Critical Borehole Orientations Rock Mechanics Aspects

Transcription

1 Critical Borehole Orientations Rock Mechanics Aspects By R. BRAUN* Abstract This article discusses rock mechanics aspects of the relationship between borehole stability and borehole orientation. Two kinds of instability are considered. One is failures resulting from normal stresses in planes perpendicular to the borehole, or along its axis. These may be tension or shear failures in the direction of the minimum and/or the maximum normal stress component related to the borehole, or peeling/loosening around the complete borehole perimeter. These can be avoided, or at least minimized, by the selection of an appropriate borehole pressure. In a borehole in an anisotropic in situ stress field, which deviates from a principal in situ effective stress direction, there occur in addition shear stresses parallel to the borehole axis. These can result in shear failures which cannot be influenced directly by the borehole inner pressure. They can therefore only be avoided by the selection of an appropriate borehole alignment. It is shown that the intensity of borehole instability increases significantly with increasing anisotropy of the effective in situ stresses. The borehole orientations most at risk are approximately the same in compact and in disturbed rock masses, however the presence of weak surfaces results in an extension of the instability. 1 Introduction The planning of a borehole s path usually depends on economic factors. These are initially defined by the drilling position and the target zone. Then, especially in cases of the development of resources in low permeability strata and in disturbed rock masses, comes a further consideration, that the part of the borehole relevant to production (often a horizontal end zone) should have an optimum orientation in relation to the preferred flow direction and the propagation direction of hydraulic fracs. Thus not only the end point but also the way to it comes into the planning. However, on understandable cost reasons, a path which is as short as possible will always be sought. With deviated boreholes instabilities occur * Dr. Roland Braun, Consultancy in Rock Mechanics, Caputh/Germany ( Dr.Roland.Braun@t-online.de) /10/II 2010 URBAN-VERLAG Hamburg/Wien GmbH again and again during the development of the inclination, and these can often only be controlled to a limited extent by classical countermeasures (variation of mud pressure and its composition). It is therefore worthwhile in every case to take a more careful look at the rock which is to be penetrated and at the possible interactions of this with the borehole. These interactions may be geological/mineralogical or geochemical, but the ones discussed here concern rock mechanics. The significant rock mechanics factors and the way in which they can influence borehole stability are discussed in the following sections. 2 Presentation of 3D Parameters Rock mechanics analyses considering three dimensions are facilitated by having a suitable method for visualizing the 3D parameters (see, top right in Fig. 1, an example of a stress state). For this the Schmidt plot is generally used (see the centre of Fig. 1). The parameters are plotted unambiguously on a 2D disc using their direction (azimuth) in relation to geographical north and their dip in relation to horizontal. Around the outside of the disc is shown the azimuth, beginning at the upper edge with 0 (north) and then increasing clockwise (as in a compass). The values of dip are shown as circles of different radii, with a vertical orientation (dip angle 90 ) corresponding to the mid-point of the disc. Parameters with a horizontal orientation (dip angle 0 ) are plotted around the outer edge of the disc. In addition, the parameter magnitude in each orientation is indicated by the colour (see the lower part of Fig. 1). Fig. 1 3 Significant Rock Mechanics Factors 3.1 In situ stresses The magnitudes and orientations of in situ stresses have a decisive influence on the borehole loadings. In order to make a reasonable assessment of these in situ loadings it is essential to distinguish between the various different components. First the external loading (also called the total loading) is to be considered. The vertical component of this is the total overburden pressure and it can therefore be determined directly (for example from the average density of the overburden). The minimum total stress component corresponds to the closure pressure of a hydraulic frac. Against these loadings acts the pore pressure as an inner loading. This is however often (above all in low porosity and low permeability rock) only partially effective and directional in its effect [1]. This behaviour is described by the pore pressure effectiveness (3D Biot coefficient). The magnitude of this depends on the rock structure and the stress situation and may have a value between 0 (no pore pres- An example of a Schmidt plot showing a 3D stress state OIL GAS European Magazine 2/2010 OG 75

