Estimation of mode I static fracture toughness of carbonate rock using small rock fragments

Abstract

Fracture toughness is the key to analysis of failure progress, which can be defined under different stress conditions as fracture toughness modes. The mode I fracture toughness (KIC) of rock is an important parameter in petroleum and reservoir engineering; however, preparation of standard samples for determination of this parameter is costly and time consuming and is not feasible under all conditions. At times, core samples are not available for such destructive tests. In such situations, the use of small rock fragments for measurement or estimation of the mechanical properties of intact rock can be a practical method. The present study evaluated the single compressive strength test (SCS) as an indirect method for estimating KIC using small rock fragments. Spherical carbonate rock samples of 2–10 mm in diameter from the Asmari Formation were produced from rock blocks and were tested. The behavior of each was studied and the fracture mechanisms were determined for calculation of the amount of stress (σSCS) at the failure point. Semicircular bending tests were used to determine the KIC for each sample and regression models were established between σSCS and KIC where R2 ≥ 0.95. Of the 16 samples studied, 10 were used to develop a regression model and six were used for validation. It was found that the regression models were able to estimate KIC with good accuracy. To evaluate the best model more accurately, the error of estimation of each model was calculated and it was found that the linear and exponential correlations had the lowest errors of estimation. Based on the range of estimated values, linear correlation was selected as the best correlation method. A correlation was made between all σSCS and KIC values for the carbonate rock samples using the linear regression model where R2 = 0.91 and a final empirical relation has been proposed.