form and can be consid- parallel to the borehole axis. In contrast to ered as weak surfaces or areas. Depending stresses normal and tangential to the bore- on the in situ stress condition and the path of hole wall, these shear stresses are not the borehole, they may also present a risk for modified by the pressure of the drilling mud stability. (for the analytical relationships see [2]).

4 Loading Conditions around 5 Typical Cases Boreholes 5.1 Input parameters and methods of The stability of a borehole is determined by calculation the in situ loading in its immediate vicinity In the following sections some typical cases Fig. 2 3D magnitudes and orientation of principal and by the pressure in the borehole. This are presented in which there may be signifi- uniaxial compressive strength (UCS) pressure, however, only has a supporting ef- cant rock mechanics influences on the sta- fect when the borehole wall is impermeable bility of boreholes. The rock behaviour is as- (filter cake or impermeable rock) or in a low sumed to be brittle (as is often encountered sure effect) and ~1. The sums of the oppos- porosity rock with a correspondingly low in oil and gas fields, deep geothermal sys- ing external and internal loadings (total pore pressure effectiveness. It must also be tems, CO2 storage and in underground stresses and effective pore pressure) are the considered that with loosening at the initia- mines). effective stresses. These are the relevant tion of instability, or following hydraulic As the in situ stresses have a considerable in- stresses for deformations and failures of the fracs, the pore pressure effectiveness influence on the borehole loadings, the case rock, and their use is therefore essential in creases. When this was previously relatively studies cover three typical situations (Fig. stability analyses. low the influence of fluid pressure then in- 3). The effective stresses used for the analy- As well as the stress magnitudes, and the dif- creases. With an unchanged borehole inner ses, as well as the pore pressure effective- ferences in these between the principal com- (e. g. mud) pressure the result is a reduction nesses, are derived from determinations ponents, the orientation of these compo- of the stabilizing effect on the borehole wall. with RACOS® [3] on cores from boreholes nents also has a significant influence on in For the assessment of the conditions in the in different geological structures. For rea- situ stability. Often encountered are trans- zone around the borehole the secondary sons of comparability they have been adverse anisotropic in situ loadings with (at stresses there must be considered. These de- justed to refer to a common depth, and to least approximately) identical horizontal pend on the in situ stresses in the unaffected have a vertical–horizontal orientation of the stresses which are generally smaller than the rock mass and on the orientation of the bore- principal stresses with the maximum hori- vertical stress. However stresses produced hole. For a borehole aligned parallel to an in zontal stress acting NW–SE. by tectonic influences can result in horizon- situ principal effective stress the normal Figure 3 shows different stress anisotropies. tal loadings which differ from each other. stresses perpendicular and parallel to the Stress state 1 has a large vertical–horizontal This can (for example, close to geologically borehole alignment are identical with the ef- anisotropy, but only a small horizontal one. young chains of mountains, like the Alps) fective principal stresses in the rock mass, It is often encountered in largely undis- result in conditions in which the maximum and there are no shear stresses in that direc- turbed reservoirs covering a large area. State stress component is approximately horizon- tion. A borehole alignment deviated from 2, in contrast, is typical for areas with fault tal. It should also be noted that the principal the orientations of the in situ principal effec- zones resulting from tectonic action. It has a components of in situ stress are not neces- tive stresses results in different magnitudes large maximum horizontal stress (but still sarily horizontal and vertical. In situ 3D and orientations of the normal stresses in re- less than the overburden pressure) with a loadings in which the components are in- lation to the borehole, and in shear stresses horizontal stress anisotropy which is only clined are also found (for example near to parallel to the borehole axis. These shear slightly less than that between the vertical faults). Fig. 1 shows a real example with the stresses around the borehole depend on the and the minimum horizontal component. In orientation of the maximum principal stress difference in magnitude of the effective the next condition, stress state 3, the maxi- moderately deviated from the vertical. Besides any direct tectonic influence, the pore pressure effectiveness can also be relevant in determining the anisotropy of the effective stresses which control stability, even when the total in situ stresses are approximately isotropic, or at least transverse isotropic. This occurs, for example, when the pore pressure effectiveness has large directional differences resulting from rock structures such as cracks and fissures.

