Effective Practices in the Abandonment of Canada's Deep Sour Gas Wells

Abstract

To meet rising global energy demand, the oil and gas industry is growing at an unprecedented rate. As mature oil and gas fields reach end-of-life, safe and cost effective abandonment techniques are now and will continue to be a critical part of the industry. Ineffective abandonment methods are a serious economic and environmental liability to an operator, particularly one having wells with surface casing vent flows. The operator's Foothills fields, located in central Alberta, Canada, are home to some of the harshest downhole producing environments in the country. Foothills wells are deep (>3500 m true vertical depth), sour (5% to 90% H2S) and mature (1947 to present) with average well reserves of over 80 million standard cubic meters of gas. They experience sulfur, iron sulfide, calcium carbonate and sodium chloride deposition. Saline water influx causes corrosion of the production tubing strings. Most of the wells have low casing cement tops leaving multiple porous geologic formations uncemented. These factors present many challenges during well abandonment operations. A well abandonment campaign for these fields began in 2011 and will continue for many years. To date over 40 wells have been abandoned. This paper will describe best practices learned, as well as "game changing" strategies that have led to significant cost savings and reduced operational risk. These include bullheading foamed cement to abandon the producing zone prior to moving in a service rig. This is followed by a "one-trip" abandonment utilizing through-tubing perforating and bradenhead cement squeezes with high pressure, slow-rate pumpers. The percentage of total well abandonment cost savings will be presented for each strategy. The principles for choosing cement squeeze intervals for the isolation of porous zones, and the success rates of surface casing vent flow shutoff will be discussed. This paper will also present data from cement plug drillouts which show that cement platforms are not required beneath a balanced cement plug in near vertical wellbores. These practical applications will reduce costs, lower risk exposure and lead to improved results in downhole abandonments.