Stability, Failure, and Measurements of Boreholes and Other Circular Openings (1993)

The stability of circular openings is a problem common to many disciplines, including geotechnical, mining, and petroleum engineering. This report attempts to discuss the current state of the art in the understanding of the mechanisms responsible for the instability of circular openings in its broadest sense. Originally, the scope of this proposed review was limited to circular boreholes, emphasizing the latest developments mainly as a result of the impetus by the petroleum and related industries this past decade. However, the panel members soon realized the similarities existing in failure observations made by various disciplines and decided to expand the scope of this report to include all sizes of circular openings. It was felt that such an interdisciplinary approach would benefit the scientific community at large. This striving for cross-fertilization also helps determine the organization of the report as the panel consciously decided to subdivide this report into chronological tasks rather than individual disciplines. Although circular openings have been studied extensively in mining and tunneling, the application to the oil and gas industries requires slight modifications because of specific and sometimes unique characteristics (Morita and Gray, 1980; Cheatham, 1984; Guenot, 1987; McLean, 1987; Klein and McLean, 1988).

In the past few years, members of the rock mechanics community have revisited the traditional approach to the problem of opening stability. This may be due to the emergence of new challenges, such as completing deviated boreholes, together with an upgrading of knowledge on rock mass behavior and analysis methodologies.

The fundamental developments for understanding the environment around a deep wellbore were established by Westergaard in 1940, who predicted the existence of a plastic state around the wellbore, based on Terzaghi 's effective stress concept. In 1941, Biot extended these considerations and clearly explained the role of pore pressure and fluid flow. Since then, numerous authors have contributed to the state of

the art (e.g., Paslay and Cheatham, 1963; Haimson and Fairhurst, 1969; Gnirk, 1972; Geertsma, 1978; Bradley, 1979; Risnes et al., 1982; and many others). Today, research and engineering efforts have been accelerated due to the need of drilling deviated and horizontal wellbores that are noncolinear with the principal stress directions. Such wellbores provide access to and increased penetration in structurally complex reservoirs (e.g., Bosio and Reiss, 1988; de Montigny and Combe, 1988; McLennan et al., 1989).

Major concern over the stability of nondeviated wells has largely been restricted to poorly consolidated formations, perforations subjected to excessive drawdown, highly compacting formations, and wellbores known to intersect tectonically active zones. In deviated wellbores, there may be greater potential risks during drilling because of the higher percentage of openhole completions and contributions to the prevailing stress concentration from a non-vanishing shear stress component at the borehold wall. During drilling of long horizontal sections, risk may be exaggerated because of the mud pressure required to maintain stability and the difficulty of pulling relatively long strings if extraordinary trouble is encountered uphole.

Exploratory boreholes or shafts also provide one of the most important means for access to deep geological formations, whether for scientific investigation, economic exploration, resource extraction, or waste disposal. The engineering aspects of these deep circular openings determine the technology required for excavation, sample extraction, possible measurements, and interpretations of rock-mass quality based on field data and core samples.

In recent years, the geotechnical community has learned to take advantage of the geological disturbance produced by the presence of the borehole itself. The stress released during drilling and the subsequent generation of a stress concentration field around the borehole serve as a carefully controlled experiment providing useful information about in-situ stress and rock properties. In addition to the results inferred from the analysis of the formation response to the introduced stress concentration field, modern investigators have the option of performing various other tests in the borehole, including dilatometer or jacking tests, micro- and mini-hydraulic fracturing tests, and fluid extraction; providing further means for the investigation of fluid chemistry, formation properties, and state of stress.

The stability of circular openings can be considered by separating the potential rock failure mechanisms into four categories: (1) failure related to the pre-existing conditions or drilling-induced (e.g., fracturing, abnormal geopressures, lack of consolidation, etc.), (2) failure caused by the introduction of a stress concentration field (e.g., shear or

tension failure resulting from intersections or presence of adjacent openings), (3) failure attributed to deliberate or unintentional introduction of additional stresses (e.g., fluid extraction or pressurizing packed off intervals), and (4) failure related to shock-wave loading (e.g., blasting, rock bursting, or earthquakes). All four of these borehole or shaft failure mechanisms have been studied extensively in terms of their consequences for engineering and/or for interpretation purposes.

In this context, one needs to introduce and define what is referred to as failure as well as the concept of abnormal stability.

This term often leads to confusion because its meaning is both a function of the ultimate goal of the engineering works as well as a question of interpretation and background of the analyst. For example, for a pure mathematician/numerist, failure usually means that the material starts experiencing some plastic yielding. At the other end of the spectrum, an experimentalist might define failure as the stage where the failed zone has reached the external boundaries of the rock sample.

For field civil engineers, failure initiation is associated with the occurrence of visible cracks at the wall. For mining engineers, failure means a major rock fall or sometimes the complete collapse of the opening. In deep mining, the rock around the openings can be stressed to the point of plastic yield; but this failed rock must be supported or contained to prevent it from being a safety hazard, and to maintain the necessary clearance and alignment for the opening to be functional.

In the oil industry, borehole stability has several meanings. Drillers are interested in safely and quickly completing their well. They do not want to reach a level of instability preventing them to “ make hole” (stuck pipe, high solid content in the mud). However, they may not be extremely demanding in requesting an intact hole, even if this would allow a better cementing job. Loggers and reservoir engineers are more demanding, because a broken, but stable, hole can be detrimental to the quality of their test data and, therefore, to the assessment of the reservoir properties. In deep burial of nuclear waste, boreholes or shafts must essentially be sealed to prevent the escape of radioactive material into adjacent water-bearing strata or the biosphere. The inability to seal these openings and the surrounding fracture zone created by the opening constitutes still another definition of failure —failure to reduce hydraulic conductivity around an opening (Coons et al., 1987; Kelsall et al., 1988).

From these observations, it is clear that several failure criteria must be established depending on the level of stability appropriate to the individual application. The common approach of equating failure with the elastic limit of the material, or even the peak, is often much too conservative.

Abnormal stability refers to a state where no signs of distress have been observed, although the level of straining has exceeded the predicted capacity of the rock formation. It usually means that the initial failure did not progress, but achieved a stable geometry (maybe partially failed), although the numerical models predicted total collapse.

When this occurs in practice, engineers usually invoke poorly developed concepts, such as progressive rupture, scale effects, stress gradient effects, or even plastic adaptation.