Downhole Casing Corrosion Monitoring and Interpretation Techniques To Evaluate Corrosion in Multiple Casing Strings

Abstract

Summary This paper reviews new developments in monitoring the corrosion of downholecasings. The results presented are based on corrosion-monitoring data from alarge number of wells. The interpretation procedure outlines techniques forestimating metal losses, corrosion rates, and detection of holes in single andmultiple casing strings. Casing corrosion monitoring and interpretation have advanced dramatically inthe past few years. New technological developments now make it possible tocarry out borehole measurements capable of distinguishing between corrosion ofinner and outer casino strings and of detecting pits and holes in the casingmetal. This paper gives a detailed description of the interpretation procedure, theequations used, and the results obtained from a study of procedure, theequations used, and the results obtained from a study of a large number of wells in the Middle East. New techniques are proposed for time-lapse corrosionmonitoring, stand-alone corrosion proposed for time-lapse corrosion monitoring, stand-alone corrosion monitoring, use of downhole casing resistivity, andmagnetic property measurements, and hole and pit evaluations on the inner andproperty measurements, and hole and pit evaluations on the inner and outercasing walls. From the results of this study, five sets of quantitative andsemiquantitative answers are obtained: percentage of metal loss on the innerand outer walls of the inner casing, percentage of metal losses on the combinedinner and outer walls of the outer casing strings, and semiquantitative values of the hole and pits sizes and their penetration into the inner and outer walls of the inner casing. These results were correlated with observations of casingsretrieved from boreholes and with production tests. Downhole corrosion monitoring, to evaluate both the extent of metal lossesand the corrosion rate, is vital because corrosion initiation and propagationcannot be predicted from theoretical estimates. Casing corrosion and failureare caused by a variety of factors. Many of these are very complex, and therelevant equations and processes governing their onset and propagation rate arepoorly understood. Experience and empirical trial-and-error approaches, however, and in the prediction of many downhole casing and completionenvironments that will initiate and accelerate corrosion. Such conditions aspoor cementing, large variations in casing metallic composition, and fluidsalinities have long been recognized as corrosion promoters. Optimize the Workover Program. The workover should address existing casingproblems and any potential problems observed during corrosion monitoring. Insome cases, a second workover is necessary as a result of a breakthrough of pits that were not completely developed during the first workover. Optimize the Initial Completion. The initial state of the casing is probablythe most important factor affecting the initiation and rate of corrosion. Poorcementing, casing stress, casing, indentation, and scratches can reduce thelife of a casing considerably. Base reference corrosion monitoring surveys canhelp identify some of these problems and can provide good references forsubsequent comparisons. Casing Protection. Steps to prevent corrosion have been used extensivelyrecently to extend casing lifetime. There are two basic techniques:the use of fluid inhibitors. aimed mainly at minimizing internal corrosion in a singlecasing and at preventing galvanic cells when used in the annulus between twocasings, andcathodic protection, which attempts to minimize the effects of anodic cells protection, which attempts to minimize the effects of anodic cellson the casing walls owing to galvanic reactions. (In essence, the use of anodebeds and a DC power generator on the surface makes the casing a cathode andhence a net receiver of current.) The three main objectives of corrosion monitoring for casing protection canbe summarized as follows. protection can be summarized as follows.Determining the areas of corrosion (e.g., inner wall, outer wall, inner casing, or outer casing).Aiding in the design of the cathodic protection system. The depth of protection, the current needed, anode locations, etc. can be determined onlyfrom pilot tests. Moreover, anodic cells are observed within the protectedintervals. This is the same as having a small AC superimposed on a large DC. Eddy current-like local cells can still create pits and holes over the areascovered by the cathodic protection current.Monitoring the effectiveness of protection which can help monitor andevaluate corrosion rates. From the results of corrosion monitoring, we are able to determine inner-and outer-wall metal loss, holes and pits on inner and outer walls, and thecorrosion rate for single casings. In multiple casings, an independent data setis needed for the second casing. Unfortunately, this doubles the number of unknown variables, limiting their evaluation in the outer casing. It is stillpossible, however, to dis-tinguish between inner and outer casing corrosion. Ifmore than two casings are present, the situation is further complicated, and itcurrently is impossible to distinguish between corrosion on the second andthird casings. More generally, these results make possible the evaluation of the locationand extent of corrosion and the prediction of future trends and metal losses. Furthermore, the most appropriate remedial action can be determined (newliners, casing retrieval and replacement, new casings, etc.), and the workoverand completion programs of new wells can be optimized. Finally, these resultsenable the design of the most effective casing protection systems. Corrosion Mechanisms Corrosion mechanisms can be sorted into three categories (see Fig. 1 and Ref. 1). Electrochemical Corrosion. This type of corrosion is caused by mechanismsthat involve the exchange of current, essentially setting up a battery-likesystem. Electrochemical corrosion occurs mainly on the outer casing walls andis responsible for the largest proportion of observed downhole casingcorrosion. This category can be subdivided into four different mechanisms. Galvanic Corrosion. This is a classic anode/cathode pair. The anode acts asthe current emitter, decreasing in mass with time, the cathode acts as thecurrent receiver. Dissimilar metals and a conducting electrolyte must bepresent for an electrochemical reaction to occur. The first condition isreadily satisfied by differences in the metals from joint to joint, joint tocollar, and even within the same joint. The presence of saline formation water, predominant in the Middle East, satisfies the second condition. Galvaniccorrosion, the most widespread form of corrosion, is generally counteractedwith cathodic protection. P. 283