Today's practice on the Norwegian Continental Shelf when designing solutions for permanent plug and abandonment (P&A) complies with NORSOK Standard D-010. This is a prescriptive approach to P&A, as opposed to a "fit for purpose" risk-based approach. A risk-based approach means that any given P&A solution is expressed in terms of the leakage risk, which can be formulated in terms of the following quantities; the probability that the (permanent) barrier system will fail in a given time period and the corresponding consequence in terms of leakage to the environment. As a part of building a leakage risk model for permanently plugged and abandoned wells, a simple leakage rate calculator has been developed for quick evaluation of the leakage potential from a given (permanent) well barrier solution. The leakage potential from the well can then be quantitatively assessed taking into account different leakage pathways including leakage through bulk cement, through cement cracks and through micro-annuli along cement interfaces. In the paper, we will provide models to estimate leakage rate for each leakage pathway and show how to integrate them in an overall leakage calculator, to obtain a description of leakage flow from the reservoir through failed barriers to the environment. The information and input parameters needed to achieve this will be discussed, and uncertain parameters will be treated probabilistically, thus allowing for expressing uncertainty in the leakage rate estimate. Results from the leakage calculator will be demonstrated on a synthetic case, showing variants of a permanently plugged and abandoned well.
Quantification of uncertain parameters in oil reservoirs is one of the major issues of concern. In underdeveloped reservoirs, there are many uncertain parameters affecting production forecast which plays a main role in reservoir management and decision making in development plan. To study the effect of uncertain parameters on the behavior of a reservoir and to forecast the probabilistic production of the reservoir, the simulator has to be run too many times with different entries for uncertain parameters. To avoid this heavy and timeconsuming process, Experimental Design methodology is used which chooses the values of uncertain parameters from their ranges in a way that the total uncertainty in the system is captured with the least number of simulator runs. In this study, Experimental Design methodology is used to observe the effect of uncertain parameters on the production of an underdeveloped oil reservoir, which is subjected to immiscible gas injection method, and to estimate the probabilistic production of the reservoir; therefore, the proper and unbiased decisions for oil reservoir development can be made. Experimental Design methodology, as a powerful and trusted method, makes it possible to choose simulator runs so as to obtain accurate probabilistic production diagrams using the least number of runs as well as to study the impact of uncertain parameters on the oil reservoir production profile, quickly.
Summary The current practice on the Norwegian Continental Shelf (NCS) when designing solutions for permanent plug and abandonment (P&A) complies with NORSOK Standard D-010 (2013). This is a prescriptive approach to P&A, as opposed to a “fit-for-purpose” risk-based approach. A risk-based approach means that any given P&A solution is expressed in terms of the leakage risk, which can be formulated in terms of the following quantities: the probability that the (permanent) barrier system will fail in a given time period, and the corresponding consequence in terms of leakage to the environment. As part of building a leakage-risk model for permanently plugged-and-abandoned wells, a simple leakage-rate calculator has been developed for quick evaluation of the leakage potential from a given (permanent) well-barrier solution. This tool is developed to serve the second aspect of the risk-based approach: the consequence in terms of leakage rate to the environment. The leakage potential from the well can then be quantitatively assessed, taking into account different leakage pathways including leakage through bulk cement, through cement cracks, and through microannuli along cement interfaces. In the paper, we will provide models to estimate leakage rate for each leakage pathway and show how to integrate them in the leakage calculator to obtain a description of leakage flow from the reservoir through failed barriers to the environment. The information and input parameters needed to achieve this will be discussed, and uncertain parameters will be treated probabilistically, thus allowing for expressing uncertainty in the leakage-rate estimate. Results from the leakage calculator will be demonstrated on a synthetic case, showing variants of a permanently plugged-and-abandoned well.
