The CTU based stimulation jobs practiced for the horizontal oil wells completed in the, LIII limestone reservoirs of giant Mumbai High field in the Western Offshore basin of India was gradually losing its desired efficacy due to the falling reservoir pressures. As an alternative, the back surging technology used for post gravel pack perforation tunnel cleaning operations in sandstone reservoir was recasted with minor modifications and applied for stimulating the horizontal wells in the saturated, multilayered LIII reservoir Wells IQ-9H and S16-P1H located in the downdipside and crestal part and of the field respectively, were identified for pilot tests so that the field trials are subjected to widely differing geological and petrophysical environment. While Well IQ-9H is placed in the thin uppermost layer of LIII limestone overlain by a huge shale sequence, well S16-P1H is placed within an intermediate layer of the same limestone formation overlain by a large gas cap in the upper eight layers and underlain by a shale band. Encouraging results in both these wells prompted more extensive use of the technology elsewhere in the heart of the field, and results of around twenty 8–1/2" horizontal sections and forty-one 6" drain holes completed in the last three years have been deemed to be successful without exceptions, both in terms of effective stimulation and rig-time savings to US $ 30,000 /well , thereby proving to be a cost effective substitute over conventional bulldozing and CTU based jobs.

The surge tool is tailormade for instantaneous drawdown across the formation face thereby removing all post-perforation fines. The assembly is run on a lock mandrel to an appropriate landing nipple profile. The tool is surface dressed with shear screws, number and type chosen to suit the pressure likely to be created across it. After the horizontal well is completed with packer, the dressed back surge assembly is installed over the landing nipple through slick line. The tubing volume above the back surge tool is knocked out for creating the differential pressure. The preinstalled shear pins gives way to the formation fluids. The back surge tool is ultimately retrieved once the stabilised flow is attained.

The paper details the design of the surge tool, its preference over other existing systems in practice and the performance enhancement profile alongwith the financial indices in the 61 wells it has been tested in the LIII reservoir of Mumbai High field.

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