Sandstone Reservoir Assessment and Production is Fundamentally Affected by Properties of a Characteristic Porous Microfabric
Authors: Ehrlich Robert , Prince Christopher , Carr Matthew B. 
Published: December 2007

Sandstone Reservoir Assessment and Production is Fundamentally Affected by Properties of a Characteristic Porous Microfabric Robert Ehrlich, SPE, U. of Utah; Christopher Prince, U. of South Carolina; Matthew B. Carr, Amoco E & P Company

All sandstones have a characteristic porous micro-fabric consisting of packing flaws and close packed domains. The packing flaws are arrayed as circuits with great areal continuity composed of oversized pores linked by oversized pore throats. The porosity involved in the packing flaws can be detected and classified by two image analytical procedures and this quantitative data can be correlated with a wide range of fluid properties (permeability, Sor, Swi, etc.). Packing flaws are preferentially preserved during compaction and chemical diagenesis. Wetting phases are preferentially segregated in the close packed domains, are relatively immobile and, in most sandstones, the porosity fraction represented by close-packed porosity is correlated with Swi. However, grain size also affects the value of Swi. In general, a coarse grained conglomerate will contain close packed domains whose throats only weakly imbibe water whereas the contrary occurs in an equivalently sorted medium or fine grained quartz-rich sandstone. For a quartz rich sandstones however, Swi is proportional to the amount of porosity in close packed domains.

Mobile non wetting phases are restricted to loose packed circuits. A subset of circuits with the aspect ratio closest to unity carries the non wetting phase at highest water saturations—the remainder of the loose packed circuits account for most of the Sor. Image analytical procedures can therefore objectively define reservoir rock types and evaluate the porous microfabrics of these rock types in terms of a more relevant reservoir description and tying rock type to wireline log response. The existence of the small scale heterogeneity consisting of packing flaws and close-packed domains may explain in some part the difficulty in scaling up permeability (a sample support problem) and may require a reevaluation of the basis of relative permeability procedures.

Introduction

Reservoir sandstones are commonly well sorted, are assembled by the process of sedimentation and the resulting depositional fabric persists over geological time scales. This depositional fabric consists of grains of variable shape and size packed in geometries dictated by the rules of sedimentation and compaction. The porosity is arrayed in a complementary fabric and consists of a three dimensional array of pores connected by pore throats. It is well known that sandstones may be internally heterogeneous at the bed level (10 cm–10 m) but, inasmuch as grain size variation is commonly minimal at the smaller scale, it is commonly assumed that the porous microstructure is uniform and homogeneous. This tacit assumption guarantees the relevance of small cylindrical plugs taken from core for measurement of physical properties. Failure of that assumption affects the ways in which we attempt to scale up core measurements and to interpret the results of physical tests.

The fundamental theory for the formation of sand and empirical evaluations of that theory, show that the assumption of homogeneity at small scale is always incorrect (figure 1). In this report we document:the existence of a universal micro-structural fabric of sandstones,the relationship of that fabric to single and multi-phase flow andsuggest ways in which present practice can be modified to take advantage of this insights.

Sandstone Microstructure. Sandstones are in fact universally heterogeneous from the mm scale upwards—with variation in packing dominating the smaller scales and grain size the higher scales. Graton and Fraser proposed that a characteristic sort of heterogeneity (or "structure") exists in all sandstones over scales of mm to cm (tens of grain diameters)-and that this fine-scaled structure is physically relevant in understanding permeability. They conjectured, based on experiments using monolayers of spheres, that:

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