A self-contained downhole recording instrument was developed and used to measure and record drilling string forces and motions. The eight signals recorded by pulse-width modulation on magnetic tape were: axial, torsional, and bending loads; axial, angular, and lateral accelerations; and internal (pipe) and external (annular) pressure. The device was used over a two-year period to collect data in fifteen wells under a wide range of drilling conditions. After about nine minutes of cumulative recording time, the tool was retrieved and brought to the surface. Data were converted from the magnetic tape to analog type oscillograph display and, in some cases, were digitized for analysis purposes. Normal variations in measured downhole bit load usually ranged between 25 and 50 percent of the mean value. Maximum bit loads reached over 3.5 times mean loads in some instances. Frequencies of weight, torque and bending traces showed evidences of rock bit tooth action, of cone action, of rotation, and also of pump pulsations. Large annular pressure variations accompanied large load variations.
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.
Laboratory drilling tests with 43/4in. hard-formation rock bits were made under rock pressure and borehole fluid pressures simulating a 3,OOO-ft borehole. The effects of bit weight and rotary speed on drilling rate and bit rotary power were determined in a hard, impermeable dolomite. With added bit weight, the drilling rate and rotary power both increased at an increasing rate; but, with added rotary speed, the rate and power both increased at a decreasing rate. The rock volume removed per unit of energy increased as weight was raised or as rotary speed was reduced. Empiric quadratic equations for drilling rate and rotary power were obtained and bit mechanical efficiency was calculated.
Bottom-hole stresses in a well bore have been calculated using the results of photoelastic analyses. Boreholes in the earth were simulated in marblette cylinders. Three hole bottom contours were stUdied, including flat and semihemispherical bottoms. Two types of surface stresses were found at or near the hole bottom, meridional and circumferential. Stress concentration factors are presented graphically as a function of overburden (geostatic) and well bore (hydrostatic) pressures. For well bore and overburden pressures encountered in normal mud drilling, radial stresses on a flat bottom hole may change from 74 per cent of overburden at the center to 192 per cent near the hole wall. Knowledge of triaxial states of stress across the hole bottom will enable a better understanding of rock strength under borehole conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.