Cement Annulus Research Articles

A Study of Completion Practices To Minimize Gas Communication

The first step towards preventing leakage of gas through a cemented annulus is recognizing that a potential problem exists. Important factors such as fluid column density and filtration control, as well as such ordinary considerations as pipe movement and centralization, must be taken carefully into account if the well completion is to be a success. Introduction The leakage of gas through a cemented annulus was first recognized as a significant problem in the mid-1960's in connection with gas storage wells. The problem was thought to be related primarily to cement-casing-formation interfaces and the bonding between them. As a result, considerable work was performed by both producing and service companies. performed by both producing and service companies. Other reasons why gas may enter a cemented annulus were not evaluated until the latter 1960's, and investigations were still predominantly concerned with bonding, density, and displacement or prevention of channeling - properties that will not be considered here. Gas communication through a cemented annulus becomes more evident in deep well completions across gas intervals, where it causes a pressure buildup in the annuli of the production and intermediate casings. To understand this occurrence of gas leakage more completely and to determine some of the factors responsible for gas migration in wells, data have been gathered from laboratory model studies simulating down-hole conditions. Variables considered in these experiments were (1) the type of cementing composition, (2) the relationship of mud and cement density, (3) temperature, and (4) pressure differential. Possible Causes of Gas Leakage Possible Causes of Gas Leakage One of the more popular explanations of why gas enters a cemented annulus is that the cement column cannot effectively transmit full hydrostatic pressure to the zone containing the gas. Gas diffuses into a cement column for many reasons, but only a few characteristics of cement are significant enough to warrant mention; for example, density, setting, gelation, dehydration, and bridging. These specific factors will be considered separately, even though more than one may actually occur in a well at one time. Density The density of the cement slurry and of other fluids in the wellbore is very important because it determines the hydrostatic pressure exerted at any particular depth. In the case of gas well completions, particular depth. In the case of gas well completions, the density of the drilling mud, chemical wash, or flush and cementing composition, either separately or in combination, must exceed the formation gas pressure to prevent the gas from entering the annulus. pressure to prevent the gas from entering the annulus. Before cementing, the well must be circulated to condition the mud and to help remove any trapped gas bubbles at the bottom of the hole. After 1 hour of circulation, the pumps should probably be shut down for a 5-minute period and then started again to help remove any microscopic gas bubbles that may be trapped in the washout areas or that may be adhering to the walls of the hole. This gas could be considered as a potential second gas kick, and if it is not removed before the cement is placed, it may lower the density of the fluid column. JPT P. 1170

A Theoretical and Laboratory Evaluation of Carbon Logging - Laboratory Evaluation

