Field Validation and Qualification of a Low-PH HF-Aminopolycarboxylic Acid Fluid for Acidization of Complex Sandstone Wells
Authors: Reyes Enrique A. , Davis John , LaBlanc Alyssa , Stimatze Russell , Beuterbaugh Aaron , Ouedraogo Martial , Williams Wade , Kebert Neal E. 
Published: November 2016

The effective stimulation of wells in offshore environments where the removal of existing completions is either not feasible or imparts significant cost implications can pose increased challenges for effective fluid delivery to the desired pay zones. This paper documents the challenges encountered in a sandstone reservoir requiring acidization and presents a hydrofluoric (HF) acid/aminopolycarboxylic acid (APCA) fluid stimulation methodology and ensuing field validation.

Frac-pack sand control is the preferred completion method in many wells located offshore West Africa. This method presents the following challenges during the life of the well: Potential for underperforming well productionScreen integrity concerns on production hot spot impact points, resulting in reduced production rates or leading to potential long-term well failureLarge screen length or multizone completions leading to diversion challenges for any potential stimulation fluidRemoval of existing completion not permitted

Wells in this area were previously stimulated using complicated fluid trains and sequences with strong HF acidizing systems; however, this stimulation approach encountered complications that included the following: Ineffective stimulation fluid placement, leaving areas under-stimulatedSpacers and pre-flushes necessitating large treatment volumesIneffective stimulation caused by fast reaction ratesFluid compatibility concernsPotential limitation of penetration of live acidPotential for rock disintegration, leading to accelerated sanding in the wellbore

The HF/APCA fluid was developed and validated for use in such reservoir types, and the fluid qualification is documented and presented. The fluid pH is low, maximizing the generation of HF acid with an acid-soluble chelant.

This chelant technology provides unique sandstone acidizing advantages, especially for mixed mineralogy (carbonate/sandstone) formations, coupled with simultaneous dissolution and/or removal of Ca, Mg, or Fe carbonate scales and fines (clay and silica) accumulated in proppant packs, perforation tunnels, tubing, and downhole tools. The fluid is designed such that historic spacer and pre-flush requirements can be eliminated, resulting in more efficient, effective treatment. The system includes health, safety, and environmental advantages; it is a readily biodegradable, nonhazardous material and is not considered a marine pollutant.

In this field, the new chelant-based stimulation fluid used alongside engineered diversionary systems provided the ability to address the demands encountered in these wells. The field validation detailed in this paper documents the successful outcome of the fluid technology. Post-stimulation evaluation showed an improved velocity profile across the screens, indicating successful diversion into the underperforming reservoir sections. This result and the overall skin reduction allowed for a production uplift of 48%.