DETERMINATION OF CHLORIDE CONCENTRATION IN DRILLING FLUIDS

Abstract

The invention relates to a system and method for analyzing drilling fluid from a drilling rig for accessing subterranean hydrocarbons. The system and method involve analysis for chloride by replacing conventional chemical titration with electrical conductivity titration.

Claims

1. A process for measuring the concentration of chloride in drilling fluid, where the process comprises: a) titrating a sample of drilling fluid against silver nitrate; b) measuring conductivity of the sample; and c) determining chloride concentration based on conductivity of the drilling fluid as silver nitrate is progressively added.

2. The process according to claim 1, wherein the drilling fluid is oil based or synthetic based drilling fluid.

3. The process according to claim 1 wherein chloride concentration is determined by observing change in slope of a line showing conductivity versus added silver nitrate.

4. The process according to claim 1, wherein a slope of conductivity versus added silver nitrate is determined before and after an approximate minimum value for conductivity, and a more accurate value of conductivity determined by determining, graphically or mathematically, an intersection of extrapolated plots of conductivity versus silver nitrate.

5. The process according to claim 1, comprising performing the titration using a conductivity probe comprising a polymer or metal protective body and at least one conductivity-detecting sensor within the protective body

6. The process according to claim 5, wherein the sensor or a major portion of the sensor comprises a substantially flat plate.

7. The process according to claim 1, wherein the drilling fluid sample is stirred or agitated during the titration.

8. The process according to claim 1, wherein the drilling fluid sample is allowed to reach equilibrium for a period selected from between 15 and 180 seconds, between 45 and 180 seconds, between 60 and 120 seconds, at least 15 seconds, at least 30 seconds, at least 45 seconds, at least 60 seconds, at least 75 seconds, at least 90 seconds, at least 105 seconds, at least 120 seconds, at least 135 seconds, at least 150 seconds, at least 165 seconds, and at least 180 second after adding each aliquot.

9. The process according to claim 1, comprising performing the titration using an automatic titration apparatus.

10. The process according to claim 9 wherein the automatic titration apparatus performs the following steps: i. adding a predetermined aliquot of silver nitrate solution to the drilling fluid sample; ii. after the addition of an aliquot, allowing the sample to equalize for a predetermined period or until an equilibrium conductivity value is obtained; iii. taking a reading from a conductivity probe in the sample; iv. repeating steps i)-iii) until an approximate end point is achieved, v. plotting the conductivity reading against total added silver nitrate before and after the approximate end point; and vi. mathematically determining an accurate end point based on calculated values of slope before and after the end point.

11. The process according to claim 10 wherein the automatic titration apparatus, in step (ii), stirs or agitates the sample.

12. The process according to claim 10, wherein, in step (ii), the sample is allowed to equalize for a period selected from between 15 and 180 seconds, between 45 and 180 seconds, between 60 and 120 seconds, at least 15 seconds, at least 30 seconds, at least 45 seconds, at least 60 seconds, at least 75 seconds, at least 90 seconds, at least 105 seconds, at least 120 seconds, at least 135 seconds, at least 150 seconds, at least 165 seconds, and at least 180 second after adding each aliquot.

13. The process according to claim 1 including, prior to performing steps a) to c), performing an intermediate titration on a sample of drilling fluid to provide an indication of calcium concentration and thereby an approximate indication of chloride concentration.

14. The process according to claim 13, wherein the intermediate titration is performed using a calcium chelating agent.

15. The process according to claim 13, wherein the intermediate titration is performed using automatic titration apparatus.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] A more complete understanding of the present invention and benefits thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings in which:

[0027] FIGS. 1a and 1b are graphs of results from Example 1, showing intersection of two sets of data to provide an end point;

[0028] FIGS. 2a and 2b are graphs of results from Example 2.

