Plug & abandonment of offshore wells: Ensuring long-term well integrity and cost-efficiency

CT-CFD integrated investigation into porosity and permeability of neat early-age well cement at downhole condition

Review of plausible chemical migration pathways in Australian coal seam gas basins

The Role of Geological Barriers in Achieving Robust Well Integrity

Well Integrity Support by Extended Cement Evaluation - Numerical Modeling of Primary Cement Jobs

It has long been recognised that during and after drilling through certain formations, the rock moves inward and begins to close off the well. Normally this phenomenon is considered undesirable since it can cause problems for drilling and casing running. It can however be put to good use as the mechanism to create an annular barrier behind casing. In order to extend the life of a number of North Sea brown fields many well slots on production platforms and sub-sea templates are being re-used. This process involves permanent plug and abandonment of the old well track prior to sidetrack drilling into a fresh area of the reservoir. Norwegian Continental Shelf (NCS) regulatory requirements dictate that compliant procedures for well abandonment require the establishment of double barriers to avoid leakage from the reservoir. With a shortage of sufficient traditional cement barriers these wells often need costly remedial work in order to meet abandonment requirements. Traditional sonic and ultrasonic azimuthal bond logging provides information on the material immediately behind the casing. Many such bond logs show solid material behind the casing far above the theoretical cement top. Clear correlation of this bonding pattern to shales, known to cause problems during drilling, indicates that the shale has sealed off the annular region and that it is the presence of such formation material that results in a good bond log response Logging and pressure testing sealed off zones in a number of wells allowed the bond log response to be qualified for a certain formation without further pressure testing. In this manner logs can provide a clear answer of whether shale successfully seals off certain zones and consequently provides a natural annular barrier. This technique has been employed successfully on over 40 wells, proving non-destructively that high quality natural annular barriers had formed, resulting in elimination of complex remedial work and substantial cost savings. Introduction Historically log responses indicating a good bond have often been observed on bond logs far above the theoretical top of cement. Many explanations exist for these responses and it is likely that there are a number of possible causes. The most frequent cause is believed to be formation displacement. This is supported by the following observations on the log: • Good bond log response far above the top of the theoretical cement. • Good quality bond correlates with shale rich intervals. • Large and sometimes frequent changes in bond log response at the same depth as geological changes. • Above the casing shoe of an outer casing string the log response changes from good quality bond to free pipe as the formation can no longer impinge onto the inner casing string. • Sinusoidal patterns on ultrasonic bond log images imply geological beds impinging on the outside of the casing. If the formation has been displaced onto the outside of the casing in a uniform manner around the circumference and over a sufficient interval along the casing, then this formation could provide an annular barrier to reservoir fluids. In order to provide an annular barrier the displaced formation must have certain physical properties. Such properties include sufficient rock

http://www.ieaghg.org/docs/General_Docs/6wellbore/Presentations/Day2/11.35%20Statoil.pdf

Identification and Qualification of Shale Annular Barriers Using Wireline Logs During Plug and Abandonment Operations

In this paper, we present new ultrasonic cement evaluation technology developed and extensively field tested over the past years. The introduction of this technology increases reliability and efficiency, and enables well integrity inspection in extreme environments with heavy muds and very large and thick casings. Such extreme conditions with heavy synthetic oil-based muds up to 20 lbm/gal and casing thicknesses up to and above 1 in. are often encountered when dealing with wells in deep water, wells used for carbon sequestration and underground gas storage, and shale oil and gas wells subject to hydraulic fracturing. Conventional ultrasonic cement evaluation technology has not been able to properly assess these cases in the past. We have addressed these challenges through new ultrasonic transducer technology with a thirtyfold increase in performance and an extended bandwidth that make it possible to log in the heaviest and most attenuative mud environments and through thick-walled casings. We have developed a calibration-free ultrasound pulse-echo processing method that capitalizes on self-compensation for mud properties and eliminates the need for separate mud property measurements. We have revised the electronics and embedded software to enable increasing the ultrasonic firing and acquisition rates leading to a threefold increase in logging speed. Finally, operating workflows have been revised to support both faster wellsite turnaround time, and robustness and the reliability of wellsite answers. This standard workflow with downhole normalizations, realtime quality control plots, and quick interactive previews provide the highest possible data quality at the wellsite. We present examples from our extensive field test campaign to demonstrate the capabilities of these new ultrasonic pulse-echo and flexural wave measurement developments. We describe how they enable cement evaluation and pipe inspection in environments of heaviest muds and large diameter/wall-thickness casings, and we show that these developments augment the ability to carry out well integrity diagnosis and help reduce uncertainty and risk in well construction or during plugging and abandonment (P&A).

Ultrasonic Cement Logging: Expanding the Operating Envelope and Efficiency

The authors would like to thank the Johan Sverdrup license Operator Equinor and Partners Lundin Energy Norway AS, Petoro, Aker BP ASA and Total E&P Norge AS for permission to publish the work. The authors would like to thank all Equinor colleagues who contributed to the development of this system, inparticular the fibra & fo.tone IT development teams.

"The views and opinions expressed in this paper are those of the Johan Sverdrup field operator and arenot necessarily shared by the license partners".

