RPSEA 

Feasibility Assessment of Early Flowback 

Water Recovery for Reuse in Subsequent 

Well Completions

Report No. 08122-05.07

Barnett and Appalachian Shale 

Water Management and Reuse 

Technologies

Contract 08122-05 

October 24, 2011

 

Principal Investigator Project Manager

L. Peter Galusky, Jr., Ph.D., P.E.

Principal Environmental Engineer 

Texerra

Monument, CO

Thomas D. Hayes, Ph.D.

Coordinator, Environmental 

Engineering Solutions

Gas Technology Institute

1700 S. Mount Prospect Rd. 

Des Plaines, IL
LEGAL NOTICE

This report was prepared by the Gas Technology Institute as an account of 

work sponsored by the Research Partnership to Secure Energy for 

America, RPSEA. Neither RPSEA members of RPSEA, the National Energy 

Technology Laboratory, the U.S. Department of Energy, nor any person 

acting on behalf of any of the entities:

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OR

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FOR ANY AND ALL DAMAGES RESULTING FROM THE USE OF, 

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DISCLOSED IN THIS DOCUMENT. 

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OR CONCLUSIONS REPORTED HEREIN SHOULD BE TREATED AS 

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OR CONSTIITUTE AND ENDORSEMENT, RECOMMENDATION, OR 

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COMMERCIAL PRODUCT, COMMODITY, OR SERVICE. 

Table of Contents

Abstract................................................................................................................................ 4

Introduction .......................................................................................................................... 6

Objectives and Scope .......................................................................................................... 7

Methods ............................................................................................................................... 7

Results and Discussion........................................................................................................ 8

Flowback Rates and Volumes .......................................................................................... 8

Flowback Chlorides and Total Dissolved Solids ............................................................. 14

Observed Relationships in Flowback Water Quality versus Flowback Quantity ............. 18

Optimizing Flowback Water Segregation for Potential Reuse ........................................ 22

Total Flowback Water Reuse – Possibilities and Constraints ......................................... 25

Conclusions........................................................................................................................ 29

Acknowledgments .............................................................................................................. 32

References......................................................................................................................... 32

Appendix ............................................................................................................................ 34

APPENDIX A – Individual Well Flowback Measurements and Model Estimates ............ 35

APPENDIX B– Flowback Chloride Concentrations vs Cumulative Frac water Recovery 

for Six Individual locations .............................................................................................. 48
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On the Reuse of Shale Gas Flowback Water

List of Figures

Figure 1: 1a (above) and 1b (below) – Daily flowback rates (Figure 1a) and accumulated 

daily volumes (Figure 1b) averaged among eleven Barnett Shale locations (black dots) and 

modeled using exponential (red lines) and hyperbolic (blue lines) functions...................... 12

Figure 2: (above) and 2b (below) – Modeled flowback rates (Figure 1a) and accumulated 

volumes (Figure 1b) estimated for eleven averaged Barnett Shale locations. Error bars 

indicate the magnitudes of +/- one standard deviations among the eleven locations......... 13

Figure 3: Flowback chloride concentration versus cumulative fraction of frac water 

recovered, averaged among six locations in the Barnett Shale.......................................... 14

Figure 4: TDS versus chloride from four Shale Barnett locations...................................... 15

Figure 5: TDS versus specific (electrical) conductance from four Shale Barnett locations.16

Figure 6: TDS concentrations shown with the drop in flowback rate, calculated for a typical 

well by modeling flowback rate using hyperbolic decline with parameters averaged among 

eleven Barnett locations. .................................................................................................... 17

Figure 7: The rate of change in TDS accumulation shown with the drop in flowback rate, 

calculated for a typical well by modeling flowback rate using hyperbolic decline with 

parameters averaged among eleven Barnett locations. ..................................................... 18

Figure 8: 8a & 8b– Flowback TDS concentrations (projected based on the average pattern 

observed from six locations) versus flowback day (8a, above) and cumulative percent frac 

water recovery (8b, below). ................................................................................................ 20

Figure 9: Cumulative mass of recovered TDS (in flowback) versus cumulative flowback 

water mass......................................................................................................................... 21

Figure 10: Potential direct reuse of flowback as a function of maximum TDS threshold. 

