Petrophysics

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Abstract

The South Tapti gas field, located in the Arabian Sea off the western coast of India, has been on production for the last three years. Drill Stem Tests (DST) conducted in five zones in the first appraisal well showed high sand production in all the tests at drawdowns as low as 20 psi. Due to the unconsolidated nature of the reservoir sand, it was decided to gravel pack all future wells.

Initially, the High Rate Water Pack (HRWP) technique was used on 13 completions including some stack and selective completions. Due to a change in the drilling fluid to an oil based mud (OBM) system and the higher damage associated with the it, it was later decided to switch the gravel packing technique to ‘Frac and Pack’. This involved creation of a small frac to bypass the near wellbore damage using a gel-based fluid, followed by gravel placement using the HRWP. The Frac & Pack technique was tried on 10 completions.

Extensive production and pressure data collected on the above completions show that the ‘Frac & Pack’ technique has led to a decrease in the mechanical skin by almost 70 %. More importantly, it has also been responsible for decreasing the Rate Dependent Skin Coefficient, D, by 50 %, which has a greater impact on the well productivity since these wells are produced at average rates of 25 MMSCFD.

This paper describes the gravel packing techniques used in the South Tapti gas field, their evolution and the results achieved.

Inroduction

The South Tapti field, located off the Western coast of India, is represented by a series of alternating sand and shale sequences. The upper reservoir intervals are dominated by tidal-bar and channel sands deposited in a deltaic environment while the lower sands represent incised valley deposits consisting largely of alluvial/fluvial channels, tidal bars and tidal creeks. The exploratory wells drilled in the field indicated strong sand production tendencies due to the unconsolidated nature of the reservoir...
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