Technological advances in all areas of oilfield development have brought about a better understanding of current practices and insight into future applications. Following this trend, computer modeling has grown to allow the application of rigorous mathematical analysis to processes which, in the past, had not been explored. A computer model, CIRCA 11, designed for coiled tubing operations, will be discussed in this paper.
Many of the early difficulties experienced in coiled tubing operations were due to physical uncertainties and the high demands placed on a relatively undeveloped process. Significant material improvements and increased operational experience have dramatically increased job success rates. Also, the ability to predict and optimize downhole operations prior to a job using a computer model takes much of the risk out of coiled tubing work.
This paper discusses some of.the practical applications of the CIRCA 11 model as it applies to circulation analysis and coiled tubing force analysis.
Development of the computer model CIRCA 11 was initiated to help answer questions about downhole flow and pressure conditions during coiled tubing operations. Utilizing accepted fluid rheology models and proven correlations for two-phase flow, thermodynamics and friction gradients, accurate circulation analysis is now possible.
Coiled tubing force and stress analysis was later developed on the model to help prevent downhole failures. While considering specific well geometry, contact friction and coiled tubing material strengths, the model also takes into account circulation effects.
In addition to circulation and force analysis, CIRCA 11 offers subroutines for sand penetration rates and nitrogen calculations.
Early versions of CIRCA were created for use on hand held computers which afforded portability to field operations. As the model's scope continued to expand, the processing requirements made it necessary to move to larger and more capable hardware systems. Currently the model is being run on the IBM PS/2 Model 70 PC with math co-processor, and on lap top computers for field use.
As a result of this evolution, CIRCA 11 has become one of the most thorough coiled tubing simulators in use to date.
Many accepted practices in coiled tubing operations have developed through trial and error rather than scientific theory. Gas lifting procedures and c1eanout techniques are often repeated if they have been successful in the past. Understandably, the complex nature of flow conditions created during coiled tubing workovers make mathematical analysis difticult without the assistance of computer technology. However, for many of the downhole operations performed with coiled tubing, ~ is often advantageous and sometimes crucial to have an understanding of the flow conditions expected.
With computer modeling, the user can now calculate whether or not a job is possible. It is also an economical means of optimizing particular job programs. For example, consider a sand cleanout in a deviated well with a water sensitive formation and low bottom hole pressure. An effective cleanout will require that the return velocity in the annulus be above certain minimums in order for sand transport to take place.
