Tracer Surveys Research Articles

Computer Study of Factors Affecting Temperature Profiles in Water Injection Wells

The computer study described here defines the main factors affecting temperature surveys in water injection wells, making possible a more accurate interpretation of these surveys. An important conclusion is that differential temperature curves are not applicable to the quantitative determination of water injection profiles. Introduction A computer study has been made of temperature behavior in and near water injection wells to improve understanding and interpretation of temperature logs in these wells. Validity of the computer model was tested by comparing computed and field-measured temperatures. Good comparisons were obtained for two West Texas water injection wells. Thus the factors affecting these temperature curves can be stated with confidence. The location of zones taking water, the determination of distribution of the injected water among the zones, the detection of channeling, etc., are fundamental to waterflood management. Radioactive-tracer surveys and, to some extent, down-hole flow meters have been used for these purposes. With an increase in the number of injection wells, there is incentive for better and more economical methods to obtain this information. Interest in well temperature surveys for this purpose has increased recently, spurred by two factors. First, absolute temperature surveys can be more economical than radioactive-tracer surveys economical enough to justify more surveying, enabling more thorough early analysis and follow-up of injection well conditions. Second, the radius of investigation by temperature surveys exceeds that for tracer surveys. Tracer surveys are limited to analysis of wellbore conditions. In contrast, the temperature profile in effect sees beyond the wellbore, since it is influenced by conditions a few feet radially from the well. The usefulness of temperature-profile measurements in wells was indicated as early as 1940 by Millikan. Guyod's discussion in 1946 of wellbore temperatures under static and producing conditions contributed to the understanding of temperature behavior. A very significant contribution to the understanding of temperature profiles in water injection wells was made by Nowak in 1953. His work dealt primarily with quantitative interpretation of absolute temperature profiles after shut-in. In 1954, Bird presented a quantitative interpretation of the injection absolute temperature profile. Several other authors have also discussed the profile. Several other authors have also discussed the interpretation of temperature profiles, providing numerous field examples. The interpretation of temperature profiles still is, of necessity, part art and part science. The shut-in temperature curves of Fig. 1 show several temperature anomalies. It is these anomalies or temperature contrasts between various vertical intervals that are informative. They permit interpretations of what has occurred down-hole. However, there have been doubts about the qualitative and quantitative validity of injection profiles determined from temperature surveys. Therefore, a need was felt for a computer study considering as thoroughly as possible the main factors affecting temperature behavior in water injection wells. The approach in studying temperature profiles was to develop a two-dimensional (radial and vertical) mathematical model to simulate heat flow in the wellbore and surrounding region. JPT P. 1447

Production Logging as Used To Solve Water Injection Problems in the Huntington Beach Offshore Field

