Summary

The variability in mechanical properties measured on sands from the Jauf and Unayzah formations of Saudi Arabia is observed to be dependent upon cementation. Understanding the role of cementation in controlling the mechanical properties can improve the design of hydraulic-fracture treatments and, hence, improve reservoir performance.

Strength measurements from triaxial-testing data and examination of core in thin sections were used to relate the detailed microstructure and cementation to the variation of mechanical properties. Strength and elastic moduli were determined for 65 samples cored from five different wells. Forty-seven samples were analyzed in thin sections and point counted to determine and quantify cementation. Cements in these two formations have variable composition and habits; both affect the mechanical properties and strength. It is not sufficient to know that cements exist; it is also necessary to know where the cement occurs.

Pure quartz over growths play a major role in increasing strength, while clay coatings play a minor role. Simple linear correlations were found relating cement concentrations to strength.

Introduction

The Jauf and Unayzah reservoirs (Ghawar field, Saudi Arabia) are deep sequences of thin sandstones and shales saturated with condensate-rich gas. These reservoir rocks display a wide variation in both permeability and porosity (Fig. 1) (Al-Qahtani and Buhidma 2001). Hydraulic fracturing has been introduced to enhance productivity in the Jauf sandstones within the Ghawar reservoir.

The complex nature of tectonic stresses, geomechanical properties, and geology leads to problems of sand production and wellbore stability in the Unayzah and Jauf formations. Such problems make the development of these reservoirs challenging. The variations of mechanical properties such as Young's modulus (E), Poisson's ratio, (u), and unconfined compressive strength (UCS) have a direct impact on hydraulic-fracture design. The variation of these properties affects fracture propagation and geometry and, consequently, gas production.

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