Abstract Results we presented from a laboratory experimental evaluation of a carbon well-logging tool based on the detection of the 4.43-MeV gamma rays produced by the inelastic scattering of 14-MeV neutrons. The success of a carbon logging tool is dependent primarily upon the gamma-ray detection scheme used, which in this investigation is a two-crystal pair spectrometer. Using such a device in a simulated reservoir with and without casing and a cement annulus, semiquantitative detection of carbon was accomplished for a fluid-filled packed sand of 35-percent porosity. Analysis of the spectral results show that the log would be sensitive to lithology, saturation and porosity as well as carbon. With the resolution of this particular gamma-ray detector scheme, the presence of the casing using seen, but did not interfere with the carbon signal. Likewise the 1-in. annulus of concrete had no effect on the carbon detection. Comparison of our results with those previously published show that the oxygen and silicon interference encountered i, other proposed logging schemes is eliminated by the two-crystal pair spectrometer. As presently envisaged, between 5 to 10 minutes per pay zone to be evaluated is required to per pay zone to be evaluated is required to accumulate raw data. However, the growth potential offered by the new Ge(Li) gamma-ray counters may well remove this restriction. Introduction Theoretical results presented in Ref. 1 have indicated that liquid hydrocarbons should be detectable in noncarbonaceous reservoirs. Although the interference problem posed by energy degraded gamma rays from omnipresent oxygen is severe, it is not severe enough to prohibit detection of hydrocarbons by nuclear means. The theoretical data also showed thermal neutron effects caused by energy degradation of gamma rays from thermal capture in silicon, chlorine and iron could be eliminated by proper choice of a neutron source gamma detector gating scheme. Indeed, this has been demonstrated experimentally. The crux of the hydrocarbon detection problem lies with finding a gamma detector system with sufficient resolution to pick out 4.43-MeV gamma rays (carbon) from the background provided by oxygen. Previous investigations have shown that single NaI(T1) crystal detectors did not have sufficient energy resolution to accomplish the task. However, energy resolution is not the whole answer when it comes to extracting a monoenergetic signal from a continuum background. In addition to the interfering gamma rays produced through Compton collisions external to the detector system, there is an in-crystal Compton background in single-crystal spectrometry. The second source of interference arises because gamma rays entering the crystal at energies higher than the discrete energy of interest will produce Compton collisions within the crystal generating electrons that have precisely the same energy as that of the "desired signal". This second source of interference significantly reduces the carbon sensitivity of a single-crystal detector. Hence, elimination of the in-crystal background from oxygen produced gamma rays would go far in improving the in-situ carbon detection picture. In searching for an acceptable detector system, this point was kept foremost in mind. It was known a three-crystal pair spectrometer would virtually eliminate the in-crystal Compton background, but limitations imposed by borehole tool size ruled out its use. However, from studying the working principle of this device the concept of the principle of this device the concept of the two-crystal pair spectrometer emerged. A description of such a device was later found in Ref. 4 although no evidence has been found that such a device has been used to record spectra in a borehole or simulated borehole environment. This report presents experimental results obtained with such a presents experimental results obtained with such a two-crystal spectrometer in various simulated reservoir conditions.

Temperature Anomalies in Wells Due to Cementing of Casing

Abstract Temperature measurements were made in the surface portion of a test well drilled in Central Texas. The well portion of a test well drilled in Central Texas. The well was drilled to 304 ft by the cable tool method and surface casing was set and cemented. Temperature observations were made in the 25-hour interval between termination of drilling and installation of casing. Subsequently, temperature data were obtained, at progressively longer time intervals, to a total time of 280 hours after cementing of the surface casing when deepening of the hole by the rotary method commenced. Field observations indicated that a hole drilled by the cable tool method is heated (the maximum observed anomaly was about 5F); and hydra- tion of the cement leads to a general heating of the well. A maximum temperature of about 15F was, observed 14 hours after cementing was complete. Field measurements were supplemented by a laboratory study. Both the thickness of the cement annulus and the thermal diffusivity of the surrounding formation were varied. Results indicated that both parameters have a pronounced influence on the resulting temperature pronounced influence on the resulting temperature anomaly. Thus, an interpretation of the temperature log in terms of cement thickness behind the casing is permissible only when the lithology of the formation is reasonably uniform. Introduction The test well, Bill Stribling No. 3 (Fig. 1 ), is located in Central Texas over the Llano uplift where the sedimentary cover over the igneous basement is about 1,240 ft. The well was cable tooled to 304 ft between Sept. 22 and Oct. 27, 1964. The cable tool method was chosen since cavernous zones posed serious problems in a previous attempt with the rotary method (the bit dropped 6 ft previous attempt with the rotary method (the bit dropped 6 ft at one stage and circulation loss could not be controlled); 13-in. casing was set to 80 ft and 10-in. casing to 128 ft.. but neither casing was cemented. At noon on Oct. 27, drilling was suspended. Instrumentation All field measurements were taken with a 1,500-ft multiconductor rubber cable. Twelve glass bead thermistors are molded into the lower part of this cable at 25-ft intervals. Over the range of temperatures encountered in the shallow subsurface, these thermistors show a change of electrical resistance of about 100 ohm/ deg. F, Resistance for each thermistor is measured while the cable is at rest. Measurements are made with a four-digit bridge and an electronic direct-current null detector. Accuracy of the measurement is within 1/2 ohm. The cable resistance is measured separately for each cable position and subtracted from the reading. After the resistance of all thermistors has been determined, the cable is moved the desired distance-generally not less than 5 ft-and the next sequence of measurements is taken. Since the thermistors are insulated from the borehole fluid by about 0.1 in. of rubber, their time constant (95 percent recovery) is several minutes. Thus, a waiting period of 5 to 10 minutes is necessary before reliable measurements can be taken. As a result, each temperature log took about an hour to complete. This makes the tool ill-suited for the observation of transient temperature conditions as described in this article. A continuous logger such as the High Resolution Thermometer* or the one described by Simmons would be superior. Field Measurements Temperature measurements in the Stribling No. 3 well are summarized in Fig. 2. JPT P. 147

Growth Layers in Dental Cement for Determining the Age of Red Deer (Cervus elaphus L.)

