Stability of Highly Inclined Boreholes (includes associated papers 18596 and 18736 )
- B.S. Aadnoy (Rogaland Regional C.) | M.E. Chenevert (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- December 1987
- Document Type
- Journal Paper
- 364 - 374
- 1987. Society of Petroleum Engineers
- 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 4.3.4 Scale, 1.14 Casing and Cementing, 1.6 Drilling Operations, 1.7.7 Cuttings Transport, 5.5.1 Simulator Development, 1.6.1 Drilling Operation Management, 1.2.3 Rock properties, 1.11 Drilling Fluids and Materials, 3 Production and Well Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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Summary. Hole inclination produces alterations in the stress state around the borehole and in the physical properties of the rock. Depending on specific conditions, such effects may lead to collapse of the borehole or a reduction in the fracture-initiation pressure. This paper shows how to determine such effects through the application of stress analysis and rock mechanics.
Stability of deviated boreholes is an important subject. Such problems as lost circulation may create hazardous conditions, while borehole collapse often results in enlargement of the borehole, causing numerous problems--e.g., poor cement displacement. The problems may exist in the producing phase of a borehole, as well as when drilling.
This paper is based on a linear elastic and isotropic model for stresses around the wellbore, with the aim of trying to understand the general behavior of inclined boreholes. The model is first used to study the two fracturing mechanisms. It was found that borehole collapse is caused mainly by shear but also by tensile failure, while fracturing of the wellbore is caused predominantly by tensile failure. Furthermore, when the wellbore is rotated from a vertical to a horizontal position, the analysis shows that the borehole becomes more sensitive toward collapse. For laminated sedimentary rocks, a weakness plane may subject the well toward collapse for hole angles between 10 and 40 deg. [0.17 and 0.7 rad]. In tectonically stressed areas, the collapse stability may be improved by choosing the proper geographic direction for the borehole. The fracturing gradient generally decreases with increased borehole inclination. A simple formula is included to estimate the fracture initiation gradients for inclined holes if data for a vertical hole are known.
The input data to the analysis are composite curves from the U.S. gulf coast area. We believe, however, that the results obtained may be applicable to any continuous depositional basin.
Borehole stability is currently being given considerable attention in Norway. With a number of offshore fields under planning and development, a substantial saving in expenditures is envisioned if a field can be drained from three platforms instead of four. This can be realized by the application of extended-reach drilling methods. An increased borehole angle, however, brings about new problems. Cuttings transport, casing setting and cementing, and drillstring friction are examples of difficulties encountered in highly deviated boreholes. Also, the formation fracturing gradient decreases with increased borehole angle. With an increased application of oil-based muds, the prediction of the fracturing gradients becomes more important than ever. Here, fracturing must be avoided during the drilling phase, and the problem is to determine the maximum values for the formation-integrity tests.
Methods to predict fracturing gradients are typically based on empirical correlations between fracturing data, overburden data, and depth. Different methods of this nature are given in Refs. 1 through 7. Daines' method in particular has been successfully applied in Norway by several oil companies. All these methods work for vertical wells. Only Bradley studied the effect of borehole inclination on the fracturing gradient. The basic difference is that while the former (Refs. 1 through 7) used empirical correlations, Bradley used equations for the stresses around the borehole. In this paper, we will continue Bradley's approach because the study of stresses around the borehole also gives us a tool to study the failure mechanisms.
Before proceeding further, we will define some of the assumptions used here. In addition to using a linear-elastic and isotropic rock model for plane-strain conditions, we assume formations where all in-situ stresses are principal and directed horizontally and vertically, respectively. Generally, no information is available regarding the relative values of the two horizontal in-situ stresses, so they are assumed equal. The key in the analysis is that when a well is drilled, the rock surrounding the hole must take the load that was previously taken by the removed rock. As a result, an increase in stress around the wall of the hole, a stress concentration, is produced. If the rock is not strong enough, the borehole will fail.
If the borehole pressure is increased too much, fracturing or splitting of the borehole will occur. Conversely, if the borehole pressure is lowered too much, the borehole will collapse because of shear failure. In this case, rock fragment will break off from the wall and fall into the wellbore. These situations are shown in Fig. 1. Finally, note that the given stress equations are valid for an intact borehole only. As soon as the borehole fails, the stress situation changes and the equations are no longer valid.
Qualitative Discussion of Mechanisms Causing Wellbore Instabilities
As stated, two main types of wellbore-stability problems occur-fracturing of the wellbore at high borehole pressure and borehole collapse at low pressure. Wellbore collapse caused by clay swelling will not be covered here.
Before we proceed further, note the peculiar characteristics of rocks. As opposed to metallic materials that have high tensile strengths, rocks will generally be very weak in tension. Bradley assumes rocks to have zero tensile strength and uses zero effective stress as his criterion for tensile failure. The reasoning is that rocks often fail along old cracks or flaws. Such an assumption is often also used by others.
In this paper, we argue that the main mechanism causing well- bore failure when the wellbore is fractured is tensile failure of the rock. For borehole collapse cases, the failure may be caused partly by tensile effects, but will be caused mainly by shear effects. This idea is illustrated in Fig. 2 and will be quantitatively shown later. A typical fracturing of the wellbore in shallow wells (horizontal fracture) is shown in Fig. 2a, where the overburden is being lifted. The axial stress, sigma z, goes tensile, while the radial and tangential stresses remain in a compressive state. Shear effects occur between (sigma theta, sigma z), (sigma theta, sigma r), and (sigma r, sigma z) because of large stress differences. These shear stresses will merely aid the fracturing process caused by the axial stress going tensile. No rock pieces will be released because both the shear and tensile stresses cause the fractures to go predominantly radially outward from the borehole. The equations used in this paper are not valid for this case. Fig. 2b illustrates the fracturing of a deeper well, where a vertical fracture produced. Here, the radial and axial stresses are compressive, while the tangential stress, co, goes tensile. Even if a rock piece should be released from the borehole wall, the high wellbore pressure would keep it in place in both cases.
A borehole collapse process is illustrated in Fig. 2c. This is a typical pressure-drawdown problem. In this case, both the axial and the tangential effective stresses are compressive, while the radial effective stress goes tensile. If linear elasticity theory is applied. the failure should occur exactly at the wellbore wall.
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