Stress Around A Wellbore

Abstract

Abstract A study was made of the stress state in a porous elastic body, with particular emphasis on variations in selected failure criteria. The objective was to relate predicted changes in stress and failure criteria with fracture initiation experienced in certain forms of lost circulation, as opposed to hydraulic fracture extension. The three principal stresses at the wellbore were expressed as functions of wellbore pressure; in addition, values of eight additional parameters were varied over realistically expected ranges to determine their possible importance on the principal stresses. These eight parameters were the three tectonic stresses, formation fluid pressure, pore pressure at the wall, Poisson's pressure, pore pressure at the wall, Poisson's ratio, the ratio of unjacketed to jacketed rock compressibilities, and position around the wellbore. Two failure criteria, the maximum principal stress and Von Mises number, were determined for the well stress condition chosen. Calculated results show that small changes (5%) in tectonic stresses can cause larger changes in wellbore stress (and potential failure) than very large (100%) changes potential failure) than very large (100%) changes in Poisson's or compressibility ratios. Introduction A theoretical investigation of the state of stress around a wellbore was made to study the problem of fracture initiation in a porous elastic body. This work was undertaken to determine the effects of rock properties, tectonic stresses, wellbore pressure, formation pressure, and mud filter cake on lost pressure, and mud filter cake on lost circulation in a drilling well. The mathematical model chosen for this investigation is a large porous body containing fluid at a particular pressure, P. The body has a cylindrical hole which contains fluid at a pressure, Pw. In addition, the porous body is acted upon by external stresses, Sx, Sy, and Sz (see Fig. 1). The quantities Sx, Sy and Sz are the tectonic stresses and are considered positive when tensile. In reality, these positive when tensile. In reality, these stresses are compressive (negative). The vertical tectonic stress, Sz, is due to the weight of all of the overburden. It is generally accepted that the principal horizontal tectonic stresses are not equal because of the preferred fracture orientation that is noted preferred fracture orientation that is noted in a specific area. The wellbore pressure, Pw, is equal to the sum of the hydrostatic Pw, is equal to the sum of the hydrostatic pressure of the column of drilling fluid and pressure of the column of drilling fluid and the annular pressure drop due to the circulating fluid.