2 faulting, distinct strength anisotropies can occur (see the example in Fig. 2). These are not necessarily identical with the stress anisotropies. In zones of looser material (bedding planes, faults, fracs etc.) the strength may be lower than in compact rock. Such features generally have a planar form and can be considered as weak surfaces or areas. Depending on the in situ stress condition and the path of the borehole, they may also present a risk for stability. principal stresses and on the deviation of the borehole path from the principal stress directions. As deviated boreholes always have (at least in part) an orientation differing from a principle effective in situ stress direction, there are (in an anisotropic in situ stress field) always sections with shear stresses parallel to the borehole axis. In contrast to stresses normal and tangential to the borehole wall, these shear stresses are not modified by the pressure of the drilling mud (for the analytical relationships see [2]). Fig. 2 3D magnitudes and orientation of principal uniaxial compressive strength (UCS) sure effect) and ~1. The sums of the opposing external and internal loadings (total stresses and effective pore pressure) are the effective stresses. These are the relevant stresses for deformations and failures of the rock, and their use is therefore essential in stability analyses. As well as the stress magnitudes, and the differences in these between the principal components, the orientation of these components also has a significant influence on in situ stability. Often encountered are transverse anisotropic in situ loadings with (at least approximately) identical horizontal stresses which are generally smaller than the vertical stress. However stresses produced by tectonic influences can result in horizontal loadings which differ from each other. This can (for example, close to geologically young chains of mountains, like the Alps) result in conditions in which the maximum stress component is approximately horizontal. It should also be noted that the principal components of in situ stress are not necessarily horizontal and vertical. In situ 3D loadings in which the components are inclined are also found (for example near to faults). Fig. 1 shows a real example with the orientation of the maximum principal stress 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. 4 Loading Conditions around Boreholes The stability of a borehole is determined by the in situ loading in its immediate vicinity and by the pressure in the borehole. This pressure, however, only has a supporting effect when the borehole wall is impermeable (filter cake or impermeable rock) or in a low porosity rock with a correspondingly low pore pressure effectiveness. It must also be considered that with loosening at the initiation of instability, or following hydraulic fracs, the pore pressure effectiveness increases. When this was previously relatively low the influence of fluid pressure then increases. With an unchanged borehole inner (e. g. mud) pressure the result is a reduction of the stabilizing effect on the borehole wall. For the assessment of the conditions in the zone around the borehole the secondary stresses there must be considered. These depend on the in situ stresses in the unaffected rock mass and on the orientation of the borehole. For a borehole aligned parallel to an in situ principal effective stress the normal stresses perpendicular and parallel to the borehole alignment are identical with the effective principal stresses in the rock mass, and there are no shear stresses in that direction. A borehole alignment deviated from the orientations of the in situ principal effective stresses results in different magnitudes and orientations of the normal stresses in relation to the borehole, and in shear stresses parallel to the borehole axis. These shear stresses around the borehole depend on the difference in magnitude of the effective 5 Typical Cases 5.1 Input parameters and methods of calculation In the following sections some typical cases are presented in which there may be significant rock mechanics influences on the stability of boreholes. The rock behaviour is assumed to be brittle (as is often encountered in oil and gas fields, deep geothermal systems, CO 2 storage and in underground mines). As the in situ stresses have a considerable influence on the borehole loadings, the case studies cover three typical situations (Fig. 3). The effective stresses used for the analyses, as well as the pore pressure effectivenesses, are derived from determinations with RACOS [3] on cores from boreholes in different geological structures. For reasons of comparability they have been adjusted to refer to a common depth, and to have a vertical horizontal orientation of the principal stresses with the maximum horizontal stress acting NW SE. Figure 3 shows different stress anisotropies. Stress state 1 has a large vertical horizontal anisotropy, but only a small horizontal one. It is often encountered in largely undisturbed reservoirs covering a large area. State 2, in contrast, is typical for areas with fault zones resulting from tectonic action. It has a large maximum horizontal stress (but still less than the overburden pressure) with a horizontal stress anisotropy which is only slightly less than that between the vertical and the minimum horizontal component. In the next condition, stress state 3, the maxi- 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 even in a compact rock, without any visible Fig. 3 3D magnitudes and orientations of principal effective in situ stresses in three different stress situations OG 76 OIL GAS European Magazine 2/2010

