Innovative Completion Approach to Develop Unconsolidated Shallow and Thin Multi-Layered Gas Reservoir: A Case Study of North Tapti Field

Abstract

The North Tapti field is located in Arabian Sea WNW of Mumbai city. The bathymetry of the area is very shallow between 8-35 m depending on the tide condition and therefore poses adverse logistic conditions due to strong bottom currents. In North Tapti field shallow gas sands of Oligo-Miocene age occurring at depth of 550- 900 m are layered reservoir with several pays. Exploratory wells produced dry gas with sand. The lithology of the reservoir sand is mainly medium to fine grained and at places coarse sandstone with fair inter-granular porosity. Initial development plan envisaged exploitation of gas through six development wells. All the six wells were envisaged as horizontal wells of about 300m drain hole with sand control completions through pre- drilled liner supported open hole gravel packs with a view to achieve higher gas productivity. The rig positioning activity in North Tapti area experienced great difficulties due to severe sand-scouring of seabed. Therefore, the first well had to be drilled as vertical well and completed with premium screen as a standalone screen completion with a view to make up lost time prior to impending monsoon. Studies based on the drilling data obtained from first development well revealed sub- surface surprises like reduction in pay thickness and occurrence of multi layered pays in Mahim and Daman formations at shallow depth. This posed considerable challenges in achieving desired high drift at shallow vertical depth of about 400m while maintaining high angle trajectory during drilling and subsequent completion programme. Hence, innovative drilling and completion approaches were adopted to tackle sub-subsurface complixities and to achieve maximum planned gas production. In this backdrop of accomplishing the onerous task to produce sand free gas in all the six wells in platforms of North Tapti which were planned to be completed with suitable sand control methode using in-house expertise was a real challenge. The methodology adopted in two wells viz. A and B of 80° and 85° inclination were single frac pack and stacked completion of both. Stand alone in the lower barefoot section and a frac pack in the upper sands which was cased cemented and perforated. The wells of P1 platform are accessible with difficulty due to severity of water depth and short tide periods. Therefore wells were completed as stand-alone screen completions through Expandable Sand Screen (ESS) in 6″ drain hole section and 7″ liner sections. This highly innovative drilling and completion approach facilitated exposure and completion of thin pay sands in multi-layered reservoirs in a deviated well. The result is stable and uninterrupted gas production for over a period of more than two and half years and continuing at a consistent rate of about 1.3 MMm3/d.