ABSTRACT

Rock mechanical properties of formation rocks play an important role in understanding the behavior of the formation rocks under stress, avoid wellbore instability, and optimizing the production of hydrocarbons from underground reservoirs. Characterizing young shallow soft rocks is very important to avoid rock instability in many engineering applications including, civil, electrical, and nuclear facility construction. In this study, cylindrical samples were cored from bulk weak sandstones collected from an outcrop area South of Riyadh, Saudi Arabia. The samples were prepared and then tested in the laboratory where many difficulties were encountered. Difficulties in characterizing the soft rock ranged from basic core measurement to triaxial mechanical testing. For example measuring permeability using gas permeameter has been difficult as the high permeability is out of the range of most of the measuring equipment. Another example is the difficulty of precisely identifying the failure situation due to two reasons; firstly, is the absence of sharp break of the rock and gradual failure of the core samples, secondly is the compaction of the sand grains when loading is continued after failure.

1. LITERATURE REVIEW

Difficulty of testing weak rocks has been recognized in the literature of geotechnical and geomechanical research. Versluis and Lindner [1] concluded that material behavior and instrumentation in hard rock media is significantly different from weak rocks. They also reported difficulties due to ineffective straingauging due to slip and rock sample irregularity [1]. The challenge of laboratory testing for soft rocks was investigated by Blumel [2], where examples of mechanical parameter determination for weak rocks were shown. In the same study[2], it was indicated that a difficulty in specimen preparation can lead to a biased selection of harder rock samples, and that standardization is viable for hard rocks and soil [2], but not weak rocks. Based on laboratory experiments studies and analysis of weak rocks that concluded that the strength of rocks depends on porosity and grain area ration, Jeng et. al. [3] developed a model to describe the effect of porosity and grain area ration on the strength of the soft rocks [3]. Shafiei and Dusseault [4] conducted a laboratory study to determine geomechanical properties of weak rocks from Iran. They indicated that little information is available in the literature about geomechanical characterization of soft rocks. They concluded that instability could occur due to weak rocks [4]. In their study of 55 samples of weak materials, Peres-Rodgigues and Sousa [5] indicated that the number of laboratory experiments must be significant in order to complement field test [5]. Weng et. al. [6] formulated a model to simulate the deformation of weak sandstone. Their model was based on linear elasticity [6]. To avoid problems inherent to laboratory testing of weak rocks, Huang et. al. [7] used in-situ tests to characterize some weak rocks of Western Taiwan northern and central regions outcrops[7].

2. INTRODUCTION

Characterizing young shallow soft rocks is very important to avoid rock instability in many engineering applications including, civil, electrical, and nuclear facility construction.

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