Water Injector Acid Stimulation: An Offshore Case Study

Waterflooding presents many unique challenges, especially in the offshore environment. Cost, slot availability, and uncertainty about return on investment limit the number of water injection wells and the use of ideal flooding patterns. Furthermore, water injectivity commonly declines with time due to formation damage. Well stimulation is a routine solution to remove such damage and recover injectivity. This case study focuses on our experience with a mud-acid stimulation of a water injector in the Gulf of Mexico (GOM). When the injectivity index of an offshore water injection well had decreased over time by a factor of 4, a mud-acid stimulation was performed, and significant injectivity was recovered. The well logs show multiple high-permeability layers, which can cause issues with waterflood conformance. A non-flowback operation, also known as bullheading, was decided upon to push insoluble fines into those high-permeability layers to improve waterflood conformance. Forgoing a post-stimulation flowback also decreased the cost of the job, reduced the risk of personnel exposure to acid, and was more favorable from an environmental viewpoint. Water injectivity was monitored with traditional diagnostic Hall plots. The efficacy of the stimulation job was evaluated through Hall plots, calculated injectivity index, and skin. Pressure transient analysis (PTA) was used to determine kh products, reservoir pressures, and skin factors before and after the mud-acid stimulation. This paper presents the successful, bullhead-style acid stimulation of a water injector supporting two oil producers in the deepwater GOM.