A Real-Time Fiber Optical System for Wellbore Monitoring: A Johan Sverdrup Case Study


 Ultrasonic and sonic logs are increasingly used to evaluate the quality of cement placement in the annulus behind the pipe and its potential to perform as a barrier. Wireline logs are carried out in widely varying conditions and attempt to evaluate a variety of cement formulations in the annulus. The annulus geometry is complex due to pipe standoff and often affects the behavior (properties) of the cement. The transformation of ultrasonic data to meaningful cement evaluation is also a complex task and requires expertise to ensure the processing is correctly carried out as well interpreted correctly.
 Cement formulations can vary from heavy weight cement to ultralight foamed cements. The ultrasonic log-based evaluation, using legacy practices, works well for cements that are well behaved and well bonded to casing. In such cases, a lightweight cement and heavyweight cement, when bonded, can be easily discriminated from gas or liquid (mud) through simple quantitative thresholds resulting in a Solid(S) - Liquid(L) - Gas(G) map. However, ultralight and foamed cements may overlap with mud in quantitative terms. Cements may debond from casing with a gap (that is either wet or dry), resulting in a very complex log response that may not be amenable to simple threshold-based discrimination of S-L-G.
 Cement sheath evaluation and the inference of the cement sheath to serve as a barrier is complex. It is therefore imperative that adequate processes mitigate errors in processing and interpretation and bring in reliability and consistency. Processing inconsistencies are caused when we are unable to correctly characterize the borehole properties either due to suboptimal measurements or assumptions of the borehole environment. Experts can and do recognize inconsistencies in processing and can advise appropriate resolution to ensure correct processing. The same decision-making criteria that experts follow can be implemented through autonomous workflows. The ability for software to autocorrect is not only possible but significantly enables the reliability of the product for wellsite decisions.
 In complex situations of debonded cements and ultralight cements, we may need to approach the interpretation from a data behavior-based approach, which can be explained by physics and modeling or through observations in the field by experts. This leads a novel seven-class annulus characterization [5S-L-G] which we expect will bring improved clarity on the annulus behavior. We explain the rationale for such an approach by providing a catalog of log response for the seven classes. In addition, we introduce the ability to carry out such analysis autonomously though machine learning. Such machine learning algorithms are best carried out after ensuring the data is correctly processed.
 We demonstrate the capability through a few field examples. The ability to emulate an "expert" through software can lead to an ability to autonomously correct processing inconsistencies prior to an autonomous interpretation, thereby significantly enhancing the reliability and consistency of cement evaluation, ruling out issues related to subjectivity, training, and competency.

Autonomous Interpretation Methods of Ultrasonic Data Through Machine Learning Facilitates Novel and Reliable Casing Annulus Characterization


 As oil and gas fields mature, many wells are scheduled for permanent well abandonment or permanent abandonment of a section for subsequent slot recovery. We present a novel data interpretation workflow that integrates pulse-echo inversion to accurately predict borehole fluid properties and sonic attenuation with ultrasonic measurement to accurately define the annular material present behind the casing.
 Per the NORSOK D-010 standard, the requirement to qualify a well section for further drilling is to evaluate any degradation of the main bore casing, along with evaluation of the well barrier element. In an example slot recovery well, pulse-echo measurement and a cement bond log were recorded in the main borehole. The data was interpreted using an industry-available cement evaluation workflow to indicate the presence of very low impedance material (gas column) between two casings up to surface. The presence of a gas column required remedial action to mitigate well integrity issues during well construction. Advanced cement interpretation was conducted using a newly developed workflow that integrates new pulse-echo inversion and sonic attenuation measurement. The novel pulse-echo inversion accurately determines azimuthal as well as depth variations in the acoustic impedance of the borehole mud. The inversion is validated using 3D models without prior knowledge of mud properties. With the help of in situ measurement of mud properties, we can now visualize features such as mud deposition and segregation in deviated pipes as well. This new processing enables easier and more accurate interpretation of the cement sheath together with providing essential information about the logging fluid. The obtained results clearly identified the presence of solids and liquids in the annular space, which helped the operator to make informed decisions for continued slot recovery and new well drilling operations, rather than spending rig time on gas column mitigation measures. Advanced cement evaluation measurement and interpretation techniques reduce risks and enable accurate and critical decision making for well construction, intervention, and plugging and abandonment in many conditions that were previously ambiguous.
 Novel three-parameter inversion for pulse-echo measurements delivers unmatched accuracy for identifying the annulus material, as well as critical information on the logging mud, leading to a high level of confidence on cement placement. This aspect is especially critical in ensuring correct evaluation in complex new well designs including vertical, deviated, or horizontal pipes with a variety of mud types.

Kevin Constable

Papers

Identification and Qualification of Shale Annular Barriers Using Wireline Logs During Plug and Abandonment Operations

Ultrasonic Cement Logging: Expanding the Operating Envelope and Efficiency

A Real-Time Fiber Optical System for Wellbore Monitoring: A Johan Sverdrup Case Study

Autonomous Interpretation Methods of Ultrasonic Data Through Machine Learning Facilitates Novel and Reliable Casing Annulus Characterization

Accurate Determination of Annular Content Behind Casing and Prevention of Remedial Action During Well Construction