Reproducing Spatial Anisotropy and Connectivity of Aeolian Systems Using Virtual Outcrops, Multiple Point Statistics and Forward-Based Process Geometrical Methods

Abstract

Aeolian reservoirs typically contain significant spatial anisotropy that is commonly not captured during conventional subsurface reservoir modeling. Current geostatistical algorithms enable the reproduction of spatial statistics, but are unable to integrate sedimentological rules that govern the spatial arrangement of deposition and stratification and therefore fluid flow. As a result, many aeolian reservoirs typically exhibit a slow or tight gas production effect at depth, with long connectivity at low levels established after a short initial plateau phase as internal heterogeneity is significantly underestimated.

Process-based geometry models produce detailed and geologically realistic models that honor depositional rules, but trade off precise conditioning. To bridge the gap between well and seismic scales, high-resolution analogue architectural studies of the Navajo, Page and Entrada Sandstone Formations provide a unique opportunity to bedform architecture and geometry at a variety of scales and to condition a series of process-based geometry models which honor the sedimentological heterogeneity observed at outcrop. Integration into conventional reservoir modeling workflows is achieved through use as training images for MPS. Four geocellular models are created using a surface-based approach coupled with MPS. They accurately and efficiently reproduce realistic aeolian bedform geometry, stacking and internal heterogeneity. Static and dynamic analyses of the geocellular models using analogue Rotliegend petrophysical values and production rates reproduce a slow gas production effect.

This study demonstrates significant advances towards conceptual understanding that the slow gas effect is inherently controlled via variable patterns of deposition, stratification and the spatial arrangement of first order bedforms. Through coupling with MPS, process-based geometrical models can be tailored from outcrop observations at both inter-well and reservoir scales. This methodology can be applied to differing aeolian systems, as well as other depositional environments including deepwater reservoirs, where similar architectural elements can be modelled accurately using only a few simple geobodies. More realistic and accurate descriptions of heterogeneity contained in aeolian systems are vital for improving recoverability in aeolian reservoirs.

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