Abstract:
Complex metal anions such as cobaltic hexacyanide are utilized as nonradioactive tracers in reservoir studies. Sensitive analytical procedures enable detection and measurement of very low tracer levels.

Description:
BACKGROUND OF THE INVENTION 
     Knowledge of reservoir behavior is important in planning and carrying out secondary and enhanced recovery processes in oil reservoirs. The use of chemical and radioactive tracers for reservoir evaluation has been thoroughly developed over the past few decades. In tracer studies, an identifiable tracer material is injected through one or more injection wells into the reservoir being studied. Water or other fluid is then injected to push the tracer to one or more recovery wells in the reservoir. The output of the recovery wells is monitored to determine tracer breakthrough and flow through the recovery well(s). Analysis of the breakthrough times and flows yields important information for use in carrying out the secondary or enhanced recovery process. 
     Most of the previously used tracers have been radioactive compounds. Output from recovery wells has been monitored for radioactivity to determine tracer breakthrough and flow. 
     In recent years, environmental and safety considerations have drastically impacted the use of radioactive tracers, and there has been a need for effective nonradioactive tracers for reservoir studies. 
     Nonradioactive tracers have been proposed, for example as described in U.S. Pat. No. 3,003,856 to Boyd, but have disadvantages in that the amount of tracer injected needs to be quite high in order to be detected during analysis of output from the recovery wells. Additionally, many chemical tracers are subject to reaction with materials in the reservoir, hindering their effectiveness. 
     There has been a need for chemical tracers that are safe, readily available, environmentally acceptable and detectible in very low amounts, so that small amounts can be injected and yet breakthrough and flow can be accurately measured. Such tracers are provided by the present invention. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, nonradioactive complex metal anions that can be detected in very low concentrations are used as tracers in reservoir studies. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The nonradioactive tracers useful in this invention are complex metal anions. Preferred tracers are the water soluble salts of cobaltic hexacyanide such as sodium, potassium or ammonium cobaltic hexacyanide. The cobalt in the tracer compound in this invention must be in the complex anion portion of the molecule, as cationic cobalt tends to react with materials in the reservoir, which leads to inaccurate analytical information. Also, the analytical procedure for detecting the cobalt involves adsorbing the cobalt on an anionic exchange resin, so use of cobalt in the cationic form would not be suitable. Other complex metal anions can be used in the process. 
     The use of nonradioactive cobaltic hexacyanide as an economic tracer material requires that a very sensitive analytical procedure be available, so that the amount of tracer injected can be low. 
     An analytical procedure that can detect cobaltic hexacyanide in amounts below 40 parts per trillion has been developed. The basic steps in the procedure are (1) adsorption of cobaltic hexacyanide complex on an anion exchange resin, (2) combustion of the anion exchange resin, (3) acid solubilization of cobalt from the combustion residue, and (4) analysis of the solubilized cobalt by inductively coupled plasma/mass spectrometry, atomic absorption, or other means. 
     The ability to detect such low amounts of cobaltic hexacyanide means that small amounts of the tracer can be used with good results. 
     The efficacy of the process has been demonstrated in actual reservoir situations, as exemplified by the following examples: 
    
    
     EXAMPLE I 
     In a reservoir targeted for carbon dioxide flooding, about 700 g of potassium cobaltic hexacyanide was injected into several injection wells where the wells were on 40-acre spacing in roughly a five spot pattern. The average thickness of the net pay reservoir was 62 feet, the average porosity was 11.7 percent, the injection rate per well was 1,000 barrels per day, and the average production rate per well was 896 barrels per day. The tracer responded in measurable and interpretable concentrations, such that reservoir characteristics could be determined without the need for use of radioactive tracers. 
     EXAMPLE II 
     In several West Texas reservoirs tested in a manner similar to that described for Example I, all but one provided measurable and interpretable concentrations of the tracer in produced fluids within four months of tracer injection, and the one that did not show tracer breakthrough in that time period had also failed to provide useful levels of tracer in produced fluid in that time period when conventional tracers were used. 
     EXAMPLE III 
     This example describes a procedure for analyzing produced fluid for cobaltic hexacyanide: 
     Step 1 
     Cobaltic Hexacvanide Concentration on Anion Exchange Resin 
     Filter 500 ml of produced water through paper and glass fiber filters. 
     Pump the 500 -ml filtrate aliquot through a 12&#34;×1/8&#34; i.d. flexible tube section packed with Bio-Rad AG 1-X8 anion exchange resin (100 mesh, chloride form). An appropriate flow rate through the column is 9 ml per minute. After the sample, rinse the resin column with 100 ml of water containing 5 ml of concentrated HCL. 
     Transfer the resin from the tube section into a glass vial and dry overnight at approximately 90°C. 
     Step 2 
     Resin Combustion 
     Combust the dry resin by placing the vial in a 550° C. furnace for four hours. 
     Step 3 
     Cobalt Solubilization 
     Cool the combusted resin vial and add 2 ml of concentrated nitric acid and 2 ml of concentrated hydrochloric acid. Cap the vial and vigorously mix for 15 seconds. 
     Uncap the vial and evaporate its contents to dryness. 
     After cooling, add 5 ml of water containing 0.1 percent concentrated hydrochloric acid to the vial. Recap the vial and vigorously mix for one minute. 
     Step 4 
     Cobalt Analysis 
     Use inductively coupled plasma/mass spectrometry (ICPMS) to determine the parts per billion cobalt in the 5-ml sample. Convert these results to parts per trillion cobaltic hexacyanide in the original sample using the following equation: ##EQU1## where, A = Cobaltic hexacyanide concentration in original sample in parts per trillion. 
     B = Cobalt analytical result in parts per billion. 
     C = Volume in liters of acidified water used to dissolve cobalt just prior to ICPMS analysis 
     D = Molecular weight of cobaltic hexacyanide. 
     E = 1000 parts per trillion/part per billion. 
     F = Original produced water sample volume (L). 
     G = Atomic weight of cobalt.