Decommissioning

Probabilistic Cost and Time Estimation of Rigless Plug and Abandonment

With a high demand for plug and abandonment (P&A) of subsea wells in the future on the Norwegian continental shelf, industry is challenged to find alternatives and rigless technologies that can make P&A operation more cost-effective and -efficient.

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With a high demand for plug and abandonment (P&A) of subsea wells in the future on the Norwegian continental shelf, industry is challenged to find alternatives and rigless technologies that can make P&A operation more cost-effective and -efficient. Light-well-intervention vessels (LWIVs) can take over some parts of P&A operations and release semisubmersible rigs to function in the drilling and completing of new wells. This paper covers potential and experience of LWIVs in P&A.

Introduction

The P&A work of a single well can be separated into three distinct phases: reservoir abandonment, intermediate abandonment, and removal of the wellhead and the casing strings a few meters below seabed (Oil and Gas UK 2011). Different vessels and technologies will be applied for these phases, which are also affected by the complexity of the well. It is also expected that these P&A phases to some extent will be performed in multiwell campaigns in which issues such as learning effects and correlations have to be considered.

The deterministic approach for time and cost estimation is a simple, traditional method, and the results can be transferred easily. There are some limitations associated with the deterministic approach, such as the yielding of biased results and, sometimes, the inability to capture the full range of outcomes.

On the other hand, the probabilistic approach has been recognized recently as a common and preferred technique for time and cost estimation, compared with the deterministic approach. This approach presents a nonbiased technique for time and cost forecasting and allows for inclusion of uncertainty into the model, covering the correct range of possible outcomes.

Oil and Gas UK (2011) provides a guideline for well-abandonment-cost estimation, aiming to provide a simple and structured approach for estimating cost and duration of field abandonment. As mentioned previously, the P&A work of a well can be divided into three distinct phases according to these guidelines. In addition, the complexity of each phase is reflected in the types of vessels and technologies that are required to perform the work scope of that specific phase. Five categories are used for this classification.

P&A operations will mostly be performed in multiwell campaigns, and different vessel technologies are used to complete distinct operations or phases. Because P&A of the wells can typically involve using the same vessels and technologies for distinct phases, performance of P&A in multiwell campaigns shares mobilization and demobilization time and cost as well as transit time from mobilization point to field and transit time among different wells.

Riserless technologies will play a significant role in reducing the cost of subsea-well P&A and releasing semisubmersible-rig time for drilling and completion of new wells. In order to quantify the benefits of moving P&A activities from semisubmersible rigs to lighter vessels, there is a need for a systematic probabilistic approach that can be used for multiwell campaigns. Light well intervention (LWI) or, more precisely, riserless well intervention (RLWI) has proved to be a cost-efficient intervention approach for subsea wells. RLWI is based on the use of monohull vessels and typically involves use of wireline for intervention operations. It should be mentioned that some vessels also have risers. This fact is reflected in the types of classification used in Oil and Gas UK (2011). For a discussion of this publication’s guidelines for well-abandonment-cost estimation, please see the complete paper.

Structuring Probabilistic Cost and Duration Estimation for P&A Campaigns

In general, when performing probabilistic duration and cost estimation, the operation is broken down into a detailed operation sequence showing the main suboperations that will be carried out. For each of these suboperations, input data revealing time and cost are given in the form of probability distributions. These distributions are based on historical data or expert judgements or a combination of both. There can also be unexpected events associated with different operations. The events have to be characterized with a certain probability of occurrence and extra time in the form of distributions, reflecting the effect these events can have.

Triangular and uniform distributions are the most common distribution shapes applied in cost and time forecasting. When the detailed suboperations with corresponding probability-­distribution curves are constructed, Monte Carlo simulation is applied. For each trial of Monte Carlo simulation, randomly selected values from each input-distribution curve are added together to yield a possible duration for the entire P&A operation. In reality, there might be dependencies among some suboperations such that the selected value for one suboperation influences the value of another suboperation. The issue of dependencies or correlations between suboperations is an important aspect to consider in acquiring a correct range of possible outcomes. The result of each trial is stored, and this procedure is repeated 100,000 times or more. The number of Monte Carlo simulations required will depend on when convergence of the resulting outcome can be observed. In the end, the stored results are presented in the form of probability-distribution curves or histograms. Cost usually is calculated by multiplying duration by the daily rate of rig or other services plus the fixed expenses, or it can be integrated into the Monte Carlo simulation process by use of input distributions for cost.

In order to implement the probabilistic approach in line with the classification system proposed in Oil and Gas UK (2011), there is a need for considering the aggregation of duration and costs on the phase level.

