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Well Integrity Analysis Applied to Workover Prediction

Profile image of Kazuo MiuraKazuo Miura

2013, OTC Brasil

https://doi.org/10.4043/24369-MS
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8 pages

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Abstract

Well integrity may be defined as the well's capability to prevent leaks to the environment and is therefore a very important feature for oil and gas wells. Its importance has grown in past years, especially after recent events have shown the impact of oil spills on the operators' financial wellness. One way to avoid leaks during well operation is performing maintenance interventions, or workovers, seeking to keep redundancy in the well's safety barriers throughout the well production life. Cost assessment regarding the wells maintenance interventions shall be done during the well design phase of the Production Development project, when the wells construction campaign is both technically and economically evaluated. This work presents a methodology for well integrity analysis based on the concept of Barrier Integrated Sets (BIS) and on Reliability Engineering techniques. Through the computation of the mean time to failure of each BIS identified in the completion configuration it is possible to estimate when the well hits a degraded status, in which it relies on a single BIS and when the well is expected to leak, i.e. there are no BIS in place. These estimations may be used for a well workover interventions resource assessment, in a well construction and operation campaign.

FAQs

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What methodology is used for well integrity assessment in workover prediction?add

The paper proposes a methodology based on Barrier Integrated Sets (BIS) to assess well integrity, utilizing reliability analysis to predict the mean time to failure for workover interventions in oil wells.

How were failure rates and MTTF analyzed for well barriers?add

Failure rates of barriers within BIS were analyzed using historical data from 135 wells, resulting in a calculated MTTF of approximately 30.6 well-years for the subsequent interventions.

What was the reliability percentage of the primary and secondary BIS?add

The reliability for the primary BIS was determined to be 71.90%, while the secondary BIS had a reliability of 74.70%, leading to an overall well reliability of 53.71%.

How does the proposed methodology compare to field data on workover interventions?add

The proposed methodology indicated a prediction of 41 failure-motivated workover interventions, demonstrating a 15% error margin compared to actual interventions during the 17-year study.

What comprises the definition of Barrier Integrated Sets (BIS) in well design?add

Barrier Integrated Sets (BIS) are defined as an envelope of well barriers, integrating multiple elements to prevent fluid leakage from reservoirs to the environment.