These projections are based on a well completed with 100,000 bbls of frac water flowing 

back at an initial rate of 2,000 bbls/d and yielding 40,000 bbls of recovered frac water as 

flowback. ............................................................................................................................ 22

Figure 11: Volume-weighted flowback dilution factors to meet specified TDS criteria in 

relation to the cumulative fraction of frac water recovered. ................................................ 24
iii

On the Reuse of Shale Gas Flowback Water

Figure 12: Cumulative flowback dilution volumes required to meet specified TDS criteria in 

relation to the cumulative volume of frac water recovered. ................................................ 24

Figure 13: Per-well modeled flowback rate and cumulative volume.................................. 26

Figure 14: Time course of per well modeled TDS and chloride concentrations................. 26

Figure 15: a (above) and b (below) - Summary of key metrics for flowback reuse for a 

hypothetical scenario in the Barnett Shale. ........................................................................ 28
Feasibility Assessment of Early Shale Gas Flowback Water 

Recovery for Reuse in Subsequent Well Completions

Abstract

Fresh water is critical for natural gas production in the Barnett Shale, where in excess of 3 

to 5 million gallons are required for the completion of each gas well. The financial costs, 

logistical challenges and public relations concerns over the use of fresh water supplies thus 

impel Barnett gas producers to use less fresh water and to use it more efficiently. The 

objectives of this work were to develop a simple, empirical modeling approach for 

characterizing and forecasting flowback water rates and total dissolved solids (TDS) and to 

use such models to characterize the efficacy of flowback water recycling based on 

predefined TDS reuse criteria. 

Approximately one third of the fracwater used for a typical Barnett Shale well is recovered 

from the initiation of blowback until the time that the well produces salable volumes of 

natural gas. Flowback water production could be efficiently described and modeled using 

either an exponential or hyperbolic equation; (these are commonly used in forecasting 

petroleum and natural gas production). Both models were found to be particularly sensitive 

to the initial flowback rate, which unfortunately tends to be rather erratic during the first few 

days of flowback production. It is suggested, therefore, that the “initial” value used not 

necessarily be that of the first day of flowback, but may be averaged among the first three 

to five days or otherwise adjusted so that the resulting flowback rate and production curves 

follow a reasonable course. 

Flowback chloride and TDS concentrations were efficiently described by a power function 

of cumulative flowback. The rate of change in flowback TDS concentration reflects the sum 

effect of exponential (or hyperbolic) decline in flowback rate and the increasing TDS 

concentration as flowback waters accumulate. Initially TDS rises rapidly because of high 

flowback rates and rising TDS concentrations. However, although TDS concentrations 
5

On the Reuse of Shale Gas Flowback Water

continue to rise, the precipitous drop in flowback caused TDS accumulation to slow and its 

rate of change to then begin to drop. The net result is that TDS concentrations begin to 

level off more or less in parallel with flowback accumulation. 

In analyzing flowback water production volumes and TDS concentration using these 

models for a hypothetical location, it was found that at the field scale that a substantial 

portion of recovered flowback could be recycled for subsequent well completions across a 

wide range of TDS reuse thresholds. This finding stems from the observations that: 1.)

only a portion (roughly a third) of the fracwater used to complete a well is typically 

recovered as flowback during the first several weeks following well completion, and 2.) The 

volume –weighted TDS concentration is low enough so that the recovered flowback can be 

diluted for effective reuse. Thus, the substantial recovery of a useful fraction of flowback 

water for reuse is technically feasible and logistically achievable. 

The primary benefits of recycling flowback water include a potentially substantial reduction 

in the volume of freshwater needed to complete future wells as well as a concomitant 

reduction in the volume of wastewater that must otherwise be disposed. 
6

On the Reuse of Shale Gas Flowback Water

Introduction

The Barnett Shale of the Fort Worth Basin is presently the largest producing natural gas 

field in Texas and the second largest in the United States. The field covers 20+ counties in 

North Texas, including the Fort Worth metropolitan area. As of May, 2011 there were more 

than 15,000 wells in the field producing nearly 5.9 billion cubic feet (BCF) per day or 

roughly 10% of U.S. natural gas production (TXRRC, 2011; US EIA 2011). The Barnett 

Shale has produced more than 10 trillion cubic feet (TCF) of natural gas thus far with peak 

gas production likely to occur over an extended period of time due to increasingly efficient 

well drilling and completion technologies (TXRRC, 2011; Powell, 2011). The Barnett Shale 

is a major economic driver on the state and local levels generating billions of dollars a year 

in economic output, including expenditures for development activities, salaries, state and 

local taxes, lease bonus and royalty payments and related economic benefits. 

Fresh water is critical for natural gas production in the Barnett Shale where in excess of 3 

to 5 million gallons of “fracwater” are required for the completion of each gas well. The 

financial costs, logistical challenges and public relations concerns over the use of fresh 

water supplies impel Barnett gas producers to use fresh water efficiently. Approximately 

90+% of the fresh water used in Barnett gas well completion is for the fracturing of the 

producing shale formation, to liberate natural gas so that it can be produced and sold. 

