protocol for non-toxic concentrations of drilling fluid additives dr. john ashworth soil science...

Protocol for Non-Toxic Concentrations of Drilling

Fluid Additives

Dr. John AshworthSoil Science Director, ALS Environmental - Edmonton

Vince WalkerDirector 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 B-11177) as a test organism

• bacteria emit light as a metabolic by-product:

Procedure• Bacteria are reconstituted from a freeze-dried

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

• 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

Control 10.2 20.4 40.9 81.8

Io 94 94 90 94 91

I5 107 100 88 76 50

I15 114 76 47 25 10

Final Sample Concentrations (%)

Raw Light Output

Readings

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.

y = 1.439x - 1.7586

R2 = 0.9998

-1.2

-0.8

-0.4

0.0

0.4

0.8

1.2

0.0 0.5 1.0 1.5 2.0 2.5

log conc

log

gam

ma

• 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

• Neglecting 2 %RSD’s over 100 caused by test liquid instability; the mean of 31 RSD values is 28%

Frequency of % RSD

0.0

10.0

20.0

30.0

40.0

0 20 40 60 80 100

% RSD

% f

req

uen

cy

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