ABSTRACT

ABSTRACT

This paper focuses on tests performed on sandstones and shales from four gas wells in the Mesaverde formation. A previous document showed a comparison of the static and dynamic laboratory moduli. This publication describes how field velocity logs were used to derive field dynamic moduli. A 3-way comparison was then performed for these stiffness properties; the results show no agreement between the three sets of values. Differences between dynamic stiffness coefficients from the field and the laboratory exceed 200%; differences between static and dynamic coefficients range up to 600%. In turn, this indicates that a reliable method of estimating mechanical rock properties for hydraulic fracturing design has not been established, and that such a task should be pursued.

1. INTRODUCTION

This research was performed as part of the Western Gas Research/Unconventional Gas Program at LLNL [1,2]. The preferred method for stimulating low permeability Western Gas Sands reservoirs is hydraulic fracturing [3]. The design of hydrofracs requires a knowledge of mechanical properties of reservoir rocks, among other things [4-8]. Such properties can be estimated in four different ways: by static or dynamic tests, either in the field or in the laboratory. Because hydraulic fracturing is a quasi-static process involving high stress and low strain rate, the most relevant of the four procedures mentioned above would appear to be the static in-situ testing. However, such tests must be conducted in uncased holes; also, the downhole jacks and dilatometers curtenfly used in the civil engineering profession typically are not tailored to gas well sizes. Hence, the static field tests are not done. Meanwhile the other three types of tests are frequently performed: velocity logs and density logs are used to calculate field dynamic moduli, and laboratory static and velocity tests are also performed to calculate the elastic moduli of cores. This study was intended to estimate the in-situ dynamic elastic moduli of the Mesaverde sandstones and shales, representative of the tight gas sands in the western U.S. This paper also summarizes the laboratory determined static and dynamic elastic moduli of these rocks reported previously [4,5], and offers a 3-way comparison of mechanical and sonic testing results on the same materials. The Laboratory determined elastic moduli were measured on cores obtained from five wells that were drilled into the Mesaverde formation in 1979. These wells were Twin Arrow C&K 4-14 (Rio Blanco County, Colorado), Pacific Transmission Supply (PTS) 24-19 Federal (Sublette County, Wyoming), PTS 22-12 Federal (Rio Blanco County, Colorado), PTS 3-10A (Sweetwater County, Wyoming) and Rainbow Resource (RR) 1-3 Federal (Sweetwater County, Wyoming). These wells were logged by Schlumberger Co. Sonic logs and density logs of the last four wells were used to estimate in-situ dynamic elastic moduli in our study. The sonic log of Twin Arrow C&K 4-14 well did not contain the Variable Density record, which was necessary for the intended estimation of shear wave velocity. The compressional wave travel time and bulk density of these four wells were directly decoded from the magnetic tapes that were made available by CER Corporation in Las Vegas, Nevada.

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