The Nature Conservancy is expected to advisethe Red Deer Commission on the scientific aspects of red deer (Cervus elaphus L.) management. Although work on the management of red deer has been in progress since 1957 on the Isle of Rhum National Nature Reserve (Lowe 196 1; Eggeling 1964), it was essential to develop methods for assessing the relevance of the results to deer populations elsewhere in Scotland (Mitchell 1965). Accurate determination of age is an important prerequisite for comparing the growth, breeding performance and survival of different deer populations. Changes in body characteristics with age can 6ften be used for the exact determination of age of short-lived animals. But for those with a long life span, the time required to collect the relevant age characteristics may be a serious restriction. Similarly, the characteristics of one population may differ from those of another, and it is reasonable to expect less accuracy with increasing age. Although annular tissues and structures have been long known in woody plants and fish, it is only in recent years that progress has been made in the quest for similar tissues among mammals. Dental tissues have attracted most attention, and whereas. the earlier studies concerned growth layers in dentine, e.g. in red deer (Eidmann 1933), Alaskan fur seals (Callorhinus ursinus (L.)) (Scheffer 1950), elephant seals (Mirounga leonina L.) (Laws 1952, 1953), harp seals (Phoca groenlandica Erxleben) (Fisher & Mackenzie 1954), bottle-nose dolphins (Tursiops truncutas (Montague)) (Sergeant 1959) and Alasklan black bears (Ursinus americanus (Pallas)) (Rausch 1961 in Mosby 1963), later work has indicated the wider potentiality of dental cement. Dentine grows internally, progressively filling the pulp cavity, but cement continues to grow n -the root surface, apparently at a constant rate, throughout life. Hence annual cement increments are relatively constant, but dentine increments tend to get smaller, and, more difficult to count, with age. The linear relationship between cement thickness and age found in human teeth (Gustafson 1950 in Miles 1961) has been suggested as a possible method of forensic age assessment. Encouraging results using annular cement layers for age determination have been published for moose (Alces alces (L.)) (Sergeant & Pimlott 1959), harbour seals (Phoca vitulina L.) (Mansfield& Fisher 1960), grey seals. (Halichoerus grypus Fabricius) (Hewer 1960, 1964), Barren Ground caribou (Rangifer tarandus groenlandicus (L.)) (McEwan 1963), beavers (Castor canadensis Kuhl) (Van Nostrand & Stephenson 1964), griizly bears (Ursus arctos L.) (Mundy & Fuller 1964) and white-tailed deer -(Odocoileus virginianus Zimmermann) (Ransom 1966; Gilbert 1966). In an earlier note (Mitchell 1963) I showed how growth layers in the dental cement of red deer could be exposed by sectioning the thick cement pad under the crown on molar

