Let Oil and Gas Talk to You: Predicting Production Performance

Let Oil and Gas Talk to You: Predicting Production Performance
SAS Global Forum 2012
Statistics and Data Analysis
Paper 342-2012
Let Oil and Gas Talk to You: Predicting Production Performance
Keith R. Holdaway, SAS Institute Inc., Cary, NC, USA
ABSTRACT
How do historical production data relate a story about the subsurface oil and gas reservoirs? Business analysts must
perform accurate analysis of reservoir behavior using only rate and pressure data as a function of time.
This paper introduces methodologies to forecast oil and gas production by exploring implementations of the
®
AUTOREG, ESM, and MODEL procedures in SAS/ETS . The AUTOREG procedure estimates linear regression
models when the errors are autocorrelated. The ESM procedure generates forecasts by using exponential smoothing
models. Examples of the MODEL procedure arising in subsurface production data analysis are discussed. In
addressing these examples, techniques for pattern recognition, implementing TREE, CLUSTER, and DISTANCE
®
procedures in SAS/STAT are highlighted to explicate the importance of oil- and gas-well profiling to characterize the
reservoir.
INTRODUCTION
The Reservoir Management Department (RMD) of Saudi ARAMCO National Oil Company (NOC) analyzes tens of
thousands of producing wells across multiple oil and gas fields that cover hundreds of heterogeneous sandstone and
carbonate reservoirs on the Arabian Peninsula. It is both complex and challenging to estimate reserves and forecast
production rates from a short- and long-term perspective in the oil and gas industry. SAS proposes an advanced
analytical methodology that encompasses a data preparation workflow to ensure integrity and reliability in the
forecasts, and a workflow that ensures models adhere to Arp’s¹ first principals that reservoir and production
engineers traditionally implement. This suite of algorithms known as Arp’s equations assume limited changes in
operating strategies and make assumptions about the pressure and drive mechanisms in the reservoir. The oil and
gas industry knows this process as Decline Curve Analysis (DCA)². Engineers determine Estimated Ultimate
Recovery (EUR) of reserves in existing reservoirs and forecast production trends implementing the equations in
Table 1.
Exponential
Hyperbolic
Harmonic
b=0
0<b<1
b=1
D¹
ln(q¹q²) / t
D¹(q¹/q²)^1/b
D¹(q¹/q²)
q¹
q¹exp(-D¹t)
q¹/(1 + btD¹)^1/b
q¹/(1 + btD¹)
Qp
q¹ - q²/D¹
q¹/D¹(1 – b)(1 - q²/q¹)^1-b
q¹/D¹ln(1 + D¹t)
t
ln(q¹q²)/D¹
(q¹/q²)^b – 1/bD¹
q¹ - q²/D¹q²
Table 1: Arp’s Equations with Varying Constants, b Values
Below you can view the glossary of terms for the Arp’s equations³ to better appreciate the first principles that
underpin current forecast model workflows. The input data are the historical production observations that engineers
collect periodically at the producing wells across the oil and gas fields. When the constant ‘b’ is equal to ‘0’, it is an
exponential type curve; when equal to ‘1’, it is a harmonic type curve and when the value of ‘b’ is greater than ‘0’, and
less than ‘1’, it is a hyperbolic type curve. Figure 1 depicts the traditional DCA methodology that embraces the
necessary steps to enable production engineers to interpret the type curves and estimate performance in the shortand long-term.

D¹ Initial rate of decline

q¹
Initial rate of production

q²
Rate of production

Qp Cumulative production

t
Time
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DCA
Rate-Time and
Rate-Cum
Oil and Water
Phases
Gas Phase
Exponential
P/Z Analysis
Hyperbolic
Harmonic
Figure 1: Decline Curve Analysis Methodology
Challenge
The challenge for Saudi ARAMCO is to develop a consistent, robust, and effective methodology to analyze across
disparate upstream silos the production data: oil, gas, and water. Such a solution enables the optimization of both
production and field development in a user-friendly Web-based experience.
Reservoir engineers analyze thousands of wells to estimate oil reserves. But owing to high volumes of data on each
well and the capabilities of existing systems, it is currently feasible to include only a limited number of wells in the
analysis. To compensate, engineers use a deterministic sampling process that increases the uncertainty of forecasts.
