Subsea Multiwell High-Flow-Rate Riserless Acid Stimulation Campaign With Two- Vessel Approach: A Comparative Case Study
Authors: Kusumawati Irma , Heigre Birger , Whitfield Hunter , Bremner Samuel , Sbordone Andrea , Nygård Kristell , Riseth Roar 
Published: December 2021

Abstract
This paper describes the utilization of a riserless light well intervention (RLWI) vessel with well control system and flexible downlines to execute a re-stimulation campaign on subsea injection wells located in the Norwegian Continental shelf in the summer of 2019 and 2020.
A riserless light well intervention (RLWI) vessel with well control system and flexible downlines was used in combination with a stimulation vessel. The objective of each campaign was to increase injectivity in the wells with high-rate acid treatments. The lessons learned from the 2019 campaign were applied to the 2020 campaign, resulting in reduced health and safety exposure, and improved operational efficiency. Analysis of the treatments and their impact on injection and field pressure support was conducted to assess the effects of these improvements and provide insights for how the treatments can be applied to vessel stimulation in general.
In each campaign, the RLWI vessel was connected to the subsea asset, and a dedicated stimulation vessel provided stimulation fluids via a high-pressure flexible hose connected between the two vessels. Both campaigns saw high treatment pump rates of up to 60 bbl/min with low-pH crosslinked gel fluids, 28% hydrochloric acid, and diverters in the form of ball sealers and rock salt. Hose deployment methodologies between the two vessels differed in the two campaigns. The 2019 campaign employed a conventional transfer utilizing the marine crane on the RLWI vessel to lift and lower the hose into a preexisting hanger. Learnings from this operation led to the development and use of a winch pull-in method in which the hose connection was accomplished with a hot stab connector on the RLWI vessel, eliminating human intervention and the use of the crane.
The 2019 and 2020 campaigns successfully stimulated five and six subsea injection wells, respectively, and realized post-stimulation improvement in injection rates of 135%. One year of field monitoring from the first campaign shows pressure support benefits with improvements in production throughout the connecting area of the field. The winch pull-in method of hose deployment between the vessels achieved time improvements of 8 hours per stimulation treatment. In addition, the added flexibility of not needing to be within crane reach gave the operation extended working weather limits. The overall result was a significant improvement in operating efficiency between the 2019 and 2020 campaigns.
The operations showed how high-rate stimulation can be achieved on subsea assets with the use of an RLWI and stimulation vessels. Detailed analysis of the operational efficiency of each campaign was performed, and the improvements from one campaign to the next documented. The winch pull-in method is a new way of high-pressure hose transfer that can be applied to future stimulation vessel operations to improve operational safety and efficiency.