Bobo project – deep water

Reducing risk in complex deepwater fields

The reservoir rocks located in deepwater settings off the coast of West Africa can be extremely complex. To increase the probability of drilling a successful well in this region, Shell applies innovative seabed-logging technologies to assess the content of potential reservoir structures before drilling into them.

Bobo field is located in deep water off the coast of Nigeria in an area of complex subsurface geology. In this setting, reservoir rocks, typically turbidite sands, can hold large volumes of oil and gas. However the huge drilling costs associated with drilling in extreme water depths (2,479m) in which these reservoirs are located, neccesitates great care in predicting the location and volumes of possible oil and gas accumulations.

Until recently, many exploration wells had failed to find significant oil accumulations in this area owing to the extreme geological complexity arising from repeated earth movements. These movements could result in oil and gas, which were previously trapped by geological structures, escaping from their traps, leaving behind only traces or residual amounts of hydrocarbons. The challenge, therefore, lies in establishing whether the hydrocarbons identified are present in commercial quantities or only residual amounts. Shell has applied a technology known as seabed logging to resolve this challenge.

Seabed logging uses receivers placed on the seabed to measure low-frequency electromagnetic waves as they pass through the rocks below. The electromagnetic waves are generated from a source device, which is towed by a ship and passes through the water close to the seabed. By measuring the electrical resistance in the rock layers, a seabed logging survey highlights the difference between rocks that hold large volumes of oil and gas and those containing very little or none.

Shell used this technology to investigate potential hydrocarbon targets at the Bobo Field: one target at an intermediate depth and another at a much greater depth. Before running the survey, the exploration team predicted how differing oil and gas volumes in the two targets might influence the survey results. When they compared the actual seabed-logging results with their predictions, they found that the survey indicated only residual hydrocarbons at the intermediate target but much greater hydrocarbon saturations at the deeper target. Drilling the Bobo-IX well confirmed the seabed logging results. At the deeper target, 140 meters of net oil sands were encountered, thereby proving the first commercial oil discovery in this geological setting.

Using seabed logging in this setting provided significant benefits by increasing the chances of drilling a successful well.