Early Insights from Various Load Fluid Recoveries after Hydraulic Fracturing Treatments: A Case History of a Developing Unconventional Oil Shale Play in Alberta

摘要

The Devonian Duvernay Formation in Alberta,characterized as a carbonate-siliceous source rock,is ramping up to be one of the largest and most prolific shale oil plays in Canada.In the southern part of the Duvernay Shale Basin(i.e.East Shale Basin),tight limestone beds are interbedded with laminated organicrich calcareous shales,which show an organic maturity ranging mostly from early oil-to condensatewindow.This new light oil shale play is still in the initial stages of development and the nature of these deposits requires hydraulic fracturing to increase stimulated rock volume.General completion programs involve ≥50 clustered plug’n’perf stages with slickwater treatments in excess of 40,000 m3 with ~4000 tonnes of proppant per well.The large water volume treatments will inevitably interact directly with the rock surface in the stimulated area and cause both oil-water and rock-water interactions.Post-hydraulic fracturing water retention is especially pronounced in light oil shale plays.The oil-wet nature of the Duvernay,along with calcareous and siliceous shale lithologies,adds to the complexity of water retention and perceived water-blockage.In addition,because of operational delays such as road bans and pipeline constraints,some wells may be shut-in after the fracturing treatment for weeks and even months,which will affect rock-oilwater behavior(i.e.production).The extent of water displacing into the matrix of the rocks of the Duvernay Formation in the East Shale Basin,as measured by load fluid recovery,varies significantly and appears to heavily rely on the choice of surfactant.Although the use of surfactants is generally accepted for this play,detailed understanding of the rock-fluid interaction mechanisms is still incomplete.This paper investigated the response of Duvernay Shale rocks from the East Shale Basin to various types of surfactants and analyzed production and fluid flowback data.Amott Cell analyses,which test for spontaneous oil displacement using various stimulation fluid types,demonstrated that in the East Shale Basin,nano-sized surfactants including multi-functional surfactants(MFS)and microemulsions significantly outperformed common surfactant chemistry when tested with mixed wettability shale core samples.The results provide an estimate as to extent of water migration into the matrix of the Duvernay as a result of the choice of surfactant.Our analysis is made possible from publicly available cores,laboratory analysis and high quality well production data from the Alberta Energy Regulator.