During drilling, there must be an evaluation of the maximum pressure that the formation can handle during a well kill scenario. This will depend on various parameters like fracture pressure, pore pressure, kick volume and several other factors. The depth of the next planned hole section will depend on if a kick of a certain size can be handled safely. This evaluation is often referred to as performing kick tolerances. When starting to drill a section, one will take a leak off test to get an indication of the fracture pressure at the last set casing shoe and this will be important information for the kick tolerance results. For HPHT wells the margin between pore and fracture pressures will be small, and one often has to resort to using transient flow models to perform the kick tolerances. However, there are many uncertain parameters that are affecting the results. Some examples here are pore pressure, type of kick and kick distribution. There is a need for trying to incorporate the uncertainty in the calculation process to give a better overview of possible outcomes. This approach has become more and more popular, and one example here is reliability based casing design. This paper will first describe the kick tolerance concept and its role in well design planning and operational follow up. An overview of all parameters that can affect the results will be given. In water based mud, the gas kick will be in free form yielding higher maximum casing shoe pressures compared to the situation when oil based mud is used where the kick can be fully dissolved. Then it will be shown how both an analytical and a transient flow model can be used in combination with the use of Monte Carlo simulations to generate a probabilistic kick tolerance calculation showing possible outcomes for maximum casing shoe pressure for different kick volumes. Here uncertain input parameters that can affect the calculation result will be drawn from statistical distributions and propagated through the flow model to estimate the casing shoe pressure. Multiple runs will be needed in the Monte Carlo simulation process to generate a distribution of the maximum casing shoe pressure. This will demand a rapid and robust flow model. The resulting maximum casing shoe pressure distribution will then be compared against the uncertainty in the fracture pressure at the last set casing shoe to yield a probability for inducing losses. The numerical approach for predicting well pressures and a schematic of the total calculation process will be given. Emphasis will also be put on discussing how this should be presented to the engineer with respect to visualization and communication. It will also be shown that one of the strengths of the probabilistic approach is that it is very useful for performing sensitivity analysis such that the most dominating factors affecting the calculation results can be identified. In that way, it can help in interpreting and improving the reliability of the kick tolerance simulation results.
Plug & abandonment (P&A) regulations on the Norwegian continental shelf are largely prescriptive, since the same requirements are applied irrespective of well conditions. A risk-based approach on the other hand, is a well-specific approach to assess the quality of a given plug and abandonment design solution. Probability of leakage and consequence, in the form of leakage rates to the environment, should be quantified for permanently plugged and abandoned wells in a risk-based approach. To address the consequence aspect of a risk-based approach, a tool for quantitative leakage assessment is essentially needed. This should cover all leakage pathways for reservoir fluids to the environment, i.e. leakage through the well and leakage outside the well through the surrounding formation. The integrity of the cement barrier could be weakened as a result of e.g. poor slurry design, tensile stresses and shrinkage, creating leakage pathways through the bulk cement, cracks and micro-annuli along cement interfaces. As for the surrounding formation, geological features such as faults and fractures, as well as the sealing ability of the cap rock, are important factors to consider from a barrier integrity perspective. Fractures or faults might intersect a permeable formation at a shallow depth, potentially enabling reservoir fluids to migrate into the wellbore or to the seabed. The authors have developed a leakage assessment simulator, to quantify leakage rates for permanently plugged and abandoned wells. The structure and models incorporated in the preliminary version of the simulator, covering only leakage pathways through the wellbore, was previously presented in SPE-185890-MS. The current study builds on the previous paper by also accounting for leakage scenarios outside the well through the surrounding formation. The structure of the leakage assessment simulator is also presented in this paper. Additionally, features that may significantly reduce leakage rates, such as barite plug are addressed. A synthetic case is presented where leakage rate is estimated for two scenarios, a) leakage through the well only and b) leakage through the well and the surrounding formation.
