Activating Shale Can Create Well Barriers
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This paper discusses shale creep and other shale-deformation mechanisms and how an understanding of these can be used to activate shale that has not contacted the casing yet to form a well barrier. The authors then explore methods of activating shale for this purpose, concluding that inducing a pressure drop in the annulus is the most-promising such method.
Creep is a well-known phenomenon in engineering in many materials. In rocks, it is related typically to grain rotation, grain sliding, cement cracking, and, in some cases, even grain cracking. The rock matrix has a viscous behavior wherein time is required to achieve new stress equilibrium at the grain-contact level as the bulk rock volume is exposed to an altered stress state. The deformation after the new load on the bulk rock volume is in place, and until the bulk rock volume is not significantly deforming in response to the altered load, is referred to as creep. Creep can interact with other physical and chemical processes, so it can be difficult to separate creep from other time-dependent deformation processes around a borehole. Fig. 1 illustrates the three main creep modes:
- Primary (transient)
- Secondary (steady state)
- Tertiary (accelerating)
Transient creep occurs after the wellbore excavation and transforms gradually into secondary creep. Secondary creep then, in some cases, can result in tertiary creep, wherein the deformation accelerates. In most cases, formations will experience steady-state creep. For plug-and-abandonment (P&A) purposes, creep can take place over several decades. Even after that much time, the creep deformation may have been too low to contact the casing, so assisting the shale to form a barrier would be beneficial. In some cases, it is best to establish shale barriers especially quickly, perhaps in a couple of days—in new well construction, for example.
Creep and Other Shale-Deformation Mechanisms
Elastic Deformation. Elastic deformation is too small generally to close an annulus around a casing. This deformation results from the excavation of the borehole and is proportional to the difference between the horizontal stresses trying to close the wellbore and the wellbore pressure trying to support the wellbore wall and inversely proportional to the shear rigidity of the rock.
Elastoplastic Deformation. This deformation, in rare cases, may be enough to close an annulus. The rock may fail in a ductile manner and reduce its stiffness by a factor of 5 to 10, inducing irrecoverable (plastic) large strains in the rock. Because rocks rarely behave as perfect plastic materials, elastoplastic deformation will seldom be enough to close the annulus.
Consolidation. Consolidation is a time-dependent deformation related to the low permeability of the shale. As the rock is exposed to an altered stress state, the deformation in the shale matrix will increase the pore pressure in the water-saturated shale and the shale will deform in a time-dependent fashion until the excessive pore pressure induced by the increased load has dissipated. The consolidation time, therefore, is highly dependent on the permeability of the shale.
Near the wellbore, both consolidation and creep can occur, and separating the two can be difficult. As the shale consolidates, it will also deform laterally but seldom enough to close the annulus.
Time-Dependent Plastic Deformation (Creep). In this subsection, the authors identify factors that have the potential to increase or enhance creep as
- High clay content, especially smectite
- High shear stresses
- Thermal deformation from heating
- Some shale/brine interaction effects
Additionally, the authors point out that pressure effects in the annulus and near-wellbore region can affect creep.
Rapid Shock-Loading of Shale. Soil liquefaction and related ground failure are associated commonly with large earthquakes. In most contexts, liquefaction refers to the loss of strength in saturated cohesionless soils because of the buildup of pore pressure during dynamic loading.
On the basis of this definition, one may conclude that liquefaction only occurs close to the surface in very weak soils. However, in the oil and gas industry, liquefaction of reservoir rocks has been observed.
A rapid shock, if resulting in liquefaction or ductile or plastic failure without macroscopic cracks and fractures, could close an annulus rapidly.
Brittle Failure. Various combinations of magnitudes of the principal stresses on the wellbore wall will result in various shapes of fractures and rock fragments (cavings) from brittle compressional shear failure. Breakouts are generated usually when the compressional stress on the borehole wall is too high because of insufficient mud weight. In some cases, cavings also can be generated by excessive compressional stress on the borehole wall because of a high mud weight. In some cases, cavings can be seen in drilling with low mud overbalance relative to the shale pore pressure (so-called pressure-induced splintery cavings). If the pressure in the borehole drops rapidly, similar cavings caused by tensile spalling of the wellbore wall can be seen.
Fracture Deformation. The authors’ findings with regard to this deformation can be summarized in the following:
- Small cracks are formed around the borehole in the drilling process.
- This cracked zone is called the excavation-damage zone in other industries such as the nuclear-waste industry.
- Cracks can form in the cement after setting (thermal and pressure loads).
- For a given normal stress on the crack, the crack in the material with the lowest Young’s modulus will be less conductive and, for the lowest unconfined compressive strength (UCS), will tend to self-heal.
- Self-healing is much more likely to take place in a weak, soft, and ductile material.
- Self-healing of cracks has been reported from nuclear-waste-storage host rocks with a UCS of several hundred psi at a depth of a few hundred meters.
Main Mechanisms for Activating Shale Barriers
On the basis of the authors’ investigations of shale-deformation mechanisms, including creation of a shale-creep predictor and a shale-barrier modeling tool detailed in the complete paper, three potential methods to activate a shale to form well barriers around a casing were identified:
- Pressure activation, inducing a rapid pressure drop in the annulus
- Temperature activation, heating the shale in the near-wellbore area
- Chemical methods, promoting shale weakening and potential swelling
These methods also can be combined to activate shales that can be difficult to activate with only one method.
The authors write that they believe that pressure reduction in the annulus is the most-effective method, followed by heating the near-wellbore area and, finally, chemical reactions. In the complete paper, the authors describe the successful pressure activation of shale barriers in many wells. The barriers have been verified using standard techniques used in the industry (i.e., cement-bond logs and pressure tests). The formation on the back of the casing can be less stiff and dense than cement, so bond-log processing and interpretation can be more challenging in some cases, especially in terms of the transition between effective and ineffective shale barriers. More work is ongoing to improve understanding of barrier-quality cutoff on the basis of bond logs.
The authors have developed prediction tools and numerical models based on first-order principles to understand better the failure mechanisms that can activate a shale to form a shale well barrier. The subsequent activation of shale for this purpose has taken place in hours or days and has been used in new wells in which well barriers have been constructed.
For pressure activation of shale barriers, modeling suggests that the shale needs to be weak. Because the failure mode involves undrained shear, a low dilation angle will be beneficial to ensure the deformation is not drained at some point. Low dilation angle is likely if the internal friction angle of the shale is low. These low-strength properties typically are related to shales with relatively high porosity and high clay content (especially smectite content) and low content of cementing minerals (e.g., quartz and carbonate). On the basis of the authors’ work, shales with very low compressional wave velocity or with wellbore-stability problems during drilling may be good candidates for activation as well barriers.
- Shale can be activated to form well barriers around casings and liners in deep wells in hours or days.
- Pressure-drop activation has been proved in the field and verified by bond logging and pressure-integrity testing.
- Shale barriers can be activated by chemical effects through exposure to water, but the process seems to be slow compared with other available methods. However, this process can be used to stimulate shale-barrier activation over time in new wells so that the well is ready for P&A.
- The authors’ understanding of temperature activation of shale barriers indicates that some shales with certain properties can be activated by temperature; the authors plan field trials to investigate this possibility further.
For a limited time, the complete paper SPE 191607 is free to SPE members.
Activating Shale Can Create Well Barriers
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