Abstract
This paper describes an electromagnetic method to facilitate drilling a well to intersect a target well casing. It has an important application in control of blown out oil and gas wells. By this method, a relief well was directionally drilled to intersect the casing of a blowout at 8,000 ft [2700 m]. The relative distance and azimuthal direction to the target casing can be determined when the relief well is up to more than 100 ft [30 m] from the blowout.
Introduction
There is a need, particularly in the control of runaway oil or gas wells, for the ability to drill a relief well to intersect a target well casing at a specified subsurface depth. Our method consists of detecting and analyzing the magnetic field generated by alternating electric current flow on a target well casing, drillstem, or fish. By comparison to the earth, steel is a very good electrical conductor; a steel well casing has a strong "short-circuiting" effect on the parallel component of electric current now in its vicinity, The magnetic field generated by current flow on the target casing and measured in the relief well can be used to determine the relative distance and direction from the relief well to the target. In this paper, we present the principles of operation along with the results of some field tests. An alternative scheme using a wireline current source is described in the Appendix.
Principle of the Method Principle of the Method Consider the apparatus shown in Fig. 1. The dimensions shown can vary greatly: those given are for reference. A low-frequency AC is injected into the ground by use of surface electrodes near the blowout. The return current is collected by remote surface electrodes. If the blowout casing were not present, this arrangement would produce a very small magnetic field response on or near the blowout axis. With the blowout casing in place, there is a large enhancement of the current flowing down the blowout axis, which results in a large enhancement of the magnetic field as indicated by Ampere's law. Considering the geometry of the magnetic field resulting from a current-carrying conductor, the apparent direction to the conductor can be deduced.
It is useful to introduce a parameter re that is the radius of a circular column of earth having the same resistance per unit length as the blown out well casing. If the conductivity of the earth is given by sigma e, that of the casing by sigma c, and the well casing has a radius rc and wall thickness hc, then re is given by
(1)
The electrical conductivity of steel is about 107 (omega.m)-1, whereas that of country rock in a petroleum environment is within an order of magnitude of 0.1 (omega.m)-1. Thus, a well casing with a 1/2-in. [1.3-cm] wall, 10 in. [25 cm] in diameter, has the same electrical resistance per unit length as a column of earth [sigma c = 1(omegam)-1] about 1,000 ft [300 m] in diameter. Such well casing has a short-circuiting effect to vertical current flow on a column of earth approximately this diameter. The sensitivity of standard magnetometers is such that after 100 seconds of signal averaging, an AC magnetic induction of less than 10–2 gammas or alternatively a magnetic field of 10–5 A/m can be detected. This corresponds to the magnetic field generated by a current of 2 mA on well casing 100 ft [30 m] away. The parameter re also indicates the scale length over which current builds up on a casing. Thus, for a semi-infinite casing surrounded by a uniform conductor of much lower conductivity, the current on the casing will build up to its asymptotic value within re of the end of the casing. Consequently, it is a valid approximation to calculate the current on the casing, Ic, by
(2)
when the electric field, E, parallel to the casing varies slowly on the scale re. This is the situation far from the surface injection electrodes. When a distance on the order of re from the blowout casing, the low-frequency magnetic field signal is caused predominantly by current flow on the blowout: predominantly by current flow on the blowout:
(3)
SPEJ
P. 269