3 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 measuredonsamplestakeninthedirectionofthe minimum uniaxial compressive strength. A Tauber failure criterion [4] was used for peak strength. This criterion considers 3D stress conditions and largely excludes the uncertainty inherent in more conventional assessment approaches [5]. For the residual strength (determined following macroscopic failure of the rock) used for the analysis of weak planes a Mohr-Coulomb criterion was used. The calculations for the cases presented here were made with the BOREHOLE program package [6]. This software is used for the assessment of the stability and the risk of failure of boreholes in any chosen orientation. First the 3D in situ stresses are calculated for the relevant zone, accounting for rock mechanics (stress conditions, rock structures etc.) and technical activity boundary conditions, and then they are evaluated in relation to a critical loading situation. The distance to the failure criterion in relation to the given 3D stress condition is defined as the Safety Factor. A Safety Factor 1 shows that the strength has been reached or exceeded in the analysed rock element and this could mean, for example, that macroscopic failure occurs in that region. For the overall assessment of the results of the analysis the individual elements around boreholes are coded with a colour corresponding to the Safety Factor. This shows the radial and tangential extent of unstable regions. The proportion of the material around the borehole which is unstable is also calculated. With an appropriate collation of individual stability analysis results, analyses can be made of the trend of the development of instability (for example resulting from variation of the borehole alignment). Fig. 4 Dependency of intensity of failure in the near-borehole zone (2 x borehole radius) on the effective borehole pressure (mud pressure) in a borehole parallel to an in situ principal effective stress 5.2 Types of failure In compact, brittle rock masses three types of failures can be distinguished. The simplest is collapse of the pore space. This results from high (approximately isotropic) compressive loading, which exceeds the loading capacity of the rock. Another type of failure (which is also relatively easy to analyse) results in cracking perpendicular to the axis of a borehole. The cause is tangential tensile stress in the borehole wall which exceeds the (generally very low) tensile strength of the rock, and these tensile stresses occur under specific combinations of the normal in situ stress perpendicular to the borehole wall and of the pressure in the borehole. The largest tensile stresses in the borehole wall occur in the direction of the maximum in situ stress perpendicular to the borehole axis. The risk of cracking is greater with high mud pressure and when the normal stress is low and/or has a high anisotropy. In the case of the vertical borehole (Fig. 4, left) under stress state 1 (with a low stress anisotropy perpendicular to the borehole wall) cracking first occurs with a high borehole pressure (the failures in Figure 4 at low mud pressure are shear failures or result from combined mechanisms). There is an analogous effect under stress states 2 and 3 (with large maximum horizontal stresses and low stress anisotropy perpendicular to the borehole axis) for a horizontal borehole in the direction of the minimum in situ stress. Crack formation occurs first at a borehole pressure larger than those shown in Figure 4. In contrast, with larger stress anisotropy perpendicular to the borehole wall (see in Fig. 4, left for stress state 2 & 3 and right for all stress states) comparatively low borehole pressures are required to initiate crack formation. The evaluations are considerably more complicated in the cases of borehole wall failures in the form of breakouts, peeling or loosening, or in combinations of failure conditions. A shear failure occurs when limiting combinations of normal and shear stresses are reached. In some cases tangential tensile stresses and crack formation can occur in combination with this condition D-failure analyses for boreholes in principal in situ stress directions The usual approach is to consider a plane perpendicular to the borehole axis. A shear failure occurs as a consequence of high tangential stress in the borehole wall and low radial support effect. This classical breakout develops in the direction of the minimum effective normal stress in relation to the borehole. With an increase in the magnitude and/ or the anisotropy of the normal effective stresses in the plane under consideration, the tangential loadings increase and so too the risk of failure. Very high magnitudes of in situ stress, but with low anisotropy, result in high tangential compressive stresses all around the borehole perimeter. These can result in peeling /loosening and so in comparatively large failures (see in Fig. 4, centre, the case of a borehole in the direction of the minimum in situ stress for stress state 3). If, in contrast, there is a very high anisotropy of the in situ stresses perpendicular to the borehole wall, this can (in addition to the greater tangential stresses in the direction of the minimum borehole loading, and therefore larger breakouts) result in tangential tensile stresses at comparatively low mud pressure and therefore to crack formation in the direction of the maximum normal stress in relation to the borehole. Reduction of the mud pressure can initiate shear failure or result in its extension. In the examples (Fig. 4, left) these effects are shown for a vertical borehole. The greatest risks occur under stress state 3, with the (perpendicular to the borehole axis) highest normal stress magnitudes and anisotropies D-failure analyses for boreholes in principal in situ stress directions The 2D approach neglects the loading parallel to the borehole axis (the 3D loading). When the difference between this and the minimum tangential or the radial stress in the borehole wall reaches a critical value, breakouts can occur also in the direction of the maximum normal stress component related to the borehole (and thus displaced 90 from the direction of classical