Engineers manually flag data errors and outliers in a time-intensive process. When Saudi ARAMCO mandate reviews
of field redevelopment plans every three months, it is time for a new system to reduce resource constraints and to
shorten the decision-making cycles.
Solution
®
The SAS for Oilfield Production Forecasting solution offers a customized data management and advanced time
series analytical approach that addresses the current inherent issues of a deterministic workflow by adopting a
probabilistic perspective.
Results
It is now easier to identify productivity improvements in mature assets. The solution is a DCA workflow that rapidly
flags problematic wells with robust data and reliable statistical accuracy indicators.
Adopting an advanced analysis of a decline curves methodology provides important forecasting results for future
production from a single well or multiple wells across reservoirs and fields. SAS approaches the problem by
delivering a product that helps you do the following:
1.
Access well and reservoir data quickly and easily with an intuitive graphical user interface that integrates
and profiles forecasting data, identifies outliers and missing values, and lets you select appropriate
treatment strategies.
2.
Apply analytical functions consistently across the organization with a forecasting solution that automatically
selects the best forecast model, but allows human intervention and benchmarking for repeatability. For gas
reservoirs, multiple embedded water encroachment models are available.
3.
Perform DCA quickly and flexibly with a robust analytical engine that provides what-if scenarios to model
EUR based on a predefined range of easily adjustable, industry-standard default values.
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4.
Estimate unconventional sources of well production more accurately using best-fit prediction that uses
smaller data sets when large volumes of historical data are not available.
SAS FOR OILFIELD PRODUCTION FORECASTING
“We are not trying to create a program that replaces applications like OFM and ARIES, but one that will automate the
DCA processes to a high enough accuracy that an engineer will only have to look into the wells (or reservoirs or
fields) that are problems. The main objective is for production optimization and development planning, not reserves
estimation.”
Gary M. Williamson, Reservoir Engineer Saudi ARAMCO
Oilfield Production Forecasting introduces a refreshingly intuitive navigation of the disparate data sources and input
parameters that are in alignment with a comprehensive suite of forecasting models. You can access these
functionalities in a Web-based solution. The user is able to analyze an extensive array of “what-if” combinations of
critical input parameters across multiple forecasting models. Whether working with oil or gas reservoirs, the user has
access to a rich compendium of forecasting techniques that detail accuracy indicators and interactive graphs.
There is a need for reservoir engineers to establish a single information analysis integrated platform, and this
requisite motivates the design and implementation of a Web-based DCA in an integrated portal. This platform
integrates different data sources and applications that make up a reservoir engineers’ toolbox for all fields and
reservoirs within the scope of study, saving them the time to search and manipulate the data.
The Web-based DCA system enables engineers to dynamically perform analyses on field and reservoir data for
individual wells or groups of wells by using different methods in an integrated portal. It provides several functions like
best-fit forecasts for Cartesian, exponential, harmonic, and hyperbolic analysis, using linear and nonlinear regression
with manual correction (both for curves and parameters). It also saves the production forecast along with parameters
in a database to perform graphical comparison between multiple forecasts and manual input of empirical parameters
(constant rate to percent reserve, reserves, decline rate, and decline type). This implementation allows engineers to
perform dynamic production analysis with more advanced forecasting techniques, which is effective in business
planning to generate the production forecast for the new wells and no drill case for the existing fields’ wells.
PRE-FORECASTING PREPARATION
The SAS solution implements several preparatory steps in order to ensure more accuracy in the PROC MODEL
execution.
The STEPAR method consists of the following computational steps:
1.
2.
3.
4.
5.
6.
7.
Fit the trend model as specified by the TREND= option using ordinary least squares regression.
This step de-trends the data. The default trend model for the STEPAR method is TREND=2, a linear trend
model.
Take the residuals from step 1 and compute the autocovariances to the number of lags specified by the
NLAGS= option.
Regress the current values against the lags, using the autocovariances from step 2 in a Yule-Walker
framework. Do not bring in any autoregressive parameter that is not significant at the level that is specified
by the SLENTRY= option. (The default is SLENTRY=0.20.) Do not bring in any autoregressive parameter
that results in a non-positive-definite Toeplitz matrix.
