The Impact of Bed-Scale Sedimentary Heterogeneities on Oil Recovery in Basin-Plain Thin-Bedded Turbidite Successions

Abstract

In turbidite reservoirs with a significant proportion of thin-bedded turbidites, low-pay sands may be bypassed during reserve estimation, consequently resulting in underestimation of in-place and/or recoverable hydrocarbon. This study focuses on the application of selected geological attributes of thin-bedded turbidites in a typical basin-plain succession, to assess the impact of bed-scale sedimentary heterogeneity on connectivity and oil recovery.

To capture variable bed-scale heterogeneities, six scenarios were defined using outcrop-based attribute data that were collected from a basin-plain succession in the Basque Basin, Spain. The variability in sand content in the six scenarios serve as a primary source of sedimentary heterogeneity, and for facies characterisation. The geological attributes of these scenarios were combined to develop their attribute indices, notably facies Net-to-Gross Index (NGI) and Sand Connectivity Index (SCI). The six scenarios serve as input for constructing 3-D high resolution models for waterflood simulation studies.

High net-to-gross scenarios 1-3 have a considerably lower degree of sedimentary heterogeneity and higher values of attribute indices. In these systems, the bed vertical stacking pattern encourages better vertical connectivity (high SCI values). These qualities translate to better conditions for displacing accumulated hydrocarbon fluids. As a result, effective oil displacement with less fingering of the waterflood front reduces the risk of early water breakthrough. By contrast, low net-to-gross scenarios 4-6 have a higher degree of sedimentary heterogeneity, which reduces their producibility, causing early water breakthrough and leaving several pockets of undrained oil. This is as a result of sand-rich, high lateral connectivity layers acting as primary conduits for injected water, supporting initially rapid oil displacement to the producer and, subsequently becoming conduits for channelling water. With the low connectivity layers largely bypassed, there is an increasingly high water production in the presence of rapid decline in oil production rate, following water breakthrough in such systems. It is therefore important to use the depletion strategy that will optimise production and minimise bypassed oil in systems characterised by scenarios 4-6, whilst reducing the risk of gravity segregation that may result in early water breakthrough in systems that are characterised by scenarios 1-3.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018