Approximately a third of the fracwater used to complete a new well is returned as 

“flowback”, at least some of which could potentially be recycled for use in subsequent well 

completions. If the amount of fresh water necessary for completing new wells can be 

reduced, the total fresh water demand required to develop the Barnett Shale will also be 

reduced. . The recycling of this “flow back” water for the completion of subsequent gas 

wells may ultimately provide a means to substantially decrease fresh water demands of gas 

operators, to reduce the volume of spent flowback that needs to be disposed and to 

concomitantly reduce the cost of drilling and completing new wells. 
7

On the Reuse of Shale Gas Flowback Water

Objectives and Scope

The objectives of this work were:

1. To develop a simple, empirical modeling approach for characterizing and forecasting 

flowback rates and volumes.

2. To characterize the relationship between flowback water recovery and total 

dissolved solids (TDS) concentration and from this develop a simple, empirical 

modeling approach for charactering and forecasting flowback TDS.

3. To develop a simple, empirical modeling approach for characterizing and estimating 

the efficacy of flowback water recycling based on pre-defined TDS reuse criteria. 

The scope of this project encompassed an evaluation of flowback data from eleven 

producing wells believed to be generally representative of the Barnett Shale of Texas. This 

work may also be found to be more or less representative of other shale gas plays in North 

America. 

Methods

Flowback water measurements and samples were taken from eleven producing natural gas 

wells in the Barnett Shale, made available by companies participating in the study. 

Flowback quantity data were measured in the field by gauging the daily accumulation of 

water in flowback tanks. Flowback quality parameters that were measured in the field 

included: chloride concentration (using a field site glass refractometer) and specific gravity 

(using a field hydrometer). 

Water samples that were collected from each location were analyzed in a commercial 

laboratory for total dissolved solids (TDS), chlorides and specific conductance. 
8

On the Reuse of Shale Gas Flowback Water

Results and Discussion

The sections to follow describe the characteristics of the flowback water with time following 

a hydraulic fracturing event in terms of volumes collected and salinity, factors that weigh 

most heavily on the logistics and economics of water management. Also described is the 

use of the flow and salt composition information base to develop models that describe the 

mass of water and salt that emerge from a completed well over the weeks following a frac 

job event. The model is then used to estimate the benefits and assess the general 

feasibility of early flowback water capture to provide a water stream that can be reused with 

minimal or no processing required for demineralization. Capture of early flowback water 

can be accomplished with an automated system consisting of in-line water flow and 

conductance measurements, real time data acquisition and analysis, and computer control 

of solenoid valves that allow the diversion of flowback water to separate storage for 

moderate and high concentration brines. A design concept for an automated system that 

captures early flowback water of low-moderate salt content separated from flowback and 

produced waters of high total dissolved solids is presented and discussed in a companion 

report submitted to RPSEA (Galusky, L.P. 2011. Report 08122-06). This report builds on 

the previous report and presents an engineering analysis of actual field data to assess the 

feasibility of early flowback water capture. 

Flowback Rates and Volumes

A substantial fraction (25 to 40+ %) of the 100+ thousand bbls of water (“fracwater”) that is 

used to create secondary porosity and to thus stimulate shale gas production returns to the 

surface under pressure during the first several weeks after well completion as “flowback” 

water. The rate of flowback is typically very high (greater than 1,000 bbls/d) during the first 

few days but drops off exponentially to very low levels (less than 100 +/- bbls/d) after 

several days. Flowback was measured at eleven locations in the Barnett Shale to 

determine general flowback trends and patterns and to determine if exponential and/or 

hyperbolic equations could be used to model flowback rate and cumulative production. 
9

On the Reuse of Shale Gas Flowback Water

It was found that Barnett Shale flowback water generation follows a generally exponential 

(and in some cases, hyperbolic) pattern as shown for the average values of flowback from 

eleven sites (Figure 1). In this graph the average daily values of flowback rates and 

cumulative volumes are given along with exponential and hyperbolic curves whose 

parameters were adjusted to provide the best fit to the data. Given in Figure 2 are daily 

flowback rates and cumulative volumes as estimated by the hyperbolic function on which 

are overlaid the standard deviations (of the variation between sites) computed from the 

eleven locations. Graphs showing measured flowback rates and accumulation for each of 

the eleven sites are given in Appendix A, along with a compilation of the respective best-fit 

model parameters used for each location (Table A-1).

