Verification of annular well barrier elements is an essential process during the well construction and plug and abandonment phases. This topic has been the subject of many publications and is governed by regulations in different countries around the world. In this paper we focus on the evaluation of shale creep barriers. We describe how sonic and ultrasonic pulse-echo and pitch-catch configuration data are used to monitor the evolution of formations creep around the casing to eventually form annular barriers. Multiple annulus logging evaluations were performed across wells from the Valhall, Ula, Alvheim, and Jette fields on the Norwegian Continental Shelf (NCS). This was done using ultrasonic pulse-echo and pitch-catch configuration data as well as traditional sonic logs. The logging data were collected, reprocessed, and compared as part of a collaborative effort between the operator and a service company. In this study we examine the progression of annular solids through time from formation creep. Our intention was to study the ability of shales creep to form barriers, and to simplify the well design and achieve isolation around the production casing without the need for conventional cementing operations. The log data provide information of annulus material with a detailed map of the axial and azimuthal variations of the annulus contents. Crossplotting state-of-the-art measurements also helps to identify the material accumulating in the annulus as published in many other industry papers. This approach has garnered interest over the years as a novel solution during well abandonment operations. The results provide solid evidence of shale creep-forming barriers and has been used by the operator to further refine their zonal isolation strategy. Comparing ultrasonic data with a particular interest on the progression of formation creep, has become a popular subject within the NCS. Innovative use of historical logging data holds a great potential to help make decisions that result in reduced cost and environmental impact. Because of this novel usage and analysis of ultrasonic logging data, the operator was able to make informed decisions faster, saving rig time and cost while reducing the carbon footprint.

Shale is an effective barrier material. It has a proven track record of acting as a seal (barrier) for oil and gas reservoirs for millions of years. Shale with high clay content and especially high smectite has low permeability, in the nanodarcy range, compared to standard class G laboratory cement that has permeability in the 10–20 microdarcy range. Weak ductile shales will also have a self-healing behavior should fractures be induced at some point. Shale is approved by regulators to be used as well barriers and part of permanent plug and abandonment (P&A) for oil and gas wells. Examples of regulations are Norsok D-010, 2013 in Norway and O&G UK, 2012. In Norsok D-010, one suggests the formation of shale barriers to happen due to creep in ductile shales. Creep occurs in many materials and is observed as deformation under constant load and is also well described in rock mechanics literature. In a previous paper (Kristiansen et al., 2018), it was discussed how shale can be activated as a barrier to form around the wellbore in some shale types. This can be done by inducing a pressure drop in the open annulus (rapid drawdown), by heating the shale by a couple of hundred degrees Celsius, or by chemical processes. In that paper, the process found most effective and practical at that time was demonstrated: the activation of shale barriers with a rapid pressure drop in the annulus. It was also shown that the barrier can be verified days after by standard verification methods used in the industry (pressure testing and bond logging). The shale barrier verification criteria are analogous to cement barriers. In this paper we share the experience from the implementation of a strategy to use shale as well barriers in new wells at Valhall and a second field, Ula, around 100 km away. The method used to activate the shale barriers has revealed some challenges from a well control point of view, but it has also shown that waiting a couple of weeks, or in some cases a couple of months, shale barriers are forming with the same quality as when they were activated or logged later as part of P&A. From this work it can be concluded that the shale barriers logged during P&A are, in some cases, in place only weeks or months after the wells have been drilled. The activation seems to induce an acceleration of time-dependent deformation that will naturally happen over longer time and is consistent with rock mechanics principles of time-dependent deformations in rocks (like creep and consolidation).

As part of plug and abandonment (P&A) operations, several acceptance criteria need to be considered by operators to qualify barrier elements. In casing annuli, highly bonded material is occasionally found far above the theoretical top of cement. This paper aims to describe how the highly bonded material can be identified using a combination of ultrasonic logging data, validated with measurements in lab experiments using reference cells and how this, in combination with data from the well construction records. can contribute to lowering the costly toll of P&A operations. Ultrasonic and sonic log data was acquired in several wells to assess the bond quality behind multiple casing sizes in an abandonment campaign. Data obtained from pulse-echo and flexural sensors was interactively analyzed with a cross-plotting technique to distinguish gas, liquid, barite, cement, and formation in the annular space. Within the methodology used, historical data on each well was considered as an integral part of the analysis. During the original well construction, either water-based or synthetic oil-based mud was used for drilling and cementing operations, and some formation intervals consistently showed high bonding signature under specific conditions, giving clear evidence of formation creep. Log data from multiple wells confirms formation behavior is influenced by the type of mud used during well construction. The log data provided information of annulus material with a detailed map of the axial and azimuthal variations of the annulus contents. In some cases, log response showed a clear indication of formation creep, evidenced by a high bond quality around the production casing where cement cannot be present. Based on observations from multiple fields in the Norwegian continental shelf, a crossplot workflow has been designed to distinguish formation from cement as the potential barrier element. NORSOK D-010 has initial verification acceptance criteria both for annulus cement and creeping formation as a well barrier element, both involving bond logs; however, in the case of creeping formation it is more stringent stating that "two independent logging measurements/tools shall be applied." This paper aims to demonstrate how this can be done with confidence utilizing ultrasonic and sonic log data, validated against reference barrier cells (SPE-199578). Logging responses like those gathered during full-scale experiment of reference barrier cells with known defects were observed in multiple wells in the field. Understanding the phenomenon of formation creep and its associated casing bond signature could have a massive impact on P&A operations. With a successful qualification of formation as an annulus barrier, significant cost and time savings can be achieved.

