The Rotliegend aeolian sandstone of Early Permian age forms the main producing horizon in the Southern North Sea gas province. Currently [1990] some 30-40 fields have been identified in a broad zone measuring 450 by 100 km stretching from the English coast, through the UK and Dutch sectors of the North Sea and on to The Netherlands and West Germany. North Sea field sizes range between 10-300 Bcm (0.3-10.5 TCF). Ultimate recoverable reserves for the province are estimated at some 4250 Bcm (150 TCF).

The Rotliegend is underlain by Carboniferous coals and carbonaceous shales, which form the source rock, while Late Permian Zechstein evaporites form the seal. The aeolian sandstones occur interbedded with wadi sandstones and sabkha siltstones. Dune sandstones with large-scale cross bedding generally form the best reservoirs, though even here reservoir quality is highly variable.

Core porosities range between 5 and 30% and permeabilities from <1 to 3000 mD. Reservoir impairment through the effects of burial diagenesis (compaction, pressure solution, clay mineral transformations and cementation) is evident in many fields. In others, the occurrence of natural fractures serves to enhance production. As a result, individual well productivities show wide variation. (see Lower Permian Rotliegend reservoir of the Southern North Sea gas province, K. W. Glennie 1, 2 and D. M. J. Provan, (© The Geological Society 1990)

Types of traps – Anticlinal
Ways the traps were discovered - Seismic Formation (s) Lower Permian Rotleigendes

Current cost-cutting practices and issues

Technologies

• Velocity strings and foam-assisted lift can help wells suffering from liquid loading.
• Fresh water treatment to mitigate halite scale formation in production wells
• Novel designs for satellite platforms. Reduced functionality, such as platforms without helicopter decks or living quarters. Rreduces CAPEX
• Incorporation of a duplex steel subsea cooling loop so as to reduce CO2 corrosion at high temperatures. As a result, a lower cost carbon steel pipeline could be used in combination with the injection of a corrosion inhibitor.
• Enhanced Oil Recovery (EOR) techniques
  • Bright Water® is a microscopic, thermally-activated particle. A BP concept. Expands in the reservoir, diverting injection water into poorly swept areas of the reservoir, increasing oil recovery.
  • LoSal® EOR is a BP breakthrough low salinity waterflooding technique. Also reduces reservoir souring risk. Mainly for sandstone reservoirs.
  • Miscible gas EOR

Business Practices (applicable in general, not just Rotliegendes). Specific ideas

• Equal time rota for offshore staff . Reduces fixed personnel costs and associated logistics costs, such as helicopters. Reduce terminal operating cost and change how costs are shared between users.
• Enhanced procurement contracts. Centralize procurement (supply chain) functions. Access lower global costs via offshoring procurement team. Consolidate purchasing volume and leverage spend across assets, with reduced transaction costs and improved quality and accessing lower cost markets. Share this set-up with other operators, increasing buying power, leverage and savings.
• Message to suppliers: money saved at each step will be reinvested in more work for the supplier. Incentivised contract structures - KPIs for service providers: payment linked to performance
• Reduce contractor rates and company headcount
• Lower unit rates - Scale treatments, subsea inspection, repairs and maintenance.
• Share infrastructure (gathering center, pipelines, etc)
• Discuss contract and leasing terms with government to seek modification of regulations to permit operators to stay "in black" without compromising public safety or environment.
• Smart use of Well Services to perform offline intervention work, de-risking rig operations

Business Practices (applicable in general, not just Rotliegendes). General ideas

• Work quickly, but safely and effectively. Short time from first spend on project to first production. Continuous improvement. Unwavering focus on delivery - keeping it simple, focusing on the detail - getting it right first time
• Identify assets not important to current owners, with declining production, unsustainably high operating costs and decommissioning costs looming. Purchase at discount and make it a primary focus.
• Excellent teamwork and clear accountabilities across Subsurface / Wells Delivery / Production team
• Intelligent hedging use of credit lines, relaxation of covenants to revolving credit facility until later. Other financial tactics

Issues

• Window of opportunity to apply new methods closing. When fields become unprofitable, they will probably be abandoned and facilities decommissioned and removed. Once facilities removed will be very hard to return to asset and make profitable.
• Experience in field abandonment and facilities decommissioning and removal is currently limited, especially in offshore setting.
• Current low oil price may create opportunities.

My information came from four sources:

1. BP 2015 annual report
2. Dutch government 2015 report of Dutch sector activity
3. Material from and about Enquest Group. They are developing a very mature field that includes a Rotliegendes reservoir. The field is now called Alma - formerly it was Argyll.
4. Material from and about Parkmead Group. They are developing some onshore fields in The Netherlands which have Rotliegendes reservoirs.

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