RIASSUNTO
Abstract
Coil tubing is a single string of pipe, which can intervene into wells and perform specific jobs without mobilizing a rig. The basic applications are cleaning wellbore/perforations, stimulating selective zones, controlling wells, milling/drilling cement/tools/fish downhole, conveying tools in deviated/horizontal wells, gas lifting, setting cement plugs below production packer, and many other applications.
Operating coiled tubing units in HPHT and ultra HPHT wells requires conformance of specific parameters for safety and successful operations. Challenges of the operations and treatment design are exacerbated considering the environment downhole. The conditions become more severe in ultra HPHT environment if the well is deviated or approaches horizontal.
Intervention and treatment in ultra HPHT and in highly deviated or in horizontal wells requires special procedures, techniques and tools. The requirements include modified procedures as well as equipment and products to withstand the BHST temperatures for the entire operation.
As an example of complications in HPHT wells, on the east coast of India, bottomhole static temperatures could go as high as 507°F and mud weight could vary between 14 and 19 ppg in deviated wells. Experience has shown that conventionally calculated bottomhole circulating temperature alone is inadequate to determine the working temperature downhole. However, with the help of computer-aided temperature simulation and all the above techniques, many successful CTU operations have been executed under these difficult conditions.
Using case histories for illustration, this paper will share best practices developed from 4 years of successful interventions in HPHT and ultra-HPHT wells. These practices will include HPHT coil tubing intervention considerations, modified procedures, special techniques and product choices.
Introduction
Coil tubing is being used by the oil industry for last 5 decades. As the technology advanced and operators are trying to penetrate deeper for hydrocarbon and pushing and extending the boundaries, challenges exacerbated while reaching HPHT and Ultra HPHT environment with safety and success. Drilling and production at east coast of India, especially at Krishna-Godavary basin, often encounters high pressure and high temperatures. Reservoir pressure and temperature may go up to 18,000 psi and 507 0F respectively as estimated. So far the maximum pressure and temperature encountered for CT operations were 13500 psi and 467 0F respectively at 5600 meter measure depth (5228m TVD) at KG basin. The case histories presented in this paper are from the KG basin located at the east coat of India. The planning for the jobs started during April 2005 and the jobs started executing from end of April 2006. The paper discusses on challenges encountered during the execution of these HPHT coil tubing intervention operations in these deviated wells. For example, at the KG field, one well had BHT of 467 0F at a MD of 5602 meter, reservoir pressure was around 13,000 psi and the mud weight was 14.8 ppg. These extreme well conditions had led to the variation in existing practices and development of modified techniques that made the operations successful. The various jobs performed on this well were: 1) Perforation wash, 2) Matrix stimulation 3) Clean up & Fishing job and 4) Abrasive Cutting of stuck pipe.
This paper illustrates on lessons learned during the design and execution of these jobs. There were some concerns on the mud properties due to suffering of severe shearing inside small diameter CT, which is discussed and resolved in the paper later. In addition, logistic and space restrictions challenges during the operations were also enumerated.