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

The definition and scope of instability encompass the localized conditions (e.g., small-scale spalling or washouts) categorized as a nuisance as well as catastrophic collapse leading to abandonment. Instabilities occur during all phases of the life of the circular opening, from excavation to utilization.

Several processes can occur simultaneously and contribute to the damage of the hole. Depending on the local conditions, one of the mechanisms will dominate. They must be accounted for in the design. The purpose of this chapter is to establish a list of the different mechanisms which could lead to a circular hole failure. This will be done by describing the different states of an opening during its life.

If the distressing caused by the approaching opening is important, fracturing of the rock mass might occur even before the hole face has reached it. This has been observed, at a fairly large scale, in deep hard-rock mining. This is also one of the mechanisms considered when analyzing core discing. Once the circular opening reaches the fractured rock mass, collapse can occur due to severe weakening. In the more favorable cases, a pattern of steeply inclined fractures can be observed along the wall of the opening.

As there is no access to the rock before it is fractured, there are only limited cures such as limiting the distressing by, for example, increasing the mud weight during the drilling process.

The excavation process and its optimization are important parameters for the ultimate stability of an opening. Making hole corresponds to a three-dimensional (3D) process as the conditions vary from the original in-situ stress state to a plane strain situation once the circular opening is made. During this transition period, the face will have a supporting role for the freshly cut wall, introducing a time-dependent effect that is a function of the excavation rate. It is usually accepted that for hole sections further than three hole diameters from the face, this three-dimensional supporting effect disappears.

However, this rather short period of time can be sufficient to provide the wall with the temporary additional support necessary to compensate for the stress redistribution which occurs instantaneously. In tunneling, this is the role of the temporary support (shotcrete or ribs) set as closely as possible to the working face. With wellbores, the pressure differential across a mudcake deposited by filtration ensures this stability. However, the stand-up time should be compared with the time required to build a sufficient cake. Depending on the filtration properties of the mud, a critical rate of penetration (i.e., the drilling rate) may exist beyond which irreversible damage can occur leading to potential instabilities.

Providing a circular opening with an efficient temporary support is always detrimental to the progression of the excavation. In drilling, an increase in the mud density or a decrease in the mud filtrate, which could be necessary for stability, reduces the rate of penetration. This is also true in mining where supporting and excavating are usually not simultaneously conducted. The use of tunnel boring machines takes full advantage of this temporary phenomenon. Therefore, optimizing the rate of advance is important in terms of the ultimate success.

A major operational difference exists with tunneling where the support system is varied along its length to be adapted to the local conditions. This is not feasible with boreholes where the mud parameters remain essentially identical for the total uncased length. An increase in mud density may indeed lead to fracturing of upper sections with possible mud-loss risks. More frequently, stuck-pipe conditions result from an excessive differential pressure across the cake near the permeable formations.

The way the stress redistribution occurs is also dependent on the “shape” of the bottom of the hole. In tunneling the working face is

usually flat, but in drilling the shape is influenced by the type of the bit. At the limit, coring operations lead to remnant stubs that minimize the distressing of the wall. A similar technique is sometimes used in tunneling where instabilities are expected. A slot is precut ahead of the face and grouted to act as a protective liner while excavation progresses.

Eventually, the excavation process has an influence on the ultimate stability. This is a complex problem in which local experience helps a great deal. A high-energy excavation process will not only comminute the rock, but also damage the formation immediately around the opening, affecting its load-carrying capabilities. On the other hand, the use of a very gentle breakage process in a highly stressed zone can be detrimental, as the intact rock has to support the full stress concentration. For example, in such environments, holes drilled with PDC bits which use a more gentle cutting action than a tricone bit, have experienced more instabilities. Here, again, a compromise has to be found to optimize the excavation process.

After excavation, temporary supports may be used for a certain period of time before a final support system is put in place. During this transition time, instabilities can also occur.

In some cases, the openings are left unsupported;the reasons for such an approach are numerous: (e.g., in tunnels or mines, if no rupture occurs at the face). In many cases (e.g., mines or storage caverns), the need for a permanent support system would add too great a cost to the project. In boreholes drilled for fluid pumping, the hole has to remain open long enough to: (1) reach the depth planned for inserting the casing, (2) perform the appropriate data acquisition (such as logging and testing), and (3) set and cement the casing.

