RPSEA 

Feasibility and Design Approach for 

Automated Classification and Segregation 

of Early Flowback Water for Reuse in 

Shale-Gas Hydraulic Fracturing

Report No. 08122-05.06

Barnett and Appalachian Shale 

Water Management and Reuse 

Technologies

Contract 08122-05 

July 21, 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
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Table of Contents Page

Introduction 1

Scope and Objectives 2

Methods 3

Results and Discussion 3

Summary 9

Potential Application 10

Conclusions 13

Acknowledgements 14
Figures Page

Figure 1 – Daily Course of flowback water quantity over time 4

Figure 2 – Cumulative flowback volume recovered from the onset of flowback 5

Figure 3 – Cumulative fraction of fracwater recovered as flowback over time 5

Figure 4 – Field-measured chloride concentration of recovered flowback water 

over time 6

Figure 5 – Cumulative chloride mass recovered in flowback water over time 7

Figure 6 – Relationship between laboratory measured total dissolved solids and 

chloride concentrations for four producing shale gas wells 8

Figure 7 – Estimated mass of total dissolved solids (TDS) recovered in flowback over 

time, calculated according to the linear regression relationship given in F 6 9

Figure 8 – Relationship between total dissolved solids (TDS) and 

specific conductance 10

Figure 9 – Simple, schematic layout of a hypothetical flowback measurement, 

separation and segregation system 12
Feasibility and Design Approach for Automated 

Classification and Segregation of Early Flowback 

Water for Reuse in Shale-Gas Hydraulic Fracturing

Abstract

Large volumes of water (1-6 million gallons containing chemical additives, friction 

reducer and sand propant) are required for the completion of a typical shale gas well. 

Approximately 10 to 50 percent of this “fracwater” is produced as “flowback” water 

during the initial weeks of fluid production. The rate of flowback water production is 

usually highest in the first day and rapidly decreases with time to a very low flow within 

a month to several weeks of a well completion. Shale gas flowback water contains 

various inorganic salts that increase in concentration over time from the onset of 

flowback production until flowback diminishes. 

An effort was made to characterize flowback waters from a limited number of shale gas 

well completion locations in the Barnett Shale. This characterization included 

measurements of cumulative volume, electrical conductivity and selected constituents of 

interest (inorganic salts) in flowback water samples taken over time from each location. 

This report summarizes these data and examines the feasibility of using real-time 

electrical conductivity measurements to identify and segregate early flowback waters 

containing relatively low concentrations of total dissolved solids (TDS). Such low TDS 

early flowback water has potential for reuse in well drilling and completion with only 

relatively minor pre-treatment for its intended purposes. This could serve to significantly 

reduce the quantity of fresh water that is presently used to drill and complete shale gas 

wells and to reduce the total quantity flowback water that needs to be disposed. This 

could lead to significant logistical efficiencies and economic savings with respect to 

water acquisition, handling, trucking and disposal. 
1

Introduction

Large volumes of water (typically 1-6 million gallons per well, along with friction 

reducers, chemicals and sand propant) must be injected into the producing formation of 

most shale gas plays in the United States to achieve effective hydraulic fracturing in 

order to stimulate and sustain natural gas production. Variable amounts (ranging from 

10 to 50 percent) of this “fracwater” are returned as “flowback” during the first several 

weeks following well completion. Flowback waters are collected under pressure at the 

wellhead and must either reused or disposed. 

The electrical conductivity (an indicator of salt content) of flowback water generally

increases over time from the onset to the end of the flowback. Conversely, the daily 

rate of flowback typically declines rapidly from an initially high rate of perhaps 

thousands of barrels per day to less than a hundred barrels per day over this period. 

The largest fraction of total flowback is thus collected early on when salt concentrations 

are relatively low. The identification, segregation and collection of early flowback may 

therefore represent an opportunity to obtain moderately concentrated (with respect to 

total dissolved solids or salts) water requiring minimal processing prior to blending with 

other water streams for reuse in subsequent frac jobs. 