In pressure transient testing, interzonal flow or the presence of just one unplugged sand tends to dominate the transient behavior and can easily mask damage in other sands. In those injectors where misleading data are suspected, combining pressure tests with spinner surveys and radioactive- tracer surveys can provide a sound basis for formulating remedial measures. Introduction The Huntington Beach oil field is approximately 41 miles southeast of Los Angeles on a high point along the Newport-Inglewood Fault trend in Orange County, Calif. (Fig. 1). The offshore portion lies in a westerly plunging anticline on the west side of the field. The heavily faulted offshore structure contains a number of thin, minor reservoirs, and three extremely thick, major reservoirs (400 to 800 ft in gross thickness). The thicker reservoirs, composed of continuous, Upper Miocene, unconsolidated sands and interbedded shales, are known as the Jones, the Upper Main and the Lower Main. Waterflooding is currently in progress in the Jones and the Lower Main. The Jones reservoir is divided vertically into two sections (each about 350 ft thick) by a 60-ft continuous shale bed located near the midpoint of the reservoir. This shale break allows independent flooding operations to be conducted in both the upper and lower zones. The Lower Main reservoir has no significant shale beds and the total interval (500 to 650 ft thick) is being flooded in a single, full-zone operation. Extremely high development costs and the low economic return generated from each barrel of secondary oil were the main reasons for developing the floods on a multiple-sand basis. The gravel-flow-packed, slotted-liner completion has been a key factor leading to the success of waterflooding in these thick, unconsolidated, multiple-sand reservoirs. The gravel flow pack provides the sand control needed to economically handle the high-rate, high-cut production arising from the unfavorable mobility ratios typical of California waterflood projects. All injection and response wells drilled projects. All injection and response wells drilled after mid-1966 in the offshore field have the gravel-flow-pack completion. The gravel-flow-packed injector wells have excellent injectivity characteristics, and generally have good injection profiles. However, they have one disadvantage in that many of the normal method such as the selective shut-off, used to control the distribution of injection water in other types of completions, cannot be easily applied to the flow pack. Some of the earlier gun-perforated injectors pack. Some of the earlier gun-perforated injectors completed before mid-1966 are still active in the Lower Jones flood. Others have been replaced with the more effective gravel-flow-packed, slotted-liner completion. Routine down-hole surveying of injector wells to check for profile changes with time has been the principal mechanism for monitoring flood principal mechanism for monitoring flood performance. The continuous spinner survey, the performance. The continuous spinner survey, the radioactive-tracer survey, and the continuous temperature survey are combined to generate this vital information. Maintaining desirable profiles is essential if multi-layered waterflooding is to be successful. Pressure fall-off testing and variable-rate testing in water injection wells are additional methods of monitoring flood performance and appraising wellbore damage. performance and appraising wellbore damage. JPT P. 1083

Radioactive Tracer SurveyingA Comprehensive Report

Presented here is a discussion of the many factors involved in planning and Presented here is a discussion of the many factors involved in planning and obtaining accurate tracer surveys, as well as brief descriptions of radioisotopes, gamma-ray detectors, tracer ejector tools, interpretation methods, and application in the fieldparticularly the Wasson San Andres field of West Texas. Introduction Radioactive tracer surveys are primarily run in production and injection wells to investigate dynamic production and injection wells to investigate dynamic conditions of fluid flow in and around a wellbore,. The greatest number of radioactive tracer surveys are run in water injection wells in secondary recovery or pressure maintenance projects as a part of a periodic pressure maintenance projects as a part of a periodic surveillance program. The surveys are run routinely to determine if the injection profile is reflecting the programmed waterflood. In producing wells, programmed waterflood. In producing wells, radioactive tracer surveys are run to locate water influx, channeling, or communication between perforations. Properties and Selection of Radioisotopes Properties and Selection of Radioisotopes Radioisotopes are classified in three categories with which the potential user of radioactive tracer surveys must be familiar: half-life value, millicurie strength, and energy level. Half-life is the length of time required for any given number of atoms to lose half their measurable intensity. Half-lives range from fractions of seconds to billions of years. The radioisotopes used in tracer surveying usually have a half-life of 8 days to 5 years. Fig. 1 compares radioisotopes and their characteristics. The millicurie strength or specific activity level of a radioisotope denotes the number of disintegations per second that are available for the gamma-ray per second that are available for the gamma-ray detector to measure (a curie being 3.70 × 10(10) radioactive disintegrations per second). For tracer surveys inside the wellbore, a radioisotope of 5 millicurie strength is sufficient. The kinetic energy level of radioisotopes is expressed in units of millions of electron volts (mev). Nearly all energies fall in the range of 0.1 to 10 mev. The higher the mev level, the greater the intensity of the gamma-ray emission per millicurie. An important property of radioisotopes is their ability to penetrate or be absorbed by most materials. The penetration qualities of gamma rays is in proportion to the specific activity or mev level of the proportion to the specific activity or mev level of the radioisotope. The ability of materials to absorb gamma rays varies and is expressed as the material's half-value thickness (HVT). Fig. 1 relates the HVT of materials common to the oil industry. HVT is the thickness of a material required to reduce the incident gamma-ray beam to half its intensity. The more electrons in the composition of the material being used to reduce an incident gamma ray, the more radiation will be stopped. Two half-value thicknesses will reduce the beam of radiation to one-fourth of its original strength. The primary concerns in the radioisotope selection are well conditions and the characteristics of the radioisotope. Because of the many aspects involved in the selection of a suitable radioisotope, there is no one material that can be used in every situation. The radioisotope selected must be completely soluble in the injection fluid. Interpretation of any surface recording will be extremely difficult or impossible if the radioisotope and the injection fluid are not compatible. JPT P. 661

