GB2246628A - Investigating subsurface formations - Google Patents

Abstract

Oxidation-reduction characteristics of porous and permeable subterranean formations in fluid communication with one or more wells are obtained by: (a) injecting into the formation an aqueous solution containing known concentrations of a chemical capable of being chemically reduced in the formation and a non-reactive and non-partitioning water-soluble tracer chemical; (b) allowing the chemical to undergo reduction (c) displacing the solution containing the unreduced chemical through the formation to a production well; (d) chemically analyzing the solution produced at the production well; and (e) calculating the extent of the chemical reduction reaction. Tracer chemical is e.g. a 1-4C alcohol or ketone or aqueous solution containing nitrate ions. Tracers can be radioactive. Reducable compound in (a) may be an oxygen containing compound of Mo, Mn or V etc.

Description

IN SITU MEASUREMENT OF CHEMICAL REDUCING CAPACITY OF POROUS MEDIA This invention provides a method of measuring the capacity of a porous and permeable subterranean formation penetrated by one or more wells to chemically react in an oxidation-reduction reaction, and, more particularly, is concerned with determining the amount of chemical reacted per unit of accessible pore volume of the medium and the rate of the reaction.

It has long been known that oxidation-reduction conditions vary in different subterranean formations in the earth. Near the surface of the earth, in the vadose zone, where air enters the pores of the rock or soil, the minerals present are in an oxidized state. As depth increases the chemical environment becomes generally less oxidizing and more reducing.

Metals in the minerals present, such as iron, may exist in more than one valence state, the valence state depending on the oxidation-reduction conditions in the zone. In zones containing hydrocarbons, reducing conditions are present and metals are in their lower valence states.

Oxidation or reduction conditions can be measured by electrodes. The measured quantity is generally referred to as a redox potential. The redox potential is an intensive measurement, or a measurement of electron concentration, just as pH is an intensive measurement of hydrogen ion concentration. The amount of oxidizing or reducing chemicals required to change the redox potential in a system, or the capacity of a system to resist change in redox potential, is an extensive measurement. This distinction between intensive and extensive quantities is important in reviewing the art related to oxidation-reduction conditions in the earth.

Various methods of locating zones of unusual redox potentials in the earth have been proposed.

U.S. Patent 3,711,765 proposed a method of measuring the redox potentials of shale cuttings recovered from a well and comparing the redox potentials as a means of locating petroleum in the earth. U.S.

Patent 4,342,222 disclosed measurements of the redox potential of drilling mud as a well is drilled to indicate the influx of fluid and the type of fluid entering a wellbore. U.S. Patent 4,365,507 proposed the stripping of drilling mud from the walls of a wellbore during drilling of the well and measuring the redox potential of the drilling mud thereafter to determine the depth of reducing beds in the earth.

U.S. Patent 3,719,453 disclosed a method to determine if reducing conditions exist in a particular subsurface formation. A transition metal salt solution, such as a vanadium salt, is introduced into a formation through a well and means are used in the same well or another well or in a sample of rock from the formation to determine if the oxidation state of the metal is reduced by contact with the rock. A lower valence state of the metal salt after contact with the rock is used to indicate the presence of crude oil-bearing strata.

Although the methods described above can be used to detect reducing conditions in a formation, there is no disclosure in this art of a method to measure the extensive quantity of reducing capacity of a formation.

U.S. Patent 4,427,235 discloses use of an oxidizing solution to oxidize certain components of a subsurface rock so that leaching of the mineral can be employed as a means of recovery from the earth. It is well-known that oxidized uranium minerals, for example, are more soluble, and the oxidation step is a part of the in situ leaching process. No tracer method of measuring in situ the reducing capacity of such ore deposits or the rate of the reduction reaction has been disclosed, however.

Various tracer or chemical techniques for obtaining in situ measurements of rock properties and fluid saturations are known. A non-reactive tracer that follows injected water and is nonretarded with respect to water movement has long been used to indicate permeability variations in subsurface rock. (See: Brigham, W.E. et al, "Tracer Testing for Reservoir Description," J. Pet. Tech., May 1987, pp. 519-527.) The use of a mixture of tracers, one completely water-soluble and one having a known degree of oil-solubility, has been used for measuring in situ saturation of oil and water in a rock. This process is described in a paper by H.A.

