The stacked sands of the South Tapti field have presented completion challenges from field start-up in 1997 to the present-day. A large part of these challenges have been caused by reactive shales interbedding the sand bodies. This has had a persistent influence on the sandface completion design and, in particular, on the drilling and completion fluid systems.
The completion design has evolved from stacked cased hole gravel pack to open hole gravel pack designs. Non-gravel pack open hole designs are also being considered for the future to meet the challenges of drilling and completion at higher well angle. The field requires a mix of all these techniques to meet well objectives.
The South Tapti gas field is located 160 km north north-west of Mumbai in the Arabian Sea off the west coast of India. The field is operated by a joint venture between BG Exploration & Production India Limited (BGEPIL), ONGC and Reliance Industries Limited (RIL). In 2002 BGEPIL acquired the interests of Enron Oil & Gas India Limited (EOGIL) who had jointly operated the field with ONGC and RIL since start-up in 1997.
The field achieved a peak gas production rate of ∼250 MMscf/d in early 2005. The field had declined during 2005/6, but the rate is expected to be restored to 250 MMscf/d by the end of 2006 where it is expected to stay until 2012. In 2007, the offset field Mid Tapti will be tied back to the South Tapti facilities. This will increase production from the combined South and Mid Tapti complex to 450 MMscf/d.
The South Tapti field produces relatively sweet and dry gas, with a methane content of ∼90%, CO2 ∼2%, no H2S and a condensate/gas ratio of ∼10 stb/MMscf.
The South Tapti reservoir consists of a stacked series of sands deposited in the Lower Miocene (Mahim formation) and Oligocene (Daman and Mahuva formations).
A wide variety of sand lithologies have produced gas since field start-up, ranging from clean, coarse grained sands to argillaceous, highly bioturbated, medium to fine grained sands. Current production is entirely from the upper Mahim sands, which are generally coarse grained (d50 300–600µ), medium to fairly well sorted (Ucf 2–5, Scf 5–10) with low fines (1–4% <44µ). The most common clay is chlorite, which coats sand grains and partly fills the pores.
In the early years of field life, a small number of grain size distributions and core analyses provided a picture of the sands being generally more poorly sorted, with high fines content, and chlorite-rich. This resulted in a conservative approach to gravel sizing and an aversion to the use of acid, which will tend to create iron precipitate gels in the presence of chlorite. In recent years, and with a greater amount of data available, the sands are now considered to be generally coarser, better sorted and less susceptible to damage when treated with acid. This has been partly responsible for the some of the changes in completion design.
The basis of design for the wells drilled from 1997 to 2002 was as follows:
Low angle wells (<45o).
Stacked cased hole gravel packs (CHGP) to provide sand control and zonal isolation capability.
Lower completion installed in 7″ liner.
Production of the lower interval until it waters out, then isolation and opening of the next shallower interval.
This basis of design was maintained throughout the period, but the completion design was changed in detail to cope with greater than expected problems with shale reactivity and formation damage.
Aspects of the experience obtained during this period in field life were reviewed by Garred et al1 and Das et al2.