Coring Deep Water Slope Valley Systems: Lessons Learned from Over a Decade of Coring “From-the-Fly” Operations in Block 14, Angola
Oscar Yepes1, Thomas H. Fate2, and William Schweller3
1Chevron, Luanda, Angola
2Chevron, Houston, TX
3Chevron, San Ramon, CA
Conventional cores have proved valuable in understanding and appraising complex reservoir architecture in Angola, Block 14 Slope Valley Systems where forty cores have been recovered from soft sediments in the last decade. These cores represent an outstanding commitment by Chevron and partners to assemble a unique (world class) core database of Miocene deep water channels in Angola overcoming technical difficulties posed by unconsolidated sediments.
Successful core operations of unconsolidated turbidite channels rely on careful planning and understanding of stratigraphy, well objectives, rock mechanics and surface handling and stabilization of the core. Detailed core procedures must be established and clearly communicated to rig personal. Due to efficient drilling rates experienced during coring operations in Block 14 and strong lateral stratigraphic variability of turbidite channels a ‘core-on-the-fly’ approach instead of drilling a pilot hole and coring a sidetrack has been used in Block 14 saving time and well cost. This approach however makes selection of core point a challenge, especially in intervals lacking reliable stratigraphic markers near the reservoir sand.
The use of resistivity at the bit and real time log data to update stratigraphic correlations and select core point are among best practices for coring ‘on-the-fly’ operations. Identification of signposts combined with close communication during operations is also important when deciding number of coring attempts and length of core barrels. Cores collected in Block 14 are primarily used to (1) gain valuable insight into the stratigraphic architecture of turbidite channels; (2) acquire rock property data used in reservoir models; (3) perform rock mechanics tests for completion design.
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