Abstract:
Integration of geomechanics test results with well log measurements is a crucial step in developing geomechanical models to estimate in situ stress state, maintaining stable wellbore and design hydraulic fracture. Limited availability of core material severely restricts design of a comprehensive geomechanics test program. Sidewall core plugs, although easier to obtain than conventional cores, suffer from being short relative to the length to diameter ratio specified by ASTM. This paper presents a solution to overcome size issues related via measuring ultrasonic velocity on sidewall core plugs at various stress paths. A unique experimental setup allows measurement of ultrasonic velocity both parallel and perpendicular to bedding. The results were then compared with velocity measurements obtained from sonic logs. It was observed that velocity measured within an isostatic stress path provides the closest match to log velocities, thus increasing confidence in dynamic geomechanics measurements on sidewall plugs and allowing geoscientists and engineers with low budgets to conduct more comprehensive geomechanics analyses.
Introduction
The value of laboratory testing to enhance our understanding of subsurface geomechanical properties to better explore and exploit a hydrocarbon reservoir is well known (Plumb et al., 2000; Cook et al., 2007). Important parameters that provide baseline information for constructing a geomechanical model of the subsurface include the in situ stress state, strength properties, and stress-strain behavior (Zoback, 2007). Among these parameters, strength and stress-strain behavior can be measured in the laboratory whereas in situ stress can be estimated via a combination of well logs, laboratory and field measurement (Thomsen, 1986; Thiercelin and Plumb, 1994). Thus it is imperative to have good quality laboratory data since that would in turn improve estimation of in situ stress state (Higgins, 2006) and help geoscientists solve wellbore stability issues and hydraulic fracture design. In this direction, ASTM and ISRM provided enormous assistance via creating standard protocols on how to conduct a test in the laboratory.
According to ASTM, core plugs used to estimate stress-strain behavior of rock should have a length to diameter ratio of 2 to 1. The process of acquiring and storing cores is a costly operation and lack of core material constrains laboratory testing and generation of geomechanical parameters. Sidewall cores, acquired by attachments to wireline logging tools provide a cost effective alternative as they can be (a) acquired relatively quickly and (b) can be acquired when there is a failure to collect core by conventional coring techniques (Agarwal et al, 2014). However, samples with a length to diameter ratio of less than 2 to 1 are less suitable for laboratory testing. We report a solution to issues related to shorter core plugs during geomechanics testing. In this paper, ultrasonic velocity was measured on sidewall core plugs through various stress paths. A unique experimental setup allows measurement of ultrasonic velocity both parallel and perpendicular to bedding. The results are then compared with velocity measurements obtained from sonic logs. It is observed that velocity measured within an isostatic stress path provides the closest match to log velocities in most of the samples. The velocities obtained throughout various stress paths were compared and efforts were made to address the discrepancies between them.
