Finite Element Studies of Wellbore Strengthening

Abstract

In drilling deep horizons, the mud weight window between pore pressure gradient and fracture gradient often narrows due to rock properties and underground stress state. Non-productive time (NPT) events such as lost circulation, wellbore instability, kicks, underground cross-flow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Plugging preexisting natural fractures or drilling-induced fractures with lost circulation materials (LCM) is often used to increase fracture gradient and widen the mud weight window. This technique, called 'wellbore strengthening', includes several strengthening methods, but there are several factors affecting these procedures are not thoroughly understood.To reduce the risk of loss circulation while drilling in formations with narrow mud weight windows such as pressure depleted reservoirs and deep-water formations, a good understanding of the mechanisms of wellbore strengthening in different downhole scenarios helps engineers to optimize the design of drilling fluids and operation procedures. This paper discusses the mechanism of wellbore strengthening in elastic and pore-elastic models, utilizing the finite element method to evaluate wellbore and near-wellbore stresses during fracture initiation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior such as formation stress anisotropy, LCM bridging location, initial pore pressure, and fluid leak-off are investigated. A better understanding of the several interacting events local to the wellbore and nearwellbore regions can result in improved operational practices related to lost circulation events.

A Numerical Study on the Application of Stress Cage Technology

“…Therefore, it can be concluded that the closer the bridging location to the fracture aperture, the more effective stress cage technology. This result is in agreement with previous studies [23,26].…”

“…Feng et al (2014) used two-dimensional elastic and poro-elastic models to evaluate the stress state at the near wellbore region before fracture initiation, during fracture propagation, and after fracture bridging with LCM. In the conclusion, authors provided a detailed description of the wellbore strengthening mechanism and affecting parameters [26].…”

Mudcake effects on wellbore stress and fracture initiation pressure and implications for wellbore strengthening

“…Lost circulation is a major costly incident in the drilling and completion phases. Existing studies have indicated that lost circulation can be effectively mitigated by wellbore strengthening (Alberty and McLean 2004;Arlanoglu et al 2014;Zhao et al 2017). The above analysis has shown that increased FIP and thus wellbore strengthening can be achieved by forming a mudcake on the wellbore wall.…”

“…The stress and rock properties are consistent with those used in Feng and Gray (2016b). For the fracture-bridging model, the commonly used 0.15-m (6-in) fracture length assumption is adopted (Alberty and McLean 2004;Arlanoglu et al 2014;Aston et al 2007;Wang et al 2009). The tensile strength is 15 MPa determined from the fracture toughness using Eq.…”

Analysis on Prediction Methods for Formation Pore Pressure under Multiple Pressure Formation Mechanisms

“…Based on the basic principles, both the pre-drilling prediction and monitoring-while-drilling methods can be divided into two categories: normal compaction trend method and direct pressure prediction method [4][5][6][7][8][9][10][11][12] . In the normal compaction trend method based on formation under-compaction mechanism, a compaction trend line equation is built within the normal section using variations of acoustic, density, resistivity and other logging data along with the depth, and the formation pressure is predicted according to deviation of the actually measured value from the trend line.…”