This invention relates to methods of evaluating the condition of oilfield tubular goods. More specifically, this invention relates to methods and apparatus for determining by measurement the extent of internal wear of subsurface pipe or casing.
A variety of problems may result from internal casing wear. Drill pipe collars may rub the casing, and possibly lead to a blowout in the casing if extremely worn. In a production well, badly worn casing may permit the undesirable "thiefing" of the flow to unwanted zones and thereby reduce surface production. In an injection well, worn casing may permit the injected fluid to flow to undesired formations.
Internal wear, which is the removal of metal from the inside casing wall occurs in three principal patterns: loss of metal from only part of the inside casing wall while the rest is unworn; uniform loss of metal around the entire inside casing wall circumference; and non-uniform loss of metal around the entire inside casing wall. In addition, there are two patterns associated with unworn casing--round internally with all of the original metal in place, and elliptical with all of the metal in place.
A number of surveys are available for assessing internal wear in downhole casing. One such survey provides a depth recording which responds to the circumferential average of the well thickness of the casing metal still in place. In particular, this survey detects changes of the wall thickness and internal diameter of downhole casing. A typical inspection instrument for performing this survey is the casing inspection instrument marketed by Dresser Atlas under the name "Magnelog tool" and disclosed in the publication "Dresser Atlas Casing Evaluation Services," pages 4-5 and 61-79; said publication being herein incorporated by reference. During operation, an inspection instrument generates an alternating (AC) magnetic field of low frequency. As the tool passes through the survey interval, the field permeates the casing wall and is detected by the receiver portion of the tool. The received electromagnetic wave is both attenuated and phase shifted by the casing wall, with the degree of phase shift in relationship to the wall thickness (weight). The degree of phase shift is detected and transmitted over the wireline for surface recording.
The phase shift curve registers the phase shift between generation of the low frequency electromagnetic wave and reception of the wave by the tool's receiver portion. The degree of phase shift increases with the increasing thickness of the casing's wall. The degree of phase shift may then be used in the calculation of the average thickness of the casing wall. While such a survey adequately calculates average thickness, it cannot detect the least wall thickness of casing or the degree of "out-of-roundness". (The term "out-of-roundness" generally refers to any non-circumferential internal wear, particularly one-sided wear) Thus, it is a distinct possibility that such a survey may fail to identify dangerously worn casing.
A second survey available for assessing internal wear in downhole casing generates a depth recording of the greatest and least internal radii of the casing, thus allowing quantitative assessment of the degree to which the casing is internally worn or out-of-round. A typical instrument for performing this survey is the casing inspection instrument marketed by Dresser Atlas under the name "Multi-Finger Caliper tool" or "MFC tool" and disclosed in the publication "Dresser Atlas Casing Evaluation Services", pages 6 and 83-103; said publication having been previously incorporated by reference into this document. During operation, the MFC tool, which may include from 40 to 60 "feeler arms" which extend outwardly from the body of the tool in all directions, is centralized and all of the feeler arms extended outwardly from the tool for contact with the entire circumference of the inside casing wall at a particular depth. As the MFC tool moves through the casing, the feeler arms follow the contour of the casing's inside wall. The feeler arm with the maximum extension operates a first potentiometer (or other alternative device) which senses the greatest measured internal radius. The feeler arm with the least extension operates a second potentiometer (or other alternative device) which senses the smallest measured internal radius. The signals from the potentiometers are coupled through the electronics section and transmitted by the wireline to a recorder for presentation. While the MFC survey allows the evaluation of the degree to which the casing is internally out of round and permits estimating the greatest and least remaining wall thickness, the MFC survey cannot distinguish casing that is seriously worn on one side from unworn casing that is merely elliptical; in other words, severe one-sided wear may not be detected at all.
Thus, while both surveys are able to provide useful information regarding the internal wear of casing, both surveys fall short of being capable of providing a comprehensive evaluation of internal wear. For these reasons, a method for a combined casing evaluation survey became necessary so that an analytical technique which provided a comprehensive evaluation of internal casing wear would be possible. While separate evaluation of the results of the two separate survey followed by integrating the information derived from the surveys is possible, such a practice proved unworkable in actual use. A better method of achieving this goal of a comprehensive survey of internal casing wear is to devise a method by which selected data of the two previous surveys are combined and subject to a single analysis technique which will yield the desired more comprehensive internal casing wear evaluation.