Case Histories of Production Logging

In injection wells, the main purpose of the production log is to assign volumes or percentages of fluid to each interval taking significant amounts of it. In producing wells the objective generally is to identify zones contributing fluid specifically the quantity of each fluid and its respective entry. Introduction Production logging techniques can be broadly Production logging techniques can be broadly considered to include all types of subsurface measurements subsequent to the initial well completion, and there are undoubtedly instances when these measurements are taken before the actual mechanical completion. There are, practically speaking, two main categories of work into which most production logging falls: injection wells and production wells. The injection wells are generally receiving water as part of a secondary recovery, a pressure maintenance, or sometimes simply a disposal system. The injection medium may also be liquid hydrocarbons, gas (including air), or a combination of liquid and gas such as saturated steam. Whatever the injection fluid, the main purpose of the production log is generally to determine the injectivity profile, that is to quantitatively assign volumes or percentages of fluid to each of the intervals taking significant amounts of fluid. While the profile is being obtained, it is often important to check also for casing failures, packer leaks, faulty cement jobs and interzonal packer leaks, faulty cement jobs and interzonal migration. In producing wells the objective generally is to identify zones contributing fluidwater, oil or gas and more specifically the quantity of each fluid and its respective entry or entries. Static or shut-in wells may also be investigated with production logging techniques. A shut-in temperature production logging techniques. A shut-in temperature profile is a recognized means of qualitatively profile is a recognized means of qualitatively indicating where injected fluids have entered a reservoir. Recent advances in temperature logging present the possibility of identifying oil, gas or water in place in possibility of identifying oil, gas or water in place in the reservoir through the differences in their respective thermal conductivities. Approach to Well Analysis Injection Wells In many instances the relatively high injection rate (1,000 or more barrels per day) and a not too large casing or liner (7 in. OD or smaller) make the continuous spinner a highly suitable tool for attaining the injection profile. Where the continuous spinner is not suited, radioactive tracers can be resorted to; or under certain conditions the "packer-type" flowmeter can be used. A temperature profile is commonly run in conjunction with the spinner or radioactive tracer survey and is particularly useful in defining the presence of a static column if such exists. Producing Wells Producing Wells Since generally at least two phases (oil and water), are present in a producing well, a somewhat different approach must usually be made from that used in a single-phase injection well. If the well conditions and flow rates are suitable, the spinner approach may be used to determine the gross rates involved. (If substantial gas production is involved, the spinner should be useful in defining the entry point or points.) JPT P. 207

