Customized Solutions for Coiled Tubing Interventions in the Bolivian Sub-Andean Basin
Authors: Carpenter Chris 
Published: August 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202620, “A Review of 25 Coiled Tubing Well Interventions: Customized Solutions for Bolivian Sub-Andean Basin,” by Jovanny A. Hernandez, SPE, Luis F. Antelo, SPE, and Carlos D. Rodriguez, Halliburton, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually 9–12 November. The paper has not been peer reviewed.
Fields in the Bolivian Sub-Andean Basin are remote and difficult to access. The producing zones include the country’s most challenging wells, with depths of up to 26,000 ft, with high pressure/high temperature (HP/HT), high gas cut, crossflow, dogleg severity, and well-access restrictions. The complete paper reviews 25 coiled tubing rigless well interventions (CTRWI) to extend the life of those wells, including operations involving nitrogen (N2) lift, acid wash, milling, shifting sleeves, setting packers, stimulation, velocity strings (VS), and fishing.

Introduction
CTRWI in Sub-Andean Basin fields had not been implemented historically because of limited road access to the fields, lack of available equipment with high technical capabilities, high pressure, and well depth. Beginning in 2017, however, operators evaluated the risk and elected to perform CTRWI involving stimulation, cleanout, N2 lift, fishing, VS jobs, and other techniques. Equipment with specialized capabilities was requested to address the operational requirements and the ability to travel on the remote roads while respecting transit regulations limiting maximum weight to 55 tons. The challenges to transporting equipment economically include
- Transportation of large reels of 2⅞-in. coiled tubing (CT) string to complete dual-sectioned VS installations
- Transportation of CT unit with 6,600 m of 1¾-in. CT string (weight of less than 55 tons)
- Continuously moving large volumes of N2 to the field
Wells in these fields were drilled no more than 10 years ago. The majority were built with smart completions, several production layers separated with swell packers and flowing through sliding sleeves configured with the same direction of actuation for opening and closure. Completions are configured with production tubing sizes of 7, 5, 4½, and 3 in. The tubing and accessories are made from chrome 13 because of the presence of carbon dioxide in the production fluids.
The region includes some exploratory fields having HP/HT conditions and mature fields with declining production. These wells are important for both the operators and the local government because their production represents more than 50% of national production.

On-Site Laboratory Testing
Characteristics of produced formation water and mineralogy tests of water samples helped identify a calcium carbonate (CaCO3) scale type. Because CaCO3 scale is known to be acid-soluble, a tailored acid-treatment system was selected with a static formation temperature between 220 to 286°F. On-site laboratory tests using a combination of 13% acetic and 9% formic retarded acid treatment systems were applied to the samples obtained, and results of a solubility test were observed.
The selected treatment was demonstrated to be highly effective at dissolving CaCO3 scale according to the test made at the wellsite. High confidence was placed in the test results of the acid-treatment sample because dissolution was above 85% in a 4-hour test period under static well temperature.