Multi-tracer logging technique
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improved method of determining sweep and effective permeabilities of fluids flowing in an underground formation and, more particularly, to such a method which utilizes one or more gamma ray emitting tracers which are logged by the same logging tool. 
     2. Setting of the Invention 
     During a secondary or tertiary oil recovery project a displacement fluid or fluids are injected into the formation through an injection well and oil in the formation is displaced towards an offset production well. The efficiency with which the injected fluid contacts the oil bearing zones is termed &#34;sweep efficiency&#34;. The percentage of available oil displaced by the injected fluid is termed &#34;displacement efficiency&#34;. Both parameters, &#34;sweep efficiency&#34; and &#34;displacement efficiency&#34;, are important in evaluating the displacement process and in obtaining a description of the reservoir. Various individual methods have been utilized to determine sweep efficiency of a waterflood project, and various individual methods have been utilized to determine the effective permeabilities of the swept formation. One such method used in the past involves injecting a first tracer fluid into the formation along with the injection fluid and at some later time injecting a second tracer fluid into the formation. A logging device run through an observation well would indicate the presence of the tracer fluids as they pass through the formation. However, experience has shown that many months after breakthrough of a tracer into a particular zone, a high background radiation of the first tracer may still be present. Thus, it has been extremely difficult to distinguish between the old tracer and the new, although an approximate evaluation could be made. If this evaluation was attempted with tracers injected several weeks apart, it would be virtually impossible to distinguish between the tracers at all. Thus, there is a need for a method of utilizing at least two radioactive tracers which can be distinguished. 
     SUMMARY OF THE INVENTION 
     Disclosed is a novel method of logging a plurality of underground intervals within a given formation, contemplated to overcome the foregoing disadvantages. A first radioactive tracer is introduced into the formation and the first tracer is logged as it passes through the formation. A second radioactive tracer is introduced in the same formation, and the second tracer is logged as it passes through the formation. The second tracer has a different energy level than the first level. The logs are then analyzed to determine changes in the effective permeabilities of the formations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a sectional view of a plurality of wells penetrating a plurality of underground formations and which illustrates the advancement of tracer fluids through the formations. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawing, reference character 10 indicates an injection well which penetrates a plurality of underground oil bearing intervals of a given formation or separate formations, 12, 14, and 16. The well 10 is comprised of a casing 18 and an inner tubing 20. A plurality of perforations 22 extend through the casing 18 and are spaced adjacent the formations 12, 14, and 16. The well 10 is completed in a conventional matter as is well known in the art. 
     The tubing 20 is set through a packer 21 above the formations of interest 12, 14, and 16, to allow any radioactive fluids to be displaced from the well 10, as will be described hereinbelow. Water is injected through the tubing 20 and into the formation 12, 14, and 16 as in a waterflood project to drive in-place oil and other hydrocarbons towards a production well 24. The produced oil from the production well 24 is collected at the surface for use elsewhere. Interposed between the injection well 10 and the production well 24 is an observation well 26. The observation well 26 is completed utilizing a nonperforated casing 28 which can be metallic or nonmetallic, such as fiber glass or plastic. 
     In order to determine vertical sweep and effective permeabilities of the formations 12, 14, and 16, the following procedure is utilized in accordance with the present invention. A first radioactive tracer is injected through the injection well 10 into the formations 12, 14, and 16. The tracers utilized in this invention produce gamma radiation. Such tracers which can be utilized include such water soluble tracers as cobalt 60, cobalt 58, cobalt 57, or strontium 85, iodine 131, and also a gas, such as Krypton 85. A predetermined period of time later, from several days to several weeks, a logging tool 30, capable of multi-channel analysis of gamma ray energy, such as a device marketed under the trademark &#34;Spectralog&#34; developed and utilized by Dresser-Atlas, is lowered into the observation well 26. Logs from the logging tool 30 obtained over a period of days record the advancement and energy levels of the tracer fluid as it passes through the formations. The waterflood project is continued and after a predetermined period of time, usually from several weeks to several months, a second radioactive tracer is then injected through the injection well 10 into the formations. The second tracer has a radioactive energy different than that from the first tracer fluid. Again a log is made utilizing the logging tool 30 through the observation well 26 as the second tracer fluid passes through the formations. 
