Detecting Microporosity To Improve Formation Evaluation
Authors: Kieke E.M. , Hartmann D.J. 
Published: December 2007

If you can't see the trees for the forest, get a scanning electron microscope. It can get you close enough to discover precisely what conditions are, and in conjunction with other devices may keep you from turning down some promising reservoirs.

Introduction

Evaluation with the most recently developed logging tools has revealed anomalous conditions of high water saturation in productive intervals and has indicated the need for a supplemental evaluation tool or technique for such circumstances. The anomalies were observed in both clastics and carbonates over a broad geographic area. The scanning electron microscope (SEM) bas been useful forgrain surface studies related to environments of deposition,microfossil investigation for biostratigraphic dating and alteration of sediments, anddocumentation of microporosity in carbonate rocks Use of the SEM has improved our understanding of the actual pore configuration of reservoir rocks characteristics that can be observed only at high magnification.

Core and cutting samples from numerous wells have been inspected with the SEM. Observations indicate that where the reservoir rock contains microporosity, the upper limits for water saturation determined by log analysis can be increased. The minimum water saturation depends in part on the relative amount of microporosity in the rock.

Detection of Microporosity

Reservoir rocks are divided into two basic categories:clastics, which include slits, sands, and gravel; andcarbonates, which encompass limestones and dolomites.

The pore space between sand or carbonate grains as originally deposited is called intergranular porosity. Vugular porosity, another well known type, porosity. Vugular porosity, another well known type, can be formed by leaching of carbonate grains or other soluble material, which results in pores larger than the grain size of the rocks. Both intergrain and vugular porosity can be detected visually or at low magnifications porosity can be detected visually or at low magnifications (50 x or less), Inspection of both clastic and carbonate samples with the SEM can reveal another type of porosity: microporosity. No limits on physical measurements have yet been established to define a micropore. Rather, it has been accepted that a micropore is so small that it cannot be seen at magnifications of less than 50 x and requires 100 x to 5,000 x to be studied in detail. The presence of microporosity in a reservoir increases the presence of microporosity in a reservoir increases the capillary attraction to water, frequently resulting in very high immobile water saturation. The microporosity is formed by one of several methods, including recrystallization of grains and oolites, dolomitization, formation of intercrystalline pores, and secondary cementation. Thus far, no completely reliable method has been established for recognizing reservoirs containing microporosity except inspection of samples with an SEM. Several techniques have, however, offered valuable assistance in revealing such reservoirs. The use of mud loggers during drilling has in some cases revealed shows where log calculations indicated high water saturation. Subsequent testing has sometimes yielded hydrocarbon production where examination of samples revealed microporosity.

JPT

P. 1080