MRA vurdering Kvitebjørn i forbindelse med PP&A av Nøkken

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1 Notat Til Eivind Ølberg Kopi Øystein Rantrud, Endre Aas Fra Vilde Krey Valle Sak MRA vurdering Kvitebjørn i forbindelse med PP&A av Nøkken 1 Bakgrunn Formålet med dette notatet er å vurdere om den planlagte permanente pluggingen (PP&A) av letebrønn Nøkken, tilhørende Kvitebjørn-lisensen, vil være dekket av miljørisikoanalysen for Kvitebjørn fra Pluggeoperasjoner er i utgangspunktet ikke beskrevet som spesifikke aktiviteter i denne analysen. En sammenligning av de viktigste parameterne som inngår i inngangsdata til miljørisikoanalysen utgjør basisen for vurderingen. Kvitebjørnfeltet er lokalisert i blokk 34/11 og produserer kondensat og gass. Vanndypet i området er 190 meter og avstanden til nærmeste land er 106 km (øygruppa Ytre Sula i Sogn og Fjordane). Vanndybden for Nøkken brønnen (subea templat) er 259 meter. Avstand mellom Nøkken og Kvitebjørn plattformen er 12.7 km. (Se Figur 1-1 for lokasjon av Nøkken i forhold til Kvitebjørnfeltet). Merk at Kvitebjørn er definert som et High Pressure High Temperature (HPHT) felt, Nøkken er ikke en HPHT brønn. Gradering: Internal Status: Draft Utløpsdato: Side 1 av 18

2 Figur 1-1 Lokasjon av Nøkken i forhold til Kvitebjørnfeltet 2 Resultater fra Miljørisikoanalysen for Kvitebjørn fra 2012 Miljørisikoanalysen for Kvitebjørn ble utført av Acona i Resultatene fra miljørisikoanalysen viser at miljørisikoen, for alle VØKer, ligger innenfor Statoils installasjonsspesifikke akseptkriterier i alle fire sesonger for både høyaktivitetsår og normalaktivitetsår. Se Figur 2-1 for høyeste miljørisiko ved et år med høy aktivitet. For år med normal aktivitet vil miljørisikoen være 55 % lavere. For flere detaljer henvises det til hovedrapporten fra Acona [1] Gradering: Internal Status: Draft Utløpsdato: Side 2 av 18

3 Figur 2-1 Miljørisiko for Kvitebjørnfeltet i år med høy aktivitet [1] Oljedriftsimuleringen viste at det ikke er sannsynlig med stranding (95-persentil) ved en utblåsning fra Kvitebjørnfeltet med drivtid mindre enn 20 døgn. 95-persentilen av korteste drivtid er 78 døgn og har en tilhørende strandet emulsjonsmengde på 29 tonn. 3 Sammenligning av inngangsdata til miljørisikoanalyse Inngangsdata til miljørisikoanalysen, blant annet oljetype, aktivitetsnivå og utblåsningssannsynlighet, -rater og -varigheter er hentet fra miljørisikoanalysen for Kvitebjørn fra 2012 [1] og Blowout Scenario Analysis for Nøkken fra 2015, se App A. Oljetype Oljetypen i Nøkkenbrønnen er den samme som for Kvitebjørnfeltet, Kvitebjørn lettolje (kondensat), se miljørisikoanalysen for flere detaljer [1]. Aktivitetsnivå Aktivitetsnivået som ligger til grunn for miljørisikoanalysen for Kvitebjørn er vist i Tabell 3-1 og aktivitetsoversikten for 2015 gitt av prosjektet er vist i Figur 3-1. Sannsynlighet for utblåsning ved pluggeoperasjoner kan antas å være tilsvarende som frekvensen benyttet for brønnoverhaling. se App A for flere detaljer. Tabell 3-1 Aktivitetsnivå for et høyaktivitetsår og et normalaktivitetsår, lagt til grunn i miljørisikoanalysen for Kvitebjørnfeltet [1] Aktivitetsnivå Deloperasjoner (antall pr. år) P olje (pr. år) Drilling (1,5) Høyaktivitetsår Completion (1,5) Workover (0,5) Wireline (1) Well intervention (4) Snubbing (1) Production (12) 4, 3x10-3 Gradering: Internal Status: Draft Utløpsdato: Side 3 av 18

