During effective acid stimulation of mature producing wells, operators often face challenges in terms of achieving sustained production increases. Such challenges in formations subjected to acid stimulation in offshore West Africa include high temperatures (305°F), low reservoir pressure (0.22 psi/ft), highly heterogeneous multilayered formations with large perforated intervals (greater than 1,300 ft net and 2,400 ft gross), and scale in the wellbore, perforations, and sometimes in the near-wellbore area (NWA). Wells in this area, when treated with conventional acid systems based on hydrochloric acid (HCl), only sustain the productivity increase for very short periods. Such conventional acid systems have been based on 7.5% HCl and 10% acetic acid (AcOH) with high loadings of corrosion inhibitor, iron sequestering agents, demulsifiers, and antisludge agents.

This approach has constraints that can potentially include the following:

  • Ineffective stimulation attributed to fast acid reaction rates

  • Limited penetration of live acid

  • Rock disintegration

  • Damage from reprecipitation of acid reaction products, such as iron and mineral fines from wellbore scale

  • Damage from high corrosion inhibitor loading

  • Fluids (stimulation and reservoir) incompatibility

The use of a newly developed aminopolycarboxylic acid (APCA) based stimulation fluid provides advantages that circumvent these constraints. The introduced technology delivers a stimulation system that provides deeper stimulation treatments because of lower reaction rates and potentially a more effective wormhole type of reaction for the reservoirs' characteristics. This stimulation fluid also chelates reaction products to help enable superior cleanup of the acid system. This system also requires lower loadings of corrosion inhibitors, demulsifiers, and antisludge agents, and is more compatible with formation lithology, lowering the risk of formation damage.

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