Implications of Cell Dimensions on Dipping Bed Continuity in Flat-Layered Geomodels

Abstract

There is a mathematical relationship between cell width/length/height, bed thickness, bed dip angle, and the degree of continuity that a horizontally-layered model framework can preserve. With very small cells, the continuity of beds at any dip angle can be approximately represented in a geologic model. However, as cell sizes get larger, bed continuity cannot be accurately represented by flat model layers. At certain critical dip angles and bed thickness, cells of a given size will fail to maintain continuity. Two types of bed continuity are important to preserve in geologic models destined to be used in flow simulation: flow barriers and flow conduits. For flow barriers, edge-to-edge continuity must be maintained to prevent flow-through in simulations. If the bed dip exceeds the threshold for a given cell size and bed thickness, gaps will form between the barrier cells, allowing flow-through. A simple trigonometric relationship exists between the bed thickness, dip angle, and the maximum allowed cell width to maintain edge-to-edge continuity of barrier beds: 1) MaxWidth = BedThickness / TAN(BedDip) For flow conduits, face-to-face continuity must be maintained to allow flow within the bed in simulation. However, the degree of face-to-face contact between adjacent cells is necessarily degraded as bed dips increase in a flat-layered framework. Beds with zero dip have 100% face to face contact, but as dip angles increase, the amount of elevation offset at cell edges causes fewer and fewer layers to communicate, until at a critical angle, only non-flowing corner-to-corner continuity exists. Beyond that critical angle, conduit beds do not touch and therefore do not flow in simulation. The algorithm for approximating the fraction of face-to-face contact of conduit beds is: 2) VertOffset = (CellWidth*TAN(RADIANS(BedDip))) 3) MinLayers = ROUNDDOWN( BedThickness / LayerThickness, 0) 4) Fraction = MAX(0, IF( OR( BedThickness / 2 < LayerThickness, (CellWidth * TAN(RADIANS(BedDip)) >= BedThickness)), 0, 1- ROUNDUP((VertOffset / LayerThickness), 0) / MinLayers)) These algorithms can be used to optimize cell sizes to a combination of bed thickness and bed dip. However, to avoid the continuity problem entirely, geologic models should be built with model layers inclined parallel to the beds. With bed-parallel layering, the occurrence of stair-step bed boundaries will be kept to a minimum.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015