A Proven Squeeze-Cementing Technique in a Dolomite Reservoir

Data on squeezed injection wells in a Grayburg-San Andres reservoir of West Texas indicate that a high final standing pressure is not a prerequisite for a successful squeeze. Profiles made a year after repair indicate that the wells have maintained the desired injectivity profile despite continued injection at fracturing pressure. Introduction The reservoir being discussed is located in West Texas and produces from the Grayburg-San Andres dolomites of Permian age at an average depth of 3,400 ft. The formation has low porosity and permeability (average 8 percent and less than 1 md, respectively) and is vertically fractured. Gross pay thickness ranges from 100 to 400 ft. Connote water saturations approximate 40 percent. The field, discovered in the 1930's, remained small until the advent of hydraulic fracturing in the 1950's. By 1960, development on 20-acre spacing was virtually complete. Completion Practices Except in the early wells, primary completion practices consisted of setting 5 1/2-in. casing at total depth and circulating cement. Float collars were used, and in many of the wells scratchers and centralizers were put over the pay section. The common cement put over the pay section. The common cement program consisted of API Class C-HSR 4-percent gel program consisted of API Class C-HSR 4-percent gel over the pay and Class C 8-percent gel as a filler to the surface. All of the wells were hydraulically fractured with lease or refined oil on completion. All of the jobs were one stage. Volumes ranged from 5,000 to 30,000 gal at rates from 15 to 30 bbl/min down the annulus between tubing and casing. Sand concentrations varied from 1 1/2 to 4 lb/gal. Acid was spotted in the hole before fracturing commenced. The majority of the wells were perforated either with one 30- to 60-ft interval at the base of the gross pay section or "blanket" perforated over the whole zone. Some wells have 4 shots/ft over intervals as great as 300 ft. Detection of Wellbore Channeling Pilot waterflooding operations in this reservoir were Pilot waterflooding operations in this reservoir were not successful. Other than two cases of direct communication to producing wells, no effect was observed. Several sets of radioactive tracer surveys were run, all of which showed that the water was confined to the desired intervals. No channeling was detected. Detailed studies were made to review primary performance and evaluate locations for one or more additional performance and evaluate locations for one or more additional pilots. In the course of these studies it became evident pilots. In the course of these studies it became evident from a review of the production history, workover success, bottom-hole pressures, and core analyses that the wells that were initially perforated low were in communication with other portions of the pay section. Failure of primary cement jobs or connection of vertical fractures in the formation were believed to be responsible for the communication. Based on the inference of wide-scale mechanical problems from these data, temperature logs were then run on the pilot water injection wells. These surveys showed that most of the injected water was being lost into nonproductive intervals in all of the injection wells. The search was then begun for a suitable squeeze-cementing technique to confine the water to the pay zone at the wellbore in hopes that this would provide the key to successful waterflooding. Subsequent performance data have shown conclusively that the problem is at or near the injection wellbore. JPT P. 1341

Selective Plugging of Water Injection Wells

Abstract A field study was made to determine the feasibility of changing the injection profile of water-input wells after water breakthrough at producing oil wells. Secondary objectives were to study the particle size and quantity of plugging material required to change the input profile and the methods of applying these materials.fifty-three injection wells were treated with various amounts and types of commercial plugging materials, and the injection profile was definitely changed in most of the wells treated. Increased efficiency of water injection after treatment was indicated by commercial tracer surveys and injection well performance. There were some indications that the rate of water production was reduced and the rate of oil production was increased as a result of selective plugging treatments, but it was found that an immediate change in the rate of oil and water production did not normally occur. Introduction The unending effort to reduce operating costs and increase ultimate oil recovery by waterflooding has required oil operators to evaluate continuously the factors affecting the performance of waterfloods. One of the most detrimental factors is by-passing of oil by the injected water between the injection and producing wells. By-passing (channeling) may be classified aspassage of water through depleted zones of high permeability andpassage of water through fractures or fissures. Oil productive reservoirs are normally composed of strata possessing wide variations in permeability, and the injected water will seek the path of least resistance to the producing well. Therefore, thief zones (zones of high permeability or fractures) will be the main conductors of injected water and will become depleted of producible oil sooner than the less permeable sections. As the more permeable zones become depleted, they will continue to accept the injected fluid at a faster rate because oil saturation is reduced and the effective permeability to water is increased. Unless water injection into the depleted zones can be curtailed and the water directed into the less permeable strata, the depleted zones will continue to transmit water from injection well to producing well during the remaining life of the waterflood. Injecting and producing a large volume of water that is doing little useful work reduces the efficiency of the operation. Various techniques have been used in an effort to isolate or plug the more permeable zones of a reservoir, includingcorrective shooting or selective perforating of injection wells,delayed drilling,multiple packer completion of injection wells,cementing injection and producing wells, andselective plugging. Cementing and selective plugging are the methods most commonly used after water breakthrough into the producing well has occurred. For the purpose of this article a selective plugging treatment is considered to be any treatment whereby a material, liquid or solid is injected into a well to change the injection profile. An ever-increasing number of waterfloods are being operated in the north Texas area. Here the formations have wide variations in permeability and many are fractured, By-passing of injected water is a constant problem, To promote conservation and efficient utilization of the nation's petroleum resources, the U. S. Bureau of Mines, in cooperation with oil companies in the north Texas area, made a field study to determine the feasibility of changing the water injection profile of a water-input well. Secondary objectives were to study the particle size and quantity of material required to change the input profile and the methods of applying these materials. This article presents a discussion of the field work performed, factors to be considered in treating wells, treating methods used and potential benefits that may be derived from selective plugging treatments. Due to the complexity of oil reservoirs and the differences between theoretical and actual performances of injection and producing wells, strict rules cannot be made to assure successful treatments. Definite answers cannot be given to some questions because of the difficulty in evaluating treatments. This article points out some of the thinking from both the theoretical and practical points of view that has developed during the past years and during this investigation. JPT P. 1550ˆ

