Controlling sulfide scavenger content of drilling fluid

The concentration of unspent zinc-based hydrogen sulfide scavenger in an aqueous drilling fluid is controlled by selectively extracting the unspent scavenger in a solvent, such as glacial acetic acid, measuring the concentration of dissolved zinc, for example, with an X-ray fluorescence spectrograph, and utilizing the results of the measurements to proportion the extent of changes to be made in the concentration of the scavenger.

BACKGROUND OF THE INVENTION 
The present invention relates to determining and utilizing the amount of 
unspent zinc-based sulfide scavenger which is present in a water-based 
drilling fluid for adjusting the scavenging capability of the drilling 
fluid to the extent desired during the drilling of a well. More 
particularly, the invention relates to a relatively quick and accurate 
procedure which can be used in field locations. 
A state of the art paper entitled "Chemical Scavengers for Sulfides in 
Water-Based Drilling Fluids" by R. L. Garrett, R. K. Clark, L. L. Carney 
and C. K. Grantham, Sr. in Journal of Petroleum Technology, June 1979, 
page 787, discusses the chemistry of commercial scavengers for water-based 
drilling fluids, the parameters that affect the reliability of such 
materials and the problems affecting scavenger use. On page 796, the 
authors point out that "From this state of the art review one can see that 
we believe more research is needed to develop scavengers and tests for 
scavenger content in muds that match more closely the qualities of an 
ideal scavenger." 
U.S. Pat. No. 3,706,532 describes a method for determining zinc 
concentrations in aqueous mediums. A sample medium is acidified, treated 
with a buffered complexing agent to complex aluminum or iron ions, treated 
with an organo sulfur compound to complex copper ions, then analyzed by 
adding an indicator the color intensity of which is calibrated with 
respect to known concentrations of zinc. U.S. Pat. No. 3,928,211 
describes a class of zinc carbonate, basic zinc carbonate and zinc 
hydroxide compounds effective for sulfide scavenging. U.S. Pat. No. 
4,252,655 describes the removal or inactivation of hydrogen sulfide 
contamination by adding at least one organic zinc chelate. 
SUMMARY OF THE INVENTION 
The present invention relates to improving a process for drilling a well 
with an aqueous drilling fluid containing a zinc-based scavenger of 
sulfide ions. A determination is made of the amount of unspent 
zinc-containing sulfide-scavenging material present in the drilling fluid. 
A measured volume of the drilling fluid is mixed with a significantly 
larger number of volumes (such as about 6 to 10) of a selective solvent 
for dissolving zinc ions and establishing within the resulting mixture a 
pH (such as a pH of from about 4 to 6) at which substantially all of the 
zinc in the drilling fluid, except for that combined into zinc sulfide 
molecules, becomes dissolved in the liquid phase of the mixture. A portion 
of the resulting liquid solution is separated from the solid components of 
the drilling fluid and the amount of zinc contained in the solids-free 
liquid is determined in order to determine the amount of unspent 
zinc-containing sulfide scavenger in the drilling fluid. The amount of 
scavenger in the mud is then adjusted to the extent required to provide a 
capability of precipitating a selected amount of sulfide ions without 
involving a solids content capable of impairing the drilling fluid 
rheology. 
In a preferred embodiment of the invention the drilling fluid sample is 
mixed with about 4 to 10 times its volume of glacial acetic acid, or a 
selective solvent which is substantially equivalent to glacial acetic acid 
with respect to selectively dissolving zinc ions which have not combined 
with sulfide ions. The concentration of zinc in the resulting solution is 
preferably measured with a portable X-ray fluorescence spectrographic unit 
which is, or is substantially equivalent to, a Portaspec Model 2501 
portable X-ray spectrograph (available from Pitchford Scientific 
Instruments Division of the Hankison Corporation). 