DETAILED DESCRIPTION

[0029] The following examples of certain embodiments of the invention are given. Each example is provided by way of explanation of the invention, one of many embodiments of the invention, and the following examples should not be read to limit, or define, the scope of the invention.

Example 1—North Sea Oil Based Mud

[0030] While titrating the silver nitrate into the solution, and in the absence of the indicator solution, a conductivity probe is inserted into the solution. The conductivity is measured with incremental additions of silver nitrate as the titrant. Once the equivalence point is reached, the conductivity of the solution should increase rapidly as more titrant is added. Plotting the volume of silver nitrate (titrant) vs measured conductivity, two distinct lines are formed. The equivalence point can be determined by plotting the data, or it can be calculated from the slopes and y-axis intercepts of the two lines. This equivalence point value is then used to calculate the concentration of chlorides, just as is done with the potassium chromate titration.

[0031] For this laboratory study, a Mettler Toledo SevenExcellence benchtop conductivity meter was used with the Mettler Toledo InLab 741-ISM conductivity probe.

[0032] The probe was first tested using stock solutions of sodium chloride and calcium chloride, using standard procedures. Errors of less than 4% were recorded in each case.

[0033] First, an oil based mud from a North Sea drilling rig was tested. Identical tests in two of the applicant's laboratories were conducted.

[0034] The oil-based mud was prepared using the standard procedure provided by API for measuring whole mud chlorides, with the exception of potassium chromate. A few milliliters of sulfuric acid were added to the solution to get the pH below 7. This pH<7.0 step is a standard part of the API procedure. Silver nitrate was added in 0.5 mL increments and given 1-2 minutes to equilibrate before the conductivity of the sample was measured. The results are shown in Tables 1 and 2 below.

TABLE-US-00001 TABLE 1 Results mL PNP 100 OBM 2 DI 200 Sulfuric Acid 4

TABLE-US-00002 TABLE 2 North Sea Oil Based Mud mL Conductivity, Conductivity, AgNO.sub.3 μS/cm (1 min) μS/cm (1 min) 0 205.5 239.8 0.5 199.1 231.7 1.0 195.4 225.9 1.5 192.6 218.7 2.0 187.9 211.9 2.5 183.7 209.2 3.0 180.1 209.0 3.5 182.7 217.4 4.0 195.0 235.6 4.5 207.8 254.3 5.0 220.1 271.4 5.5 231.8 289.5 6.0 248.7 317.9

[0035] These results are presented graphically in FIGS. 1a and 1b. The final results are presented in Table 3 below.

TABLE-US-00003 TABLE 3 Final Results North Sea 1 min 2 min mL AgNO.sub.3 3.45 3.4 mol AgNO.sub.3 0.282 0.282 mL Sample 2 2 Cl.sup.− mg/L 17,250 17,000

[0036] Comparable results were achieved using the conductivity probe to measure the concentration of chloride versus the standard API method of titrating with a color changing indicator solution. Repeat testing using the same 0.5 mL increments and 1-2 minute measurements yielded similar results.

[0037] A test conducted to determine if the amount of time between taking conductivity measurements could be shortened while still giving comparable results: a 30 second interval test was used and it was determined that 30 seconds was not long enough for the sample and titrant to come to equilibrium. the decision was made to stay with the 1 minute interval that had proven to be as effective as 2 minutes in previous testing.

Example 2—Eagle Ford/Bakken Oil Based Mud

[0038] A field sample of a mud type known as from the Eagle Ford and Bakken fields in the USA was tested. A sample of this mud was measured via standard titration and the conductivity probe method. Results were compared to the chloride value reported in the field (titration method). Considerable difficulty was noted in determining a color change with the potassium chromate indicator for this mud.

[0039] Results of the conductivity test with the oil-based system from the Eagle Ford/Bakken show that the values generated are within 12% error of the value generated from standard titration in the laboratory. However, the difference between conductivity generated values and that of the field titration is 25% error, showing the inaccuracy with the standard color change titration when using the current industry standard potassium chromate method.