3.2 Rock strength The strength of a rock is determined by the mineral components and the intergranular cement, but also to a significant degree by in situ structural features. This means that Fig. 3 3D magnitudes and orientations of principal effective in situ stresses in three different stress even in a compact rock, without any visible situations

OG 76 OIL GAS European Magazine 2/2010 DRILLING mum horizontal stress component is the largest of all. Such horizontal anisotropy maxima are encountered above all close to zones of active mountain building. The triaxial strength of undisturbed material was assumed to be the same in all the cases. It is for clayey, silty sandstone with low porosity and low permeability and was mea- suredonsamplestakeninthedirectionofthe minimum uniaxial compressive strength. A Tauber failure criterion [4] was used for peak strength. This criterion considers 3D stress conditions and largely excludes the Fig. 4 Dependency of intensity of failure in the near-borehole zone (2 x borehole radius) on the uncertainty inherent in more conventional effective borehole pressure (mud pressure) in a borehole parallel to an in situ principal assessment approaches [5]. For the residual effective stress strength (determined following macro- pendicular to the borehole axis. The risk of contrast, there is a very high anisotropy of scopic failure of the rock) used for the analy- cracking is greater with high mud pressure the in situ stresses perpendicular to the bore- sis of weak planes a Mohr-Coulomb and when the normal stress is low and/or has hole wall, this can (in addition to the greater criterion was used. a high anisotropy. In the case of the vertical tangential stresses in the direction of the The calculations for the cases presented here borehole (Fig. 4, left) under stress state 1 minimum borehole loading, and therefore were made with the BOREHOLE program (with a low stress anisotropy perpendicular larger breakouts) result in tangential tensile package [6]. This software is used for the as- to the borehole wall) cracking first occurs stresses at comparatively low mud pressure sessment of the stability and the risk of fail- with a high borehole pressure (the failures in and therefore to crack formation in the ure of boreholes in any chosen orientation. Figure 4 at low mud pressure are shear fail- direction of the maximum normal stress in First the 3D in situ stresses are calculated for ures or result from combined mechanisms). relation to the borehole. the relevant zone, accounting for rock me- There is an analogous effect under stress Reduction of the mud pressure can initiate chanics (stress conditions, rock structures states 2 and 3 (with large maximum horizon- shear failure or result in its extension. In the etc.) and technical activity boundary condi- tal stresses and low stress anisotropy perpen- examples (Fig. 4, left) these effects are tions, and then they are evaluated in relation dicular to the borehole axis) for a horizontal shown for a vertical borehole. The greatest to a critical loading situation. The distance borehole in the direction of the minimum in risks occur under stress state 3, with the (per- to the failure criterion in relation to the given situ stress. Crack formation occurs first at a pendicular to the borehole axis) highest nor- 3D stress condition is defined as the Safety borehole pressure larger than those shown in mal stress magnitudes and anisotropies. Factor. A Safety Factor ≤1 shows that the Figure 4. In contrast, with larger stress ani- strength has been reached or exceeded in the sotropy perpendicular to the borehole wall 5.4 3D-failure analyses for boreholes analysed rock element and this could mean, (see in Fig. 4, left for stress state 2 & 3 and in principal in situ stress directions for example, that macroscopic failure occurs right for all stress states) comparatively low The 2D approach neglects the loading paral- in that region. For the overall assessment of borehole pressures are required to initiate lel to the borehole axis (the 3D loading). the results of the analysis the individual ele- crack formation. When the difference between this and the ments around boreholes are coded with a The evaluations are considerably more com- minimum tangential or the radial stress in colour corresponding to the Safety Factor. plicated in the cases of borehole wall fail- the borehole wall reaches a critical value, This shows the radial and tangential extent ures in the form of breakouts, peeling or breakouts can occur also in the direction of of unstable regions. The proportion of the loosening, or in combinations of failure con- the maximum normal stress component re- material around the borehole which is unsta- ditions. A shear failure occurs when limiting lated to the borehole (and thus displaced 90° ble is also calculated. With an appropriate combinations of normal and shear stresses from the direction of “classical” breakouts). collation of individual stability analysis re- are reached. In some cases tangential tensile As the stabilizing effects of the constraining sults, analyses can be made of the trend of stresses and crack formation can occur in stresses around the borehole are missing in the development of instability (for example combination with this condition. this case, the magnitude of the stress parallel resulting from variation of the borehole to the borehole axis required for failure (for alignment). 