Reducing Costs of Well Plugging and Abandonment While Verifying Risk
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A risk-based approach to well plugging and abandonment (P&A) has been developed and successfully applied. The recommended method is based on research that allows the modeling of fluid flow through microcracks through a range of failure modes in downhole components, the determination of the effect on the environment through dispersion modeling, and identification of the basis for acceptance criteria. The complete paper describes how various P&A designs can be compared by use of a risk methodology that takes account of degradation mechanisms, potential flow rates, and the effect on the environment.
The application of the proposed method to fields suggests that alternative plugging solutions with fewer barriers than prescribed by the standard NORSOK D-010 guidelines result in the same low level of environmental risk. The method accounts for uncertainty related to input parameters and can be refined further if these uncertainties are reduced over time by field observations and testing.
The proposed approach to quantifying the environmental risk associated with minor, long-term leakage from P&A barriers, overburden formations, and natural seepage is to frame the issue in terms of potential modifications to valued environmental resources. This permits a degree of differentiation between alternative plugging designs, none of which may be physically capable of leading to the type of major leakage treated by standard environmental-risk-analysis approaches.
Description of Well-Abandonment Designs
General. Per NORSOK D-010, a P&A job should be planned and performed with an eternal perspective. Numerous regulations and requirements for different countries and areas of operation exist, but these generally state that there should be at least one permanent well barrier between a potential source of inflow and the surface.
A potential source of inflow is defined in the guideline as a formation with permeability, but not necessarily a reservoir. This requirement does not apply to the case in which the formation is a reservoir, with a reservoir defined as a permeable formation or group of formation zones originally within the same pressure regime, with a flow potential or hydrocarbons present or likely present in the future; in this case, the requirement is two permanent well barriers. These requirements can lead to confusion because NORSOK D-010 does not provide any further explanation of the terms “inflow” and “flow potential.” Some major operators define “flow potential” as a formation with permeability and overpressure, meaning that a reservoir can be a formation containing hydrocarbons, a formation with permeability and overpressure, or a combination of both. Furthermore, NORSOK D-010 holds that the last openhole section of a wellbore should not be abandoned permanently without installation of a permanent well barrier, often known as the surface P&A barrier.
Well Barriers. Per NORSOK D-010, well barriers should be installed as closely as possible to the potential source of inflow. However, if the well barriers need to be installed at a shallower depth, the requirement is that the estimated formation-fracture pressure at the base of the plugs be higher than the potential internal pressure, for both the primary and the secondary plugs. The potential internal pressure is the reservoir pressure minus the reservoir fluid’s hydrostatic pressure. The reservoir pressure is not defined specifically in NORSOK D-010, but initial or virgin pressure can be used.
The only requirement for well-barrier materials in the guideline is that materials used in well barriers must withstand the load or environmental conditions as long as the well is to be abandoned; for permanent P&A, this means forever.
Failure-Modes Effect and Criticality Analysis (FMECA)
The FMECA is a systematic approach that steps through the permanent barriers between the hydrocarbon zones and the environment to establish how that barrier might fail, the cause of the failure or the degradation mechanism, and the expected consequence of the failure. Each potential failure mode is assessed in the FMECA, to create an understanding of how the permanent barriers can fail. The study is supported by design information, historical logs, and subject-matter experts. The outcome is a qualitative assessment of risk. To convert the FMECA qualitative assessment into a probabilistic estimate, a fault-tree analysis is used.
Downhole Leakage Modeling
The objective of this step is to analyze the initiating events identified, assess the probability or frequency, and identify potential causes. The initiating events to be analyzed are to be determined by the hazard identification and can be summarized as leakage through a microannulus that bypasses the shallowest cement plug in the well.
Performing Risk Evaluation
The risk assessment is based on the criteria established for the specific well to be analyzed. Factors affecting the established criteria are the well location, the operator, and the well type.
For risk comparison, it is recommended to divide the risks into short-term and long-term risk implications, such that the overall risk picture is easier to understand and comparisons are simplified. Operational-risk criteria can be established by use of industry standards such as NORSOK D-010 or, alternatively, by use of operator-specific risk-tolerance levels. Typically, operational risk is evaluated in terms of well integrity and the potential loss of well barriers during P&A operations. Similarly, platform safety criteria are available.
Environmental-risk criteria are also established. These might be operator-, well-, or area-specific and can be used from both a probabilistic and consequence perspective. In order to allow comparison, various risk levels, both short-term and long-term, are typically summarized in tabular format.
Case Application of Risk-Based Well Abandonment
The complete paper describes three case studies based on work performed by DNV GL in the North Sea; one of these cases is summarized here. The cases illustrate the application of the risk-based well-abandonment method described in DNVGL-RP-E103 to select and optimize well-abandonment design.
The well in this case (Case C in the complete paper) was located on a fixed platform with a dry tree in the Norwegian North Sea, with a water depth of 150 m. The well was one of multiple wells on the platform for which the lower section would be plugged and abandoned and a sidetrack of the well drilled. The majority of the other wells on the platform would continue production and operation.
Case C had two reservoir zones. The two reservoir zones consisted primarily of oil and had pressures of 240 to 280 bar before the planning of abandonment. The overburden involved one gas overburden zone; the volume of the overburden zone was uncertain but was conservatively taken to be moderate.
During the operation of the wells, leakages had been observed and recorded in the A annulus with a known rate. This had been controlled by bleedoff during the operation of the well.
The first well-abandonment design consisted of setting individual double permanent well barriers for each reservoir zone and one double permanent well barrier for the overburden zone. The second well-abandonment design included a double permanent well barrier above the reservoir zones and a double permanent well barrier above the overburden zone. Illustrations of the two design alternatives are shown in Fig. 1 above and Fig. 2.
The risk-based well assessment involved quantitative downhole modeling and site-specific environmental mapping and sensitivity evaluation. However, the operational-risk evaluation was given emphasis. The site-specific environmental mapping and sensitivity evaluation were able to establish thresholds for low, medium, and high risk levels. Quantitative downhole modeling proved to be complex because the two reservoir formations needed to be analyzed together for Design C2. The results were established in the form of probability distributions of the potential consequences for both designs. It was concluded that Design C2 had a slightly higher exposure to the long-term environment, and thus was assessed as a medium risk.
A detailed operational-risk analysis of both designs was performed. It was concluded that Design C1 had a high operational risk and Design C2 had a low operational risk. The risks were combined, which involved a breakdown of the short-term and long-term risks and inclusion of the quantitative analyses into the risk results and presentation. The risks were cross-checked to confirm their validity. With the quantitative-analysis results, the acceptance criteria, and the comparative differences in the operational aspects determined, the risk levels of the two designs were set.
In this case, the risks had to be balanced between the two designs, so that the short-term high operational risks were weighed against a slight increase in long-term environmental risk. Because the medium value was assessed for the long-term risk of Design C2, an as-low-as-reasonably-practicable (ALARP) perspective was applied, which included a time and cost comparison of the two designs.
Although no optimal selection was available, Design C2 was selected as the most appropriate from an ALARP perspective. This provided a major advantage in the short-term health, safety, and environmental operational risk as well as from a time-and-cost perspective, while the long-term effects were not considerably higher.
From the results of the case studies presented in the complete paper, the alternative P&A designs were chosen instead of the prescriptive P&A concepts. DNVGL-RP-E103 provides the framework for establishing and evaluating P&A wells individually by use of a risk-based perspective, aligning with a growing focus in the industry and leading to adoption of such an approach by regulators.
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