Abstract

There are numerous design criteria available for gravel packs based On the well-known gravel to sand grain size ratio. These rules are mainly derived from the interpretation of compatibility experiments. The favorable gravel to sand ratios are determined within the limits set by successful and unsuccessful tests. Because of empirical approach various influences have to be considered due to different test setup and evaluation of experiments. In this paper, the types of design criteria, the similarities and differences between the various methods, the uncertainties in the use of these design criteria, and the gaps in current knowledge are described. Applicability of these purely geometrical methods under fluctuating flow, methods relating the sand retention, permeability impairment, clogging and acceptable quantity of sand within a certain depth of penetration into the gravel are discussed.

Introduction

Since gravel-packs are recognized as the most popular sand prevention method, numerous gravel-pack design criteria have been developed roughly a quarter of a century ago. These methods have ranged from using simple geometrical approaches on easily measurable sand and gravel properties to performing filtration tests with representative reservoir sand and the proposed gravel-pack. The latter have generally been regarded as being better indicators of future gravel-pack performance. However, as sand particles of reservoirs are structured at random, it is almost impossible to describe the interaction between sand and gravel theoretically in a rigorous manner. All existing gravel-pack design criteria are more at less based on laboratory tests with defined sandgravel combinations and can therefore only be applied in connection with the respective combinations and test conditions. However, despite careful application of these rules, higb pressure drop and reduction in production rate are still associated with this exclusion technique. Consequently, a gravel-pack seems to change its structure, permeability and pressure differential over the course of a filtration period Thus. there is a certain unpredictability about the process of sand exclusion due to the inability of fully describing the mechanisms and the properties of gravel-sand fluids system and of relating properties and mechanisms to the performance of materials. So far, there is not much known about the reliability of gravel-packs that are designed on the basis of different geometrical criteria; in particular in cases where the grain size distribution of the produced formations is not well known in additional performance tests only qualitatively evaluate the gravel-packs, and therefore discourage improvement or gravel-pack design by not providing quantitative performance measures.

Gravel-pack design methods based on gravel to sand Size ratio result have been marc readily accepted because they are easy to use and easily implemented into practice. However. there is considerable variability between these methods, and such methods do not adequately address sand migration, clogging and permeability impairment. In addition, these conventional design criteria rely completely on the geometrical condition that the pores or the gravel have to be smaller than the sand particles to prevent sand from being transported through the pores of the gravel-pack.

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