In a small percentage of wells, open-hole testing is conducted. This may include permeability determination, sampling of formation fluids, and in-situ stress determination. Economic factors are an important consideration in performing these tests, as are concerns over the maintenance of stability while the tools are in the hole. Hesitancy to run such tests has an impact on the availability of reliable formation information which generally influences the type and success of subsequent operations performed on a well (such as hydraulic fracturing). Furthermore, even if testing is performed, hole conditions may influence the fundamental reliability of test interpretation. Breakouts are a good example of the progressive deterioration of a borehole.

Delayed instabilities could have serious consequences in drilling such as:

• squeezing rock conditions prevent the proper transfer of weight and torque to the bit;

• annulus packing off with debris results in intolerable mud losses;

• reluctancy or impossibility to lower the tools and acquire data (logs), cores, and pressure information, for which the hole was drilled;

• poor cementing of the casing leading to possible pressure buildup behind the casings (i.e., channeling); and,

• stuck pipe or stuck casing conditions that could lead to the abandonment of the hole.

Disturbance of the original equilibrium can result from: (1) stress conditions in the rock mass that are more severe than anticipated, (2) weakened rock, and/or (3) reduced temporary support capability.

An increase in the stress magnitudes at the wall of the cavity can be caused by:

• Coupled effects. A change of pressure or temperature in the opening will generate corresponding gradients in its surrounding. Those, in turn, will generate additional stresses at the wall which, when superimposed to the original stress condition, could induce fracturing. The collapse of a heated mine adit, due to an excessive temperature gradient, has already been reported. Similar effects can be used to explain delayed instabilities of the upper part of an open hole during drilling, as well as the increase of the fracturing pressure with depth.

• Delayed volume changes. Shale swelling, rock shrinkage due to dehydration, and time-dependent rheological effects can cause substantial delayed deformations. The circular opening will have to accommodate these volume changes, sometimes resulting in an increase of the rock stresses to the point that it could ultimately lead to a delayed rupture.

The weakening of rocks can be caused by different factors such as:

• rheological time-dependent deformation that affects the structure and therefore the strength;

• shale hydration and swelling that decrease rock strength and stiffness;

• hydrocarbon production wells that require acid injection (acidizing) or production tests that result in bond destruction and fine migration; and,

• cyclic effects that result from pressure and temperature changes.

Decreases in support capacity occur primarily in tunneling where the support might yield and lose its strength if excessive deformation of the hole had to be contained. Actions that could damage the efficiency of the mud cake can also decrease support capacity such as backflow during production or gas kick, mechanical action of the drilling assembly on the cake, and slug circulation.

Cased holes might experience the following different types of operational problems of geomechanical origin:

• Hole collapse/support failure. The hole might still be open, but excessive deformations prevent sufficient access.

• Insufficient bonding/channeling. This is essentially a problem for wellbores, where one of the roles of the casing is to isolate the various formations from each other. Improper sealing could: (1) lead to early water production coming from underlying horizons even though they are not perforated, (2) create pressure buildup in the annulus above the cemented part, and (3) be a serious problem when the borehole has been designed for nuclear waste disposal and isolation.

• Formation damage/stimulation. Excessive strains of a well during drilling, completion, testing, and production might lead to a significant reduction in the well productivity (or injectivity) and seriously reduce its performance. These strains can also reduce the

performance of any hydraulic fracturing stimulation treatment. The causes for these phenomena are multiple, and are beyond the scope of this report as they are specifically oil-related problems.

The same factors that result in temporary support collapse can cause permanent support collapse. Poor packing (i.e., poor contact between the support and the opening) can gradually load the distressed rock mass. Collapse can also occur in cases of poorly cemented wellbore casings.

Shear failure can result from large rock movements or fault activation when shearing energy is transferred to the casing. This can be caused by large compaction-subsidence phenomena, or by pore-pressure changes.

Excessive creep associated with particular formations such as salt and potash is also known to be the origin of serious casing deformations.

Although an excavation can be completed without a major collapse or even without major problems during drilling, local instabilities can cause delayed problems if good bonding is not achieved between the formation and the support.

This is a typical wellbore problem where excessive strain resulting from the drilling operation is due to an aggressive pressure history (e.g., rapid large drawdown). The stability of the perforations, ¹ and therefore the performance of the well, may also largely suffer.

For certain types of problems, such as waste disposal, long-term stability of the opening is critical. Designs have to include the time-dependent properties of the formation, even if they are not showing creep at shorter time scales. The determination of such pertinent parameters such as the proper equation of state is a problem in itself as minor parametric variations lead to large consequences over long periods of time.