Core Analysis of Unconsolidated and Friable Sands
Authors: Mattax C.C. , McKinley R.M. , Clothier A.T. 
Published: December 2007

Techniques for handling and testing unconsolidated sands are discussed. Evidence from acoustic and pressure-pulse field tests suggests that high first-cycle compressibility observed in laboratory, experiments is a realistic reservoir phenomenon.

Introduction

During the past decade the rubber-sleeve core barrel has been used very effectively in recovering cores from unconsolidated and friable formations. Complete core analyses of incompetent sands are now possible with either plugs or full-diameter samples using techniques that, for the most part, are simple extensions of techniques used for consolidated core plugs. This paper discusses typical results of a complete set of paper discusses typical results of a complete set of analyses run on a large number of cores during the last several years. Both friable and unconsolidated samples, including a wide assortment of rock types ranging from well sorted sands through conglomerates, were included in the test samples. Although no particular set of data can be considered "typical" of a specific unconsolidated formation, it seems desirable to have readily available in the literature a complete "set" of test results for comparison with other data or for use in the absence of other data.

In addition to a number of well known uncertainties that remain in the interpretation of core tests, even on competent cores, there are some uncertainties that are unique to unconsolidated sands. This paper alerts the engineer to these uncertainties, clarifies some of them, and discusses the over-all reliability of data obtained in the measurement of the properties of unconsolidated and friable sands.

The most difficult question to resolve is simply, "are consolidated sand cores recovered without significant disturbance, or are sand grains dislocated to the extent that no laboratory technique is capable of reestablishing conditions acceptably close to actual reservoir conditions?" This question is addressed in the discussion of compressibility.

Only brief descriptions of test procedures are presented; these are included to insure understanding of presented; these are included to insure understanding of any special techniques needed in unconsolidated sand tests. Procedures not described in detail either are referenced or may be assumed to be identical with those used in tests of competent cores.

Core Handling and Test Procedures

Other than the actual coring operation, the most critical step in collecting unconsolidated cores is the removal of the core, encased in the rubber sleeve, from the core barrel. Care must be taken to prevent the flexible rubber sleeve from buckling and seriously damaging the core. When the core is safely out of the barrel, it can be cut into sections that can be handled conveniently for shipment. The cut sections should be at least 2 to 3 ft long because the sand in the vicinity of the cut will be disturbed and some material will be lost. Rubber caps, taped in place as soon as the core is cut, adequately seal the ends of the cut sections and prevent exposure to air and loss of core fluids. The sections of the rubber sleeve core, properly boxed for protection, should be shipped to the laboratory without delay.

Two procedures Will produce acceptable test plugs from unconsolidated core material in nearly all cases. If the sand is completely unconsolidated it can be plugged with a thin-walled tube. plugged with a thin-walled tube. JPT

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