Experimental study of hydraulic fracture initiation and propagation in unconsolidated sand with the injection of temporary plugging agent

Abstract

High fluid leak-off rate and low pressurization rate are great challenges for hydraulic fracturing in unconsolidated sands. In this paper, the design of an injection plugging agent as a pad for fracture initiation aid in unconsolidated sands is presented. We have carried out tri-axial hydraulic fracturing tests, focusing on super-high- and high-permeability unconsolidated sands with AE (acoustic emission) monitoring. Powdered oil-soluble materials are used as temporary plugging agents. The laboratory tests demonstrated that three types of fracture geometry, multiple fracture, bi-wing fracture and single-wing fracture, are created. Furthermore, there are continuous AE energy signals with the increase of plugging agent injection, which is a good match with the fluctuated pressure. Moreover, multiple peak fracture points can be observed due to intense grains interaction. In addition, the breakdown pressure has positive relationships with the pump rate and concentration of injection of plugging agents. Multiple fractures only exist for super-high-permeability unconsolidated sands with high pumping rate and low plugging agent concentration, which is primarily due to the plugging agent seal for both hydraulic fracture and the rock matrix. A low injection rate with low plugging agent concentration led to single-wing fracture for high-permeability unconsolidated sands, whereas a high injection rate with high plugging agent concentration led to single-wing fracture for low-permeability unconsolidated sands. Low plugging agent concentration is beneficial for aiding hydraulic fracture propagation along the maximum horizontal stress direction. In addition, the high-pump-rate and low-plugging-agent-concentration scenario and low-pump-rate and high-plugging-agent-concentration scenario have a greater chance of creating bi-wing fracture for super-high-permeability unconsolidated sands, whereas low pump rate and high plugging agent concentration are beneficial for bi-wing fracture for high-permeability unconsolidated sands. Local tortuous fracture path indicate tensile and fracture both exist during unconsolidated sands fracturing. With respect to sand control and large contact area, these treatments should be the better fracturing scenario for unconsolidated sand fracturing design with injection of a plugging agent. The findings of this study can help for better understanding of the fracturing mechanism and optimizing treatment parameters in unconsolidated sands fracturing.