Deans and C.T. Carlisle, "Single Well Tracer Test in Complex Pore Systems," SPE/DOE Fifth Symposium on Enhanced Oil Recovery, paper SPE/DOE 14886, April, 1986. A single well test for measuring chemical retention in a reservoir and employing a nonadsorbing tracer is described by A. Satter et al, "Single Well Cyclic Method for Determining in situ Chemical Retention," SPE 8837, 1980. The use of a mixture of tracers for measuring oil saturation around a single well is the subject of U.S. Patent 3,623,842 and the use of tracers for measuring oil saturation between wells is the subject of U.S.

Patent 3,590,923. The use of a mixture of tracers for measuring fractional flow and corresponding saturations of fluids is disclosed in U.S. Patent 3,990,298. Computer programs which take into account the dispersive effects of fluid flow, permeability variations, drift velocities of underground fluids, reaction rates and other factors have been developed and applied to the measurements with tracers to determine fluid saturations, as described, for example, in the above-identified paper by Deans and Carlisle.

There is a widespread need for an in situ technique to determine and quantitate the capacity of a formation for reduction of different oxidizing chemicals and the rate of the reaction. Such a technique can be used in formations which may or may not contain hydrocarbons along with an aqueous solution. Normally, the aqueous solution will be a naturally occurring brine, but it may alternately be a solution vhich has previously been injected into the formation.

This invention relates to a method of measuring the chemical reducing capacity of porous and permeable subterranean formations by injecting a mixture of known concentration of a selected oxidizing chemical, such as a chromate, and at least one non-reactive water-soluble tracer, such as methanol, into the formation through one or more wells, allowing a selected time for the chemical reaction to occur between the oxidizing chemical and the formation, displacing the fluid from the formation through the same well or from wells in fluid communication with, and distant from, the injection well, and analyzing the produced fluid for content of the injected chemicals or any reaction products thereof. Oxidizing chemicals of different oxidizing potential may be used.In addition to the water-soluble non-reactive tracer, a second, nonreactive tracer which is partially oil-soluble may be used, and the saturation of oil in the rock may be measured, using known techniques, at the same time the test for determination of reductive capacity of the formation is made.

The rate of the oxidation-reduction reaction in the pore spaces of the formation can be determined by comparing the measured concentrations of oxidizing chemical and non-reactive water-soluble tracer to calculated curves of concentrations of oxidant and tracer versus volume of solution produced from a well. The calculated curves may be obtained using well-known techniques by assuming different rates of reaction in the formation, considering rates of flow, and including the effect of mixing of fluids in the formation. A best fit of the calculated curves and experimental data is obtained by well-known curve-fitting techniques used in interpreting experimental data. The best fit of the data determines the value of the rate constant and the order of the reaction.

The total reductive capacity of the formation may be calculated as weight of oxidant per volume of pore space and the rate constant can be measured as weight per volume per time. Preferably, a computerbased mathematical model is utilized to aid in performing the calculations, but this is not necessary for use of the method.

This invention is applicable to any porous and permeable medium, but will normally be applied to subterranean rock or soil where the test fluid is injected through one or more wells. The invention is useful in the oil industry, for underground reservoirs, and in other industries. In the oil industry, the determination of reducing capacity and rate of reaction within a formation determined by this invention will be used to design a process for cross-linking polymers using reduced metal compounds. This process is the subject of our copending patent application filed on even date herewith. This invention may also be used in determining the properties of formations or soil used for disposal of liquid waste. For example, waste solutions in the earth may contain metals which can be immobilized by oxidation-reduction reactions with the formation, preventing contamination of ground water.The capacity of the formation for reducing the solutions is an important property which can be determined by the method of the present invention. Other applications will be apparent to one skilled in the art.