     As can be seen from the drawing, the advancement of the second tracer fluid through the formation over the same given period of time is effectively greater than that of the first tracer due to the fact that the permeability of the intervals are different as the fluid saturation within the formations changes as the flood project progresses. As shown in the drawing and specifically referring to formation 14, the second tracer over the same period of time has traveled a greater distance than first tracer fluid and effectively has progressed much further than the fluids through the other formations 12 and 16. 
     After the second log is produced, the two logs are compared so that the effective permeabilities in the formations and the changes therein may be determined. Various methods for comparing the logs may be utilized including graphical, analytical, and digital analysis, as by utilizing a computer. From this data, a determination may be made of (1) the zones which are invaded by the injected fluid or fluids and the vertical extent to which these zones are swept; (2) the continuity of the &#34;pay zones&#34; between the injection, observation and producing wells in the same flood patterns; (3) the effective permeability of the flooded zones; (4) the relative vertical sweep efficiency of the various sequentially injected fluids; and (5) the evaluation of sweep improvement treatments or other enhanced oil recovery processes. 
     The above information is obtained through the observation well 26 in the interwell area and thus should be more representative of the flood pattern than the same information obtained at a production well or injection well which may have been subjected to various stimulation treatments. 
     The described method may be utilized in a five-spot or multi-well injection program wherein a different tracer is injected into each separate injection well, and these tracers are logged through an observation or observation wells adjacent the production well. Alternately, a different energy tracer may be injected into each separate well and logged which would not only indicate sweep efficiency and permeability change, but also whether one zone or formation is in communication with another. 
     The above technique lends itself readily to evaluation of several areas of enhanced recovery where methods of tracing and tracking the relative rates of advancement of injected fluids is needed. Areas that are visualized for using this method of evaluation are water-gas-water slug injections to determine the relative rates of fluid advancement, presence of fingering, which zones are being swept, whether both fluids sweep the same zone and/or influence of different fluids on effective permeabilities. The above method may be used in preflush water-micellar-polymer injection banks to determine relative rates of advance and which zones are being swept. Also, the above method may be utilized in sweep efficiency tests where a polymer, foam or other similar materials used for sweep control are injected. 
     The radioactive tracers may be utilized to accompany injected flooding fluid or sweep control fluid to determine which zones are being swept, and whether the same zones are still being swept, relative rates of advance and/or effective permeabilities of flooding fluids before and after sweep control has been instigated. The above method may also be utilized in stimulation tests where the injection fluids are traced before and after stimulation to determine which zones are being swept, whether the same zones are being swept, relative rates of advance, and/or effective permeabilities to injected fluids before and after stimulation. 
     The above method may also be utilized without the requirement of an observation well. The requirement for an observation well to log through is necessitated by the fact that a wellbore to be utilized must be isolated from the reservoir so that any gamma ray increase over previously measured background ratings will indicate the passage of trace fluids past the well in that zone. If the casing was perforated or the well was not cased, the trace fluids could enter the wellbore and diffuse with the wellbore fluids, thereby smearing the identification of the intervals. Cross flow of fluids from one interval to another by flow through the wellbore could also be misinterpreted as an interval invaded by interwell flow. Therefore, to eliminate these wellbore effects, it has been necessary to use cased observation wells for this type of monitor logging. Preferably, the observation well would be located in the interwell area between an injection and a producing well. 
     One method of logging through a production well which could eliminate the requirement of logging through an observation well is accomplished by replacing the wellbore fluids in the production well with a nonradioactive water. This procedure involves producing the production well to establish representative radioactive tracer concentrations in the formation in the vicinity of the well, obtaining a base log of the well while producing, and injecting water down the annulus (between the casing and the tubing with the bottom of the tubing set below the formations of interest) to displace the radioactive traced water out of the hole while continuing the production of the well. The injection rate of water into the annulus is varied until the well hydrostatic pressure just balances the formation pressure. The injection-production procedure of the well is continued until all the radioactive fluids have been produced from the well. While this injection-production continues, a log of the well is obtained utilizing a gamma ray device capable of multichannel analysis and is compared to the base gamma ray log to determine the intervals containing the different gamma ray trace materials. Next the injection of the water into the annulus is stopped, but the production of fluids is continued. A log is then obtained and increases in log radioactivity due to fluids being produced into the well should better define the tops of the productive intervals. The production of the well should then be shut down with continued logging of the well. This would aid in defining the bottom of the production intervals. 
     Whereas the present invention has been described in particular relation to the drawing attached hereto, it should be understood that other and further modifications, apart from those shown and suggested herein, may be made within the scope and spirit of this invention.