A Novel Approach for Predicting Sand Stringers: A Case Study of the Baka Field Offshore Nigeria

This paper details the seismic reservoir characterization study, aimed at predicting isolated sand lenses or stringers. Sand stringers or lenses are laterally discontinuous bodies that are encased in a different lithological body, they appear isolated and variable in lateral extent and thickness but mostly occur as thin beds. The thickness of these stringers makes it difficult for pre-drill predictions. One complicating factor of sand stringer is its potential of being isolated overpressured ramps which if unexpectedly encountered during drilling can cause kicks and if severe a blow-out is inevitable.

The Baka field is located some 120km off the coast of Nigeria in water depths of 2600ft (800m) to 3900ft (1200 m) with several wells drilled to date. Two of these wells have encountered unpredicted sand bodies that range in thickness from 15-25ft. These sand bodies were encountered within the Baka reservoir play interval of 5000 to 10000ft and were not predicted from conventional seismic reflectivity data, inverted seismic and offset well correlation. To obtain a better prediction of this stringers, alternative solutions are required. For this reason, a novel methodology - Geo-body constrained inversion approach was developed in 2017 to enable accurate prediction of stringer sands. Using this technique, a high-resolution acoustic impedance volume was built. Result of this study allowed the identification of gas bearing sand stringers encountered in previously drilled wells, thereby calibrating the model. The Geo-body inversion model proved a reliable tool for safe drilling of wells in 2018, ensuring that sand stingers were predicted at pre-drill phase in the Baka field.