Tectonically Influenced Regions See Complex Stress States, Casing Failures
In tectonically influenced regions, potential hydrocarbon traps are subject to complex states of stress. This scenario often translates into severe strike/slip (SS) and thrust-fault or reverse-fault (TF/RF) stress states.
In tectonically influenced regions, potential hydrocarbon traps are subject to complex states of stress. This scenario often translates into severe strike/slip (SS) and thrust-fault or reverse-fault (TF/RF) stress states. The complete paper demonstrates that such complex stress states will affect well completions and hydraulic fracturing directly in a multitude of ways but that, often, some of the more influential consequences are severe casing failures, production-liner restrictions, and complex-fracture-initiation scenarios. Unconventional-resource development in such environments requires that a renewed focus be made in all phases of well design and construction.
Distinguishing clearly between the two separate issues of well-integrity loss and reservoir-accessibility loss is critical when the presence of casing deformation or failure is considered. When describing casing deformation or failure, it is important to understand that not all of these failures are associated with a loss of well integrity. This is particularly true of any casing failure or deformation within the productive target formation because the target formation is fully outside the primary well barrier envelope.
In the complete paper, the authors refer only to those casing deformations and failures across and within the target productive intervals. Therefore, even if casing deformation results in loss of pressure integrity or partial or complete casing separation as a result of the event itself or because of the remedial actions implemented, this will not compromise actual well integrity. Consider that if the casing deformation occurs in a section that would or could normally be completed with perforations or fracturing, then the casing deformation is outside the primary wellbore barrier.
When such casing deformation occurs in the target area, well accessibility will be affected with regard to time, economics, and production but without health, safety, or environmental consequences. The operational effect can be significant up to and including a total loss of the well and a need to redrill through abandonment or sidetracking and is, therefore, a probabilistic part of development economics. The occurrence of extreme casing deformation within the primary well barrier envelope is extremely rare. Regardless, occurrence of such an event is treated as a major incident and involves all necessary regulatory bodies.
Casing Deformation Types
The following is only a partial list of possible casing deformation types but includes some of the more common encountered during well-intervention operations.
Failure Influenced by Tubulars
- Exceeding tubular pressure differential
- Treatment pressure causing connection or coupling failure
- Treatment pressure bypassing plug
Failure Influenced by Poor Cement
- Point load—direct contact with formation
- Point load—nonuniform formation load distribution
Failure Influenced by Tectonics or Rock Properties
- Casing deformation because of formation compaction
- Point load—fault, fracture, layer, or bedding-plane slippage
- Other point load—creeping formations
Failure Influenced by Operations
- Casing deformation—thermal effect
The occurrence of casing deformation in hydraulically fractured wells is a global phenomenon. However, data suggest that such incidents are often clustered in such a manner as to appear to be influenced significantly by specific local characteristics. Considerable evidence suggests that such occurrences are affected by the dominant local in-situ stress state. Stress states could affect stability scenarios in many ways, not the least of which would be resulting treating pressure magnitudes; pore pressures; and the potential for fault, fracture, and bedding-plane reactivation. The complete paper describes some of the reported occurrences of casing deformation and fracture-related issues reported in the literature on a global basis.
Sichuan Basin. Casing deformation in the Sichuan Basin is extremely well-documented. Indeed, it appears that casing-deformation events may be seen in 30 to 40% of treated wells. Activation of tectonic forces in the high-pressure/high-temperature and highly stressed formations such as the Longmaxi shale induced by hydraulic fracturing is widely accepted as the root cause for most casing deformations.
Reservoir-access case histories provided in the complete paper relate to several wells drilled in the Sichuan Basin as part of a comprehensive appraisal program over several blocks. While issues related to well access were problematic initially, they were resolved and managed eventually, and the resulting accessibility was sufficient to allow the program to be completed effectively. Much of the complete paper is devoted to data analysis of these Sichuan Basin case histories.
