In the early days of the oilfield, matrix acidizing was considered useful only for stimulating carbonate formations. However it has been known for many years that with proper job design, acid and additive choices, and in-depth study of potential formation damage sources, matrix acidizing can safely and successfully stimulate production in sandstone formations. By incorporating carefully designed tubing pickle, pre-flush, main acid system and post-flush (including optimum doses of corrosion inhibitors, mutual solvents, iron control agents, clay control additives, surfactants, etc.), matrix acidizing can provide excellent productivity improvements even in sandstone formations with complex mineralogy.
This paper describes a matrix acidizing campaign executed successfully in the Gulf of Cambay (Khambhat) on the west coast of India. In initial laboratory tests and during simulation runs, it was aimed to design a pre-flush, acid system and post-flush to best suit the challenging reservoir conditions while also considering offshore logistics. Further as the formation had iron-rich minerals like chlorite clay, siderite, etc., a chemically modified organic sandstone acid system was selected as opposed to conventional hydrofluoric acid system. Moreover a spacer system was designed and pumped to push any iron precipitation far away from the near-wellbore area and to clean the area for the organic acid system.
After pumping the system in one well, the system design, pumping procedures and volumes were modified to further improve results in the next well. Fines-stabilizing agents were introduced to avoid clay migration or swelling that may otherwise jeopardize a successful treatment. The paper will explain the basis for modifying the design and pumping procedure, based on lessons learned from the prior operation.