A large number of offshore oil and gas wells need to be plugged and abandoned on the Norwegian Continental Shelf in the coming decades, implying significant costs to the industry. To develop a methodology for evaluation of the quality of the barrier system of permanently plugged and abandoned wells from a risk perspective, the research project "Leakage risk assessment for plugged and abandoned oil and gas wells" has been initiated. The chosen quality measure to quantify containment risk in this context is the "leakage risk" expressed in terms of probability of barrier failure and potential future leakage rates. Reviewed papers consist of fragmented contributions to a complete framework for leakage risk assessment of plugged and abandoned wells. However, they do not provide a complete risk assessment framework with a corresponding detailed analysis. Without such a framework, regulations will be limited to prescriptive approaches. As an alternative to the currently prescriptive approach to plug and abandonment operation, this article presents how a risk-based approach can be used in practice by incorporating assessments of probability of leakage and the consequence (leakage rate), as well as uncertainty quantification. A comprehensive methodology and workflow, encompassing required steps for risk-based evaluation of containment performance, is established and presented in the paper. It is shown that this approach can provide a quality measure for a given plug and abandonment design and subsurface conditions.
Summary In a few years, there will be a need for performing a considerable number of subsea plug-and-abandonment (P&A) operations on the Norwegian continental shelf. There are certain challenges associated with this. These include more-difficult access to subsea wells compared with ordinary platform wells, high daily rates of semisubmersible rigs, and the fact that these rigs will be allocated for drilling new exploration-and-production wells to sustain the hydrocarbon production. At the moment, there is large focus on finding rigless technologies to reduce P&A cost and use of rig time. A probabilistic approach should be used to assess the cost- and duration-saving potential of such technologies relative to semisubmersible rigs. In this paper, we will consider rigless technologies for performing parts of the P&A operation. For the first time, a probabilistic approach, including learning curves, correlations, and possible risk events, is used to evaluate subsea batch-operated P&A. A realistic example that includes the use of a semisubmersible rig for batch-operated P&A of subsea production wells is used to show how a probabilistic methodology can be implemented for obtaining probabilistic estimates of P&A cost and duration. Inclusion of learning curves and correlations makes the application of a probabilistic approach for multiwell campaigns challenging. It is demonstrated how to incorporate learning curves, correlations, and possible risk events to capture a realistic range of cost and duration for multiwell P&A operations. In a second operational example, a light-well-intervention vessel is used to accomplish preparatory work and wellhead cutting and removal for P&A of the subsea production wells in batch campaigns. These two scenarios will be compared from a cost and duration point of view. Moreover, the advantages associated with riserless technologies for multiwell abandonment are discussed.
High demand for plug and abandonment (P&A) of subsea wells in the forthcoming years on Norwegian Continental Shelf, low availability and high daily rate of semisubmersible rigs as well as high proportion of P&A cost to total cost of drilling exploration wells challenge industry and intensify looking for alternatives and rigless technologies which can make P&A operation more cost effective and release time of semisubmersible rig earlier. Light well intervention vessels currently can take over some parts of P&A operation and release time of semisubmersible rigs in order to function in drilling and completing of new wells. In addition these vessels have a low daily rate compared to semisubmersible rigs and decrease cost associated with P&A. A systematic yet simple approach is required to acquire a correct picture for expenditure and time of these alternative technologies. In this paper, potentials and experiences of light well intervention vessels for P&A and benefits associated with them will be discussed. An example case is considered wherein a new technology is deployed from a light well intervention vessel to set surface plugs in a multi-well campaign. Thereafter, benefits associated with application of this rigless technology are discussed and highlighted. The paper will discuss different approaches for structuring probabilistic time and cost estimation of multi-well P&A campaigns discussing at which level these should be performed and what is the most appropriate approach with respect to modularity and efficiency. This will also be seen in relation to the Oil and Gas UK cost estimation guideline for P&A and the NORSOK technical specification. The objective is to keep the simulations simple while still acquiring the correct range of time and cost for multi-well P&A operation in order to make the proposed approach applicable for the industry.
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