5.3 2D-failure analyses for boreholes identical mud pressure) is lower than the in principal in situ stress directions maximum tangential compressive stress in 5.2 Types of failure The usual approach is to consider a plane the borehole wall required for failure in the In compact, brittle rock masses three types perpendicular to the borehole axis. A shear 2D case. In critical conditions with a high of failures can be distinguished. The sim- failure occurs as a consequence of high tan- principal stress parallel to the borehole axis plest is collapse of the pore space. This re- gential stress in the borehole wall and low ra- and low mud pressure this shear failure sults from high (approximately isotropic) dial support effect. This classical “breakout” results in loosening/peeling all around the compressive loading, which exceeds the develops in the direction of the minimum ef- borehole perimeter. loading capacity of the rock. fective normal stress in relation to the bore- In boreholes parallel to an in situ principal Another type of failure (which is also rela- hole. With an increase in the magnitude and/ effective stress these tensile and shear fail- tively easy to analyse) results in cracking or the anisotropy of the normal effective ures can be prevented or at least minimized perpendicular to the axis of a borehole. The stresses in the plane under consideration, the by the selection of an appropriate mud cause is tangential tensile stress in the bore- tangential loadings increase and so too the pressure. hole wall which exceeds the (generally very risk of failure. Very high magnitudes of in low) tensile strength of the rock, and these situ stress, but with low anisotropy, result in 5.5 3D-failure analyses for boreholes tensile stresses occur under specific combi- high tangential compressive stresses all deviated from principal in situ stress nations of the normal in situ stress perpen- around the borehole perimeter. These can re- directions dicular to the borehole wall and of the pres- sult in peeling /loosening and so in compara- For the stability analysis of boreholes which sure in the borehole. The largest tensile tively large failures (see in Fig. 4, centre, the are not aligned with a principal in situ stress stresses in the borehole wall occur in the di- case of a borehole in the direction of the min- the effects of the normal stresses parallel and rection of the maximum in situ stress per- imum in situ stress for stress state 3). If, in perpendicular to the borehole wall must still

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Stress state 3 With stress state 3 (with the largest stress difference between the horizontal principal stresses and only a slightly smaller difference between the overburden pressure and the minimum horizontal stress) boreholes are most at risk which are horizontal and aligned with the bisector of the angle between the principal horizontal in situ stresses. The main cause are the (in a comparison of all the cases, the largest) shear stresses in the horizontal plane. These lead to the development of shear failures in the direction of the overburden pressure (Fig. 6, right). As there is (analogous to stress state Fig. 5 Dependence of borehole stability on drill path in three different situations of situ stresses in an 2) also a large difference between the over- intact rock mass burden stress and the minimum horizontal stress, another risky orientation is the be considered. Their magnitudes are less nent in this case acts along the borehole axis. bisector of the angle between them. than the principal stresses, but all the mecha- A shear failure results from the superposi- As the mud pressure has no effect on the nisms described above still apply. In addition of these loadings (at the side of the bore- shear stresses, it also has no influence on the tion, in anisotropic stress conditions, there hole wall) in the direction of the maximum failures resulting from them. Rather, with are shear stresses. These have maxima in the horizontal in situ stress component (Fig. 6, changes of the mud pressure, and therefore direction of the bisector of the angle between left). variation of the radial and tangential stresses each pair of principal stresses. The results in the borehole wall, other additional failure are complex inter-relationships and a wide Stress state 2 conditions (as described above) can occur. range of potential failure conditions. For stress state 2, with a large difference be- Instabilities resulting from shear loadings Some examples are shown in