Model for fracture initiation and propagation pressure calculation in poorly consolidated sandstone during waterflooding

• A new model for fracture initiation pressure calculation in poorly consolidated rock is studied.
• The model takes the poroelastic and thermal effect for consideration.
• The effective stresses under different conditions around the water injection well are studied.
• The criterion for shear fracture initiation and propagation is proposed.

During the process of waterflooding, the water injection pressure needs to be increased to improve the water absorption capability of formation. However, the injection pressure should be less than the fracturing pressure. Otherwise, it will cause lots of problems, such as casing failure, caprock damage and the reactivation of faults. The past works indicate that fracturing in poorly consolidated sandstone is different from fracture growth in brittle competent rocks due to strong poroelastic effect and low strength. This paper describes a new model with the consideration of fluid flow and thermal effect to calculate the fracture initiation and propagation pressure. The fracture initiation and propagation criterion for poorly consolidated sandstone is different from the conventional stress intensity factor approach and a new shear criterion with the consideration of the conjugate shear plane direction is proposed. Base on the mechanical and physical properties parameters from Bohai Bay in China, the stresses around the wells during water injection are calculated to determine the failure mode. And the fracture initiation and propagation pressure are obtained with the model. The results of the study show that the fracture in poorly consolidated sandstone during water injection is dominated by shear failure, and the fracture is perpendicular to the minimum horizontal stress. The fracture initiation pressure is less than that calculated by the traditional approaches and is related to the in-situ stress, rock strength, water temperature, water injection rate and Biot's constant. The fracture propagation pressure is related to the in-situ stress, fracture length, rock strength, water temperature, water injection rate and permeability of the fracture. Through this study, the maximum injection pressure can be determined during water injection in poorly consolidated sandstone. In addition, this study could also have application in other areas, such as wellbore stability, sand production, frac-packing.