LWIV Technologies for P&A Operations

LWIVs can take over some parts of P&A operations. For instance, they have been used for executing the preparatory work of P&A, such as killing the well, punching the tubing, circulating heavy fluids down the tubing and up the annulus, securing the well by temporary plugs, and pulling the tree. A permanent barrier needs to isolate laterally across the full cross section of the well. Hence, the quality of cement behind the casing should be checked to achieve full-cross-section isolation. To verify the quality of cement behind 9⅝-in. casing, either old logging data are used or new logging has to be performed. Current logging technology is unable to log through multiple casings. Hence, tubing has to be retrieved to surface in order to log behind the 9⅝‑in. casing. In addition, if tubing has control lines attached to itself, it is necessary to pull the tubing to remove the control lines where the barriers will be positioned. If a well is equipped with a vertical tree, it is necessary to retrieve the tree before pulling the tubing. Before retrieving the tubing and tree, the well needs to be secured by temporary barriers. These operations can be accomplished by an LWIV. LWIVs also have been used in the UK sector of the North Sea to set the surface plug.

The complete paper provides a case example in which the proposed structure for cost and duration forecasting of P&A in a subsea multiwell campaign is implemented and tested.

Abandonment-Cost and -Duration Estimation

It is assumed that an LWIV completes preparatory work for all wells sequentially. Then a semisubmersible rig completes Phase 1, setting the primary and secondary barriers for all wells in the field. After setting the primary and secondary barriers, an LWIV is transferred to the location to complete Phase 2 of P&A for the field by use of riserless technology. Phase 3 of P&A, cutting and retrieval of the wellhead and the casing strings a few meters below the seabed, is completed by use of the abrasive cutting technology deployed from an LWIV.

Given the case description, the next step will be to establish the classification proposed in Oil and Gas UK (2011). This guideline provides different tables that can be used for determining the complexity of each phase, and thereby indicates the type of vessel required for each phase.

According to these guidelines, the likely duration associated with each phase can be determined by probabilistic or deterministic modeling, although in this study, probabilistic modeling is applied. On the basis of the proposed classification, Monte Carlo simulations were performed at the phase level. It was chosen to use 1,000,000 simulations, and triangular input distributions were used for all suboperations.

In cost and duration estimation of a multiwell campaign, one of the most common mistakes is to add mean or percentile values of individual distributions to acquire mean or percentile values for the entire field. This is correct for mean value but not for percentile values. To acquire cost and time-distribution curves for the entire field, Monte Carlo simulations are run 1,000,000 times such that in each run the randomly selected results from each phase with different complexity are aggregated. The duration-distribution curve resulting from aggregation of individual distributions is shown in Fig. 1.

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Fig. 1—Time-distribution curve for P&A of six subsea production wells.

 

Probability-density function (PDF) describes the likelihood of a random variable to take a certain value. This determines the likelihood of the P&A operation to have a certain duration. In addition, the cumulative-distribution function (CDF) describes the likelihood of a random variable to be less than or equal to a certain value. This can be used for determining the probability of completing the P&A operation within a certain time frame.

Cost Efficiency of Riserless Technology for Phase 2 of P&A—Setting the Surface Plug

To highlight the benefits associated with using a riserless technology for performing Phase 2 of P&A, a second scenario is considered wherein a semisubmersible rig sets the surface plug for all six wells in the field. Hence, upon completing preparatory work by an LWIV, a semisubmersible rig is transferred to the location to complete Phases 1 and 2, setting primary, secondary, and surface plugs for all wells in the field sequentially. Then, an LWIV completes Phase 3 of P&A—cutting and removal of the wellhead and conductor and casing strings a few meters below the seabed.

To establish a full cross-section surface barrier inside the 20-in. casing, both the 13⅜- and the 9⅝-in. casing have to be cut and removed by use of the semisubmersible rig. A schematic of the wells after P&A has been shown in Fig. 2. The operation sequence for this scenario has been listed in Appendix B of the complete paper. The sequence of operations for Phases 1 and 3 and for preparatory work is identical to that of the previous ­scenario; for brevity, they are not repeated. Use of a semisubmersible rig for removal of casings to set the surface plug is assumed to be a complex rig-based process.

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Fig. 2—Well schematic after P&A.

 

It is concluded that application of a riserless technology for setting the surface plug takes slightly less time compared with the use of a semisubmersible rig. In addition, this releases approximately 35 days of semisubmersible-rig time for this multiwell campaign.

Reference

Oil and Gas UK. Guideline on Well Abandonment Cost Estimation. 2011. Issue 1.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 169203, “An Investigation of Different Approaches for Probabilistic Cost and Time Estimation of Rigless Plug and Abandonment in a Subsea Multiwell Campaign,” by Fatemeh Moeinikia and Kjell Kåre Fjelde, SPE, University of Stavanger; Arild Saasen, SPE, Det Norske Oljeselskap ASA and the University of Stavanger; and Torbjørn Vrålstad, SPE, SINTEF, prepared for the 2014 SPE Bergen One Day Seminar, Bergen, Norway, 2 April. The paper has not been peer reviewed.