Cementing and Casing Deformation
Forces associated with layer slippage are geological by nature and so large that it is questionable whether any kind of material—cement or steel—would be able to withstand them. Cement typically is the weakest point of the link; steel has much better resistance to shear, compressional, or tensional forces. The same can be said for formation rock if not fractured. Therefore, the notion that a good cement and bond could mitigate the effect of wellbore deformation in every situation effectively is unrealistic.
Determination of whether the point load is the mechanism causing the deformation (as in the case of layer slippage) or as a direct consequence of poor cement (as in the case of a compaction event) is critical. In the latter case, good cement potentially will help reduce the effects of deformation by distributing the load. In the former, good cement is unlikely to have any major mitigation effect; on the contrary, it can result in amplification of the deformation.
The general concept is that uncemented sections are the preferred means of dealing with casing deformation caused by geomechanical forces. Therefore, rather than attempting to strengthen the tubulars and using increasingly expensive techniques to achieve an excellent cement placement to build a very rigid and tough wellbore, it may be more effective to consider a wellbore design that allows a certain degree of flexibility.
In unconventional completions, this can reasonably be achieved using the technique of openhole completion systems, which are entirely uncemented, with some form of openhole mechanical or swellable packer approach.
Mitigations and Contingency Plans
The mitigations that can be put in place to reduce the risk of casing deformation are numerous. However, in the case of tectonically or geomechanically driven casing deformations, a perfect solution likely does not exist because of the magnitude of the forces in play. It is more likely in such situations that the most-important mitigation consists of a well-structured suite of operational contingency plans. Such plans are not, and should not, be fixed, but should evolve on the basis of local experience gathered in each well, formation, play, and basin while progressing individual stage operations. Several such types of plan are detailed in the complete paper.
Access Management and Mitigation
In the case of the Sichuan Basin case histories, these wells were among several drilled and completed as part of a specific appraisal program. Once the problem had presented itself on the first well, it was effectively too late in the project-execution process to change the well design subsequently, so the actions became focused primarily on mitigating the possible consequences of such deformation rather than addressing its root causes.
These approaches revolved around a well-considered contingency plan in place to avoid unnecessary nonproductive time caused by lengthy decision-making processes. This preplanning also allowed the team to leverage local experience and develop a plan to minimize pressure and volume stress factors considered contributary to the deformation progression. Coiled tubing was available on standby, with milling tools and assemblies also ready and available. A prefracture baseline caliper log also was implemented.
Judicious application of these approaches, as can be seen with the case-history wells detailed in the complete paper, allowed completion and subsequent flowback and cleanup operations to proceed. By the end of the appraisal program, the casing-deformation effect was considered more of an annoyance than a hindrance to operations.
Potential approaches to address casing deformation in tectonically active areas include the following:
- Lateral position
- Cemented vs. uncemented lower completion approaches
- Use of hybrid solutions
- Use of concentric pipes combining an outer rigid structure with a flexible inner approach
- Plugless completions
The authors acknowledge that actual root-cause manageability, as well as any potential proactive solutions to occurrences of hydraulic-fracturing-induced casing failure, are case-specific and should be considered as such.
Unconventional plays require that careful attention be paid to the structural geological setting.
- This challenge is exacerbated by basins broadly characterized by extensive normal or relatively passive SS stress regimes.
- When working in such new areas, it is imperative that early studies classify the structural geology and in-situ stress state and that gathering of related information is prioritized.
- Deep and high-pressure unconventional wells located in active tectonic regions with dominant SS or TF/RF stress regimes can be prone to casing deformation.
- Wellbore construction should consider that, under certain conditions, stiffer and more-rigid completions may not always be the best choice.
- Extensive, tailored surveillance, including prefracturing baseline measurements, for potential casing, cement, and formation features should play a substantial role when proving the multifractured horizontal well concept.
- While the plug-and-perforation approach may have been the most-popular method to stage fractures during the development of most North American shale plays, an assumption that this will transfer readily to other basins may be premature.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 199710, “The Unconventional Unconventionals: Tectonically Influenced Regions, Stress States, and Casing Failures,” by A. Casero, SPE, and M. Rylance, SPE, BP, prepared for the 2020 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, 4–6 February. The paper has not been peer reviewed.