Both equations do a good job of fitting to the data and can thus presumably be used for 

projecting flowback rates and volumes provided that accurate values are used for their 

respective input parameters. Exponential and hyperbolic equations and their respective 

input parameters are given below: 

Exponential Functions

This equation estimates daily flowback production (bbls/d):

q = qie

−Dt

This equation estimates cumulative daily flowback production (bbls):

V =(qi –q)/ D

Where:

q = daily flowback rate (bbls/d)

qi = initial flowback rate (bbls/d)

D = exponential decline exponent (day-1)

t = flowback day

V = cumulative flowback volume (bbls)
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On the Reuse of Shale Gas Flowback Water

Hyperbolic Functions

This equation estimates daily flowback production (bbls/d).

q = q(1+bDit)-(1/b)

This equation estimates cumulative daily flowback production (bbls):

V = (qib/(D(1-b))*(qi

(1-b)

- q

(1-b)) 

Where (these parameters being the same as for the exponential function):

q = daily flowback rate (bbls/d)

qi = initial flowback rate (bbls/d)

D = exponential decline exponent (day-1)

t = flowback day

V = cumulative flowback volume (bbls)

And (this parameter being unique to the hyperbolic functions):

b = hyperbolic function exponent (fraction <= 1)

The potential advantage of the hyperbolic equation is that it allows for an increasingly more 

gradual rate of flowback decline over time, and this model appears to be a better fit for 

many of the Barnett Shale analyzed sites in this study. 

It is easy enough to fit one of these equations to accumulated data once the general 

pattern is apparent. It is another matter to use them to forecast future flowback when the 

input parameters are unknown but must be estimated. However, as with using 

mathematical models for projecting any unknown future quantity, parameter values can be 

estimated using surrogate data from representative locations and adjusted early on by 

modifying their values based on real-time flowback performance.
11

On the Reuse of Shale Gas Flowback Water

In this regard it should be noted that both models are particularly sensitive to the initial 

flowback rate, qi, and unfortunately the initial flowback rates during the first few days are 

highly erratic. It is suggested, therefore, that the “initial” value that is used represent an 

average of the first three to five days or otherwise adjusted so that the resulting flowback 

rate and production curves follow a reasonable course. 

In reviewing the graphs for the eleven individual sites in the Appendix it is noteworthy that 

for many sites flowback was interrupted for one or more extended periods. Such 

seemingly random interruptions pose obvious difficulties in using predictive models. 

Nevertheless, if the purpose of modeling flowback is primarily to assist in the better 

management of these waters then it becomes a matter of operational judgment as to 

whether it is better to predict high or low. The overestimation of flowback will ensure that 

enough operational infrastructure and capacity are available, but this needs to be balanced 

against the financial cost of doing this. Extensive practice with using mathematical 

models to predict flowback behavior in a given area will enable engineers to use them 

wisely. There is no substitute for such experience nor can any mathematical model or 

software product compensate for its lack. 
12

On the Reuse of Shale Gas Flowback Water

Figure 1: 1a (above) and 1b (below) – Daily flowback rates (Figure 1a) and accumulated daily volumes (Figure 1b) 

averaged among eleven Barnett Shale locations (black dots) and modeled using exponential (red lines) and hyperbolic 

(blue lines) functions.
13

On the Reuse of Shale Gas Flowback Water

Figure 2: (above) and 2b (below) – Modeled flowback rates (Figure 1a) and accumulated volumes (Figure 1b) estimated 

for eleven averaged Barnett Shale locations. Error bars indicate the magnitudes of +/- one standard deviations among 

the eleven locations.
14

On the Reuse of Shale Gas Flowback Water

Flowback Chlorides and Total Dissolved Solids

Chlorides and total dissolved solids (TDS) are two parameters that alone or together 

provide a simple indication of the ionic strength of shale flowback waters. As flowback 

water is recovered both parameters generally increase from relatively negligible 

concentrations in fresh influent frac water to levels commonly found in oil field brines. This 

is illustrated in Figure 3, where the chloride concentrations of frac water are averaged 

among six sites in the study area. The relationships between flowback chloride 

concentrations versus cumulative percent of flowback recovery are shown for individual 

sites in Appendix B. 

Figure 3: Flowback chloride concentration versus cumulative fraction of frac water recovered, averaged among six 

locations in the Barnett Shale.
15

On the Reuse of Shale Gas Flowback Water

It is not immediately straightforward to directly measure TDS concentrations in the field. 

However, chlorides and electrical conductivity, both of which can be accurately and reliably 

measured in the field using simple instruments, provided excellent surrogate measures for 

TDS as shown by their excellent linear relationships with TDS (Figure 4 and Figure 5). 

Figure 4: TDS versus chloride from four Shale Barnett locations.
16

On the Reuse of Shale Gas Flowback Water

Figure 5: TDS versus specific (electrical) conductance from four Shale Barnett locations.

Flowback chloride concentrations and electrical conductivity can both be readily measured 

in the field using a hand-held optical refractometer or an electronic chloride probe (the latter 

being similar in size and operation to a pH meter). Electrical conductance can be similarly 

measured using a hand-held instrument. 

TDS concentrations rise asymptotically as flowback rate declines exponentially (Figure 6). 