Figure 5. For tween the magnitudes of the overburden can therefore only be avoided by the selec- these the mud pressure was used for which pressure and the minimum horizontal stress, tion of an appropriate borehole alignment. there is no instability for that case for a bore- and only a slightly smaller difference be- For stress states 2 & 3 this is parallel to the hole parallel to a principal in situ stress. This tween the horizontal stresses, the most criti- maximum horizontal stress. In the plane de- provides a good indication of the influence cal conditions were found for a borehole in- fined by this and the overburden pressure the of the shear stresses and altered normal clination of 45° and an azimuth deviated smallest shear stresses occur for all borehole stresses which apply to a borehole deviated just a little from the direction of the mini- inclinations. This path can also be recom- from such a direction. mum in situ stress. The main reasons for this mended for stress state 1, as in that case the are the maximum shear stresses in the plane shear stresses do not reach critical values. In Stress state 1 perpendicular to the maximum horizontal in all cases the mud pressures must still be se- In the case of stress state 1, with the large dif- situ stress component (also the maximum lected to avoid failures resulting from ference between the vertical and the hori- normal stress referred to the borehole). mechanisms related to normal stress. zontal stresses, the most critical borehole These lead to the development of shear fail- orientation can be expected to be with azi- ures in this direction (Fig. 6, centre). The az- 5.6. Influence of weak surfaces muth and inclination both at 45°. With the imuth for the most critical borehole direc- Borehole stability is not only influenced by selected conditions no failure is indicated. tion is also influenced by the (in this case in situ loading and rock strength, but also by However, in the case of lower rock strength subordinate) “classical” breakout mecha- surfaces with lower strength and rigidity, if and/or greater difference between the verti- nism. such are present. Considering in detail their cal and horizontal stresses, failures resulting With the large difference between the princi- many possible orientations with respect to from shear stresses would occur, these ex- pal horizontal in situ stresses there are also the in situ stresses and the path of a borehole tending in the direction of the maximum large shear stress related instabilities around would exceed the space available in this horizontal stress component (which is also boreholes in the direction of the bisector of article. the minimum normal stress in relation to the the angle between them (analogous to stress For this reason only a few typical situations borehole), which acts on the sides of the state 3). are described in which the failure direction borehole. In addition, breakouts would occur in this direction because of the normal stress condition. It can be seen that there are critical conditions for a moderate deviation of the borehole in the direction of the minimum principal in situ stress (Fig. 5, left). This failure results from the combination of two different loadings, which individually would not result in any instability. One is the shear stress parallel to the borehole axis which, for this borehole orientation in stress state 1, acts in a plane perpendicular to the maximum horizontal in situ stress. In the direction of this principal in situ stress component (under these boundary conditions) is also found the minimum tangential stress in the borehole Fig. 6 Safety factor and failure area around deviated boreholes in three different in situ stress wall. The maximum normal stress compo- configurations

OG 78 OIL GAS European Magazine 2/2010 DRILLING is directly related to the in For boreholes in an anisotropic in situ stress situ stresses (Fig. 7). This field, deviation from one of the principal takes into account that the stress directions can result in additional in- orientation of a failure stability caused by shear stresses parallel to surface deviates from that the borehole axis. As borehole pressure has of the (usually) maximum no effect on these shear stresses, it also has stress component by an no influence on the failures resulting from amount depending on the them. Instabilities resulting from shear load- angle of internal friction ings can therefore only be avoided by the se- of the rock. In the case of lection of an appropriate borehole align- a normal fault we find a ment. surface dipping in the di- In a comparison of all borehole orientations rection of the minimum in the greatest intensities of instability occur situ stress with a strike with in situ loading with the greatest stress parallel to the maximum Fig. 7 Fault types assumed from a description in [2] (a – Normal fault, anisotropy. This result applies to both intact σ horizontal stress (itself b – Thrust fault, c – Strike-slip fault