Effects of Dynamic Surge Pressure on Wellbore Stability

Abstract

ABSTRACT This study presents a coupled poroelastodynamic model for wellbore stability analysis considering the effects of tripping operation and the flow communication between wellbore and formation. First, a transient hydraulic model is developed based on transient pressure propagation in the wellbore to predict the generated surge pressures during tripping operation. The transient hydraulic model is transformed into ordinary differential equations using the method of characteristics and is finally solved through the finite difference method. Then, the results are coupled with a wellbore stability model to include the effect of wellbore pressure variation with time. The developed transient hydraulic model is validated through comparisons with available field data and modeling results in the literature. Comparing the surge pressure predictions of the model with field data indicates a consistent and accurate prediction of the transient surge pressure. The results further show that a maximum surge pressure can be expected before approaching an equilibrium surge pressure, which could not be predicted by the previous surge models due to ignorance of the acceleration terms. The total radial and tangential stresses are calculated and shown to vary versus time. The results indicate that the induced pressure initially rises to a maximum value and then decays with time, but the maximum value is not necessarily always at the wellbore wall. The results of tensile and shear failure analysis indicate time-dependent failures can occur in the vicinity of the borehole depending on the magnitude of the tripping velocity. INTRODUCTION Tripping is a frequent operation that is running pipes into or out of a well for different reasons, e.g., replacing a dull bit, replacing bottom hole assembly, running logging tools, running casings or liners, and wellbore conditioning. It has long been known that the tripping operation can induce surge or swab pressure if running into or out of the well, respectively. In fact, the axial movement of a drill string, like a piston, in the wellbore results in pressure perturbations. Pioneer studies such as Cannon (1934) and Goins et al. (1951) show that a high tripping velocity can cause drilling problems such as formation fracture or gas kicks. The magnitude of surge pressures typically may not exceed the safe pressure limit in most wells. However, there are critical wells such as deep wells or depleted reservoirs where the surge and swab pressures shall be maintained within a narrow pressure limit. Although a low tripping velocity can avoid borehole problems, such practice will ultimately increase the Non-Productive Time (NPT) and hence the total drilling cost. Accurate prediction of surge and swab pressures helps to find the maximum allowable tripping speed and reduce the NPT, which all optimize the drilling performance. In the first part of this study, a very brief review of fundamental studies about surge/swab modeling and associated wellbore stability concerns is presented.