Transcript Protocol for Non-Toxic Concentrations of Drilling Fluid

Protocol for Non-Toxic
Concentrations of Drilling
Fluid Additives
Dr. John Ashworth
Vince Walker
Soil Science Director, ALS
Environmental - Edmonton
Director of Operations, ALS
Environmental - Fort St. John
Formerly
Overview
• Introduction and Significance
• Background
• Method Description (Microtox®
Acute Toxicity Analysis)
• Determination of Threshold Values
• Conclusion and Acknowledgements
Introduction and Significance
• Averaging 300 wells drilled/week in
western Canada
• Alberta produces 70% of Canada’s crude
and 80% of its natural gas
• in 2004/2005 fiscal year, revenues from oil
and gas accounted for more than 34% of
Alberta’s total revenues (ie. $10 billion)
• WCSB...
Drilling and Disposal
• Total of 19,365 (including dry and service)
wells drilled in Alberta in 2004
• Alberta Energy and Utilities Board (EUB)
permits on-site disposal of generated
drilling waste provided criteria are met
(Guide 50; EUB 1996)
• Disposal methods require quantification of
toxicity of waste using Microtox® bioassay
Background
• Petroleum Services Association of Canada
(PSAC) was developed in 1981 to represent
upstream oil and gas industry sectors (in
response to National Energy Program)
• PSAC’s Mud List - drilling fluid additive
product listing for potential toxicity:
Toxicity Thresholds
• To be listed, a product’s toxic rate of
application/addition must be known
• PSAC asked the Western Canada Microtox
Users Committee (WCMUC) to establish
toxic rates for new additives
WCMUC
• Resource group formed in 1987, consisting
of various members dedicated to the
standardization of Microtox® testing
• To maintain performance standards, an
inter-laboratory quality control Round
Robin program is run twice a year
• At present, the group consists of 17
members with 13 laboratories participating
in Round Robin studies
Microtox® Acute Toxicity
Assessment
Photoluminescent Bacteria
• Uses a strain of Vibrio fisheri (NRRL B11177) as a test organism
• bacteria emit light as a metabolic byproduct:
Procedure
• Bacteria are reconstituted from a freezedried state, and initial light outputs are
measured from homogenized suspensions
• Maintained at 15°C, suspensions are
exposed to serially-diluted (2-fold)
concentrations of osmotically-adjusted test
sample
• Light output readings are taken at specified
time intervals (usually 5 and 15 minutes)
EC50(15 min)
• EC - effective concentration of a test sample
that reduces light emission by a specific
amount under defined conditions of time
and temperature (also called Inhibitory
Concentration, or IC)
• EC50(15 min) = effective concentration of a
test sample that reduces light emission by
50% at 15 minutes at 15°C
• NOTE: EUB defines non-toxic substances
as those with EC50(15 min) > 75%
Determination of EC
Final Sample Concentrations (%)
Control 10.2 20.4 40.9 81.8
Raw Light
Output
Readings
Io
94
94
90
94
91
I5
107
100
88
76
50
I15
114
76
47
25
10
• Correction factor (Rt) = ratio of light output of
control at time t to initial light output of control
(used to correct for time-dependent changes):
Rt = It/Io
• Gamma (Gt) = ratio of light lost at time t to light
remaining at time t (calculated for each sample
dilution): Gt = [(Rt x Io)/It] - 1
Determination of EC
• The log of Gamma values are plotted against the log of
concentrations for each respective time t:
Log Gamma vs. Log Conc.
1.2
y = 1.439x - 1.7586
R2 = 0.9998
0.8
log gamma
0.4
0.0
0.0
0.5
1.0
1.5
2.0
2.5
-0.4
-0.8
-1.2
log conc
• Therefore, when log gamma = 0 (x-intercept), this is the
point where light output is halved, and represents the EC50
concentration at time t after the anti-logarithm is applied.
Measures of Uncertainty
• Confidence limits (CL’s) are estimated for
every analysis performed, based on the
deviation of light output readings obtained
(derivation of R2 values)
• IMPORTANT - this is only a partial
measure of within-lab uncertainty, and
DOES NOT represent inter-lab uncertainty
(critical in determining safe rates of additive
use)
Determination of Threshold Rates
• Can be made from absolute EC values, but
allowances need to be made for uncertainty
in test results
• Confidence limits (CL’s) are normally set at
2 standard deviations (sd) from the mean
• To be conservative, we would use the lower
confidence limit (ie. replicates displaying
higher toxicities) to derive threshold rates
Lower Confidence Limit
• Since % relative standard deviation
(%RSD) = 100 x (sd/mean), we arrive at the
following equation:
lower CL = mean EC50(15min) - 2 x (%RSD x
mean/100)
• Modified, we get the following:
lower CL = mean EC50(15 min) x (1 - 2 x %RSD/100)
Volume and Threshold Conversion
• This lower CL is expressed as a percentage
of the original sample concentration
(1/100); to convert to L/m3 (1/1000), we
apply a factor of 10
• as well, since the EUB EC50(15 min) “pass”
threshold is set at 75% of the original
concentration of sample, a factor of 4/3 is
applied to the lower CL to meet this
criterion
Equation for a Non-Toxic
Threshold Rate
L/m3 = (4/3) x 10 x mean EC50(15 min) x (1 - 2 x %RSD/100)
%RSD and WCMUC
• Since it’s not feasible to subject all drilling
fluid additives to WCMUC round robin
studies, how can we derive an appropriate
%RSD for every additive to obtain a
probable non-toxic rate of application?
• The examination of WCMUC Round Robin
data from 2000-2005 revealed a skewed
frequency distribution of %RSD values
Frequency of % RSD
40.0
% frequency
30.0
20.0
10.0
0.0
0
20
40
60
80
100
% RSD
• Neglecting 2 %RSD’s over 100 caused by test liquid
instability; the mean of 31 RSD values is 28%
New Threshold Equation
• Inserting a %RSD of 28 into the equation
for determining a non-toxic threshold rate,
we derive the following:
L/m3 = 5.867 x EC50(15 min) ,
Or simply:
L/m3 = 6 x EC50(15 min)
Conclusion
• Of course, depending on the stability of
additives and consistency in results which
they yield, % RSD will vary; it is
recommended that this conservative
threshold equation is used in cases where
the additive is only tested at one laboratory
• Likewise, coloured samples display wider
scatter of data, and thus higher %RSD’s; in
these cases, using a factor smaller than 6 is
advisable
Acknowledgements
• Dave Horton of Brine-Add Fluids
(representing PSAC) for providing various
drilling fluid additives
• Dave Wong of Epcor Canada for
distributing test liquids and for collation and
statistical analysis of WCMUC Round
Robin data