4 Normalaktivitetsår Well intervention (2) Production (10) 2,2x10-3 Figur 3-1 Aktivitetsoversikt for Kvitebjørnfeltet for 2015, App A. Ut fra aktivitetsoversikten for 2015 (Figur 3-1) er det planlagt 1 brønnintervensjon samt boring av 1 brønn. En kan dermed konkludere med at PP&A av Nøkken vil være godt innenfor aktivitetsnivået lagt til grunn i miljørisikoanalysen for Kvitebjørnfeltet. Utblåsningsrater og -varigheter Tabell 3-2 og Tabell 3-3 gir utblåsningsrater og varigheter som ligger til grunn for hhv. Kvitebjørnfeltet og Nøkken. Tabell 3-2 Utblåsningsrate og -varighet for Kvitebjørnfeltet [1] Sannsynlighetsfordeling Rate (Sm 3 /d) Sannsynlighetsfordeling - varighet Sannsynlighet for raten Overflate ,66 0,14 0,09 0, Tabell 3-3 Rate og varighetsfordeling for Nøkken, App A. Sannsynlighetsfordeling Rate (Sm 3 /d) Sannsynlighetsfordeling - varighet Sannsynlighet for raten Overflate 0, ,66 0,14 0,09 0,03 0, Sjøbunn 0, ,49 0,16 0,14 0,06 0, Utblåsningsraten for Nøkken er beregnet til 3300 Sm 3 /d, dette er godt under utblåsningsraten for Kvitebjørn som er estimert til 5300 Sm 3 /dag. Gradering: Internal Status: Draft Utløpsdato: Side 4 av 18

5 Utblåsningsvarigheten for Kvitebjørn er vurdert av prosjektet og funnet gyldig for PP&A av Nøkken. Kvitebjørn er definert som et High Pressure High Temperature (HPHT) felt og utblåsningsvarigheten for Kvitebjørn vil trolig være høyere enn for Nøkken som ikke er en HPHT brønn. Modelleringsverktøy, miljøressursgrunnlag og akseptkriterier Miljørisikoanalysen for Kvitebjørn (2012) ble utført med OSCAR. Det er ikke betydelige endringer i miljøressursgrunnlaget fra Akseptkriteriene som ble benyttet i 2009 analysen er uendret. 4 Vurdering av fortsatt gyldighet av miljørisikoanalysen fra konklusjon En gjennomgang av inngangsdata viser at alle parameterne til PP&A av Nøkken er godt innenfor forutsetningene som ligger til grunn i miljørisikoanalysen for Kvitebjørnfeltet. Miljørisikoanalysen for Kvitebjørn vil derfor være dekkende for PP&A av Nøkken brønnen. 5 Referanser [1] Acona (2012) Stokastiske oljedriftsimulering og miljørisikoanalyse for planlagt aktivitet på feltet Kvitebjørn (PL 193) En analyse for Statoil ASA. Rapport nr: Gradering: Internal Status: Draft Utløpsdato: Side 5 av 18

6 App A Technical note Blowout Scenario Analysis Input to evaluation of whether the PP&A operation on Nøkken is covered by the environmental risk analysis for Kvitebjørn. Alexander Solberg, TPD TEX SST ST January 12 th 2015 Summary This note presents a quantitative assessment of blowout risk related to the PP&A operation on Nøkken to evaluate if the operation is covered by the ERA for the Kvitebjørn field (HPHT). Blowout probability, flow rates and duration are quantified for comparison. The overall blowout probability of the well is judged to be The potential oil blowout rates range up to 3300 Sm 3 /d. Note that this is based on the conservative assumption that unrestricted flow will be possible through the cement plugs placed in the well that, given their integrity, should ensure zero flow potential during the PP&A operation. It is found that the duration of a blowout could potentially amount to 98 days with a 1 % probability. Nøkken Probability top/ sub Rate (Sm3/d) Probability distribution - duration Scenario probability Topside Subsea Nøkken data compared to Kvitebjørn ERA Activity level The activity level for Kvitebjørn used in the current ERA (year of high activity) is presented below. Table 1: Activity level 2012, year of peak activity Activity Number of operations Gas well Drilling 2 Completion 2 Workover 1 Gradering: Internal Status: Draft Utløpsdato: Side 6 av 18

