Water Injection Enhancement in Nimr Fields Sandstone Formations: Restoration and Lessons Learnt
Authors: Al Kalbani Muhannad , Al Barwani Asma , Al Farsi Malak , Al Kindi Said 
Published: March 2023

Acid matrix stimulation in Nimr fields located in the South of the Sultanate of Oman has historically been utilized with caution for production enhancement. This is due to the prevalence of clastic rock as the main rock type coupled with the lack of precise minerology data from all wells with some drilled in the 70s’ and 80’ of the last century. This rendered matrix stimulation a risky endeavor due to the perceived lack of damage mechanism understanding and the occurrence of potential damaging precipitation.

Acid treatments are however readily used to treat skin buildup over time due to scale deposition, and this is a common theme across Nimr fields’ injector wells. These wells suffer from significant scale deposition over time caused by water incompatibilities and pressure/temperature changes. Hence these wells are often treated with hydrochloric acid (HCl) to dissolve deposited calcium carbonate, yet due to the presence of HCl insoluble mineral deposits, these treatments were found inadequate and only provided temporary solutions to a much larger problem. Thus, this study attempted formulating a fit for purpose acid treatment strategy, and new recipes were attempted on both injector and producer wells using available field level minerology data to set a standard for acid stimulation and scale treatments in Nimr fields.

Laboratory tests were done in-house and at third party laboratories to investigate several suitable acid systems which would aid in near wellbore skin removal from producer wells, as well as scale removal from injector wells. These tests consisted of emulsion tendency tests, X-ray fluorescence (XRF) analysis, X-ray diffraction (XRD) analysis of scale, and solubility tests. Other tests were done concurrently such as scale inhibition tests at various conditions to investigate the best sustainable solution for scale inhibition in injector wells to avoid adhoc stimulation treatments. Based on these laboratory tests, several trials were attempted in producer and injector wells across Nimr showing positive results.

Laboratory results showed that using minerology data from analogous wells, mud acid (9%Hydrochloric acid-1%Hydroflouric acid) can be safely used for stimulating poorly performing injectors and producers. This is due to the relatively high clays content and lack of calcite which can otherwise be treated by HCl alone. Tests have also shown that chealant based solvents along with organic acid blends produce the best scale removal from Nimr wells, where scale content was confirmed via XRD analysis. Once these test results were verified, trials at target well level were attempted and showed considerable improvement in skin removal. This resulted in enhanced production and injection rates of up to 20% and 50% respectively.

The case studies proven by tests and field trials have reversed skepticism regarding the utilization of non-traditional acid and solvent systems in sandstone formations in the Southern fields of the Sultanate of Oman.