In contrast, the rate of accumulation of TDS mass initially rises rapidly because of high 

flowback rates and rising TDS concentrations but then declines due to the drop-off in 

flowback (Figure 7). 

Taken together these analyses indicate that the opportunity for identifying and segregating 

relatively low TDS flowback waters is limited to the first few days in the flowback process. 
17

On the Reuse of Shale Gas Flowback Water

Figure 6: TDS concentrations shown with the drop in flowback rate, calculated for a typical well by modeling flowback 

rate using hyperbolic decline with parameters averaged among eleven Barnett locations.
18

On the Reuse of Shale Gas Flowback Water

Figure 7: The rate of change in TDS accumulation shown with the drop in flowback rate, calculated for a typical well by 

modeling flowback rate using hyperbolic decline with parameters averaged among eleven Barnett locations. 

Observed Relationships in Flowback Water Quality versus Flowback Quantity 

On first consideration it would seem that the present practice of disposing flowback waters

from Barnett Shale wells is simply wasteful, that these waters could be collected and 

readily recycled for use in subsequent frac jobs. There are, however, substantial 

concerns, which have made completion engineers reluctant to reuse frac water in the 

Barnett Shale. TDS increases the downhole friction of the frac water with the target 

formation causing a substantial increase in the compressive horsepower required to push 

the frac water into the shale rock. There are also concerns over scale formation in the well 

bore and shale rock as well as other potential geochemical incompatibilities in high TDS 

waters with the producing formation. The safe handling and transport of large volumes of 

these highly saline waters from site to site poses additional risks and logistical challenges 

of its own. 
19

On the Reuse of Shale Gas Flowback Water

Nevertheless, and although the use of freshwater for well drilling and completion represents 

a relatively small fraction of the available supply (Galusky, 2007 & 2009), Barnett operators 

are concerned about optimizing the use of this valuable resource and in reducing costs 

wherever possible. Since the costs of acquiring, storing and disposing of frac water and 

flowback water are considerable there is a clear economic incentive for operators to 

consider the reuse of flowback water for subsequent frac jobs wherever this is logistically 

and economically feasible. In this regard it is relevant to mention that most of the frac 

water in the Marcellus shale play is now recycled (Rassenfoss, 2011).

Approximately one third of the frac water used for a typical Barnett Shale well is recovered 

from the initiation of flowback until the time that the well produces salable volumes of 

natural gas, typically an interval of several weeks. After this point, any further flowback that 

occurs is comingled with produced formation water (particularly if the underlying water 

bearing rock is penetrated during the frac job) such that the ultimate volume of water that is 

recovered may approach or even be greater than that used to originally frac the well. In 

any case, the maximum potential for reducing freshwater use to complete new wells will not 

exceed the quantity of water produced by the wells that have previously been completed. 

In order to consider and to plan for the potential reuse of flowback waters it is necessary to 

understand the relationships between flowback water quality (which will be considered here 

simply as TDS concentration) and flowback rates and cumulative volumes. It was found 

that flowback chloride concentration was a power function of cumulative flowback (Figure 

3) and that TDS was linearly related to chloride concentration (Figure 4). Thus the 

relationship between TDS and flowback may also be described by a power function similar 

to that of chlorides versus flowback (Figure 8 a & b and Figure 9). 
20

On the Reuse of Shale Gas Flowback Water

Figure 8: 8a & 8b– Flowback TDS concentrations (projected based on the average pattern observed from six locations) 

versus flowback day (8a, above) and cumulative percent frac water recovery (8b, below). 
21

On the Reuse of Shale Gas Flowback Water

Figure 9: Cumulative mass of recovered TDS (in flowback) versus cumulative flowback water mass. 

Water sampled from the flowback pipe has a higher TDS than the (calculated) cumulative 

volume (Figure 8 a & b). This is because TDS from the pipe was always rising even as 

flowback rates were falling over time (Figure 6). In reality, flowback is typically not mixed in 

a single reservoir but is collected in a series of frac tanks. This potentially provides a 

convenient basis for segregating flowback according to TDS for reuse in subsequent frac 

jobs, which will be discussed subsequently. 

TDS mass accumulates more quickly, and at an accelerating pace, compared to the mass 

of recovered flowback (Figure 9). This simply reflects the increasing concentration of 

flowback TDS as the rate of flowback recovery decreases over time. 

It should be noted that flowback waters become saline early on, exceeding 20,000 mg/l 

TDS during the first day.
22

On the Reuse of Shale Gas Flowback Water

Optimizing Flowback Water Segregation for Potential Reuse

Although all flowback waters could potentially be reused if they were diluted to the requisite 

water quality criteria, of immediate interest is the fraction of flowback that could be reused 

directly without dilution or other pre-treatment. It stands to reason that the more lenient 

the water quality requirements are the greater the proportion of flowback that could be 

reused directly. This is illustrated in Figure 10, where TDS is used as the measure of 

acceptable limits for flowback water reuse. Summarized briefly, if completions engineers 

can accommodate the use of high TDS flowback then most (potentially all) of it can be 

directly reused for subsequent well completions. 