When one well is used for practicing the present invention, the test fluid containing the oxidizing chemical and tracer or tracers is normally injected and subsequently produced from the same permeable zone in the well. Sufficient time may be allowed for reaction to take place in the rock prior to commencement of production. The time of reaction with the rock will depend on the rate of injection, the amount of fluid injected, the time allowed for reaction before production, and the rate of fluid production. The tests can also be performed in a single well when flow is between two zones or two depths in the well. The test procedure is then similar to that when determinations are made between wells, and techniques well-known in the art are used to prevent flow in the wellbore 'between the two zones.When two or more wells are utilized, fluid containing the oxidizing chemical and tracer or tracers is injected into one well and produced from one or more different wells at an appropriate distance from the injection well. The chemical mixture is then displaced between the wells. The displacing fluid may be the same chemical mixture.

Alternatively, a drive fluid of another composition may be used.

Wells are often drilled in regular patterns for fluid injection and production - such as five-spot patterns or line drives. In a five-spot pattern, for example, the fluid is produced from a center well and fluid is injected into four wells in a square pattern with the production well at the center of the square. The present invention can be used in any pattern of wells and for any well spacing. For multiple wells, the time of reaction with the rock will depend on the well spacing, injection and production rates of the wells and the uniformity of flow through the rock.

For a single well test in a single zone, reductive capacity of the formation is measured by injecting a mixture of a non-reactive tracer and a known concentration of an oxidizer into a well, allowing the mixture to remain in the formation for a pre-selected time, and producing fluids from the formation through the same well. The concentration of non-reactive tracer in each sample of produced fluid is determined by analyzing the prcduced fluids. This concentration is normalized by dividing by the injected concentration of tracer. A similar procedure and calculation is followed for the reactive oxidant. Plots of the normalized concentrations of tracer and oxidant as a function of produced volumes are prepared and a comparison of these plots allows determination of the amount of oxidant removed from the injected solution by a material balance calculation.This amount may readily be converted to amount of oxidizer reacted per unit of pore volume contacted by the solution.

While a material balance calculation which uses total volumes and concentrations is satisfactory, more detailed information can be obtained using mathematical models which take into account different layers or formations, drift of fluid in the formation from large-scale pressure gradients, diffusion of injected tracers to pores where flow is not occurring and rate of the chemical reaction. We have found that the material balance methods discussed in the paper by Deans and Carlisle regarding measurements of oil saturation, referenced above, are applicable to the experimental results from the present invention. The distribution of the amount of a chemical product of an oxidationreduction reaction can also be calculated by such mathematical simulations.

For a test involving two or more wells, or between two points along the wellbore in a sincle well, a mixture of a non-reactive tracer and an oxidizing chemical are injected into one well and produced fluids are analyzed from adjacent producing wells. Normalized curves of the concentration of non-reactive tracer and reactive chemical are plotted, as for the single well test, and the amount of reactive chemical removed from the fluid by flow through the formation is calculated by a material balance calculation.For example, if 50 per cent of the non-reactive tracer is recovered by displacement of the mixture through the formation and only 10 per cent of the reactive chemical is recovered, then recovery of the reactive chemical is normalized by dividing recovery of the reactive chemical by the per cent recovery of the non-reactive tracer, such that the amount of reactive chemical recovered is 20 per cent. Thus, 80 per cent of the reactive chemical injected was reacted in the formation.

When this amount of chemical is divided by the volume of chemical mixture injected, the amount of chemical reacted per unit of pore volume of the formation is determined.

In formations where hydrocarbons are present, the methods described above for either a single-well test or multiple-well tests can be used, if desired, along with the use of an additional tracer mixed with the oxidizing chemical and the inert tracer, the additional tracer or a component of the additional tracer being partly soluble in the hydrocarbon phase. A component of the additional tracer is defined as a product which forms as a result of a hydrolysis reaction of the additional tracer in the formation. In a single well test, an ester is used as the additional tracer. The ester hydrolyzes in the formation and the alcohol formed by hydrolysis partitions into the hydrocarbon phase from the injected aqueous phase. For example, a tracer often used is n-propyl formate, which will hydrolyze in the formation to form n-propyl alcohol and formic acid. The component of the additional tracer which partially partitions into the oil is npropyl alcohol. In a test between wells, the additional tracer may be n-propyl alcohol, since the hydrolysis reaction is not necessary to measure oil saturation. The combined use of partitioning and non-partitioning tracers allows the evaluation of the residual hydrocarbon saturation in the formation using the techniques described in the paper by H.A.

