Effective Sandstone Acidizing of Horizontal Openhole Subsea Wells from Intervention Vessel: Challenges, Lessons Learned and Results
Authors: Barradas Orlanda , Viegas Donato , Cabuco Antonio , Staltari Daniele , Jesus Carlos De , Cazeze Osvaldo , Eduardo Nadia , Gianvittorio Domenico Di , Vescera Gaetano , Chikezie Emmanuel , Bernardone Aurelio , Stimatze Russell , Reilly Brett , Ilyasov Robert 
Published: February 2022

Abstract
Gravel-packed wells in the "C" field located in offshore Angola are prone to damage by a variety of factors including scales, fines migration, paraffin and asphaltene deposition resulting in skin values of 45-95. This paper focuses on the approach used for 2 subsea open hole gravel packed wells located within "C" field. Rigless subsea stimulations in approximately 470 m of water using an intervention vessel with the downline deployed via the vessel moonpool. Additionally, a stimulation vessel was utilized to provide pumping and fluid capacity without disturbing the primary intervention operations.
This paper documents the efforts made to restore the wells forecasted production by bullheading the acid stimulation treatment from the stimulation vessel through the open-water hydraulic access system installed on the intervention vessel. Well history attributes the impairment to fines migration accumulation and scale and paraffins deposition. The proposed stimulation fluids were designed to treat as many damage mechanisms as possible during a single intervention. The basis for design incorporated a primary solvent pre-flush to clean possible paraffin and asphaltene deposition as well as prepare the reservoir and proppant pack for further stimulation fluids by stripping away hydrocarbon residue. The preflush was followed by a second treatment fluid consisting of HCl acid to remove any carbonate-based damage. The final treatment fluid utilizing a combination of HCl acid and hydrofluoric acid (HF) was specifically designed to remove fines contained in the gravel pack and screens. Injectivity tests were performed to evaluate the reservoir prior to and after the acid treatment as well as to help understand the damage mechanism.
Based on the bottomhole pressure response during acid-treatment stages, measurable improvements were evident on both wells, which supports the pre-treatment damage diagnosis. The efficient and cost-effective execution of the treatment campaign, combined with the conclusive post-stimulation production data, confirms the effectiveness of open-water hydraulic access by utilizing an intervention vessel and a stimulation vessel, allowing to provide pumping and fluid capacity without disturbing the primary intervention operations on complex subsea wells.
Post-stimulation results after the successful removal of wellbore scale and formation damage in the two subsea wells showed an average increase in oil production of 60%. Skin damage was reduced by 66% on Well A and a complete removal of skin on Well B. The results confirm the effectiveness of cost-driven acid stimulations on complex subsea wells without the use of a drilling rig as well as demonstrating the ability to address multiple damage mechanisms from a single intervention.