Figure 10: Potential direct reuse of flowback as a function of maximum TDS threshold. These projections are based on 

a well completed with 100,000 bbls of frac water flowing back at an initial rate of 2,000 bbls/d and yielding 40,000 bbls of 

recovered frac water as flowback.
23

On the Reuse of Shale Gas Flowback Water

Dilution to achieve a desired TDS concentration is not the only criterion on which frac water 

reuse will be considered. There are other criteria, particularly suspended solids, floc-iron 

and the like which may impose additional constraints and/or require varying degrees of pretreatment before frac water can be reused. 

If the maximum TDS concentration that can be accommodated for new well completions is 

less than the highest flowback TDS concentration for a given well, then only some fraction 

of that flowback may be used directly (pre-treatment for solids and other constituents 

notwithstanding) for subsequent well completions. The question then turns to the level of 

dilution required for flowback reuse once a defined TDS reuse threshold is exceeded. 

Flowback dilution factors are presented in Figure 11, which indicate the relative magnitude 

of flowback dilution required to meet specified TDS criteria at specific points of flowback 

recovery1. (A dilution factor of 1.0 indicates that no dilution is required. A dilution factor of 

2.0 indicates that one volume of flowback must be diluted with one volume of freshwater). 

The most stringent TDS requirement (20,000 mg/l) imposes a substantially greater degree 

of dilution than higher TDS thresholds and this continues to increase as flowback is 

recovered. The 40,000 mg/l TDS threshold requires no dilution (indicated by the dilution 

factor of 1.0) until approximately 10% of the frac water is recovered. The highest TDS 

thresholds (80,000 and 100,000 TDS) retain a dilution factor of 1.0 (meaning no dilution is 

required for flowback reuse) throughout the range of frac water recoveries because the 

volume-weighted TDS concentration of the flowback is less than 80,000 mg/l (Figure 8b). 

Using these dilution factors, the cumulative volume of freshwater required to effect 

specified TDS given the cumulative quantity of flowback recovered is given in Figure 12. 

Whereas approximately 90,000 bbls of freshwater would be required to dilute 40,000 bbls 

of flowback to 20,000 TDS only approximately 25,000 bbls of freshwater would be required 

to achieve a TDS threshold of 40,000 mg/l. 

 

1

These projections follow from the patterns illustrated in Figures 6 through 8 and are based on a hypothetical 

well completed with 100,000 bbls of frac water flowing back at an initial rate of 2,000 bbls/d and exhibiting 

TDS versus flowback characteristics averaged from six Barnett Shale locations.
24

On the Reuse of Shale Gas Flowback Water

Figure 11: Volume-weighted flowback dilution factors to meet specified TDS criteria in relation to the cumulative fraction 

of frac water recovered.

Figure 12: Cumulative flowback dilution volumes required to meet specified TDS criteria in relation to the cumulative 

volume of frac water recovered.
25

On the Reuse of Shale Gas Flowback Water

These projections are based on a well completed with 100,000 bbls of frac water flowing 

back at an initial rate of 2,000 bbls/d and yielding 40% (40,000 bbls) of recovered frac 

water as flowback. 

If logistics and cost were not an issue it would be reasonable to simple ask as to why we 

would not simply recover, dilute and reuse all of the flowback. The constraints to this are 

addressed in the next section. 

Total Flowback Water Reuse – Possibilities and Constraints

In the following discussion we present an analysis of projected (modeled) flowback reuse 

statistics considered at the field scale for a hypothetical drilling project consisting of 25 

wells. In this exercise we applied the hyperbolic model of flowback recovery basing this on 

an initial flowback recovery rate of 2,000 bbls/day, a daily exponential decline exponent of 

0.06/day and a hyperbolic coefficient of 0.5/day. We applied the same TDS versus 

cumulative flowback relationship used previously. We assumed that each well will use

100,000 bbls for well completion. The per-well modeled daily and cumulative flowback are 

given in Figure 13 and the time course of TDS and Cl- concentrations in Figure 14. 
26

On the Reuse of Shale Gas Flowback Water

Figure 13: Per-well modeled flowback rate and cumulative volume.

