Fractured Wellbore Stress Analysis: Sealing Cracks to Strengthen a Wellbore

Abstract

Abstract Wellbore pressure containment, the maximum pressure that a wellbore can withstand before whole drilling mud begins to leak off into formations, defines the upper bound of the mud weight window. Drilling costs are heavily associated with this mud weight window in terms of expensive casing programs or nonproductive time because of narrow mud weight windows. Although various methods have been developed using different materials for curing mud losses, the performance has been poor as a result of the lack of engineering design for the materials and placement techniques. The commonly used "trial and error" approach reveals inadequate understanding of the challenge, although a number of advances have been accomplished in recent years. In this paper, results are reported from a fundamental study aimed at revealing the basic physics that control the success of treatments strengthening wellbores or widening the mud weight window. It first focuses on reviewing and identifying factors that can reduce the upper bound of this mud weight window, followed by analysis of wellbore strengthening that can increase this upper bound of mud weight window, especially by sealing hydraulically conductive cracks at the wellbore. Introduction With the move to deeper reservoirs that requires penetrating depleted formations or those located in deeper water, the safe drilling mud weight window is becoming smaller. This has been translated into much higher costs because of either expensive casing programs or nonproductive time (NPT). More than 12% of NPT has been reported1 for Gulf of Mexico (GOM) area shelf drilling as a result of lost circulation alone. It is a common practice to use lost circulation materials to cure mud losses to drill ahead2. However, because of a lack of a clear understanding of mechanisms and loss conditions, a "trial and error" approach is commonly used by following a predetermined flowchart. This is obviously not the most efficient approach. In recent years, work3–16 has been performed to address this problem and the concept of "wellbore strengthening" has been gradually emerging, replacing the old concept of "plugging a hole." This important change reveals that the industry is now embracing the understanding of the technology and focusing on the physics behind wellbore problems and potential solutions. Among these solutions, it is important to discuss wellbore strengthening with particulate lost circulation material (LCM) treated drilling fluids. An example of this technique is the "stress cage" method12,13. This approach shows great potential for solving lost circulation, especially when drilling depleted formations. Although some report success, the stress cage method has been challenged by others17. Some issues, such as fracture stability and the maximum wellbore pressure containment, have been reviewed in a previous publication18. A detailed study of this method is probably needed to better understand its effect on the physics of wellbore strengthening. This paper begins with the analysis of factors that weaken wellbore pressure containment at or near the wellbore. It appears obvious that reversing of some of those factors could strengthen the wellbore. However, this is not so apparent for other factors, such as fractured wellbores. Therefore, this paper uses a boundary element analysis method, a numerical approach, to demonstrate how a wellbore can be strengthened by sealing the fractures. A similar analysis that involves the far field stress affected by pore pressure depletion would reveal the mechanism for lost circulation in depleted formations. This will be discussed in a separate paper with an analysis of a different strengthening approach. Wellbore Pressure Containment and Wellbore Weakening Wellbore pressure containment (WPC) is defined as the maximum pressure that a wellbore can withstand before the wellbore starts to leak its mud into the formation16. When large pores, vugs, or open natural fractures exist, WPC is defined primarily by the formation pressure. The loss of mud into large pores, vugs, or open natural fractures is beyond the scope of this paper. Because hydraulic fracturing is a tensile failure process, the near wellbore stress concentration pertinent to WPC is the tangential stress around the wellbore, sometimes called hoop stress. Sedimentary rock has a very small tensile strength, usually not exceeding 200 psi. It is often neglected to simplify stress analyses.