In a preferred procedure, for example, in situations in which the 
proportions found of unspent zinc based scavenger are relatively low, an 
augmentive test for total zinc (including that combined into zinc sulfide 
molecules) can be performed by (a) an X-ray fluorescence measurement, or 
equivalent measurement, of the zinc in the unleached drilling fluid or, 
(b) using as the solvent for dissolving zinc from the drilling fluid a 
strong acid, such as hydrochloric acid, as a solvent, for combined and 
non-combined zinc, prior to measuring the concentration of the zinc 
solution. Such an acid preferably has a normality of from about 1 to 3. 
The difference between the prior and augmentative tests will indicate 
whether the scavenger concentration was reduced by dilution of the 
drilling fluid or by combination with sulfide.

DESCRIPTION OF THE INVENTION 
Applicants have discovered that possible needs for changing the 
concentration of zinc base scavenger in a drilling fluid can be accurately 
monitored at the well site so the corrections in the rate of scavenger 
addition can be properly initiated. This can be effected by utilization of 
the present process. This process enables the drilling fluid to be sampled 
at a selected frequency with the results of determinations of the 
concentrations of unspent scavenger promptly available to the mud 
engineers. For example, within about 30 minutes or so, based on such 
information, increases and decreases can be made in the rate of scavenger 
addition and for additions of scavenger-free fluid to the extent needed to 
quickly change that connection to either avoid an impairment of the 
drilling fluid rheology or to quickly scavenge a sudden encounter of 
sulfide. 
Experiments were conducted using samples of an aqueous drilling fluid 
typical of that used in drilling operations. Quadruplicate examples were 
performed on samples of that mud spiked with proportions of 1 lb. per 
barrel (ppb) of Sulf-X (a zinc based sulfide scavenger available from Imco 
Services, a Halliburton Company). The tests employed the following 
procedures, which are preferred procedures for use in the present 
invention. 
SAMPLE PRESENTATION 
1. Measure 10 ml of stirred mud into a 10 ml graduated cylinder using a 
pipet with the end of the pipet cut off to minimize any particle size 
exclusion. 
2. Transfer the measured mud sample to a 150 ml beaker. 
3. Add 60 ml of glacial acetic acid to the mud sample. 
4. Heat at about 110.degree. C. with frequent stirring for 10-15 minutes. 
5. Allow the solution to cool sufficiently to prevent damage to a plastic 
centrifuge tube. 
6. Place a portion of the mud-acetic acid mixture into a plastic 50 ml 
centrifuge tube. 
7. Centrifuge so that all the mud is firmly packed at the bottom of the 
centrifuge tube. 
8. Accurately pipet 10 ml of the centrifuge solution into a Chemplex X-ray 
fluorescence counting vial using a 5 ml Finnpipette. 
9. Cover the counting vial with polypropylene film, brace the film onto the 
vial with a small collar, and fix the film onto the vial with a large 
collar. 
INSTRUMENTAL MEASUREMENTS BY X-RAY FLUORESCENCE 
1. Position the element selector to Zn using the sidearm lever. 
2. Open the sample compartment door. 
3. Plug into a 110 V outlet and engage "Power" button. Wait for the "ready" 
light and let warm 10 minutes. 
4. Place sample counting vials in the spring-loaded mount. Insert the mount 
into the sample chamber with the rounded edge of the stainless mount 
facing inward. [Note: Make sure no droplets are present on the 
undersurface of the polypropylene film. These droplets will cause an 
errant increase in count rate.] 
5. Close the sample chamber door and check to see if the "X-rays on" 
indicator is illuminated. If it is not illuminated, the stainless planchet 
holder should be reinserted in the other direction. 
6. With "X-rays on", adjust the current to read 0.5 milliamps. 
7. Set the counting scaler on the front panel to 60 seconds. 
8. Engage count pushbutton and record the final gross X-ray intensity 
counts on the digital readout. 
9. Obtain gross X-ray counts for the glacial acetic acid blank and a 
calibration standard prepared y the dissolution of ZnO in glacial acetic 
acid. 
10. To leave instrument in standby position, open the sample compartment 
door. 
11. For longer periods of inactivity, turn down the current, turn off main 
power and unplug. 