Some Case Histories of Remedial Work Resulting from Water Tracer Surveys

Abstract Use of water tracer surveys to improve the over-all performance of waterfloods has long been recognized. Remedial work to water injection wells is becoming more important as the ratio of secondary reserves to primary reserves increases. While the proper use of tracer surveys does not necessarily mean a workover after every survey, too little significance is often given surveys where they could be of real productive use. This paper lists several case histories where tracer surveys were run and properly analyzed, resulting in thoughtful remedial work. What actual effect this remedial work had on production in the immediate area it also shown, when available. Introduction During the past 10 years, waterflood operators have tended to rely more on the use of quantitative water injection profiles. Though there are several different types and methods being used today to obtain this quantitative information, this paper will not discuss their principles or merits; rather, the assumption is made that quantitative water injection profiles can be run by a variety of service companies. Concern over what can and has happened from this point is the basis of this paper. This work was undertaken to present evidence from actual case histories where water tracer surveys were used, relied upon and found capable of being highly informative and of utmost practical value. During the search for such histories, an effort was made to stay away from examples where evaluation of remedial work could only be done at the injection well. Instead, examples are sought where direct production results confirmed proper water tracer survey evaluation of injection well remedial work, regardless of its success. This had a tendency to lead the author away from any but the most positive cases-where any cause for production changes, other than the success or failure of the remedial work, was extremely remote. Case History 5 is somewhat of an exception to this aim. Case History 1 This case history was taken from a dolomite flood in which a northwest-southeast breakthrough system had been indicated (Fig. 1). Well 63 was converted to water injection in March, 1964. By June, 1964, Well 124 started to increase in water production with no increase in oil production (Fig. 2). This trend continued until Well 124 was producing over 100 BWPD and very little oil (2 to 3 B/D). JPT P. 791ˆ

A Continuous Multistage Fracing Technique

Abstract Wells with multiple producing zones can be fractured one zone at a time during continuous pumping with a high degree of probability of treating all zones by [1] perforating with charges designed to produce round, burr-free holes; [21 perforating the same low number of holes [usually eight] into each zone; [3] perforating in acid; and [4] using sealer balls between frac stages exactly matching in number the holes in each zone. During field trials of the process, recovered fluid-cut sealer balls indicated that quite often jet perforated holes had poor roundness and ragged burrs preventing good seal. It was necessary to have jet charges and guns redesigned to provide consistently round and burr-free entrance holes. Perforating the same number of holes per zone eliminates the guesswork when determining the number of ball sealers to use per stage. Experience has shown that perforating in acid reduces breakdown pressures, allowing the process to be used in areas where previous fracing indicated the method could not be applied. Using sufficient holes per zone to handle the total pumping rate at a low differential pressure prevents breaking down more than one zone at a time. High differential pressure from too few holes consumes energy and adds to the cost of the treatment. A calculation procedure is used to determine the holes required per zone based on flow rates and total cross sectional area of the perforations. Radioactive frac sand and gamma ray tracer surveys have been used in conjunction with straddle packers to determine the zones treated. Success ratios calculated as the number of zones fractured divided by the number of fractures attempted have been found quite high in 7–8 zone wells and still greater in wells with fewer zones. The method is less expensive than treating one zone at a time using isolating packers and is much surer of producing multiple fractures than simultaneously fracing generously perforated multiple zones. Continuous multistage fracturing produces maximum fracture area for a given injection rate as compared with simultaneous injection into multiple zones. The multistage technique also allows use of all the steps considered essential to good, single-stage treating. Results of field trials have consistently indicated vertical fractures. The fracture orientation has been evidenced by low treating pressure gradients and by gamma ray tracer surveys. Field work with radioactive propping agents provided a means of checking the quality of the cement job after fracturing.

