Stability of Highly Inclined Boreholes (includes associated papers 18596 and 18736 )

Abstract

Summary. Hole inclination produces alterations in the stress state aroundthe borehole and in the physical properties of the rock. Depending onspecific conditions, such effects may lead to collapse of the borehole or areduction in the fracture-initiation pressure. This paper shows how todetermine such effects through the application of stress analysis and rockmechanics. Introduction Stability of deviated boreholes is an important subject. Suchproblems as lost circulation may create hazardous conditions, whileborehole collapse often results in enlargement of the borehole, causingnumerous problems--e.g., poor cement displacement. The problems may existin the producing phase of a borehole, as well aswhen drilling. This paper is based on a linear elastic and isotropic model forstresses around the wellbore, with the aim of trying to understandthe general behavior of inclined boreholes. The model is first usedto study the two fracturing mechanisms. It was found that boreholecollapse is caused mainly by shear but also by tensile failure, whilefracturing of the wellbore is caused predominantly by tensile failure. Furthermore, when the wellbore is rotated from a vertical to ahorizontal position, the analysis shows that the borehole becomesmore sensitive toward collapse. For laminated sedimentary rocks, a weakness plane may subject the well toward collapse for holeangles between 10 and 40 deg. [0.17 and 0.7 rad]. In tectonicallystressed areas, the collapse stability may be improved by choosingthe proper geographic direction for the borehole. The fracturinggradient generally decreases with increased borehole inclination. A simple formula is included to estimate the fracture initiationgradients 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 maybe applicable to any continuous depositional basin. Borehole stability is currently being given considerable attention in Norway. With a number of offshore fields under planning anddevelopment, a substantial saving in expenditures is envisioned ifa field can be drained from three platforms instead of four. Thiscan be realized by the application of extended-reach drillingmethods. An increased borehole angle, however, brings about newproblems. Cuttings transport, casing setting and cementing, anddrillstring friction are examples of difficulties encountered in highly deviated boreholes. Also, the formation fracturing gradientdecreases with increased borehole angle. With an increased applicationof oil-based muds, the prediction of the fracturing gradientsbecomes more important than ever. Here, fracturing must be avoidedduring the drilling phase, and the problem is to determine themaximum values for the formation-integrity tests. Methods to predict fracturing gradients are typically based onempirical correlations between fracturing data, overburden data, and depth. Different methods of this nature are given in Refs. 1through 7. Daines' method in particular has been successfullyapplied in Norway by several oil companies. All these methods workfor vertical wells. Only Bradley studied the effect of boreholeinclination on the fracturing gradient. The basic difference is thatwhile the former (Refs. 1 through 7) used empirical correlations, Bradleyused equations for the stresses around the borehole. In this paper, we will continue Bradley's approach because the study of stressesaround the borehole also gives us a tool to study the failuremechanisms. Before proceeding further, we will define some of the assumptions used here. In addition to using a linear-elastic and isotropic rockmodel for plane-strain conditions, we assume formations where allin-situ stresses are principal and directed horizontally and vertically, respectively. Generally, no information is available regarding therelative values of the two horizontal in-situ stresses, so they areassumed equal. The key in the analysis is that when a well is drilled, the rock surrounding the hole must take the load that waspreviously taken by the removed rock. As a result, an increase in stressaround 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 orsplitting of the borehole will occur. Conversely, if the boreholepressure is lowered too much, the borehole will collapse because of shearfailure. In this case, rock fragment will break off from the walland fall into the wellbore. These situations are shown in Fig. 1. Finally, note that the given stress equations are valid for an intactborehole only. As soon as the borehole fails, the stress situationchanges and the equations are no longer valid. Qualitative Discussion of Mechanisms CausingWellbore Instabilities As stated, two main types of wellbore-stability problemsoccur-fracturing of the wellbore at high borehole pressure and boreholecollapse at low pressure. Wellbore collapse caused by clay swellingwill not be covered here. Before we proceed further, note the peculiar characteristics ofrocks. As opposed to metallic materials that have high tensilestrengths, rocks will generally be very weak in tension. Bradleyassumes rocks to have zero tensile strength and uses zero effectivestress as his criterion for tensile failure. The reasoning is that rocksoften fail along old cracks or flaws. Such an assumption is oftenalso used by others. In this paper, we argue that the main mechanism causingwell- bore failure when the wellbore is fractured is tensile failure of therock. For borehole collapse cases, the failure may be caused partlyby tensile effects, but will be caused mainly by shear effects. Thisidea is illustrated in Fig. 2 and will be quantitatively shown later. A typical fracturing of the wellbore in shallow wells (horizontalfracture) is shown in Fig. 2a, where the overburden is being lifted. The axial stress, sigma z, goes tensile, while the radial and tangentialstresses 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 aidthe fracturing process caused by the axial stress going tensile. No rockpieces will be released because both the shear and tensile stresses causethe fractures to go predominantly radially outward from the borehole. Theequations used in this paper are not valid for this case. Fig. 2billustrates the fracturing of a deeper well, where a vertical fractureproduced. Here, the radial and axial stresses are compressive, whilethe tangential stress, co, goes tensile. Even if a rock piece should be released from the borehole wall, the high wellbore pressure wouldkeep 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 axialand the tangential effective stresses are compressive, while the radialeffective stress goes tensile. If linear elasticity theory is applied. the failure should occur exactly at the wellbore wall. SPEDE P. 261^