Figure 14: Time course of per well modeled TDS and chloride concentrations.
27

On the Reuse of Shale Gas Flowback Water

We first consider the total volume of frac water needed (and able to be accommodated) for 

the field, which is 25 wells * 100,000 bbls/d = 2,500,000 bbls . This represents the 

maximum amount of water that can be injected down-hole for use in fracturing and 

completing the 25 wells in the field. Any water above this volume produced in the field 

must, for the purposes our discussion, be treated and disposed as waste. We now 

consider the relationships between key flowback statistics and the TDS threshold for in-field 

reuse, which are summarized in Figure 15a and Figure 15b. 

The total recovered flowback volume in our hypothetical example is approximately 

1,000,000 bbls, which represents 40% of the total volume of frac water used to complete 

the twenty-five wells in the field. The maximum projected flowback TDS concentration is 

approximately 65,000 mg/l. Therefore, if the TDS threshold criterion is above this value all 

of the flowback water can be reused (since its total volume is less than that which is 

needed to frac all of the wells). However, if the TDS threshold for reuse is too low, say 

20,000 mg/kg, then it becomes impossible to reuse all of the flowback water since the 

combined volumes of the flowback (1,000,000 bbls) and the water needed for its dilution 

(2,000,000 bbls) exceed the total volume of frac water (2,500,000 bbls) needed to complete 

all of the wells in the field. 

In our hypothetical example, the fraction of flowback that can be recycled reaches 100% 

above a TDS threshold of approximately 40,000 mg/l. Above this level less the combined 

volumes of recovered flowback and freshwater needed for dilution is less than the total 

volume needed to complete all of the wells in the field ... thus all of the flowback (1,000,000 

bbls in this case) is potentially recycled. The difference (1,500,000 bbls) between the 

recycled flowback (1,000,000 bbls) and the total volume of water needed to complete the 

field (2,500,000 bbls) 
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On the Reuse of Shale Gas Flowback Water

Figure 15: a (above) and b (below) - Summary of key metrics for flowback reuse for a hypothetical scenario in the Barnett 

Shale. 
29

On the Reuse of Shale Gas Flowback Water

must be provided from other sources, this being indicted as “make-up frac water” in Figure 

15a. Nevertheless, the potential for reducing freshwater demand for completing wells in 

the field rises from approximately 20% at a TDS threshold of 20,000 mg/l to 40% at a TDS 

reuse threshold of 40,000 mg/l or higher. 

Conclusions 

Patterns of water and salt generation in the period following hydraulic fracturing of a shale

gas well indicate a large fraction (>50%) of the flowback water is likely to be generated in

the first weeks following the frac job event; salt concentrations in the water stream

increase steadily from below 20,000 mg/l to high concentrations over a period of months to

a level above 60,000 mg/l. There may be considerable variation in these characteristics

from site to site, but the general pattern of decreased flow and increased salt concentration

with time is generally observed in a preponderance of cases. This presents the opportunity

of collecting the initial fractions of flowback water that have low concentrations of salts that

can be reused with minimal need for demineralization. In this circumstance of water

management, the early flowback water can be taken directly to the next shale gas well for

blending with freshwater for subsequent use in the next completion. The remainder of the

water emerging from the original shale gas well can be demineralized for recovery of

additional water for reuse – or the more concentrated flowback water can be disposed of in

a Class II well (which is a common disposal practice in the Barnett Region).

Considering only TDS dilution as the sole limiting factor, and leaving logistical and 

economic considerations aside, it should be theoretically possible to recycle a substantial 

portion of flowback waters from the Barnett Shale, assuming that the limited data upon 

which this study and modeling exercise were based is generally representative of the field. 

The primary reasons that this is possible are: 1- only a portion (25-40%) of the frac water 

used to complete a well is typically recovered as flowback during the first several weeks 

following well completion, and 2 – The volume-weighted TDS concentration is low enough 

so that the recovered flowback can be diluted for effective reuse. The primary benefits of 
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On the Reuse of Shale Gas Flowback Water

recycling flowback water is a potentially substantial reduction in the volume of freshwater 

needed to complete future wells as well as a concomitant reduction in the volume of 

wastewater that must otherwise be disposed. 

The modeling results of this report indicate that, in many cases, the collection of early

flowback water can be an effective approach to obtaining relatively low-to-moderate 

strength flowback water for reuse with minimal or no processing for demineralization. 

Mathematical models, like the one described in this report, can be employed to predict the 

benefits of early flowback water capture prior to determine potential benefits of the 

approach before implementing such systems.

This study is not intended to be definitive for the purpose of demonstrating technical 

feasibility for any given project, but is offered as an approach for considering flowback 

water recycling based on site-specific data. We did not address here the considerable 

technical and logistical difficulties of segregating, collecting and transporting high TDS 

flowback waters. Nor did we address regulatory permitting or economic factors. Nor did 

we consider potential geochemistry problems associated with the reuse of high TDS 

flowback waters. However, the modeling indicates that the level of potential benefits of 

early flowback water recovery may result in reducing fresh water demand by 5-10% or 

more for shale gas completions with minimal cost required for water demineralization. 

