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Abstract: Matrix Acidizing of Water Injectors in a Sandstone Field in Saudi Arabia: A Case Study

AL-ANAZI, HAMOUD, Saudi Aramco, Saudi Arabia

The aims of this study were to design a cost-effective stimulation treatment to remove formation damage from water injectors in a recently developed sandstone field in Saudi Arabia and enhance well injectivity, while maintaining formation integrity. Several water injectors were damaged due to the formation of drilling mud filter cake (mainly calcium carbonate particles). The formation rock is highly unconsolidated and contains several iron-rich minerals (e.g. chlorite). This paper examines the design of the treatment, field monitoring, and analysis of acid returns.

Core flow experiments were performed to screen various acid formulations, and evaluate acid additives. The experiments were conducted using reservoir core plugs at reservoir temperature and pressure. The designed treatment was applied on several water injectors. Acid returns were analyzed to evaluate the effectiveness of each treatment.

It was found that a large preflush of 3 wt% NH4Cl solution minimized secondary precipitation induced by HF-based acids. Ammonium chloride solution was effective in displacing potassium and sodium ions from feldspars and clay minerals compared to a preflush that contained hydrochloric acid only. A second preflush of 7.5 wt% HCl enhanced the performance of a retarded HF acid that was based on AlCl3. It was also found that some of the acid additives (an iron reducing agent) caused significant loss of core permeability. Consequently, these additives were not used in the field.

A multi-stage treatment was designed to remove formation damage encountered in several water injectors in this field. The treatment significantly improved the injectivity index of the treated wells. Analysis of acid returns confirmed that these wells were damaged by calcium carbonate particles used in drilling operations. The concentration of total iron in the acid returns reached a maximum of 14,300 mg/L. This high amount of iron was kept in solution by using an effective iron control agent and maintaining low pH in most of the treatment.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California