Innovative Approach to Effectively Stimulate Openhole Sections of Horizontal Laterals

Abstract

This paper describes design methodology, execution details, and production results of stimulation treatments in openhole sections of laterals in the Williston Basin. It will be demonstrated that a sequenced fracturing technique utilizing appropriate hydraulic fracturing design with proprietary diversion material can effectively stimulate 900 to 2,500 ft. long barefoot intervals of horizontal laterals drilled in the Bakken group of formations. In order to achieve multiple hydraulic fractures in an openhole environment, it is necessary to have a diversion material that bridges effectively in the current fracture, and can withstand enough pressure to allow initiation and propagation of subsequent fractures. In this case, a blend of four distinct particle sizes and fibers made from degradable synthetic polymer was pumped following each fracturing stage in order to divert subsequent treatments from previously created fractures. The number of stimulation stages in the design has been selected based on openhole interval length and the number of designed fractures. Two wells have been completed in the Williston Basin with TVD's between 9,800 and 11,200 ft. and fracture gradients between 0.85 and 0.97 psi/ft. Due to operational difficulties while running casing, approximately 900 ft and 2,500 ft of the toe sections were left as an openhole completion. This represented 10% and 29% of respective lateral lengths. Both openhole intervals were stimulated with a bullhead sequence fracturing technique utilizing 11 and 24 stimulation stages respectively, pumped sequentially and separated by diversion pills. During stimulation operations the instantaneous shut-in pressures (ISIPs) were used as an indicator of diversion efficiency. Based on the overall trend it can be concluded that the early fractures initiated in the lowest stress areas and then were diverted to higher stress sections reaching the highest local fracture gradients by the end of the treatment. The ISIP increases were 1,400 and 2,100 psi for the respective laterals. The values of ISIP in the openhole intervals correspond very closely to the lowest and highest pressures seen in the cased portion of respective laterals. This is a strong indication that multiple fractures were created over the lengthy openhole intervals. Production results at 270 days and 100 days indicate that these wells are among the top producers in their areas. This paper describes the first application of the sequence fracturing technique in horizontal wells with openhole completions. The ability to create multiple fractures in an openhole environment without packers or other mechanical isolation opens options for improving operational efficiency in any un-cemented environment. This technique provides an alternative completion method for these types of wells which can lead to significant production improvement because no part of the drilled lateral is left unstimulated. Adding effective diversion to this type of completion could significantly increase the number of productive fractures that are created.