Figures (8)
Figure 1:leakage pathways from the reservoir to the environment In order to define the BIS, Miura et al. (2006) adopted of pathways to be considered connecting the reservoir to the environment and Da Fonseca (2012) proposed the use of four main pathways: rock, wellbore annulus, wellbore and string (Figure 1).
Figure 1:leakage pathways from the reservoir to the environment In order to define the BIS, Miura et al. (2006) adopted of pathways to be considered connecting the reservoir to the environment and Da Fonseca (2012) proposed the use of four main pathways: rock, wellbore annulus, wellbore and string (Figure 1).
Figure 2:representation of the SSSV as a barrier and its components A set graph is a tool used to map the relation between elements of a set, making their relationship explicit. The se elements are represented by nodes and their relationship by archs. Set graphs may be used to denote the relationship betweer the well barrier components reliability and the well barrier reliability. In that sense, the Subsurface Safety Valve (SSSV) may be represented as shown in Figure 2, where each component reliability is described as “F” (false or zero) or “T” (true o: greater than zero).
Figure 2:representation of the SSSV as a barrier and its components A set graph is a tool used to map the relation between elements of a set, making their relationship explicit. The se elements are represented by nodes and their relationship by archs. Set graphs may be used to denote the relationship betweer the well barrier components reliability and the well barrier reliability. In that sense, the Subsurface Safety Valve (SSSV) may be represented as shown in Figure 2, where each component reliability is described as “F” (false or zero) or “T” (true o: greater than zero).
Figure 3: (a) completion schematic for the reliability analysis, (b) primary BIS and (c) secondary BIS The next step is identifying the Barriers Integrated Sets in the completion schematic. Figure 3b and Figure 3c demonstrate the two BIS identified for the completion schematic assumed.
Figure 3: (a) completion schematic for the reliability analysis, (b) primary BIS and (c) secondary BIS The next step is identifying the Barriers Integrated Sets in the completion schematic. Figure 3b and Figure 3c demonstrate the two BIS identified for the completion schematic assumed.
Table 1: BIS composition, failure modes, failure rates and reliability for 27 years The failure rates for the salt formations were considered to be 0 and therefore it is assumed that the formations will not fail. The gas-lift mandrel (and valve) is the most critical equipment in the completion design, with failure rates two or more orders of magnitude greater than the other equipment, which has a clear impact on this component’s MTTF.
Table 1: BIS composition, failure modes, failure rates and reliability for 27 years The failure rates for the salt formations were considered to be 0 and therefore it is assumed that the formations will not fail. The gas-lift mandrel (and valve) is the most critical equipment in the completion design, with failure rates two or more orders of magnitude greater than the other equipment, which has a clear impact on this component’s MTTF.
Figure 4: primary BIS graph with barriers relationships In order to quantify the reliability and mean time to failure of the BIS and of the well, the relationship between the barriers to form the BIS must be mapped. This is done through the barriers graphs, demonstrated in Figure 4 for the primary BIS.
Figure 4: primary BIS graph with barriers relationships In order to quantify the reliability and mean time to failure of the BIS and of the well, the relationship between the barriers to form the BIS must be mapped. This is done through the barriers graphs, demonstrated in Figure 4 for the primary BIS.
Table 2: results for the primary and secondary BIS and for the completed well From Table 2, the reliability for the primary BIS is significantly lower than the reliability for the secondary BIS and for the well. This is explained by the presence of the gas-lift mandrel (and valve) in the primary BIS: the MTTF of the primary BIS is very close to the MTTF of the gas-lift mandrel MTTF shown in Table 1. The MTTF shown in Table 2 are in well-years, which means that the cumulative time for each well is computed in order to assess the expected time to failure. In order to compute the reliability of the well, the well integrity was analyzed. In that sense, either the primary or the secondary BIS would need to be in place to consider the well integrity reliability. This is thus an assessment of how reliable the well is in terms of preventing leakage to the environment.
Table 2: results for the primary and secondary BIS and for the completed well From Table 2, the reliability for the primary BIS is significantly lower than the reliability for the secondary BIS and for the well. This is explained by the presence of the gas-lift mandrel (and valve) in the primary BIS: the MTTF of the primary BIS is very close to the MTTF of the gas-lift mandrel MTTF shown in Table 1. The MTTF shown in Table 2 are in well-years, which means that the cumulative time for each well is computed in order to assess the expected time to failure. In order to compute the reliability of the well, the well integrity was analyzed. In that sense, either the primary or the secondary BIS would need to be in place to consider the well integrity reliability. This is thus an assessment of how reliable the well is in terms of preventing leakage to the environment.
Table 3: results for the primary and secondary BIS and for the completed well During the 17 years of data collection, there were 36 workover interventions motivated by barrier failures. These interventions were motivated by failures of barriers from either the primary BIS (e.g. SSSV repair) or the secondary BIS (e.g. PAB repair). Furthermore, the analysis of the available data showed that there was not any workover intervention motivated by
Table 3: results for the primary and secondary BIS and for the completed well During the 17 years of data collection, there were 36 workover interventions motivated by barrier failures. These interventions were motivated by failures of barriers from either the primary BIS (e.g. SSSV repair) or the secondary BIS (e.g. PAB repair). Furthermore, the analysis of the available data showed that there was not any workover intervention motivated by
The methodology was then applied to the well completion schematic for the type-well used in the construction campaign. The reliability computed for the primary BIS was 71.90% and the reliability computed for the secondary BIS was 74.70%. Considering that both BIS must be in place in order to keep the production without an intervention, the well reliability considered the reliability of the primary and the secondary BIS, and the well reliability was 53.71%. The equivalent failure rate for this reliability is approximately 4.1707 x 10° failures/hour and the calculated MTTF is 30.6 well-years. Distributing the MTTF along the 1,284 well-years of the campaign, the methodology indicated 41 failure-motivated workover interventions would be performed. Figure 5 shows the workover intervention distribution along the 17 years of well construction and operation, from the methodology and the real field data. The results shown in Figure 5 demonstrate that the methodology was capable of predicting the overall tendency of failure- motivated workovers during the 17 years of well construction and operation. It is important to notice that the distribution extracted from the methodology and shown in Figure 5 was obtained from the well completion schematic and the respective failure rates, and therefore might be performed in the early stages of the production development project. The overall error of the method was around 15% (for the MTTF and the number of workover interventios), which is an acceptable figure in the early project phases. Furthermore, the result from the methodology is more conservative than the field data, since the method previewed more interventions than were actually performed.
The methodology was then applied to the well completion schematic for the type-well used in the construction campaign. The reliability computed for the primary BIS was 71.90% and the reliability computed for the secondary BIS was 74.70%. Considering that both BIS must be in place in order to keep the production without an intervention, the well reliability considered the reliability of the primary and the secondary BIS, and the well reliability was 53.71%. The equivalent failure rate for this reliability is approximately 4.1707 x 10° failures/hour and the calculated MTTF is 30.6 well-years. Distributing the MTTF along the 1,284 well-years of the campaign, the methodology indicated 41 failure-motivated workover interventions would be performed. Figure 5 shows the workover intervention distribution along the 17 years of well construction and operation, from the methodology and the real field data. The results shown in Figure 5 demonstrate that the methodology was capable of predicting the overall tendency of failure- motivated workovers during the 17 years of well construction and operation. It is important to notice that the distribution extracted from the methodology and shown in Figure 5 was obtained from the well completion schematic and the respective failure rates, and therefore might be performed in the early stages of the production development project. The overall error of the method was around 15% (for the MTTF and the number of workover interventios), which is an acceptable figure in the early project phases. Furthermore, the result from the methodology is more conservative than the field data, since the method previewed more interventions than were actually performed.

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