breakouts). As the stabilizing effects of the constraining stresses around the borehole are missing in this case, the magnitude of the stress parallel to the borehole axis required for failure (for identical mud pressure) is lower than the maximum tangential compressive stress in the borehole wall required for failure in the 2D case. In critical conditions with a high principal stress parallel to the borehole axis and low mud pressure this shear failure results in loosening/peeling all around the borehole perimeter. In boreholes parallel to an in situ principal effective stress these tensile and shear failures can be prevented or at least minimized by the selection of an appropriate mud pressure D-failure analyses for boreholes deviated from principal in situ stress directions For the stability analysis of boreholes which are not aligned with a principal in situ stress the effects of the normal stresses parallel and perpendicular to the borehole wall must still OIL GAS European Magazine 2/2010 OG 77

4 Fig. 5 Dependence of borehole stability on drill path in three different situations of situ stresses in an intact rock mass be considered. Their magnitudes are less than the principal stresses, but all the mechanisms described above still apply. In addition, in anisotropic stress conditions, there are shear stresses. These have maxima in the direction of the bisector of the angle between each pair of principal stresses. The results are complex inter-relationships and a wide range of potential failure conditions. Some examples are shown in Figure 5. For these the mud pressure was used for which there is no instability for that case for a borehole parallel to a principal in situ stress. This provides a good indication of the influence of the shear stresses and altered normal stresses which apply to a borehole deviated from such a direction. Stress state 1 In the case of stress state 1, with the large difference between the vertical and the horizontal stresses, the most critical borehole orientation can be expected to be with azimuth and inclination both at 45. With the selected conditions no failure is indicated. However, in the case of lower rock strength and/or greater difference between the vertical and horizontal stresses, failures resulting from shear stresses would occur, these extending in the direction of the maximum horizontal stress component (which is also the minimum normal stress in relation to the borehole), which acts on the sides of the 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 wall. The maximum normal stress component in this case acts along the borehole axis. A shear failure results from the superposition of these loadings (at the side of the borehole wall) in the direction of the maximum horizontal in situ stress component (Fig. 6, left). Stress state 2 For stress state 2, with a large difference between the magnitudes of the overburden pressure and the minimum horizontal stress, and only a slightly smaller difference between the horizontal stresses, the most critical conditions were found for a borehole inclination of 45 and an azimuth deviated just a little from the direction of the minimum in situ stress. The main reasons for this are the maximum shear stresses in the plane perpendicular to the maximum horizontal in situ stress component (also the maximum normal stress referred to the borehole). These lead to the development of shear failures in this direction (Fig. 6, centre). The azimuth for the most critical borehole direction is also influenced by the (in this case subordinate) classical breakout mechanism. With the large difference between the principal horizontal in situ stresses there are also large shear stress related instabilities around boreholes in the direction of the bisector of the angle between them (analogous to stress state 3). Fig. 6 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 2) also a large difference between the overburden stress and the minimum horizontal stress, another risky orientation is the bisector of the angle between them. As the mud pressure has no effect on the shear stresses, it also has no influence on the failures resulting from them. Rather, with changes of the mud pressure, and therefore variation of the radial and tangential stresses in the borehole wall, other additional failure conditions (as described above) can occur. Instabilities resulting from shear loadings can therefore only be avoided by the selection of an appropriate borehole alignment. For stress states 2 & 3 this is parallel to the maximum horizontal stress. In the plane defined by this and the overburden pressure the smallest shear stresses occur for all borehole inclinations. This path can also be recommended for stress state 1, as in that case the shear stresses do not reach critical values. In all cases the mud pressures must still be selected to avoid failures resulting from mechanisms related to normal stress Influence of weak surfaces Borehole stability is not only influenced by in situ loading and rock strength, but also by surfaces with lower strength and rigidity, if such are present. Considering in detail their many possible orientations with respect to the in situ stresses and the path of a borehole would exceed the space available in this article. For this reason only a few typical situations are described in which the failure direction Safety factor and failure area around deviated boreholes in three different in situ stress configurations OG 78 OIL GAS European Magazine 2/2010