Deans and C.T. Carlisle, "Single Well Tracer Test in Complex Pore Systems," SPE/DOE 14886, April, 1986.

At the same time the reducing capacity of the reservoir rock is determined by the measurements of concentrations of the oxidant in the solution and the non-reactive water-soluble tracer.

If the wettability and pore structure characteristics of a particular reservoir rock are known, this information may be used to infer the surface area exposed to the oil and water phases in the reservoir and the reductive capacity per unit area of rock surface can be evaluated.

If the formation contains hydrogen sulfide, the hydrogen sulfide will affect the determination of reductive capacity of the formation, and it can be accounted for by determining the amount of hydrogen sulfide contacted by the oxidant-containing solution. Preferably, the amount of hydrogen sulfide contacted by oxidant is minimized by preflushing the formation with an aqueous solution free of oxidant or hydrogen sulfide before oxidant is injected for a determination of reductive capacity of the formation.

Metal-containing chemicals suitable for use as oxidizing agents in this invention are compounds of chromium, such as chromates or dichromates. Other suitable compounds having a metal that can be reduced are compounds of vanadium, molybdenum, manganese, niobium, ruthenium, iridium, cerium and bismuth. Non-metallic oxidizing compounds include compounds which ionize in aqueous solution to form hypochlorite, perchlorate, iodate, periodate, and bromate anions.

Since the potential reductive species in a reservoir are many, including hydrogen sulfide, Group III metals, bacteria, and organic compounds within crude oil, it will be useful at times to use different oxidizing agents in sequential injection and production tests. Oxidizing agents may be used with sequentially increasing oxidizing potentials.

The sequential employment of different oxidizing chemicals leads to a more complete analysis of the reductive capacity of the reservoir volume tested.

Both the reductive capacity and the rate of the reaction can be obtained for compounds of differing oxidizing potential. This information may be used in selection of a metal compound having an oxidizing potential such that it will be reduced and the metal will be deposited over the optimum distance from a well in a reservoir.

Chemicals suitable for use as the non-reactive or inert tracer in this invention are methanol, ethanol, isopropanol, and compounds which ionize in aqueous solution to form nitrate or thiocyanate anions. Radioactive tracers such as tritium or water-soluble cobalt isotope compounds may also be used. The non-reactive tracer should also exhibit very low adsorption on the solids present in the formation and should not partition into any nonaqueous fluid phase present in the formation.

Analyses of solutions to determine chemical concentrations are made by standard techniques known to those skilled in the art of analytical chemistry.

Concentrations of tracers and oxidants used are selected to be easily determinable using these standard techniques, even when the chemicals have been substantially diluted by mixing with water or brine present in the formations being tested. The volumes of fluids containing.tracers or oxidizing chemical and the concentrations of chemicals should not be so great that the chemical cost becomes prohibitive or that the length of time for producing the tracers is excessive.

Field Example The reductive capacity of the Caddo Limestone formation in Central Texas was measured by a singlewell test. The thickness of the zone at the well was 26.5 ft and the average porosity was 17 per cent. A volume of 11,000 barrels of fresh water was injected into the well, followed by 200 barrels of fresh water containing 100 ppm Cr+6 (hereinafter referred to as Chromium VI) in the form of sodium dichromate and also containing 3500 ppm methanol.

The chemical oxidant and methanol were added to the injection water by small chemical injection pumps. Injection water samples were analyzed every 15 minutes to verify tracer concentrations. Water injection rates and volumes were monitored by flowmeters. Injection was down the annulus between tubing and casing. The well was equipped with a submersible pump without a production packer. The annulus volume was 99 barrels to the standing fluid level of the well under shut-in conditions. For each injection, this 99 barrels of water was added to the end of the injection volume to place the designed volume of fluids into the reservoir.This 99 barrels of displacement water was labeled during each injection with 300 to 500 ppm methyl ethyl ketone and this tracer was used to correct the volume and concentration data of each tracer used during the production phase, using methods known to one skilled in the art. In each case, the annulus was exhausted after 350 barrels of water was produced.