CALCULATIONS 
(based on the following conditions) 
10 ml mud, 60 ml acetic acid, 10 ml aliquots in counting vial. 
Calculations are not valid for variations from these amounts. 
1. Determine net counts for samples and ZnO calibration standard by 
subtracting the glacial acetic acid blank counts. 
2. Determine the mg of Zn in 10 ml mud sample by the following ratio: 
##EQU1## 
3. Determine ppb (pounds per barrel) Zn by multiplying the mg Zn in the 10 
ml mud sample by 0.035. The factor 0.035 is derived from the following 
conversion: 
##EQU2## 
4. Determine ppb Sulf-X by multiplying ppb Zn by 1.67 [Sulf-X contains 
60.0% Zn]. 
TEST RESULTS 
The tests indicated by the following: 
Sulf-X was experimentally determined to be present at 0.97.+-.0.09 ppb. 
These results indicate the accuracy and precision of the method to be 
within the 10 percent relative objective. 
Additional experiments were designed to simulate situations where the 
scavenger containing mud had been totally exhausted by hydrogen sulfide 
intrusion. This was accomplished by spiking mud with 1 ppb zinc sulfide 
which is the product from the reaction of the zinc scavenger with sulfide. 
Duplicate analyses yielded unspent scavenger concentrations of 
0.02.+-.0.01 ppb indicating that the acetic acid leach is effective at 
differentiating spent and unspent zinc scavenger. 
In a third experiment, unspiked mud was found to have 0.03 ppb unspent zinc 
scavenger which indicates that potential interferents inherent to the mud 
are virtually non-existent. 
In general, the selective solvent for zinc ions can comprise substantially 
any buffered liquid having a composition and concentration capable of 
providing a pH of about 4 to 6 when one part by volume of a drilling fluid 
having a pH in the range of from about 9 to 12 is mixed with about 4 to 10 
parts by volume of said liquid. Examples of suitable selective solvent 
solutions include: glacial acetic acid, 10% formic acid, and 0.0001 M 
hydrochloric acid. 
In general, the concentration of zinc which becomes dissolved in the 
selective solvent can be measured by substantially any suitably accurate 
procedure. Procedures capable of being conducted in field locations are 
preferred. An example of such a procedure is described in "Colorimetric 
Determinations of Elements" by G. Charlot, Elsevier Publishing Company, 
1964. 
Suitable Compositions and Procedures for Use in the Invention 
The present invention is applicable to substantially any process for 
drilling the borehole of a well with an aqueous drilling fluid in a 
location in which the wellbore may encounter water soluble sulfide ions 
such as those in hydrogen sulfide or salts containing HS.sup.- or 
S.sup.--. Such acids and salts commonly coexist in a subterranean 
sulfide-containing water system. 
In a preferred embodiment of the invention, the above described analyses 
are conducted at the drilling site with a frequency which increases with 
the likelihood of the borehole encountering sulfide ions and/or increases 
in the extent by which the zinc-based scavenger is found to have been 
depleted by round trips of the circulating drilling fluid. The zinc-based 
sulfide scavengers are generally available as solids and can be added as 
dry solids through a hopper for mixing solids with the circulating 
drilling fluid. But, in a preferred procedure, the scavengers are 
preferably added in the form of slurries in aqueous liquids. In addition, 
as known in the art, a lignosulfonate treatment of the drilling fluid can 
be utilized for controlling any undesirable zinc-induced flocculation of 
mud components. 
In general, the most commonly used zinc based sulfide scavenger is a basic 
zinc carbonate. It is a manufactured compound having a formula averaging 
about 3Zn(OH).sub.2.2Zn Co.sub.3. As known in the art, where desirable to 
minimize any adverse effects of zinc ions, those ions can be loosely 
bonded with organic compounds into the form of metal chelates. 
Commercially available zinc chelates are based on aliphatic amino acids or 
their salts. Such chelated zinc ions tend to avoid being captured on clay 
surfaces in a manner causing flocculation while still being available for 
precipitating sulfide ions.