[0040] Two titrations were conducted and the results shown in Table 4 below.

TABLE-US-00004 TABLE 4 Eagle Ford/Bakken Oil-Based System mL Conductivity, Conductivity, AgNO.sub.3 μS/cm μS/cm 0 256.6 232.5 0.5 255.4 228.5 1.0 253.6 220.3 1.5 251.5 218.9 2.0 250.0 214.7 2.5 248.7 211.2 3.0 248.5 208.7 3.5 250.4 205.5 4.0 265.2 220.4 4.5 293.5 247.5 5.0 313.6 268.2 5.5 333.9 285.9 6.0 357.1 314.0

[0041] The final results are shown in Table 5 below.

TABLE-US-00005 TABLE 5 Results Eagle Ford/Bakken mL 3.6 3.6 AgNO.sub.3 mol 0.282 0.282 AgNO.sub.3 mL 2 2 Sample Cl.sup.− 18,000 18,000 mg/L

[0042] The examples above are laboratory procedures using manual probes. The manual procedure is in principle feasible for use in the field, but it is preferable to use an automatic titration apparatus. Apparatus such as the auto titrator produced by a large number of commercial providers including LABTRONICS®, METTLER-TOLEDO®, HIRSCHMANN-OPUS®, THERMO-SCIENTIFIC®, HANNA®, COLE-PARMER®, METROHM®, and others. One example from METROHM® (www.metrohm.com/en-us/products-overview/titration/eco-titrator/210083010) would be suitable to perform the analysis in a way which would free up an operator's time, but is a piece of equipment designed for use in a laboratory not on an offshore oil platform. The inventors believe that, by adaptation of equipment such as this by the provision of suitable protective casings and robust screens and controls, it would be possible to produce a piece of apparatus suitable for use in the field.

[0043] Using automatic titration apparatus with conductivity probes will, in the inventors' view, address the problems of a toxic chemical indicator and a color change which can be hard to observe in the field. In one embodiment, the automatic titrator may be a field titrator having a durable case and reagents prepared for use in a remote location. In another embodiment, the automatic titrator may have an automatic sampler and be configured to retrieve and process samples automatically from a container in a laboratory, a mud logging trailer, or other location where analysis may be conducted. The sample location may be connected to or integrated with drilling equipment including but not limited to a stand pipe, mud pump, mud pit, shaker table, mud systems trailer, water processing trailer, or other oilfield equipment.

[0044] An automatic titration apparatus may have several components that may be stored in a computer readable media (e.g., memory) and executed on a processing system. The processing system may include instructions that may be executed in an operating system environment, such as a MICROSOFT WINDOWS™ operating system, a LINUX® operating system, or a UNIX® operating system environment. The computer readable medium includes volatile media, nonvolatile media, removable media, non-removable media, and/or another available medium. By way of example and not limitation, non-transitory computer readable medium comprises computer storage media, such as non-transient storage memory, volatile media, nonvolatile media, removable media, and/or non-removable media implemented in a method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data. The processing system may also utilize a data source of the computer readable media for storage of data and associated information. In one embodiment, data from the automated titration apparatus may be transmitted to a central location for analysis, processing, and modeling. In another embodiment the automated titrator may perform titrations, blanks, probe cleaning, probe calibration and determine concentration.

[0045] In closing, it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. At the same time, each and every claim below is hereby incorporated into this detailed description or specification as additional embodiments of the present invention.

[0046] Although the systems and processes described herein have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

REFERENCES

[0047] All of the references cited herein are expressly incorporated by reference. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication data after the priority date of this application. Incorporated references are listed again here for convenience: [0048] 1. API Recommended Practice for Field Testing Oil-Based Drilling Fluids 13B-2, 5.sup.th edition, 2014 [0049] 2. www.metrohm.com/en-us/products-overview/titration/eco-titrator/210083010