Dynamic Rock Mechanics Testing for Optimized Fracture Designs
Authors: Lacy Lewis L. 
Published: December 2007

To optimize fracture designs, rock mechanics data are needed at multiple locations in the formation and adjacent zones. This paper will review a laboratory technique that reduces testing time and cost by 60 to 80%. The technique has been successfully used on a wide variety of core and also reduces core-size requirements. Ultrasonic (dynamic) test equipment and procedures are discussed to standardize the method for petroleum industry applications and provide reliable data for fracture designs. The primary data provided are Young's modulus and Poisson's ratio. Dynamic testing has been performed on 600 cores from about 60 formations. The data are also compared to static uniaxial and triaxial data on the same cores to determine correlation coefficients between the static and dynamic data.

Procedures and apparatus for performing ultrasonic testing have been successfully developed that determine the dynamic Young's moduli for weakly consolidated cores, with Young's moduli of 60 thousand psi, to hard limestone with Young's moduli of 14 million psi. Several equations are also provided that have applications to sonic logging for mechanical property evaluation of formations. The same equipment has been used to determine fracture azimuth from oriented core at significant cost savings over other techniques. The paper will also review the relative importance of rock mechanics data on optimized fracture designs.

Introduction

Optimized fracture design treatments require data on rock properties, fracturing fluid properties and proppant properties. Our rock mechanics laboratory routinely supplies data to customers on rock and proppant properties. Optimized hydraulic and acid fracture designs require rock properties data in multiple locations in the producing interval and adjacent formations. The use of 3-D and pseudo-3-D fracture design models in recent years has increased the importance of acquiring inexpensive and rapid rock mechanics data from cores and logs. Optimized designs require, as design input parameters, Young's modulus, Poisson's ratio, fracture toughness or tensile strength and estimates of the in situ stress versus depth.

Rock mechanics data, such as Young's modulus and Poisson's ratio, can be estimated from dipole or long space sonic logs or measured in the laboratory using uniaxial, triaxial or ultrasonic testing. The preferred laboratory technique is to perform triaxial testing by simulating the in situ stress and fluid saturation conditions that exist down hole. Triaxial testing is considered as a petroleum industry standard. The primary limitations of triaxial tests is the extra cost and time to perform 8 to 15 tests on cores from one or more wells. Dynamic or ultrasonic testing is also sometimes used in lieu of triaxial testing but has never been accepted as a standard in the oil and gas industry 1.

The advantages of dynamic versus static testing can be summarized as follows:–The testing time and cost are reduced by 60 to 80%.–A nondestructive testing technique for cores that allows other cores property measurements such permeability.–Smaller samples can be tested thus reducing the minimum sample length from 2 inches to 0.5 inches for 1-inch diameter samples.–A wide variety of cores can be tested from weakly consolidated samples to hard rocks with dynamic Young's moduli between 60 thousand psi and 14 million psi.–More samples can be tested in the same zone to provide multiple values in the formation and adjacent layers in a rapid response system needed for typical fracture designs.

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