5 is directly related to the in situ stresses (Fig. 7). This takes into account that the orientation of a failure surface deviates from that of the (usually) maximum stress component by an amount depending on the angle of internal friction of the rock. In the case of a normal fault we find a surface dipping in the direction of the minimum in situ stress with a strike parallel to the maximum horizontal stress (itself the intermediate principal stress). In a strike slip Fig. 7 Fault types assumed from a description in [2] (a Normal fault, b Thrust fault, c Strike-slip fault and σ 1 maximal in situ stress, σ 2 intermediate in situ stress, σ 3 minimum in situ stress) failure a steeply dipping surface forms striking deviated from the direction of the (horizontal) maximum in situ stress. Here as a further significant weak surface a horizontal plane (generally resulting from deposition) is considered. The largest intensity of failure for all borehole directions (analogous to conditions in intact rock, see Fig. 5) is with the largest stress anisotropies vertical horizontal and also in the horizontal (stress state 3) (Fig. 8). The boreholes most at risk, independent of the weak surfaces, have orientations similar to those in intact rock. This shows that the stress state has the greatest influence on stability even in the presence of weak surfaces. However the failure intensity is considerably greater than in intact rock. For faults in the direction of the maximum horizontal stress (with the minimum horizontal stress normal to the fault plane) there is the largest failure intensity, independent of the borehole orientation, and for horizontal surfaces (with the overburden stress as the normal stress) there is by far the lowest failure intensity. The intensities increase with moderate deviation of the weak surface from the maximum in situ stress direction. Shear failures in the zone around the borehole, resulting from marked shear stresses parallel to the borehole axis, cannot be influenced directly by modifications of the borehole pressure. However, increasing the pressure, and thus the infiltration of fluid into the weak surface, may reduce the effective stress normal to the surface and result in an extension of the instability. For all the investigated stress states the most favourable borehole path (analogous to in intact rock) is parallel to the maximum horizontal stress. This will not always prevent instability (especially in adverse in situ conditions) but it will at least minimize it. 6 Conclusions The stability of the wall of a borehole may be modified by the pressure within the borehole, but it is influenced above all by the effective in situ stresses (the combination of the total loading and effective pore pressure). The decisive factors are the loadings which result from the orientation of the borehole. In a borehole parallel to one of the three principal in situ stresses the failures which occur result from normal stresses in planes perpendicular to the borehole and along the borehole axis. These may be tension or shear failures in the direction of the minimum and/or the maximum normal stress component related to the borehole, or peeling/loosening around the complete borehole perimeter. These can be prevented, or at least minimized, by the choice of an appropriate pressure (e. g. mud pressure) inside the borehole. For boreholes in an anisotropic in situ stress field, deviation from one of the principal stress directions can result in additional instability caused by shear stresses parallel to the borehole axis. As borehole pressure has no effect on these shear stresses, it also has no influence on the failures resulting from them. Instabilities resulting from shear loadings can therefore only be avoided by the selection of an appropriate borehole alignment. In a comparison of all borehole orientations the greatest intensities of instability occur with in situ loading with the greatest stress anisotropy. This result applies to both intact and disturbed rock masses. In the latter case (independent of the specific orientations of weak surfaces) the borehole orientations most at risk are similar to those for the undisturbed case, but the presence of the weak surfaces increases the tendency for instabilities to occur. All these results show that selection of appropriate methods for avoiding or minimizing borehole instability must be based on an evaluation of the in situ conditions. Most important is a reliable determination of all three principal in situ effective stress magnitudes and their geographical orientations. References [1] Braun, R.:A Commonly Neglected Factor in Rock Mass and Borehole Stability. OIL GAS European Magazine, 2/2007, pp. OG79 OG82. [2] Fjaer, E.; Holt, R. M.; Horsrund, P.; Raaen, A. M.; Risnes, R.: Petroleum related rock mechanics. Developments in petroleum science 33, Elsevier [3] Braun, R.: Predicting Production Induced Changes in Reservoirs. OIL GAS European Magazine, 3/2006, pp. OG124 OG129. [4] Tauber, F.: A triaxial empirical failure criterion for rocks a contribution to safety calculations. 7th Int. Cong. on Rock Mechanics Vol. 1, pp Aachen (1991). [5] Braun, R.: Consideration of 3D Rock Data for Improved Analysis of Stability and Sanding. OIL GAS European Magazine, 2/2008, pp. OG64 OG68. [6] Braun, R.; Stromeyer, D.; Tauber, F.: Method for calculating borehole stability. ERDÖL ERDGAS KOHLE 111, 10/1992, pp 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 particular to questions of rock mass and borehole stability, in situ stresses and reservoir deformation. He holds a Diploma in Petroleum Engineering and a PhD in Rock Mechanics from the Technical University 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) OIL GAS European Magazine 2/2010 OG 79