Study on Fracture Propagation Law of Cement Sheath Interface During Fracturing Process

Abstract

ABSTRACT: In the process of fracturing construction, the injection of high-pressure fluid will lead to the propagation of fractures at the first and second interfaces of the wellbore cement sheath, which seriously affects the integrity of the wellbore, and even cause the interlayer channeling of the fracturing fluid. In this paper, a three-dimensional finite element model of fracture propagation at cement sheath interface is established based on the Cohesive Element Method. The numerical model can quantitatively analyze the influencing factors of fracture propagation at cement sheath interface, including mechanical properties of cement sheath, horizontal in-situ stress difference, initial fracture shape, injection pressure and time of fracturing fluid. The results show that the wellbore integrity can be maintained and the interface cementation strength can be improved by optimizing the appropriate cement slurry system and the elastic modulus of cement sheath. Before fracturing, the selection of perforation location needs to fully consider the heterogeneity of the formation. Selecting the formation with relatively low horizontal in-situ stress difference will effectively slow down the fracture propagation at the cement sheath interface. When the injection pressure of the fracturing fluid exceeds the propagation pressure of the interface fracture, it will lead to the interlayer channeling of the fracturing fluid, and even fracturing failure. In summary, in order to improve the wellbore integrity in the fracturing production process, it is very important to optimize the appropriate cement slurry system and fracturing design scheme. 1. INTRODUCTION Wellbore integrity is an important attribute of wellbore to resist structural damage and maintain wellbore function. It can ensure the safety of drilling engineering and is very important to the whole oil and gas production process (Tian et al., 2015; Song et al., 2016). The cement sheath is the core of wellbore integrity, providing interlayer isolation and support for the casing to ensure its integrity throughout the life of the wellbore (Gray et al., 2009; Feng, 2016). At present, hydraulic fracturing technology is a major means of increasing production in the oil field. This technology is to inject the fracturing fluid (mainly water, sand or other proppants suspended with the help of thickeners) into the wellbore through a high-pressure pump to form artificial fractures in the deep strata, so that oil and gas can flow more freely in the reservoir. When the fluid pressure is removed from the well, the small particle hydraulic fracturing proppant (sand or alumina) maintains the fracture in an open state to increase the reservoir permeability (Xu et al., 2019; Biao, 2011). In the process of fracturing construction, the injection of high-pressure fluid will lead to the propagation of fractures at the first and second interfaces of the wellbore cement sheath, which seriously affects the integrity of the wellbore.