Applications of a Subsurface Solid-State Isotope Injector To Nuclear-Tracer Survey Methods

Abstract The techniques of following the movement of dispensed isotopes in a wellbore with a gamma-ray detector have been described in previous literature. The method of determining fluid-movement profiles and velocity readings in wellbores has proved valuable for defining producing zones and thief zones, for water flooding for secondary recovery programs and for solving lost-circulation problems. The Electric isotope injector utilizes a new principle to eliminate many of the disadvantages of previous down-hole dispensers. This method injects radioisotope from a solid state directly into the surrounding fluid. The radioactivity is "fixed" on a metal "button" carried by the tool; when ready to perform an injection operation, the button is lowered to the appropriate depth in the well. The isotope is dispensed from the button by energizing the proper circuits in the logging system from the control panel in the logging service truck. The isotope remains in stable solution in the well fluid. The injector is run in combination with the Scintillometer gamma-ray logging tool for tracing the movement of the fluid. Using isotopes in solid-state form is a major improvement from the standpoint of radiation safety. The problems of radioactive-material spillage and contamination have been virtually eliminated. This new tool has been field-tested under high-temperature and high-pressure well conditions and has proved very reliable and dependable under various well conditions. The substantial saving in operating time and the more reliable control permitted provides an economical and efficient subsurface tracer-survey system. Introduction Numerous techniques have been developed in the literature regarding the dispensing and tracing of radioisotopes in subsurface operations. The solution of problems relating to the detection of fluid movement in the well and subsequently into subsurface strata can become complex by using radioisotopes as tracer material. Probably the earliest application of radioactive materials in oilfield operations was the addition of carnotite to cement slurry and the subsequent detecting of the height of the cement column through location of the gamma-ray emissions. Possibly the most important and widely developed use of radioisotopes has taken place in injectivity-profile studies. The success of injection and permeability profiles in waterflood operations has been amply demonstrated for a number of years. Oil- or water-injection profiles, made both before and after fracturing operations, are extremely beneficial in demonstrating the efficiency of fracture treatments. Additional applications for tracer surveys have been developed for locating lost-circulation zones in drilling operations, locating casing leaks, locating cement-channeling behind casing and for determining the effects of vertical permeability on fluid movement through a reservoir. Techniques have been developed and utilized for almost any application in which the detection of fluid movement is essential for sound drilling or production practices. Prior to the development of the Electric Isotope Injector, utilization of radioisotopes in either oil- or water-soluble states necessitated injecting the liquid tracer either at the wellhead or from a container lowered on a cable into the wellbore. This type of downhole dispenser depended entirely upon electro-mechanical and/or hydraulic action to evacuate the isotope from the container. Injecting radioisotopes through wellhead equipment demands a high degree of control in handling procedures, in preventing surface radioactive contamination, and in monitoring practices. Operations with older down-hole dispensers have been plagued by a number of serious problems, including radioactive-material leakage at undesired depths and the failure of tools to operate at high temperatures and high pressures. To overcome many of these problems, the McCullough Tool Co. designed and has successfully utilized the Electric Isotope Injector. JPT P. 121^

