Multistage Controlled Acid Fracturing: A Unique Acid-Stimulation System Proved Efficient in a Field of Bombay Offshore Basin-A Case Study

摘要

The pay zones of one of the Bombay offshore field are thin, tight and heterogeneous carbonate reservoirs with typical lithological pattern. The production declined sharply in all the wells within a short period. Achieving an effective stimulation treatment for the wells of this field has long been a recognized challenge. No conventional stimulation methods have succeeded. With unique characteristics, the multi-stage controlled acid fracturing treatment is proved to be highly efficient in this formation. Studies were carried out to determine the formation rock characteristics and finding a suitable acid system. Acid etching patterns in the formation rocks were seen for different acid systems. Studies demonstrate that a multistage fracturing by viscous frac fluid and alternately acidizing the fracture surface by the improved acid system are giving better etching pattern. Finally, closing the fracture surface and injecting large volume of nitrified acid at a very slow rate further enhances the permeability generated. In this technique, the farthest part of the fractures, the middle part and the fractures near the well bore as well are acidized and etched uniformly by controlling the acid reaction rate at different stages. Laboratory test results on formation cores and field results verify this technique. Temperature survey and pressure transient tests are done before and after stimulation treatment for post job evaluation. This paper describes the technical basis of the improved acid system. Laboratory test results are included. It also discuses the treatment design, execution and evaluates the fracture geometry. Production comparisons indicates a substantial improvement in well performance following fracture acidizing treatments. The significance of the paper can be attributed to achieving longer fracture length and creating higher conductivity by better etching through controlled reaction rate of acid for tight and heterogeneous carbonate reservoir.