DRILLING Critical Borehole Orientations – Rock Mechanics Aspects 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 OIL GAS European Magazine 2/2010 OG 75 DRILLING 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 in- fluence 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 ad- verse 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.