The objective of this paper is to present experimental results and to investigate the correlation between tensile strength and mode-l fracture toughness of rocks. Rock specimens were obtained from Saudi Arabia, from both an outcrop and a petroleum reservoir at a depth of about 3.5 km below the ground surface. Other material and mechanical properties of this rock are also presented in this paper. Tests were conducted in the rock mechanics laboratory to characterize these rocks, using Brazilian disk and notched Brazilian disk specimens. Specimens were diametrically loaded using a strain-controlled loading frame equipped with a load cell to measure the applied load. The data for the investigated rocks were analyzed for a relationship between fracture toughness and tensile strength. The findings were also compared with similar results available from the literature.
When a reservoir rock formation has low permeability, the recovery of oil or gas becomes very difficult. To enhance the recovery from such a formation, cracks or fractures are created deliberately by injecting high pressure fluid into the reservoir. This process is known as hydraulic fracturing. The pressurized fluid creates the fractures by overcoming the in situ stresses in the reservoir and the tensile strength of the rock at the reservoir temperature. Due to these new cracks, the surface area of the rock is significantly increased thereby enhancing the flow of oil or gas towards the well. The creation of fractures depends, apart from magnitude and orientation of in situ stresses, on the intrinsic properties of the rock formation. Rock fracture toughness and tensile strength are two basic and fundamental factors to take into account when designing a hydraulic fracturing program in a particular formation at a given depth (Shlyapobersky & Chudnovsky 1992; Jeffrey & Settari 2000). Although a number of studies have been focused on the tensile strength of rocks and rock-like materials (Andreev 1995: Pells 1993), the tensile strength and fracture toughness behavior of reservoir rocks of Saudi Arabia, particularly its variation with depth, has not been fully investigated. The study is valuable in terms of estimating the pressure required to carry out the process of hydraulic fracturing successfully at the desired depth. In this investigation, an experimental program was conducted on reservoir rock specimens collected from a local well drilled in the Eastern Province of Saudi Arabia, as well as on specimens from an outcrop of the same formation as that of the reservoir. Tensile strength and fracture toughness tests were conducted using Brazilian disk specimens and notched Brazilian disk specimens, respectively.
Rock samples were obtained from the "Khuff" formation in the Eastern Province of Saudi Arabia. Geologically, the Khuff formation relates to the early Triassic to late Permian age (215 to 270 M.Y.B.P.). The structural geology for this formation indicates that It outcrops at various places in the Central Province of Saudi Arabia, with an altitude reaching hundreds of meters above the sea level.