Abstract Safety is a primary concern at underground gas storage (UGS) sites. Thorough evaluation of all potential migration pathways is critical to ensure UGS containment and public safety. Substantial risk is directly associated with inaccurate technical evaluations, that may result in subsequent gas migration from a UGS facility. Existing wellbores, including abandoned oil and gas wells, old dry exploration wells and water wells represent primary potential vertical gas migration conduits, which are not always thoroughly analysed during UGS site evaluation studies. Most abandoned oil and gas wells develop leaks over time, even when plugged in accordance with current (US) government regulations. Leaking wells in urban areas represent significant health and safety hazards. Many cities including Los Angeles permit construction of new homes directly over abandoned wells, even though state agencies recommend against this practice, thus placing residents at risk. Explosions and fires, along with possible exposure to substances such as benzene and toluene, are possible when gas reaches the surface through leaking wells and accumulates inside building voids. These potential problems are amplified with UGS fields, where operating pressures, already raised above the declining pressures of the field, fluctuate when alternating gas injection and extraction induce cyclic stress on wellbores and caprock sequences.

Abstract The Erskine high-pressure–high-temperature gas condensate field was the first such field developed in the UK Continental Shelf. Since production started in 1997, the field has produced over 350 bcf of gas and 70 MMbbl of condensate. The reservoir pressure has depleted from an initial pressure of 960 bar (13 920 psi) down to 140–400 bar (2030–5800 psi), resulting in some compaction and sand production in some of the wells. Free water production has led to the formation of wellbore scale, which has required interventions to remove. The reservoirs are sandstones of the Jurassic Puffin, Pentland and Heather formations. Estimates of hydrocarbons in place made using production and pressure data compare favourably with the initial estimates made during field development planning, although the Pentland Formation volume is some 20% below the sanction estimate. Several major field outages have occurred, such as a condensate fire in 2010 and a blockage of the multiphase export pipeline in 2007. In addition, the field has experienced flow assurance problems related to scale and wax deposition. A new pipeline section was installed in 2018 to bypass a full pipeline blockage which occurred due to wax deposition.