7 Wireline 2 Well intervention 1 4 Snubbing 1 Coiled Tubing 1 Production 12 For comparison the drilling and well master schedule for 2015 is shown below It is observed that only 1 well intervention campaign is planned for 2015 and one well to be drilled (A16). Thus, it is evaluated that the activity level presented in the ERA is well within the expected activity level for Kvitebjørn, also when including the PP&A job. Rate A oil blowout rate of 3,300 Sm 3 /day is estimated for the Nøkken well. This is well below the 5,300 Sm 3 /day oil blowout rate used in the Kvitebjørn study for workover activities. This is conservative as the well is currently plugged and, given the integrity of the cement plugs, should ensure zero flow potential during the PP&A operation. Duration The duration data for Kvitebjørn has been reviewed by the project and is found valid for the Nøkken PP&A operation. As Kvitebjørn is a HTHP Field the duration is likely higher than for Nøkken (not HTHP). Based on the input and evaluations performed by the project is is found reasonable that the parameters used for Kvitebjørt wrt. frequency, rate and duration also covers the Nøkken PP&A operation. 1 Well Intervention is given the same blowout probability as a workover operation. Gradering: Internal Status: Draft Utløpsdato: Side 7 av 18

8 6 Introduction The purpose of this note is to provide input to the environmental risk analysis for the PP&A operation on Nøkken to evaluate wheter it can be considered covered by the Kvitebjørn ERA. Blowout probability, rates and duration will be evaluated. Kvitebjørn is a gas/ condensate field located in the Southern Central part of block 34/11, 41 km south-east of Statfjord B. The Kvitebjørn platform consists of a topside structure which is supported by a four legged slender steel jacket structure. The water depth at the field is 190m and the air gap is 24.5 m. The installation can produce from up to 16 wells. The Nøkken well is situated on a subsea template at water depth 259 meters MSL. Note that Nøkken is not a HTHP well as opposed to the Kvitebjørn wells. The assessment of risk figures in this note is based on: Historical blowout statistics /1/ Blowout and well leak frequencies /2/ Calculated blowout rates from the reservoir, surface and seabed /5/ Project specific input /3/, /4/ & /6/ Judgements and considerations in TPD TEX HSEC ST and in dialogue with the relevant organisation. Gradering: Internal Status: Draft Utløpsdato: Side 8 av 18

9 7 Well specific information Water depth at well location is 261 meters MSL. The distance RT-MSL is 25 meters. A well schematics of the well is shown belowin Figure 1. Figure 1: Well Schematic for well 34/11-2S P&A Expected reservoir data and fluid properties are found in Gradering: Internal Status: Draft Utløpsdato: Side 9 av 18

10 Table 1 and Error! Reference source not found. below. Gradering: Internal Status: Draft Utløpsdato: Side 10 av 18

11 Table 1: Reservoir data for well 34/11-2S, ref /4/ Reservoir Data Unit Perforation interval 1 Perforation interval 2 Tarbert Ness Sum Ness Etive Sum Top perforation intervall m TVD RKB Bottom perforation intervall Total formation thickness m TVT Net formation thickness m TVT Net/Gross v/v Porosity v/v Permeability md Kv/kh ratio Temperature (top res) C Pressure at top of reservoir bar Reservoir length along well m Reservoir width across well m X-position of well within reservoir Y-position of well within reservoir m m Sw % Table 2: Fluid properties for the expected fluid from well 34/11-2S, ref /4/. Fluid data Unit Perforation interval 1 (Tarbert / Ness) FLUID PROPERTIES AT SURFACE CONDITIONS Perforation interval 2 (Ness/ Etive) Oil density kg/m Gas gravity sg GOR Sm3/Sm FLUID PROPERTIES AT RESERVOIR CONDITIONS Fluid type gas/oil/cond gas/ cond gas/ cond Gas density Oil density Viscosity g/cc g/cc cp CO2 % 4 4 N2 % Formation Gas Volume Factor, Bg Formation Oil Volume Factor, Bo Rm3/Sm3 Rm3/Sm3 Gradering: Internal Status: Draft Utløpsdato: Side 11 av 18

