A Practical Approach For Predicting Casing Running Speed From Couette Flow Of Non-Newtonian Power-Law Fluids

Abstract

Godwin A. Chukwu* Abstract The displacement of drilling fluid by the movement of the drill string or casing pipes produces pressure variations in the borehole. The velocity of this displacement governs the magnitude of the pressure change which can add or subtract to the hydrostatic pressure of the mud to produce surge or swab pressure, respectively. The surge or swab pressure computation techniques presented in this paper are based on sound, easy to follow, step-by-step analysis of steady laminar couette flow of non-Newtonian power-law fluids through a concentric annular wellbore. The annular geometry is made of a stationary outer pipe and a steady axial motion of an inner pipe. By employing the computed surge pressures, in combination with family of curves, the maximum velocities at which the casing pipes can be run in the hole without the danger of fracturing the formation are obtained. The results obtained from these analysis will aid in proper design and optimization of drilling programs. Introduction Couette flow phenomenon occurs when two co-axial cylinders are placed such that one of which is stationary and the other is moving at a specified velocity. This flow characteristic is representative of flow in the borehole annulus where the wall of the wellbore is represented by the stationary cylinder, and the drill string or casing is represented by the moving cylinder. The fluid local velocity is dependent on the velocity of the moving cylinder. The geometric representation of a concentric annulus is as shown in Figure 1. In drilling operations, the displacement of fluid encountered when running or pulling the drill string or casing in the annulus produces momentary variations in fluid pressure. This pressure change is known as surge or swab pressure, the magnitude of which is governed by the movement of the inner pipe, the annular geometry, and the drilling fluid properties. Surge or swab pressure evaluation is important in the general planning of drilling a well. Excessive surge or swab pressure can lead to formation fluid invading the well annulus, causing a threatened blowout. High surge pressure can lead to loss of mud, either suddenly by fracturing the formation or slowly by continuous efflux to permeable formation, The drilling fluid enters the fractures and this produces a drop in the fluid level, causing a reduction in mud hydrostatic pressure in the borehole. The reduction in mud hydrostatic pressure allows formation fluids to enter the wellbore, which may result to blowout. PREVIOUS WORK Several investigators have developed methods for predicting pressure surges in drilling wells. P. 263