The total injection time was 20 hours. The well was shut in for 48 hours and then produced for 8 hours, during which time 976 barrels was recovered. Samples of the produced water were taken at 1 to 5 minute intervals and immediately analyzed for chemical content. Organic analysis was carried out by gas chromatography and Chromium VI content was measured by standard ASTM procedures. A material balance for the methanol showed that 81 per cent of the methanol injected was recovered. None of the Chromium VI was recovered. Calculations showed that the chemical mixture penetrated the formation around this well to a radius of 10.6 ft.

A second test was performed in the same manner using the same well. A much higher Chromium VI concentration of 830 ppm was injected and a larger volume of fluid (837 barrels) was used. Two inert tracers were used---1940 ppm methanol and 2467 ppm isopropanol. After the fluid was injected and the well shut in for one hour, production of the fluids began. A total of 2760 barrels of fluid was produced in 27 hours of production. Samples of produced fluid were taken every 2 to 6 barrels and immediately analyzed as before.

Peak Chromium VI concentration was 444 ppm.

Analysis results indicated that about 30 or 40 ppm of Chromium III was present in the early samples.

Only 40.5 per cent of the injected Chromium VI was recovered, but 96 per cent of the methanol and 89 per cent of the isopropanol were recovered. The material balance of the isopropanol was used to normalize the per cent Chromium VI recovered to 45.5 per cent. The amount of Chromium VI converted to Chromium III was applied to the total weight of chromium VI injected per barrel of water to yield the reductive capacity of the reservoir, which was 0.16 lbs chromium per barrel of available pore space. The amount of chromium converted in the first test was 0.03 lbs per barrel of available pore volume, so the total chromium converted in the volume of rock extending out to a calculated radius of 21.8 ft around the well was 0.19 lbs chromium per barrel of available pore volume.

A third test was run to determine the residual oil saturation. A volume of 26.5 barrels of fresh water containing 4450 ppm methanol, 7310 ppm of isopropanol and 7378 ppm of n-propyl formate was injected at a rate of 425 barrels per day. The well was shut in for only 22.5 hours to lessen the extent of the conversion of ester to alcohol. The well was then produced at a rate of 2516 barrels per day.

The recoveries of methanol and isopropanol were both 99 per cent of that injected and the recovery of npropyl formate was 82 per cent of the amount injected.

A computer model which simulates fluid migration in the reservoir and includes pore diffusion, the rate of the chemical reaction, deadend pore spaces and fluid drift was used to evaluate the results of the tests. The simulation was based on modeling concepts reported in "Single Well Tracer Tests in Complex Pore Systems," by H.A. Deans and C.T. Carlisle, SPE/DOE 14886, April, 1986. The simulator was modified to include the chrome reductive mechanism. Models of the complex pore system were matched to actual field data from the nonreactive tracer, and then the best fit model was compared to the reactive tracer data. The modeling of the reductive capacity test in the Caddo Limestone gave a residual oil saturation of 30 per cent pore volume, a dead-end pore space value of 40 per cent pore volume and a radial distribution profile of Chromium III in the reservoir.

The rate of the chromium reduction reaction can be calculated by two independent material balance calculation methods. The first method assumes the reaction to be or order one and a curve fit of calculated chromium III concentrations to the measured values yields the rate of reaction. The second method uses a two variable minimization curve-fitting technique, well-known in the art, to determine both the order of the reaction and the rate of reaction from the measured data. Using the second method, the order of the reaction calculated to be 1.4 and the reaction rate constant was determined to be 0.6 gram-mole/liter-second.

Having described the invention above, various modifications of the techniques, procedures, material and equipment will be apparent to those in the art. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.

Claims (25)

We claim:

1. A method of determining the oxidationreduction characteristics of a porous and permeable subterranean formation in fluid communication with one or more wells comprising: (a) injecting into the formation an aqueous solution containing known concentrations of a chemical capable of being chemically reduced in the formation and a non-reactive and nonpartitioning water-soluble tracer chemical; (b) permitting the solution to remain in contact with the formation for a time sufficient to allow the chemical partly to undergo a chemical reduction reaction; (c) displacing the solution containing the unreduced chemical through the formation to a production well; (d) chemically analyzing the solution produced at the production well; and (e) calculating the extent of the chemical reduction reaction.

