Successful Hydraulic Fracture Stimulation of Yoredale Carboniferous Sands in the UKCS

Abstract

Abstract The Breagh gas field lies in the UK Southern North Sea, 100 km east of Teesside in Blocks 42/13, 42/12a and 42/8a. The normally-unmanned gas field came on stream in October 2013 and is the first offshore development of the Yoredale formation of the Lower Carboniferous. The reservoir comprises a mixture of sand sheets and channels with permeabilities predominantly in the range 0.1 to 10 md. The plan for the first phase of Breagh Field development was to drill 10 conventionally completed 65° angle deviated wells, through the reservoir, from a 12 slot platform. Analysis of log and PLT data from some of the initial wells identified channel sands, tighter than expected, yet containing significant GIIP. Wells 7 and 8 of the first phase of drilling were completed using hydraulic fracturing to evaluate the potential for improvement in productivity and drainage from the lower permeability intervals, although the execution of the stimulation programme brought significant challenges. This paper introduces the Breagh Field, discusses the reasons behind the change in completion design to include hydraulic fracture stimulation, considers the challenges associated with fracture placement in high angle wells and solutions implemented. The paper also addresses the importance of the clean-up phase, and reviews the lessons learned. In support of this programme, a number of hydraulic fracture designs and modelling methods were evaluated and used to make real time decisions during stimulation operations. These methods are discussed and used to estimate the value of hydraulic fracture stimulation in the Breagh field to date. The first two wells to be stimulated on Breagh have been successful, yielding initial increases in productivity of the order of 4–10 fold. The magnitude of this increase has led to a forecast of significantly higher recovery from the lower permeability sands and from the total field. Important learnings from the subsurface characterisation, fracture planning and execution phases have been made. As a result of the success to date, hydraulic fracture stimulation is being integrated into the well completion planning and facilities design for the next phase of Breagh Field development.