Annular Frac operations performed with Coiled Tubing (CT) offer many advantages for unconventional completions, particularly wells with long laterals and many pay zones (frequently in excess of 100 stages). The primary disadvantage to annular frac is erosion of the coiled tubing due to impinged, high pressure fluids containing abrasive frac sand. This paper will describe a methodology to detect erosion of the CT to provide consistent, reliable operations.
When designing equipment and writing operational procedures to detect erosion in annular frac through coiled tubing, the following considerations may be considered: CT string design and pumped fluid flow rate when fracturing. From a CT service company perspective, the fluid dynamics of the pumped fluid may contain uncontrolled variables such as fluid density, viscosity, and slurry from job-to-job.
As a result of the limitations noted above, the onset of erosion may be difficult to predict. However non-destructive electromagnetic inspection can be utilized to highlight possible locations of erosion within CT strings to develop "field-tested" guidelines for pumping against tubing size.
Electromagnetic inspection using Magnetic Flux Leakage (MFL) and/or Hall Effect Sensors can highlight localized variations in wall thickness. However, this information alone does not give a clear indication if the tubing has been damaged by erosion without a baseline inspection to compare to, since there can be variations in wall due to the CT manufacturing process and the prevalence of tapered CT string designs.
If the CT string is inspected either when new or very early on in its life, a comparison of wall variation by electromagnetic methods can "rule out" wall thickness variations that were present at the time of manufacturing. Evaluation of CT strings with electromagnetic inspections performed when new and after retirement will be presented in this paper. The inspection results will then be supplemented by pumping parameters from annular frac jobs performed with these strings.
This paper describes a methodology of verifying that CT strings have not been subject to erosion due to annular frac operations. An exploration of pumping rates used on these strings in operations also provides some "field-tested" practical guidelines for avoiding erosion when performing annular frac jobs.