Find the autoregressive parameter that is least significant. If the significance level is greater than the
SLSTAY= value, remove the parameter from the model. (The default is SLSTAY=0.05.) Continue this
process until only significant autoregressive parameters remain.
If the OUTEST= option is specified, write the estimates to the OUTEST= data set.
Generate the forecasts using the estimated model and output to the OUT= data set. Form the confidence
limits by combining the trend variances with the autoregressive variances.
The solution tolerates missing values in the series and estimates autocorrelations from the available data. This
method requires at least three passes through the data: two passes to fit the model and a third pass to initialize the
autoregressive process and write to the output data set.
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/* Run a Default Linear Model */
PROC FORECAST
DATA = WORK.TMP0TempTableInput
OUT=WORK.LinearForecast_INPUT(LABEL="Forecast of WORK.FORECAST_INPUT")
OUTALL
OUTEST=work.FCS1PARAMETERESTIMATESINPUT_0002
METHOD=STEPAR
INTERVAL=MONTH
/* Forecast Period default is 240 Months */
LEAD=&FORECASTPERIOD.
TREND=2
ALPHA=0.05;
ID Time;
VAR MEASURE;
RUN;
/* Define Arps’ Equations and Models Calculation */
PROC SQL;
CREATE TABLE WORK.PRED_OUTPUT AS
SELECT PRED_Table.T ,
PRED_Table.Time,
PRED_Table.MEASURE,
(&Lin_Expo_crate. * EXP(&Lin_Expo_Di. * PRED_Table.T )) AS L_PRED_EXP,
(&NLin_Expo_crate. * EXP(&NLin_Expo_Di. * PRED_Table.T )) AS N_PRED_EXP,
(&Lin_Harm_crate. / (1- &Lin_Harm_Di. * PRED_Table.T )) AS L_PRED_HAR,
(&NLin_Harm_crate. / (1- &NLin_Harm_Di. * PRED_Table.T )) AS N_PRED_HAR,
(&Lin_Hyp_crate. / (1- &Lin_Hyp_n. * &Lin_Hyp_Di. * PRED_Table.T) ** (1/ &Lin_Hyp_n.))
AS L_PRED_HYP,
(&NLin_Hyp_crate. / (1- &NLin_Hyp_n. * &NLin_Hyp_Di. * PRED_Table.T) **
(1/ &NLin_Hyp_n.)) AS N_PRED_HYP
FROM Work.PRED_Table AS PRED_Table;
QUIT;
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Let Oil and Gas Talk to You: Predicting Production Performance, continued
Figure 2: SAS for Oilfield Production Forecasting
In Figure 2, you see an example of the visualization that enables engineers to compare rates of oil production for a
single well over a finite period of time. Engineers adopt both linear and non-linear models to fit the monthly oil
production volumes input into the PROC MODEL code.
The graphical output in Figure 2 illustrates a non-linear harmonic (b=1), a non-linear exponential (b=0) and a nonlinear hyperbolic (0<b<1) as well as a linear harmonic suite of trends. You can select the optimum or best fit
trend/curve based on deterministic experience. You can also endorse a priori interpretation by studying the statistical
accuracy indicators that the SAS for Oilfield Production Forecasting solution determines post executing the following
macro code:
%MACRO accuracyIndicators();
%IF &HANDLER eq forecastRvTResultHandler %THEN %DO;
/* ------------------------------------------------------------------Forecasting accuracy indicators calculation
------------------------------------------------------------------- */
/*L_PRED_CART*/
data Pred_output_L_CART (keep=Indicator AI_L_PRED_CART);
set Work.forecast_output_final end=eof;
Length Indicator $4;
if missing(Time)=0 then do;
retain Sum_E_L_PRED_CART Sum_E2_L_PRED_CART Sum_E3_L_PRED_CART
Sum_E4_L_PRED_CART n;
n+1;
Error = MEASURE - L_PRED_CART;
/* ------------------------------------------------------------------1- Mean Absolute Error (MAE)
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------------------------------------------------------------------- */
/* MAE */
E_L_PRED_CART = ABS(Error);
Sum_E_L_PRED_CART + E_L_PRED_CART;
/* ------------------------------------------------------------------2- Mean Absolute Percentage Error (MAPE)
------------------------------------------------------------------- */
/* MAPE */
if ABS(MEASURE) NE 0 then do;
E2_L_PRED_CART = E_L_PRED_CART / ABS(MEASURE);
end;
else E2_L_PRED_CART = E_L_PRED_CART;
Sum_E2_L_PRED_CART + E2_L_PRED_CART;
/* ------------------------------------------------------------------3- Percent Mean Absolute Deviation (PMAD)
