Assessment of induced fracturing and fracture sealing during drilling

Abstract

Laboratory testing of lost circulation or loss prevention materials are typically studied either using permeable ceramic discs or sealing of slotted steel discs. Such studies may well reflect the sealing of a permeable formation or the plugging of an existing fracture. When drilling a permeable formation with a high overbalance, the pressure from the wellbore sometimes induces fractures and thereby cause the potential for severe lost circulation incidents. In such situations, it is important that the drilling fluid is composed such that the fracture is sealed rapidly, and that fracture propagation does not occur.Testing of induced fracturing is very limited due to the complexity of the equipment and access to cores which may adequately represent a formation. The objective of the study was to design an experimental method for assessing the functionality of a drilling fluid to seal induced fractures in a porous formation, and to apply the method for evaluating different fluid compositions. A method was developed using ceramic discs and adapting a conventional permeability plugging apparatus to induce fractures in the disc during a high-pressure fluid-loss test. This permitted a study of the rupturing and re-sealing of the filter-cake when a fracture was induced. The method was applied to water-based (WBM) and oil-based fluids (OBM) containing different loss prevention materials (LPM), and the method facilitated studying the permeability of filter-cakes as well as the fluid loss occurring at the fracturing stage. The method thereby enabled to clearly separate the performance of different fluids for sealing of induced fractures even when the fluids showed similar permeability plugging characteristics.The loss prevention materials used for the study was selected among mixtures typically applied for drilling campaigns in the North Sea, and cross references to field applications are included. The study showed that lower fluid loss was observed with oil-based fluids than with water-based fluids. However, once the discs fractured, the water-based fluids treated with loss prevention materials sealed the fractures in the discs with lower fluid-loss than similarly treated oil-based fluids.