2. The method of claim 1 further comprising adding to the mixture injected a known amount of a second water-soluble tracer, a component of which is capable of partially partitioning between water and hydrocarbons, and chemically analyzing the solution for the partially partitioning tracer to determine hydrocarbon saturation.

3. The method of claim 1 or claim 2 wherein the nonreactive and non-partitioning water-soluble tracer chemical is an alcohol or ketone having from 1 to 4 carbon atoms.

4. The method of claim 3 wherein the tracer chemical is methanol, ethanol or isopropanol.

5. The method of claim 1 wherein the nonreactive and non-partitioning water-soluble tracer chemical is a compound having in aqueous solution a nitrate or thiocyanate anion.

6. The method of any preceding claim wherein the nonreactive and non-partitioning water-soluble tracer chemical is a radioactive compound.

7. The method of any preceding claim wherein the chemical capable of being chemically reduced in the formation is a water-soluble compound containing oxygen and a metal.

8. The method of claim 7 wherein the metal is chromium.

9. The method of claim 7 wherein the metal is vanadium, molybdenum, manganese, niobium, ruthenium, iridium, cerium or bismuth.

10. The method of any one of claims 1 to 6 wherein the chemical which is capable of being chemically reduced in the formation is a water-soluble inorganic oxidizing compound not containing a metal.

11. The method of claim 10 wherein the compound forms a hypochlorite, perchlorate, iodate, periodate or bromate anion in solution.

12. The method of any preceding claim wherein the mixture of chemicals is injected into the formation at a location in a well and is withdrawn from the same well.

13. The method of any one of claims 1 to 11 wherein the chemical mixture is injected into the formation at one well and is withdrawn from the formation at a second well which is spaced a distance from the first well.

14. The method of any preceding claim wherein the formation contains a metal from the disposal of liquid waste.

15. The method of any preceding claim wherein the order of the oxidation-reduction chemical reaction is assumed to be one and the rate constant for the reaction is determined by comparing measured and calculated concentrations of chemicals displaced from the formation.

16. The method of any preceding claim wherein the concentrations of chemicals displaced from a well are compared with calculated curves describing the displacement such that the order of the chemical reaction and the rate constant of the chemical reaction are determined.

17. A method of determining the amount and rate of a chemical reaction of an oxidizing chemical in the pore spaces of a subterranean formation comprising: (a) injecting the oxidizing chemical at a known concentration through one or more wells into the formation, the oxidizing chemical being mixed with a known concentration of a non-reactive tracer; (b) permitting the oxidizing chemical to react with the subterranean formation for a known time, then recovering at least a portion of the injected mixture through one or more wells; (c) measuring the concentrations of the oxidizing chemical and the non-reactive tracer in the produced fluids; and (d) using the measured concentrations in a material balance calculation to determine the amount and rate of the oxidation-reduction reaction in the pore spaces of the formation.

18. The method of claim 17 wherein the formation contains an oil phase.

19. The method of claim 18 further comprising adding a hydrolyzable ester to at least a portion of the mixture of oxidizing chemical and non-reactive tracer and chemically analyzing the mixture produced for a product of hydrolysis to determine hydrocarbon saturation.

20. The method of any one of claims 17 to 19 wherein the oxidizing chemical contains chromium.

21. The method of any one of claims 17 to 20 wherein the nonreactive tracer is an alcohol or ketone having no more than three carbon atoms.

22. The method of any one of claims 17 to 20 wherein the nonreactive tracer is an inorganic compound which exhibits low adsorption on the solids of the formation.

23. The method of any one of claims 17 to 22 wherein the mixture is injected and produced through the same well.

24. The method of any one of claims 17 to 22 wherein the mixture is injected into a first well and produced from a second well which is spaced apart from the first well.

25. A method as claimed in claim 1 substantially as hereinbefore described.