------------------------------------------------------------------- */
/* PMAD */
E3_L_PRED_CART = ABS(MEASURE);
Sum_E3_L_PRED_CART + E3_L_PRED_CART;
/* ------------------------------------------------------------------4- Root Mean squared Error (RMSE)
------------------------------------------------------------------- */
/* MSE & RMSE */
E4_L_PRED_CART = (Error**2);
Sum_E4_L_PRED_CART + E4_L_PRED_CART;
end;
MODEL-BASED BOOTSTRAP
Saudi ARAMCO is seeking an improved methodology for probabilistic quantification of reserves estimates using
DCA. To evaluate the uncertainty in reserves estimates using a probabilistic approach, it is reasonable to provide a
distribution of reserves estimates with three confidence levels (P10, P50, and P90) and a corresponding 80%
confidence interval. You might ask: how reliable is this 80% confidence interval? Is the true value of reserves
contained within this interval 80% of the time? Using traditional statistical analyses, it is apparent that true values of
reserves estimates lie outside the 80% confidence interval much more than 20% of the time. Thus, you are
underestimating uncertainty, often significantly. The challenge then is to determine not only how to appropriately
characterize probabilistic properties of a complex production data set, but also how to improve the reliability of the
uncertainty quantifications. In order to preserve the data structure, Figure 3 employs a rigorous model-based
bootstrap algorithm. It uses the decline models (hyperbolic or exponential equations) to fit the production data and
constructs residuals from the fitted model and observed data. You then generate a new series by incorporating
random samples from the residuals into the fitted model. To consider correlation within the residuals and to preserve
data structure, you use a block resampling approach to generate residual realizations. To determine the size of the
blocks, you use the autocorrelation plot of residuals, which can help to detect the randomness or possible
correlations within residual data and confidence band that can help to detect significantly nonzero points out of the
band of a particular confidence level on the autocorrelation plot.
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Figure 3: Model-based Bootstrap Methodology
/* Estimating Linear Trend Model */
PROC AUTOREG data=WORK.Selection;
model y = t / nlag=2 method=ml
run;
/* Forecast each of the monthly time series */
PROC ESM data=WORK.Selection out=outesm print=forecasts;
id DATE interval=month;
forecast y;
run;
PROC MODEL data=WORK.Selection(where=(rate > 0));
%if &lin_Fix_QI eq Y %then %do;
parms a=0;
if &lin_Fixed_QI**2-2*production*a < 0 then
predicted_rate = 0;
else
predicted_rate = (&lin_Fixed_QI**2-2*production*a)**0.5;
outvars qi a;
%end; %else %if &lin_Fix_A eq Y %then %do;
parms qi=&qi;
bounds qi >= 0;
if (qi**2-2*production*&lin_Fixed_A < 0) then
predicted_rate = 0;
else
predicted_rate=(qi**22*production*&lin_Fixed_A)**0.5;
outvars qi;
%end; %else %do;
parms a=0 qi=&qi;
bounds qi >= 0;
if qi**2-2*production*a < 0 then
predicted_rate = 0;
else
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predicted_rate=(qi**2-2*production*a)**0.5;
outvars qi a;
%end;
rate=predicted_rate ;
fit rate / out = ModelEst;
run;
SEGMENTATION OF AN OILFIELD
Geologists, reservoir, and petroleum engineers and geophysicists attempt to breakdown the geo-scientific silos and
through collaborative methodologies aggregate the disparate data sets and determine ideal locations for field
reengineering in mature or brown fields.
The Mature Fields Evaluation consists of these three steps:

Implement graphical user interfaces in the preliminary stages (control and data validation) to aid the global field
analysis and diagnose the producing wells.

Transform the data (modeling, smoothing, extraction of production indicators) to make data conducive to the
multivariate analysis (MVA) techniques.

Use statistical clustering procedure to segment the field into different zones based on well profiling.
