This paper presents the case history of a successful acid stimulation treatment in sandstone formation of Tapti gas field offshore western India, which greatly changed the perceptions and provided a new beginning for production enhancement in the field. Over the past decade, the sands were considered acid prohibitive in this field. Test data from core samples showed chlorite as one of the cementing material along with other iron bearing minerals making formation sensitive to inorganic acids. Considering solubility data of the core in hydrochloric acid in absence of calcite also supported the perception that acid may not help in stimulation, and might exacerbate the problems of un-consolidated sands in this field.
Tapti gas wells have internal gravel pack completions. Reduced gas deliverability from relatively high permeability formations in Tapti wells were caused either by formation damage in the near wellbore area or sub-optimal tubing size. Major factor contributing to near wellbore damage was considered to be the use of oil -based mud during drilling in order to stabilize hole conditions. Frac pack completions helped in improving the initial deliverability in wells completed towards the later part of field development. However, quest was on for serious stimulation options for rest of the wells.
Well selection was based on modeling work done to identify stimulation potential in candidate wells. Extensive core flow tests were carried out in the laboratory with specialty system including blend of organic and weak hydrofluoric acid along with appropriate additives. Solubility data of formulations indicated improved regained permeability with no reduction in core compressive strength. Field trial in the first well which exhibited large skin caused by formation damage during completion was successfully carried out. The well showed sustained post-stimulation deliverability gain making a step change for future production enhancement opportunities.
The field is located in the Arabian Sea off the western coast of India and has a series of alternating sand and shale sequences. The unconsolidated nature of the reservoir rock requires some form of sand control to prevent sand ingress. The wells completed in the early development phase were gravel packed using high rate water pack technique. These completions were successful in controlling sand but caused additional skin. The problem was accentuated when the drilling mud system was changed to an oil based system.
The mineralogy of the reservoir rock consists of sand grains and a variety of clay minerals. The sand grain composition typically has predominantly quartz and minor shale fragments and feldspar. The clays have significant amount of iron rich chlorite, which has been a major source of concern for planning any acid stimulation job.
