Influence of Common Sedimentary Structures on Fluid Flow in Reservoir Models
Authors: Weber K.J. 
Published: December 2007

This paper summarizes the state of the art of deriving detailed permeability-distribution models on the basis of cores, sidewall samples, and logs. Reservoir heterogeneities such as clay drapes and intercalations, crossbedding, sand laminations, slumping and burrowing in various major depositional environments are examined.

Introduction

The rapid increase in the number of secondary- and tertiary-recovery projects has resulted in a greater need for detailed geological modeling. Especially when expensive chemicals are injected into a reservoir, one wants to know how they will be distributed away from the wellbore.

Nearly all reservoirs are heterogeneous to such a degree that realistic reservoir simulation cannot be carried out on the basis of homogeneous prototypes. The overall heterogeneity can be dominated by open or sealing faults, fractures, contrasting lithologies, diagenesis, or sedimentological complexity. The influence of such features is described in a relatively large number of papers, while there are few that have concentrated on the internal heterogeneity within a genetically defined sand body caused by sedimentary structures and intercalations.

There is a fair understanding of the overall shape and continuity of some of the more important genetic reservoir sand-body types. At the other end of the scale, use of the scanning electron microscope (SEM) has revealed much information on pore shape and diagenesis. This paper focuses On the sedimentary heterogeneities on a scale between an entire, genetically defined, sand body and a small volume of matrix.

Although more realistic than glass packs, very homogeneous sandstone for flow and displacement experiments in laboratories - Berea sandstone in the U.S. and Bentheim sandstone in Europe - has been detrimental to the understanding of the influence of medium-scale heterogeneities. There are few examples in the literature of experiments taking into account sedimentary structures and clay laminations. Pettijohn et al1 give good annotated references of such literature and also discuss the hierarchical sequence of primary controls on permeability; They state that because cross-bedding with preferred orientation is the dominant structure in many, if not most, sand reservoirs, it is the prime suspect for depositional control of anisotropic horizontal permeability in a reservoir. The use of internal directional structures for predicting the trend or orientation of sand bodies is beginning to be well understood. Numerous examples are given by Potter and Pettijohn.2

There is nothing new in the statement that the internal permeability distribution in a sand body Will influence fluid flow and sweep efficiency. A literature survey yielded several early papers describing flow experiments on heterogeneous sand models. Mills carried out such experiments as early as 1920.3 He was particularly interested in the trapping of oil in fine-grained sand lenses embedded in coarser sand during a waterflood. It is stated that there is first a rapid movement of water along the paths of least resistance under the laws of hydraulic flow of liquids, followed by readjustment under the laws of capillarity. Unfortunately, the understanding of the influence of sedimentary structures of the scale used in the experiments has not advanced much beyond this statement of 60 years ago.

A second early (1939) paper worth mentioning is by Illing.4 He states that the most difficult zone to flush with water is the coarse/fine interface; thus, where there are numerous textural changes in the reservoir rock, it will be increasingly difficult to obtain a high percentage of oil extraction. All such coarse/fine interfaces tend to lock up oil in the strata.