Radioactive Tracer Techniques

Abstract This paper summarizes the principal applications of radioactive tracer materials in the oil field, and describes the different tools and techniques used in conducting various sub-surface surveys. Radioactive tracer surveys have been used to locate thief zones during drilling, and are unaffected by lost circulation agents present in the mud. During long string casing cementing operations, they have been used to locate the top of the cement, and the effectiveness of the cement job has been determined by channel location surveys through perforations. The success of remedial cement squeeze jobs also has been evaluated by this method. Radioactive tracers have been used to control two-pump selective acidizing treatments, as well as determining which zones have been stimulated, during conventional acidizing operations. Considerable experimental work has been conducted in connection with fracturing operations to locate and determine the orientation of any fractures so formed. Other multi-tracer experiments have been conducted in to determine the extent of co-mingling the various fracturing materials undergo in the formation, and in what order they return to the wellbore following the treatment. Other applications for radioactive tracers include the performance of relative permeability surveys, injectional capacity surveys and water location surveys; the detection of casing or tubing leaks, or leaks around casing seats or packers; evaluation of shaped charge perforating jobs, and subsequent acidizing treatments; and the identification of produced brine following break-through in secondary recovery projects. Introduction The oil industry has employed gamma ray detectors to log formation characteristics since the early 1940's. With the development and exploitation of atomic energy following World War II, the availability of various radioactive isotopes made possible their utilization as tracing agents. They have since proven a valuable aid to the oil operator in solving his drilling, completion and remedial problems. Radioactive tracers, as used in oilfield applications, may be defined as chemical materials that emit gamma rays capable of being detected by suitable electronic equipment. A number of artificially produced radioactive materials have been used for a number of years. The choice of which type tracer to use for any given application depends on a number of factors: availability, cost, solubility, radioactive intensity and halflife. The half-life of a radioactive compound is the time required for the gamma ray emission to decrease by half. Materials in use may have half-lives ranging from a few minutes to several thousand years. One of the radioactive isotopes that is popular for oilfield work at the present time is Iodine-131. It emits gamma rays that can penetrate steel tubing, casing and several inches of cement, and is readily located by means of a subsurface Geiger counter. This material is chemically versatile, in that it can be modified to be water-, acid-, or oil-soluble. Thus, it can be readily incorporated into any of these carrying media, without "hot spots" that might complicate the interpretation of the surveys. Once mixed in, the isotope does not settle out of solution, neither does it "plate out" on the formation.

The hepatic radioactivity survey.

It is the purpose of this communication to report our experiences with the hepatic radioactivity survey for the diagnosis of cancer of the liver. The technic of the procedure and the results in a preliminary series of 65 patients have been described in a previous publication (2). The radioactivity survey has been performed on a total of 210 patients to date and has proved to be more accurate and reliable for the diagnosis of hepatic metastases than either liver-function test or needle biopsy of the liver. Iodinated human serum albumin (I*HSA) has been employed as the tracer agent, and the scintillation counter as the detector of the gamma quanta emanating from the I131. Method The I *HSA used in this study was obtained from Abbott Laboratories. I131 has a half-life of eight days and emits a gamma ray sufficiently intense to be recorded externally by the scintillation counter. Radioactive human serum albumin has been used extensively in other investigations (5), and no toxic effects have been encountered from its use in the doses required for radioactivity tracer surveys. I*HSA is a homologous serum protein and contains 5 mg. of human serum albumin per millicurie of solution (3). The wide-angle scintillation counter (1) has been modified for this application by adding 2.5 cm. of lead around the sides of the tube and 3 cm. of lead around the bottom of the tube, leaving a 3/4-inch aperture. This additional shielding minimizes background gamma radiation from parts of the body other than that under survey and is essential for the success of the procedure. It also provides collimation so that the incident angle of radiation is limited to approximately 30 degrees. The counter is mounted on a portable x-ray unit with a special adapter in order to facilitate positioning of the tube during the survey (Fig. 1). The patient is given a single sterile intravenous dose of 300 microcuries of I*HSA, in an average volume of 2 c.c. Twenty-four hours after the injection the patient is placed upon an ordinary examining table and a careful abdominal examination is done to determine the anatomic location of the costal margins, the xiphoid process, the liver edge, and any palpable masses or organs. With the aid of a perforated guide centered over the xiphoid process, co-ordinate rectolinear points are marked on the patient's skin at intervals of 5 cm., extending from the nipple line to below the umbilicus and from flank to flank. A total of 42 points are thus marked and the guide is removed. The aperture of the tube is placed flush with the patient's skin and successive counts are obtained at each point. Care is taken to maintain the counter perpendicular to the long axis of the body at all times. A sufficient number of counts are observed at each point to insure a counting error of less than 3 per cent. Approximately 5 per cent of the radioactive iodine contained in the I*HSA is present as free iodide and may be absorbed by the thyroid gland.