Dalia Development - Subsurface, Drilling, and Well Completions Engineering
Authors: Caie D. , Cassé E. , Elias B. 
Published: October 2008

Integrating, from the preliminary development study phase, the subsurface engineering team in the Dalia project group has permitted the development of a highly evolved and optimal field development plan.

Integration from the onset of all drilling, subsea, surface constraints and specific objectives while meeting subsurface data acquisition requirements, has allowed opportunities for well count and pattern together with field layout architecture optimisation, until design freeze.

This paper describes the philosophy and main elements of the development for this deep offshore, turbidites, heavy and acidic oil field. It will review how plans, during the early phase, have been adapted to events and changes. In the context of operational constraints during the run up to first oil in 2006, the robustness and flexibility of Dalia's Reservoir Management Plan have proved valuable for better and timely field understanding and reduction of subsurface uncertainties. Shallower than Girassol, the channel and sheet like type, Dalia reservoirs require shallow kick-off and high inclination in unconsolidated formation allowing the drilling of horizontal drains. Issues of well bore stability and hole cleaning required close monitoring. Mud and cement have been specially engineered. Trajectory control, specific bit-BHA designs together with drilling procedures has been developed. Well completions designs and cost reduction efforts with the novel drill-through horizontal trees will be presented.

Dalia Overview

For more details refer to Dalia Paper OTC N°18538. Refer to Figures 1 and 2 for the general field lay-out and a graphical representation of the reservoir and some of the wells below the mud line.

The Dalia reservoirs cover an area of about 230 km2, and the subsea production system covers an area of about 100 km2. When the 2,700 days drilling programme is completed in 2011 the two rigs will have completed a total of 71 subsea wells with 37 producers, 31 injectors and 3 gas injectors. The production system has been designed from the beginning to handle up to 100 wells, allowing for future infill drilling. Dalia is estimated to contain close to 1 billion barrels of recoverable oil.

Reservoir Description and Fluid Characteristics

The reservoirs on the Dalia field were formed about 25 million years ago by the accumulation of sediments at the mouth of the Congo River. These sediments were pushed out into the deep offshore by gigantic subsea avalanches and subsequently deposited in and along the edges of seabed channels. Today they lie not very deep (about 800 metres) below the seabed, forming reservoirs that contain a viscous oil (21° to 23°API) at relatively low temperatures of between 46 and 56°C and pressure of 215/235 bars.

Appraising these extensive so-called "turbiditic", unconsolidated and heterogeneous oil reservoirs is a complex business. One of the key challenges is to resolve the sedimentological uncertainties, because the reservoir sands are compartimentalised by clay deposits that inhibit connectivity and disrupt the oil flow.

The Dalia field is in fact a group of four turbiditic complexes from Lower Miocene to Middle Miocene age and called;Lower Main Channel,Lower Flanks (with distinct east and west parts),Upper Main Channel and - Camelia.