Hydraulic Frac Stimulation of the Sarir Sandstone in Nakhla Oilfield

The Nakhla Oilfield is located in concession 97 within the Sirt Basin about 60 km south of concession 96. The field was discovered by drilling well G1-97 in 1970. 13 wells were drilled to date: nine of them are producing on natural flow, three found volcanics or volcanoclastic sediments replacing the entire or at least the oil-bearing, upper part of the reservoir, one technically failed in the Zelten Formation.
All wells are producing from the Lower Cretaceous Sarir Sandstone. Low porosities of 10 to 14% and poor permeability ranging from 0.1 to 20 md only allow a moderate production. Therefore, hydraulic frac stimulation of seven vertical wells was performed between 1995 and 2002.

State-of-the-art real-time analysis was performed in all hydraulic frac stimulations to evaluate and optimise the frac operation on-the-fly. Hydraulic frac simulation findings were compared to post-frac well test results obtained from pressure buildup analysis. Within their limits of accuracy, the frac parameters (e.g., frac half-length) obtained by both methods match reasonably well.

This paper also summarises the lessons learned from the treatment executed and their impact on future operations and field development.

The production behaviour of fractured and unfractured wells and the resulting additional production capacity is presented and discussed. To date, about 45% of the cumulative oil produced from Nakhla oilfield can be assigned to the hydraulic frac treatments.

Especially during the last two years, a lot of effort was made to understand and describe the reservoir and its rock mechanical parameters better than before. The outcome was also implemented to improve the approach to plan and optimise future hydraulic frac stimulations.