The goal of the analytical process is to understand the distribution of the Water Cut (Sw) values in relationship to the
cumulative liquid production across the field.
By profiling the individual wells according to certain criteria such as Water Cut, minimum Free Water Level distance
(FWL), cumulative liquid, well bore type within different time phases and incremental geographic regions, it is
possible to classify those wells and appreciate through the analytical indicators of similarity/dissimilarity a potential
segmentation of the field.
Geoscientists map this appreciation of the statistical results to identify the production mechanisms such as best
producers, depletion, pressure maintenance, and thus locate poorly drained zones and potentially lucrative field
reengineering tactics and strategies based on a more complete understanding of the distribution of water throughout
the field.
After normalizing the Water Cut curves with the cumulative liquid, the problem of finding similarities between wells
basically becomes finding a similarity between the water cut and cumulative production scatter plots.
There is a wide variety of distance and similarity measures available in the cluster analysis technique, but as the well
data are now in a coordinate form, it is appropriate to use a non-Euclidean distance for clustering, computing a
distance matrix by using the DISTANCE procedure.
You can then convert similarity measures to dissimilarities prior to running the PROC CLUSTER. You implement the
DISTANCE procedure to compute the Jaccard coefficient between each pair of wells. The Jaccard coefficient is
defined as the number of variables that are coded as 1 for both wells divided by the number of variables that are
coded as 1 for either or both wells. The CLUSTER procedure requires dissimilarity measures, and you then select the
DJACCARD coefficient.
Figure 4 illustrates the dissimilarity matrix. A small value means that two wells are similar with respect to their water
cut mark.
proc distance data=ARAMCO.M_ClusteringInput
(where= (TimeWindow='Window1'))
method=DJaccard absent=0 out=ARAMCO.M_DistanceMatW1;
var anominal(G1-G100) anominal(WellType) interval(DistFWL)
id WellBoreName;
run;
;
proc cluster data=ARAMCO.M_DistanceMatW1 method=Ward outtree=ARAMCO.M_treeW1;
id WellBoreName;
run;
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Figure 4: Dissimilarity Matrix Using Jaccard Metric
The hierarchical cluster analysis successively joins data points to form groups of similar behavior until all records are
joined in one group.
You can see in Figure 5 the default resulting graph of the hierarchical cluster analysis. It is similar to a tree structure
starting with the single records at the bottom and finishing with all records joined into one single group at the top.
This scales the graph based on the distances of the groups and makes dissimilarities more visible. The higher the
distance is to the next join (represented by the lines in the graphs that join previous groups), the larger the distance
between the groups and the more dissimilar the groups. You can draw conclusions from the dendrogram such as the
presence of two distinct groups in the data around the water cut and cumulative liquid behavior of the 43 wells that
constitute the cluster in time-window 1, thus generating two clusters. It is then possible to profile the two clusters to
identify the main characteristics of each group. You obtain the clusters by implementing the Ward algorithm for
minimum variance.
proc tree data=ARAMCO.M_treeW1
id WellBoreName;
run;
n=2 out=ARAMCO.M_outW1;
proc sort;
by WellBoreName;
run;
data ARAMCO.M_clusW1;
merge ARAMCO.M_ClusteringInput(where=(TimeWindow='Window1') ) ARAMCO.M_outW1;
by WellBoreName;
run;
proc sort;
by cluster;
run;
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Figure 5: Dendrogram of Wells in One Cluster
Type Wells
Engineers often use type well methodologies to predict production so as to determine the performance of future wells.
Although predicting production based on analog can be considerably uncertain, it might be the only method available
when specific information such as porosity, reservoir thickness, and saturations about a location is unknown. One of
the objectives is to provide a workflow that develops type wells based on unique statistically valid selection methods.
You can then predict the production performance in development planning using the type wells.
This application allows the automated grouping of wells based on multiple selection criteria (rates, water cut, etc.) into
Type Well classes. You then obtain a statistically based selection of the valid production for each type well grouping
and then develop a production forecast using DCA.
An important step in the process of DCA is the selection of production data used for the forecast. Besides the decline
of the reservoir, there can be technical or economic reasons to cut back production for a well or a group of wells. The
engineer must decide based on his or her knowledge which data to use for the forecast. An automated filtering and
selection of data can support the decision process.