Recovery of relatively low salt content flowback water can be achieved using readily 

available equipment comprising a separation system that is capable of realtime, in-line 

measurement of conductance and flow controls to achieve the capture of early, low-salinity 

flowback water; the conceptual design for this automated early flowback water system is 

described in a previous report submitted to RPSEA (Galusky, 2011).

The information developed in this report suggests that in many shale gas plays it may be

worthwhile for drilling companies to take a fresh look at early flowback water capture for the

purpose of recycling flowback waters. Preliminary modeling to forecast potential water

conservation benefits could utilize readily available well field design data and cost

information. Further, although the total water use by Barnett shale operators has been

demonstrated to be small relative to other users (Galusky, 2007, 2009), the development of
31

On the Reuse of Shale Gas Flowback Water

pro-active water conservation measures would likely engender good will with industry

stakeholders in the face of the current and extensive drought presently gripping the State of

Texas.

Sealed

L. Peter Galusky, Jr. Ph.D., P.G., P.E.
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On the Reuse of Shale Gas Flowback Water

Acknowledgments

The author would like to thank the Gas Technology Institute and the Barnett Shale Water 

Conservation and Management Committee, whose member companies provided the fracwater 

samples analyzed for this report.

Laboratory analyses were performed by Test America, Inc. Test America Pittsburgh 301 Alpha 

Drive RIDC Park Pittsburgh, PA 15238. Tel: (412) 963-7058 Fax: (412) 963-2468 

www.testamericainc.com .

References

Galusky, L.P. 2007. Fort Worth Basin/Barnett Shale Natural Gas Play: An Assessment of Present 

and Projected Fresh Water Use. 

http://www.barnettshalewater.org/documents/Barnett_Shale_Regional_Assessment_of_Water_Use 

_Apr_3_2007.pdf

Galusky, L.P. 2009. An Update and Prognosis on the Use of Fresh Water Resources in the 

Development of Fort Worth Basin Barnett Shale Natural Gas Reserves. 

http://www.barnettshalewater.org/documents/Barnett_hydro_update_Nov_4_2010.pdf

Galusky, L.P. 2011. Feasibility and Design Approach for Automated Classification and Segregation 

of Early Flowback Water for Reuse in Shale Gas Hydraulic Fracturing. RPSEA-NETL Report No. 

08122-05. 

Powell, 2011. Powell Shale Digest. August 29th, 2011 issue. 

http://www.shaledigest.com/newsletter/newsletter.php?accesscheck=%2Fnewsletter%2Fnewsletter.

php

Rassenfoss, S. 2011. From Flowback to Fracturing: Water Recycling Grows in the Marcellus 

Shale. JPT Online July, 2011. http://www.spe.org/jpt/print/archives/2011/07/12Marcellus.pdf
33

On the Reuse of Shale Gas Flowback Water

Texas Railroad Commission, 2011. http://www.rrc.state.tx.us/barnettshale/index.php

US Energy Information Administration. July, 2011. Natural Gas Monthly. 

http://205.254.135.24/oil_gas/natural_gas/data_publications/natural_gas_monthly/ngm.html
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On the Reuse of Shale Gas Flowback Water

Appendix
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On the Reuse of Shale Gas Flowback Water

APPENDIX A – Individual Well Flowback Measurements and Model Estimates
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On the Reuse of Shale Gas Flowback Water
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Table A-1: Best-fit model parameter estimates for exponential and hyperbolic flowback equations

Site ID Initial Flowback 

Rate (bbls/d)

Exponential 

Function 

Exponent (day-1)

Hyperbolic 

Function 

Exponent

lpg1 2,500 4.0 % 0.30

lpg2 1,200 5.0 % 0.10

lpg3 2,000 5.0 % 0.20

lpg4 1,000 6.0 % 0.20

lpg5 1,250 3.0 % 0.20

lpg6 1,250 4.0 % 0.60

lpg7 3,000 10.0% 0.10

lpg8 2,500 10,0% 0.40

lpg9 2,250 10,0% 0.50

lpg10 2,500 5.5% 0.25

lpg11 1,000 3.0% 0.60

average 1,859 5.1% 0.31

std deviation 732 2.1% 0.18
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On the Reuse of Shale Gas Flowback Water

APPENDIX B– Flowback Chloride Concentrations2vs Cumulative Frac water 

Recovery for Six Individual locations

 

2

Flowback chloride concentrations are the “instantaneous” values from the pipe. (They are 

not the chloride concentrations in the “mixed”, cumulating flowback tanks). 
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On the Reuse of Shale Gas Flowback Water
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