Huldra PP&A project. - from five to one double barrier. Audun Golberg / Jan Reidar Johnsen

Transcription

1 Huldra PP&A project - from five to one double barrier Audun Golberg / Jan Reidar Johnsen PAF Seminar, Stavanger Classification: Internal

2 The Huldra Field Huldra is a gas/condensate field Top reservoir: 3500m TVD. Reservoir pressure: Initially: 675 bar Today: bar (3-400 bar on A-11) Overburden gas pressures above normal (charged formations) Production start: 2001 Production end: wells to be permanently plugged and abandoned. PP&A start: License participants % Statoil (operator) 19,87 Petoro 31,95 Total E&P Norge 24,33 ConocoPhilips 23,33 Repsol 0,49 2 Classification: Internal

3 Pore pressures and formation strengths Predicted matrix pore pressure from logs Pore pressure in gas charged layers 3 Classification: Internal

4 Traditional approach The Huldra motivation Motivations for fewer barriers on Huldra Gas that migrates along the annulus will expand into the formation and loose pressure accordinggly. Good caprock and buffer sands above the overburden gas No leaks observed from old exploration wells Low gas rates observed at B-annulus 4 Classification: Internal

5 The other motivation Huge cost savings -for the license, but even more for the society. 5 Classification: Internal

6 Huldra PP&A subsurface studies Basis for establishing a safe and cost efficient PP&A solution for Huldra Outline Challenges Subsurface evaluation Main Conclusions Recommendation 6 Classification: Internal

7 Schematic illustration of Huldra P&A challenges 7 Classification: Internal

8 Gas chimney Clue to the Huldra pressure system and overburden gas fill mechanism Balder/Lista 8 Classification: Internal Shetland Shows gas leakage mechanism from chimney into Balder/Lista and partly into Shetland Balder/Lista keeps gas volumes in place below the Green Clay caprock, and has large storage capacity compared to gas vol in Shetland More limited (discontinuous) gas volumes available in Shetland compared to upper Rogaland.

9 Huldra Pressure system - post PP&A a 1. Competent natural formation barrier Dual Plug holds Balder/Lista pressure 3+5 Lower PP 3. Most leak-off Isolated PP Closed fractures in Brent caprock. Natural geological hydraulic leakage terminated due to depletion 4 4 Higher PP 3. Probably weak pressure depletion unless reservoir fill system is reestablished over geological time. 3. Less leak-off Brent reservoir isolated by plug, casing and cement 1. Future Partly reestablishing of Brent pressure: Pressure buildup from compaction, acquifer influx/expansion and continued very slow gas migration into depleted Brent reservoir P&A - cross flow process in well analogue to geological cross flow process in chimney 9 Classification: Internal

10 Overburden dynamics B-annulus pressure buildups in the Huldra wells Pressure buildup comparison Historical pressure buildups A-9 A-9 Lista Lista Other wells-shetland Historical pressure buildups A-5 Shetland 10 Classification: Internal

11 Inflow measurements Lista (A-9) A-9 flow data, 2011 Initial rate 500 Sm3/hr 26th March 2015 Initial rate 372 Sm3/hr 4th April 2015 Duration: 3 hrs 15 mins 13 hrs 15 mins 4,15 Sm3 Core from Balder confirms permeability in tuff layers (< 1-3 md) Adding inflow & x-flow capacity into Rogaland 11 Classification: Internal

12 Inflow potential Shetland (examplified by Well A-5) Inflow tests show: Very limited inflow capacities, declining with time after a limited gas expansion from a minor volume Reflects very low permeability, and loss of energy with time Duration: Average for 5 wells ca. 2 hrs Effective X-flow from Shetland to Rogaland: 0,002-0,007 m3/hr 12 Classification: Internal

13 Capacity of Green Clay caprock to withstand pressures from below Static concept A: Dynamic concept Geological cross-flow process Traditional way of static column thinking: - Barrier may break gas B: Dynamic concept: - Pressure release (leak-off) - Barrier will hold The dynamic concept applies the same mechanism that has naturally been ongoing in the gas chimney. Stimplan simulation of 1000 downhole m3/d injected in Kyrre predicts efficient leak-off below top Rogaland with only 1mD in upper layer and 0 perm. in layers between 1000 m3/d is almost 6000 times larger than highest rate expected from Shetland No fracture growth through the Green Clay 13 Classification: Internal

14 Huldra reservoir pressure prognosis Introduction This work is based on Statoils internal guidelines for determining reservoir pressure in foreseeable time after PP&A The main process ongoing is pressure equilibration between higher pressured Oseberg Formation and Brent in the A-11 area. Also considered: Water influx from chemical compaction in drainage area, Cross flow from Shetland/Rogaland into Brent within the wells Both have only insignificant influence 14 Classification: Internal

15 Historical vs. Future Pressure Development in Huldra Historical pressure data: Future in a 500 years perspective: Pressure buildup to max. 150 bar predicted in reservoir within a 500 years perspective. - This implies that in 5 wells, only one dual barrier plug above gas bearing formations in overburden is needed. The reservoir dual plug is then combined with the overburden dual plug. Due to higher pressure in the A-11 (Oseberg Fm.) an additional dual barrier plug above reservoir is planned for 15 Classification: Internal

16 Subsurface evaluation of the Huldra PP&A solution Main conclusions Detailed cross-disciplinary studies of all relevant data supports: Sealing capacity of Green Clay (caprock) Pressure buildup to above formation integrity of Green Clay will, due to cross flow, not occur Brent reservoir pressure will not exceed 150 bars (initial 675 bars) within foreseeable time (500 years) One dual barrier plug at top Rogaland does not increase the leakage risk compared to a solution with larger number of plugs. An additional dual plug above reservoir in A-11 is planned for as precaution for higher reservoir pressure (Oseberg Fm.) in that well. 16 Classification: Internal

17 Recommended solution based on subsurface studies The 1 dual barrier plug solution has been chosen as both a safe and cost efficient concept for PP&A on Huldra Future Brent pressures (500 years) will not exceed 150 bar Dual plug at top Rogaland will withstand 320 bar, equivalent to a Brent pressure of 330 bar 17 Classification: Internal

18 Recommendation based on Huldra PP&A Subsurface Studies In future P&A barrier concepts - it is important to include dynamic subsurface relationships (cross flow conditions) as part of the barrier integrity evaluations (new concept) It is recommended that the PP&A concept from Huldra also can be used in other areas with comparable geological conditions. However, this recommendation must be based on a thorough field specific subsurface evaluation of both the reservoir and the overburden. 18 Classification: Internal

19 Presentation title Presenters name Audun Golberg, Jan R. Johnsen Presenters title address Tel: Classification: Restricted