12 8 Blowout scenarios and probabilities During a plugging operation a blowout may result if the well pressure is in under balance with the formation pore pressure (well pressure < reservoir pressure), and a loss of well control follows. As the well is already drilled one scenario is defined: 1. Tripping Kick and loss of well control through wellbore, typically due to swabbing during tripping. The overview of blowout causes given in /1/ (Table 4.9) combined with an assumption of open hole flow do, in our opinion, justify the following probabilities: P(Tripping blowout) = 1,00 The blowout frequencies found in Scandpower /2/ are the outset of our assessment. Plug and abandonment is not defined as a separate activity in /2/ and workover is evaluated to be the most appropriate category. As the expected fluid is oil, an oil blowout frequency is used below; P(blowout, heavy workover, gas well) = per well The frequency relate to an average well with blowout potential from a deep reservoir (shallow gas not included). Such blowout frequencies are typically applied to describe heavy workover. The frequency is considered conservative for the P&A on Nøkken and an adjustment of the average frequency has been performed based on the following arguments: - The well has two tested plugs installed. The plug will be in place for the duration of the P&A operation. Thus, potential surge and/ or swab effects resulting from the operation on the wellbore should not affect the reservoir. The cement plug has not been tested to the current requirement of 70 bar above the fracture pressure of the casing shoe 2 but no indication that the integrity of the plugs are weakened has been observed. Appropriate frequency reduction factor: 50%. Based on the arguments above it is evaluated that a 50% reduction of the average heavy workover blowout probability is justified. P(blowout, P&A operation Nøkken, gas well) = per well Songa Trym will be used for plugging the well. This is a semi-submersible drill rig that will operate on thruster assisted mooring. Based on information in Table 6.2 /2/ and an overall evaluation of different scenarios and sort of vessel a probability distribution between seabed and surface release scenarios is set to 75% and 25% in order of appearance. This results in the following probabilities: P(blowout with seabed release) = ,75 = P(blowout with surface release) = ,25 = The pressure test performed was to 175 bar SF w/ 1.59 sg mud (DBR: ). Gradering: Internal Status: Draft Utløpsdato: Side 12 av 18

13 8.1 Blowout Frequency, Kvitebjørn The Kvitebjørn field is in a development phase. High well shut-in pressures and temperatures are expected and the wells are categorized as HPHT. The area of development is considered well known. No particular risk factors beyond the HPHT properties that might complicate the drilling operation have been identified. The activity level on Kvitebjørn was evaluated by the project based on information regarding the planned operations for the next 5 year period. A year of peak activity is described by the project /4/ as presented in Table 1. Table 1: Activity level 2012, year of peak activity Activity Number of operations Gas well Drilling 2 Completion 2 Workover 1 Wireline 2 Well intervention 3 4 Snubbing 1 Coiled Tubing 1 Production 12 The resulting blowout probability relative to a year of peak activity is: P(blowout, development drilling, gas well) = = P(blowout, completion, oil well) = = P(blowout, workover, oil well) = = P(blowout, wireline, oil well) = = P(blowout, well intervention, oil well) = = P(blowout, snubbing, oil well) = = P(blowout, coiled tubing, oil well) = = P(blowout, production, oil well) = = = P(blowout in a year of peak activity) The expected acitivity level for 2015 on Kvitebjørn includes 1 well intervention campaign. Thus, the addition of the Nøkken PP&A operation is within the premise of the current ERA for the Field. 3 Well Intervention is given the same blowout probability as a workover operation. Gradering: Internal Status: Draft Utløpsdato: Side 13 av 18