Summary As part of plug and abandonment (P&A) operations, several acceptance criteria need to be considered by operators to qualify barrier elements. In casing annuli, highly bonded material is occasionally found far above the theoretical top of cement. This paper aims to describe how the highly bonded material can be identified using a combination of ultrasonic logging data, validated with measurements in laboratory experiments using reference cells and how this, in combination with data from the well construction records, can contribute to lowering the costly toll of P&A operations. Ultrasonic and sonic log data were acquired in several wells to assess the bond quality behind multiple casing sizes in an abandonment campaign. Data obtained from pulse-echo and flexural sensors were interactively analyzed with a crossplotting technique to distinguish gas, liquid, barite, cement, and formation in the annular space. Within the methodology used, historical data on each well were considered as an integral part of the analysis. During the original well construction, either water-based mud (WBM) or synthetic oil-based mud (OBM) was used for drilling and cementing operations, and some formation intervals consistently showed high bonding signatures under specific conditions, giving clear evidence of formation creep. Log data from multiple wells confirm that formation behavior is influenced by the type of mud used during well construction. The log data provided information of annulus material with a detailed map of the axial and azimuthal variations of the annulus contents. In some cases, log response showed a clear indication of formation creep, evidenced by a high bond quality around the production casing where cement cannot be present. Based on observations from multiple fields in the Norwegian continental shelf, a crossplot workflow has been designed to distinguish formation from cement as the potential barrier element. NORSOK Standard D-010 (2013) has initial verification acceptance criteria both for annulus cement and creeping formation as a well barrier element, both involving bond logs; however, in the case of creeping formation, it is more stringent stating that “two independent logging measurements/tools shall be applied.” This paper aims to demonstrate how this can be done with confidence using ultrasonic and sonic log data, validated against reference barrier cells (Govil et al. 2020). Logging responses like those gathered during full-scale experiments of reference barrier cells with known defects were observed in multiple wells in the field. Understanding the phenomenon of formation creep and its associated casing bond signature could have a massive impact on P&A operations. With a successful qualification of formation as an annulus barrier, significant cost and time savings can be achieved.

Thousands of wells will enter the plug and abandonment (P&A) phase across the Norwegian Continental Shelf (NCS), either for permanent well abandonment or section abandonment with subsequent sidetracks. In the medium and long term, more wells will be added to follow the same path as exploration, drilling, and production continues. The cost of abandonment operations demands improvement of how P&A operations are performed. A critical, and often time-consuming operation, of well or section abandonment is to cut and pull (C&P) some of the casing strings. Uncertainties about the status of the annular contents and the material within it, such as settled solids, contaminated cement, or well geometry might pose restraints that could hinder the C&P efficiency. The uncertainties may cause operations to deviate from the plan, increasing the time and the costs required. New-generation ultrasonic tools, in combination with sonic tools, provide information about the annulus material with a detailed map of the axial and azimuthal variations of the annulus contents. The geometric position of the inner pipe can be determined relative to the outer casing or borehole using advanced measurements. Logging with ultrasonic and sonic tools is a noninvasive method that can increase the efficiency of C&P operations. In this paper we discuss three case studies of wells ranging from 2 to 40 years old. Some of the wells have reached the end of their economic life and are now ready for permanent plug and abandonment (PP&A) or slot recovery. Each case is unique with different casing sizes being retrieved, along with varied annulus contents observed from ultrasonic and sonic log data. The innovative use of the data interpretation with advanced workflows decreased uncertainties about the annulus contents and enabled following an informed C&P strategy. In all three cases, the casing sections were retrieved without difficulties from the recommended depths of the analysis. Casing milling was performed in intervals where C&P was not supported by the data analysis.