Souring of Kaybob Duvernay Wells: Investigation of Frac Barrier Effectiveness, Completions Design and Pre-Duvernay Structural Features

Abstract

The Devonian Duvernay Formation is an organic-rich mudrock that is a prolific self-sourcing light oil to condensate-rich gas reservoir in the Kaybob region of Central Alberta. Well tests show initial rates up to 3900 boe/d with EUR values less than 1.6 MMBOE. As the Duvernay play appraisal progressed since 2010, more wells are testing or producing sour hydrocarbons (0.5-8000 ppm) from a sweet hydrocarbon play. A need to understand and model the processes that sour Duvernay wells is crucial as souring impacts the economics, health and safety of operations. Both the underlying Beaverhill Lake Group and the Duvernay laterally equivalent Leduc Formation contain conventional oil and gas pools that are sour (< 45%). These sour pools are the most probable source of the H2S seen in ∼10% of drilled wells (June, 2015) within the Kaybob Duvernay play. The possible reasons for souring include artificial and natural fracture conduits, proximal Leduc reef facies changes, communication between existing sour Duvernay and Beaverhill Lake wells or in-situ thermal sulfate reduction processes. The most likely cause of this souring is artificial fracturing and the increase in sour wells over time is due to, in part, the increase in frac fluid volumes from 5-10 m³/m to 25-35 m³/m (2012-2015). Companies have increased in frac volume designs based on observation that the EUR increases with increasing frac fluid volumes (including proppant loading). Cross plotting the frac fluid volume (m³/m) and the basal frac barrier thickness indicates that as the basal frac barrier thins (< 18m) the barrier becomes less effective and the risk of souring increases. The basal frac barrier consists of the Duvernay middle carbonate, Majeau Lake and Waterways formations. Increasing the frac fluid volume (i.e., from 15m³/m to 30m³/m) will reduce the effectiveness of the basal frac barrier and also increase the risk of souring Duvernay wells in areas where the frac barrier is greater than 18 m thick. Sour risking map has been created for the Kaybob Duvernay play based the basal frac barrier thickness and mapping of Beaverhill Lake pools and Leduc Reefs.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016