In recent years, in order to improve the oil production of unconsolidated sand reservoirs, hydraulic fracture through the screen liner has been carried out in the Bohai oilfield. Traditional hydraulic fracture design methods usually assume the rock is elastic, while the weak sands are often nonlinear elastic rocks. This study investigates how to optimize the mechanical parameters used in the hydraulic fracture design to best approximate the rock elastic properties under in-situ formation conditions, and show how to derive them from well logs.

We performed uniaxial strain and triaxial stress compression experiments on five and seven groups of core samples, which were from well P6 and well P8, respectively. Each group of samples had five plugs with similar depth, one of which was designed for the uniaxial strain experiment and the other four for the triaxial stress experiment. Linear regression analyses and extrapolations were carried out for each set of core data to find the proper mechanical parameters for the fracturing design. The quantitative conversion formulas between core analysis and well log-derived results of these moduli were eventually established.

The reservoir rocks of P oilfield located in Bohai are relatively weak and have low stiffness. Mechanical experiments show that there is a linear correlation between deformation modulus and effective confining pressure. Core data analyses indicate that constrained modulus provides the best approximation of rock modulus under initial reservoir conditions. And its value is the ratio of stress to strain (from initial linear data) in the uniaxial strain experiment. Therefore, it is recommended to use the constrained modulus as the mechanical parameter in hydraulic fracture design. Meanwhile, there is a strong linear relationship between constrained modulus and Young's modulus, and the former is roughly six times greater than the latter. Combining the conversion between core and well log-derived Young's modulus, it is possible to estimate constrained modulus from well logs. The method proposed in this research was used for the fracturing design of well P45. After fracturing, the daily oil production increased from 399.3 to 2,462.9 ft³, and the liquid production increased four times.

Based on the studies of rock mechanics laboratory data, we propose that constrained modulus is the appropriate parameter in the hydraulic fracture design of soft sediments. And it has been verified by actual production data. The new method provides a reliable reference for the hydraulic fracture design of weakly consolidated sands in Bohai and other similar oilfields.

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