Multizonal coiled tubing fracturing (CT frac) services have been successfully performed in shallow gas fields in western Canada for more than 1 year. Properly managing and designing the serviceability of CT strings is critical to these operations.
This paper discusses the practical and theoretical aspects of optimizing the frac-string design, changing the equipment geometry of the CT units, and using a CT fatigue model and fatigue-simulation software. The paper also discusses factors affecting the longevity and performance of frac strings, programs for continuously monitoring the service life of the strings, and actions considered for recovering and extending the fatigue life.
With a properly designed and implemented comprehensive tubing management program, service operators in western Canada have helped ensure a high degree of personnel safety during fracturing while minimizing the risk of CT failure and maximizing pipe life.
Fracture stimulation is required for economic gas production in the shallow gas fields of southeastern Alberta and southwestern Saskatchewan. In these fields, gas is produced from the Second White Specs, Medicine Hat, and the Lower and Upper Milk River formations. These formations are medium- to fine-grained sandstones and siltstones interbedded with shales and mudstones.
The wells in these formations have traditionally been perforated and fractured with multiple operations at the wellsite; one operation is performed for every zone completed. The fracturing fluids used were predominantly crosslinked water energized with carbon dioxide (CO2) or nitrogen (N2). Previous fracture designs typically consisted of a small pad (3.0 to 5.0 m3, 20 to 30 tonnes of 20/40-mesh frac sand placed at an average concentration of approximately 1500 kg/m3).
In the late 1990's, a new method of fracturing these wells was developed. This method, which involves CT and a selective-injection packer assembly, allows multiple fracture stimulations in one operation at the wellsite. The fluids (energized, crosslinked-water gels) and the fracture designs are similar. When this method is used, the service company typically fracture-stimulates more sections with smaller volumes of sand per fracture. Generally, the total volume of sand placed is similar to that for previous techniques. The pad volumes are generally limited to the CT volume, and sand concentrations of approximately 1500 kg/m3 are placed. The volume of proppant placed in these fractures ranged from 5.0 to 30 tonnes. The fractures were pumped down CT at rates of 1.5 to 2.0 m3/min and surface pressures of 35 to 40 MPa.
Designing strings for fracturing operations involves evaluating the different criteria the CT should meet, including fracture-stimulation design, CT mechanical parameters, and economics. The ultimate design goal is to meet these requirements while ensuring that the service string has adequate stress/force capabilities throughout its life, extending fatigue life and string revenue.
High-pressure fracturing treatments with energized fluids can compromise personnel safety and potentially lead to catastrophic string failure. Because large-diameter CT has a shorter fatigue life than small-diameter CT, the service company had to continually investigate string serviceability and search for ways to optimize the parameters that influence the operational longevity of the CT frac strings.