A Progress Report on Cement Bond Logging

PUTMAN, LLOYD, OIL WELL PERFORATORS, INC., CASPER, WYO. MEMBER AIME Abstract During the past year notable progress has been made in acoustic cement bond tool design as well as improvements in operating techniques. These developments have been a direct result of suggestions and challenges presented in previous papers by such authors as Pickett, Bade, Walker, Anderson, Riddle and others. These writers noted the many variables and inherent difficulties involved in running and interpreting cement bond logs. During the development process of combating these problems, more than 100 cement bond logs were run in the Wyoming and Montana areas. On the majority of these surveys a development engineer was present and prepared to make design changes under field operating conditions on a trial and error basis. In each case the well operator allowed extra rig time or else a portable mast was used. In addition to commercial logs, an 8,900-ft test well on the Cedar Creek anticline of Montana areas used to perfect techniques of calibration and make other more difficult design and circuit changes. Illustrations and field examples are presented and discussed to support conclusions reached. Introduction Since late 1962 a field development program involving more than 100 acoustic cement bond logs has been underway in fields of Wyoming and eastern Montana. This effort has been toward developing a practical and routinely usable logging system which incorporates a maximum number of desirable qualities mentioned in previous papers. With the vast amount of actual information available through writers such as Walker, Bade, Anderson, Pickett, Riddle and others, it is not surprising that a vigorous field development program would produce a refined logging instrument. Results have shown that with proper tool design many of the mechanical and electrical problems now existing can be overcome. Also, it appears evident that evaluation of the effectiveness of cementing under most of the known conditions is now reasonably reliable. Quite unique in this development program was the presence of a development engineer on commercial jobs who was prepared to make design changes under field operating conditions. On many occasions numerous runs were made on a trial and error basis to achieve desired results. The advantage of this type arrangement is apparent because the engineer could run a survey, come out of the hole and make simple or major modifications, go back and re-run the log and repeat as many times as necessary until desired adjustments and responses were effected. In addition to commercial logs run, an 8,900-ft test well on the Cedar Creek anticline of Montana was used to perfect calibration techniques and make more difficult design and circuit changes. Progress is reported concerningfast formation response;use of shear waves in bond logging;logging boreholes containing oil and water; andlog calibration suggestions. The fundamentals of acoustic cement bond logging are by this time well known. For the purpose of this paper, bond logging implies an attempt to correlate acoustic coupling of casing to cement and formation to a hydraulic bond. Although no attempt is made in this paper to ascertain the indicated degree of bond to actual hydraulic seal by production testing, work is progressing satisfactorily along this line. For bond logging to be completely successful this work is gravely needed to properly determine when remedial action should be initiated from the bond log. New steps and refinements in techniques toward achieving this goal are discussed individually. Fast Formation Response The term "fast" or "early formation arrival" has become a popular term, and normally is used to describe an interpretation problem that has existed in the past when logging hard formations. Simply, it implies that a portion of the energy from an acoustic transmitter may be transmitted through casing, cement and formation and arrive at a receiver simultaneously or faster than energy from the same source can travel a physically shorter path by way of the casing and cement annulus (illustrated in Fig. 1, Path B over A).Presently the method for detecting this situation is to run a single receiver-time curve simultaneously with the bond log. For some time this method has been accepted as a satisfactory solution to the problem, however, recent study indicates a more direct solution. JPT P. 1117ˆ

Chlorine Logging in Cased Holes

Abstract A new chlorine logging tool, developed primarily for the detection of oil saturation behind casing, is described. The principle involved is the selective detection of gamma radiation from neutron capture in chlorine. The tool is relatively insensitive to porosity, responding mainly to the concentration of chlorine in the interstitial liquid rather than to the total chlorine content in the formation. A brief review of the fundamentals of chlorine logging is given. This includes a discussion of the partition of neutron captures in clean sand and limestone formations, and the spectra of gamma rays obtained. The effect of neutron capture in iron is shown to be important. Following a short description of the logging tool, the laboratory-determined response of the instrument in cased sandstone formations is presented. It is shown that better results can be obtained by logging in empty casing than in casing filled with oil, fresh water, or salt water. Effects of neutron capture in the casing, casing liquid, cement annulus and formation matrix are discussed. A method of interpreting the Chlorine log, based on the use of the response curves, is explained. This is illustrated with several field examples which also show comparisons of electrical and Chlorine logs. In Gulf Coast formations, the Chlorine log often bears a marked resemblance to the SP curve, and in some cases water-oil contacts have been identified by a visual comparison of the two curves. Comparison of the Chlorine log with a neutron log is also valuable in interpretation. The role of the neutron log is to give a porosity (which need be approximate only), to indicate formation shaliness and, most important, to identify gas which is generally indistinguishable from oil on the Chlorine log. The Chlorine log should prove most useful in old wells where adequate open-hole logs do not exist, or where saturation conditions in the producing zone have changed. The combination of the Chlorine log with a Cement Bond log and the wireline formation tester should aid materially in successful reworking of such wells.