Normally the reservoir engineer selects a time period where a constant production rate and decline is visible.
Sometimes the minimum number of valid data points of month-on-month production rates for a time series analysis
cannot be represented with the data available. An automated data selection supports the engineer to solve the
following two basic issues:
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1.
2.
replacing or excluding zero production rates
smoothing extreme variance in the data (for example, production cuts due to economic reasons, well tests)
While excluding zero production rates is straightforward, mathematics helps in the detection of outliers. In probability
theory and statistics, the standard deviation of a statistical population, a data set, or a probability distribution is the
square root of its variance. A data point is statistically considered an outlier if the value is more than two times the
standard deviation from the mean. These are possible automatic treatments of detected outliers in a given data
population:



filtering out of all records with outliers
filtering out records with zero production rates only
replacing all outliers with the mean or median production rate
The value of mean and median of the data selected has been calculated excluding zero production rates. That
ensures that the result of the mean and median calculation creates equal results with or without removing the zero
production rates first.
For time series data used for forecasting, the identity and order of the observations are crucial. A time series is a set
of observations made at a succession of equally spaced points in time. If records have been filtered out, this rule of
thumb has been violated. The simple solution to the problem is to keep the order of the observations and to adjust the
time ID of the records in the data population.
The system provides interpolation methods in its analytical software (PROC EXPAND) that could be used to
interpolate missing values or to generate lower frequency output from higher frequency data (for example, generating
quarterly estimates from monthly production data).
CONCLUSION
The Web-based DCA system’s current broad range of time series and forecasting capabilities enables users to
model, forecast, and simulate business processes for improved field redevelopment strategies and well management.
Users can model complex business scenarios and analyze the dynamic impact-specific events might have on the
lifespan and production of an asset, be that a field or an individual well.
The system provides an intelligent application environment in which data from unrelated systems can be gathered,
stored, analyzed, and distributed in a simple and timely manner. These technologies allow disparate systems to
contribute data and information to an integrated, enterprise-wide business intelligence strategy. This ensures that the
production data and all requisite reservoir data can be aggregated to provide a robust forecasting experience.
In the future, the addition of grid computing will offer a cost-effective solution for customers who want to accelerate
the forecasting process or increase the scale or scope (number of users, size of data sets, and the frequency of
analysis) of the DCA.
The flexibility of the DCA solution enables future access to data mining processes to create highly accurate predictive
and descriptive models based on vast amounts of data gathered from across an enterprise. It will also open up work
flows for interactive data analysis and visualization capabilities, broad data preparation features, and deep analytics,
thus enhancing a multi-faceted experience to field management and robust short- and long-term forecasts.
The system provides statistical tools that include the ability to automate the DCA process by working backwards
through the data. It can measure the compatibility of fit of the data and then select only the data that fits into one
production profile and do the decline on that data. This will allow us to compute declines and production forecasts for
every producing well in the database with marginal impact on manpower.
REFERENCES
1.
2.
3.
J.J. Arps, “Analysis of Decline Curves,” Trans., AIME 160 (1944): 228-247.
L. Kewen and N.H. Roland. 2003. “A Decline Curve Analysis Model Based on Fluid Flow Mechanisms.”
83470-MS. Proceeding of the SPE Western Regional/AAPG Pacific Section Joint Meeting. Society of
Petroleum Engineers.
SPEE. 2000. “SPEE Recommended Evaluation Practice #6 – Definition of Decline Curve Parameters.”
Available at http://www.spee.org/images/PDFs/ReferencesResources/REP06-DeclineCurves.pdf.
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ACKNOWLEDGMENTS
The author wishes to acknowledge the research and contributions of Gary M. Williamson, Dennis Seemann, and
Husameddin Madani, Saudi ARAMCO.
CONTACT INFORMATION
Your comments and questions are valued and encouraged. Contact the author:
Keith R. Holdaway
SAS Institute Inc.
C5188 SAS Campus Drive
Cary, NC, 27513
Work Phone: +1 919 531 2034
Fax: +1 919 531 2034
E-mail: keith.holdaway@sas.com
SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS
Institute Inc. in the USA and other countries. ® indicates USA registration.
Other brand and product names are trademarks of their respective companies.
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