Improved Magnetic Model for Determination of Range and Direction to a Blowout Well

Abstract

Summary. Magnetic logs are commonly used in wild-well kill operations to guide a relief well to a blowout well. Estimates of distance and direction to the blowout well are highly dependent on the assumed magnetic model of the blowout well casing. Theoretical and experimental results show that the magnetic poles of a long magnetized cylinder fit an exponentially distributed model rather than the standard impulsive-pole model. Magnetic techniques for determining range and direction from a relief well to a blowout well are explained, and modifications of these techniques based on the new magnetic model are developed. The techniques presented here can be used during drilling operations with data from most measurement while drilling (MWD) tools. Substantial improvements in range estimates are achieved for near- and medium-range targets. Introduction Drilling a relief well to intersect the borehole of a blowout well is often the only practical method for bringing a wild well under control. Inaccuracies in the well surveys make the chances of intersection remote without some means for determining the relative distance and direction of the two wells at a given depth. Measurements of the magnetic field in the relief well detect the presence of such iron objects as casing or drillpipe in the blowout well, and these data are used to estimate the relative locations of the two wells. Although current magnetic ranging techniques often result in an eventual intersection with the blowout well, estimates of the distance between the wells may substantially exceed the actual range when the relief well is close (within 20 ft [6 m]) to the blowout wellbore. This error results from the assumption of infinitely con-centrated (impulse) magnetic poles in long iron cylinders. The pole is actually distributed, or smeared, along the magnetized cylinder, and the smoothed features give the appearance of greater distance. A magnetic model has been derived that distributes a magnetic pole along the magnetized cylinder with an exponential distribution. Two different experiments confirm the accuracy of this model. In this report, we outline the techniques used to estimate distance and direction from a relief well to a blowout well and modify these methods to include the distributed magnetic model. We present experimental results that confirm the exponentially distributemagnetic-pole model and data describing the actual pole width for various grades and diameters of steel pipe. The new ranging techniques differ from older methods at the interpretation stage and do not require the development of new logging equipment. Most current MWD tools can provide sufficient information to apply the magnetic ranging techniques developed in this paper, thus allowing distance and direction estimates to be made during drilling. A substantial reduction in the number of relief-well redirections required to intersect a blowout well should be obtained with the new techniques. Description of Magnetic Methods for Blowout Ranging It is well known that iron objects possess a significant degree of magnetization unless they are frequently demagnetized. Unless iron tubing is artificially magnetized, each section acts as a single magnetic dipole, and after assembly into a long casing or drillstring, each joint roughly maintains its previous magnetization. A pipe string can be described magnetically as a line of magnetic dipoles of random strength evenly spaced at intervals equal to the length of a single section. Characteristics of Magnetic Monopoles. Although each pipe is an exact magnetic dipole, it is often more convenient to regard a dipole as two monopoles of equal but opposite strengths. The following discussion will concentrate largely on the ranging of magnetic monopoles. If we consider moving a three-component vector magnetometer past an impulse magnetic monopole of strength M and with distance of closest approach L, the measured magnetic field will be a vector sum of the earth's magnetic field and the field of the magnetic monopole. After the earth's field is subtracted, the axial and radial components, Fa and Fr, of the magnetic field that is caused by the monopole are determined from the inverse-square law describing the magnetic field caused by a concentrated pole: (1)(2) where s is the distance along the relief-well axis from the point of closest approach (see Fig. 1). The amplitude of the axial and radial fields depends on the total pole strength, but the shapes of these fields depend on only L. This fact is fundamental to magnetic ranging techniques. The range L can be determined from the separation between the maximum and minimum of the axial magnetic field (see Fig. 2a) by the relation (3) The range can be determined in a similar manner from the half-width of the radial field, (see Fig. 2b): (4) SPEDE p. 316