and 1 – maximal in situ and disturbed rock masses. In the latter case stress, σ2 – intermediate in situ stress, σ3 – minimum in situ the intermediate principal stress) (independent of the specific orientations of stress). In a strike slip weak surfaces) the borehole orientations failure a steeply dipping surface forms strik- sure, and thus the infiltration of fluid into the most at risk are similar to those for the undis- ing deviated from the direction of the (hori- weak surface, may reduce the effective stress turbed case, but the presence of the weak zontal) maximum in situ stress. Here as a normal to the surface and result in an surfaces increases the tendency for instabili- further significant weak surface a horizontal extension of the instability. ties to occur. plane (generally resulting from deposition) For all the investigated stress states the most All these results show that selection of ap- is considered. favourable borehole path (analogous to in propriate methods for avoiding or minimiz- The largest intensity of failure for all bore- intact rock) is parallel to the maximum hori- ing borehole instability must be based on an hole directions (analogous to conditions in zontal stress. This will not always prevent in- evaluation of the in situ conditions. Most im- intact rock, see Fig. 5) is with the largest stability (especially in adverse in situ condi- portant is a reliable determination of all stress anisotropies vertical–horizontal and tions) but it will at least minimize it. three principal in situ effective stress magni- also in the horizontal (stress state 3) (Fig. 8). tudes and their geographical orientations. The boreholes most at risk, independent of the weak surfaces, have orientations similar 6 Conclusions References to those in intact rock. This shows that the The stability of the wall of a borehole may be [1] Braun, R.:A Commonly Neglected Factor in Rock stress state has the greatest influence on sta- modified by the pressure within the bore- Mass and Borehole Stability.OIL GAS European bility even in the presence of weak surfaces. hole, but it is influenced above all by the ef- Magazine, 2/2007, pp. OG79–OG82. [2] Fjaer, E.; Holt, R. M.; Horsrund, P.; Raaen, A. M.; However the failure intensity is considerably fective in situ stresses (the combination of Risnes, R.: Petroleum related rock mechanics. greater than in intact rock. the total loading and effective pore pres- Developments in petroleum science 33, Elsevier For faults in the direction of the maximum sure). The decisive factors are the loadings 1992. horizontal stress (with the minimum hori- which result from the orientation of the [3] Braun, R.: Predicting Production Induced zontal stress normal to the fault plane) there borehole. In a borehole parallel to one of the Changes in Reservoirs. OIL GAS European is the largest failure intensity, independent three principal in situ stresses the failures Magazine, 3/2006, pp. OG124–OG129. [4] Tauber, F.:A triaxial empirical failure criterion for of the borehole orientation, and for horizon- which occur result from normal stresses in rocks – a contribution to safety calculations. 7th tal surfaces (with the overburden stress as planes perpendicular to the borehole and Int. Cong. on Rock Mechanics Vol. 1, pp. 351– the normal stress) there is by far the lowest along the borehole axis. These may be ten- 354 Aachen (1991). failure intensity. The intensities increase sion or shear failures in the direction of the [5] Braun, R.: Consideration of 3D Rock Data for Im- with moderate deviation of the weak surface minimum and/or the maximum normal proved Analysis of Stability and Sanding. OIL from the maximum in situ stress direction. stress component related to the borehole, or GAS European Magazine, 2/2008, pp. OG64– OG68. Shear failures in the zone around the bore- peeling/loosening around the complete [6] Braun, R.; Stromeyer, D.; Tauber, F.: Method for hole, resulting from marked shear stresses borehole perimeter. These can be prevented, calculating borehole stability. ERDÖL ERDGAS parallel to the borehole axis, cannot be influ- or at least minimized, by the choice of an KOHLE 111, 10/1992, pp. 345–347. enced directly by modifications of the bore- appropriate pressure (e. g. mud pressure) hole pressure. However, increasing the pres- inside the borehole.

Roland Braun is an independent consultant. For more than 30 years he has specialised in the application of rock mechanics in the petroleum and mining industries and in partic- ular to questions of rock mass and borehole stability, in situ stresses and reservoir defor- mation. He holds a Diploma in Petroleum Engineering and a PhD in Rock Mechanics from the Technical Uni- versity Bergakademie Freiberg, Germany.

Fig. 8 Dependence of borehole stability on drill path in three different in situ stress states in a disturbed rock mass (note that the scale of intensity of failure is modified from that in Fig. 5)

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