14 9 Blowout rates A scenario for blowout rate calculations has been defined and scenario probability distribution adjusted based on the above and well specific information. Blowout rates and scenario probabilities combined describe a well specific risk picture. Blowout rates to surface and seabed has been calculated in GAP/ Propser by ST, ref /3/. The simulated scenario consider an open hole flow with the finished well design as shown in Figure 1, and the simulation results are shown below in Table 3. Table 3: Simulated blowout oil rates (Sm 3 /d) and probabilities. Scenarios Scenario probability Blowout rates*, (Sm3/d) Seabed Surface Tripping 100% Average rate * Adjusted towards the nearest hundred. It is assumed that the scenarios involve free, unrestricted open hole flow and a total failure of the BOP. The well is currently plugged with 2 cement plugs. The deep set cement plugs will be in place for the duration of the P&A operation. Given the integrity of the cement plugs the well will not have a flow potential, i.e. the premise of the simulations regarding free, unrestricted flow is very conservative. The simulations were performed by TPD TEX SST ST based on the input presented in Error! Reference source not found. and Error! Reference source not found.. For a full description of the rate calculations see the GAP/ Prosper simulation files /3/. Gradering: Internal Status: Draft Utløpsdato: Side 14 av 18

15 10 Blowout duration A condensate blowout can be stopped by: 1. Operator actions mechanical (capping) 2. Wellbore collapse and/or rock material plugging the well (bridging) 3. Altered fluid characteristics resulting from water or gas coning during a blowout 4. Drilling a relief well and applying kill mud The probability distribution of the duration of a possible blowout is derived by way of the approach utilised in /2/. Water and gas coning are not considered in the assessment. Well specific input about time to drill two relief wells /3/, was given by the project for Kvitebjørn and presented in Table 4. The input is evaluated to be conservative as Kvitebjørn is a HTHP Field and expected to have a longer duration than do Nøkken PP&A. Table 4: Time to drill a relief well (days) Time to: Minimum: Most likely: Maximum: - make decisions mobilise a rig; transfer, anchoring, supply of equipment and preparations drilling 8 ½ geomagnetic steering into the well killing the well The required time to drill a relief well and kill a blowout is judged by the project to be between 56 and 112 days. A Monte Carlo simulation is performed to produce a duration distribution from the well specific input in Table 4.1. The expected time found is 81 days. A probability distribution is presented in Figure 2. Gradering: Internal Status: Draft Utløpsdato: Side 15 av 18

16 Probability 0,45 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0, Time to drill a relief well (days) Figure 2: Duration distribution, Time to drill a relief well The probability distribution, found in Table 5 below, is constructed by combination of the well specific duration distribution and probabilities that a blowout will end by the mechanisms capping and bridging /2/. Based on Table 5 maximum blowout duration is suggested to be 98 days. Table 5: Probability distribution for a blowout to end as a function of time (days) Duration (days) Surface blowout Seabed blowout Duration (days) Surface blowout Seabed blowout 0,5 0,41 0, ,00 0,01 1 0,12 0, ,00 0,00 2 0,13 0, ,00 0,00 5 0,14 0, ,00 0,00 7 0,04 0, ,004 0, ,03 0, ,016 0, ,02 0, ,027 0, ,02 0, ,018 0, ,01 0, ,005* 0,011* 35 0,00 0,01 *Probabilities in the tail end of the duration distribution (< 0,003) are added to the probability of the preceding duration category. Different probability descriptions of the duration of a seabed or surface blowout are produced. Possible durations of a seabed or surface blowout are described by probabilities in Figure 3. In Error! Reference source not found. seabed and surface blowout duration and time to drill a relief well are described by cumulative probability curves. Gradering: Internal Status: Draft Utløpsdato: Side 16 av 18

17 0,60 0,50 0,40 ility b a0,30 b ro P 0,20 Surface Seabed 0,10 0, Blowout duration (days) Figure 3: Blowout duration described by proba bility distributions 11 Gradering: Internal Status: Draft Utløpsdato: Side 17 av 18

18 Referanser /1/ Holand, Per: Blowout and Well Release Characteristics and Frequencies, 2013, Sintef Technology and Society, Report no F25705, rev Final Report, December 19th /2/ Scandpower: Blowout and Well Release Frequencies based on SINTEF Offshore Blowout Database 2013, report no /2014/R3, ref Final, /3/ 2012 Inputdata for oppdatering av miljørisikoanalyse - Kvitebjørn, Preben Harlo, /4/ Reference documents are located on team site /5/ Simulation files are located on team site /6/ The Nøkken project have a team site and older documents located on the G:\ drive (G:\E\EPDS\DATA\WELLS\NO_0034\ _s) Gradering: Internal Status: Draft Utløpsdato: Side 18 av 18