The possibility of achieving significant water conservation through early flowback water 

capture and reuse was discussed by the Barnett Shale Water Conservation and 

Management Committee (BSWCMC), a group of shale gas developer companies within 

the Barnett Shale Play whose aim is to identify solutions and best practices in water 

management. Within the committee, the concept was thought to have merit as an 

approach for future assessment. 

If, for example, it was important that chloride levels of 10,000 mg/l or 16,500 mg/l of 

total dissolved solids (TDS) were the desired specifications for the final blend of influent 

water used in future frac jobs (with the aim of avoiding adverse ionic strength effects to 

friction reducer performance), an early flowback water volume with an average 
2

concentration of over 60,000 could be blended with most types of surface waters at a 

4:1 ratio (i.e. comprising a blend with 25% recycled flowback water content) and still 

meet the TDS spec for influent water quality. This, of course, is only an example 

reflecting industry thinking in 2007; over the past three years, a new class of friction 

reducers has been identified that perform well at twice the TDS concentrations. 

The purpose of this project was to obtain flow and salt concentration data from 

representative fracturing operations in the Barnett Shale and to conduct an assessment 

of the feasibility and potential benefits of capturing early flowback water for reuse in 

future frac jobs. This report presents data from a limited number of locations and 

summarizes a general design approach for an automated system for classification and 

segregation of moderately concentrated flowback water streams from highly 

concentrated brines that are released following hydraulic fracturing performed for a 

shale gas well completion. A second report will be generated from this effort that will 

provide more detailed characterization of flowback water from locations in the Barnett 

Shale. 

Scope and Objectives

Fracwater flowback volumes and field-measured parameters were obtained from 

multiple gas wells in the Barnett Shale of Texas over the period 2010 into 2011. These 

data were collected and analyzed with the following objectives: 

1- To elucidate relationships between simple field-measured (or field measurable) 

inorganic parameters (chloride concentration, total dissolved solids, specific gravity 

and specific conductance) and flowback rates and cumulative volumes; and 

2- To suggest a field measurement and monitoring protocol whereby flowback waters 

could be characterized for the purpose of classifying and segregating brines (i.e. 

early flowback water of low salt concentration from flowback and produced waters of 

higher salt concentrations), in real time, in order to facilitate the recycling and best 

management of these waters.
3

3- To describe a process flowsheet that could potentially be used to effectively 

segregate early flowback water of moderate TDS levels to improve reuse costs. 

Methods

Flowback quantity data were measured in the field by gaging 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). Waters were analyzed in the laboratory for specific 

conductance and total dissolved solids. 

The graphs presented below present and summarize data for a single well that was 

selected among approximately a dozen sites as being most representative of them all. 

A subsequent report will present a wider analysis of inorganic and organic analytes 

among all of the sites considered together, focusing on their common, general 

attributes.

Results and Discussion

The daily course of flowback for the selected site is given in Figure 1. Despite the 

significant variation in flowback rate from day to day there is a clear pattern of 

exponential decline from the initial high rate. (Note that the days with zero flowback are 

indicative of operational vagaries, with the well being shut in for several days around 

day 60). 
4

Figure 1 – Daily course of flowback water quantity over time.

The cumulative flowback over time and the fraction of fracwater recovered as flowback

are given in Figures 2 and 3. The cumulative fraction of fracwater recovered (as 

flowback) in this well was approximately 33% of the total volume of fracwater used to 

frac the well. This is somewhat higher than the 20 to 25% fraction of fracwater that is 

normally recovered from most Barnett wells. However, the fact that fracwater 

production continues to decline after day 60 (Figure 1) indicates it is unlikely that an 

underlying water bearing formation has been penetrated during well fracturing. 
5

Figure 2 – Cumulative flowback volume recovered from the onset of flowback.

Figure 3 – Cumulative fraction of fracwater recovered as flowback over time. 
6

The field-measured chloride concentration of fracwater over time indicates a rapidly 

rising initial trend that asymptotes according to a logarithmic decline curve (Figure 4). 

This indicates that fracwater that has penetrated further into the shale formation and 

has thus been resident there longer has entrained more soluble salts than the fracwater 

that was “last in and first out”. The fact that the chloride concentration levels off over 

time suggests that the soluble constituents in the fracwater come into equilibrium with 

the formation over time, taking longer than approximately 45 days to do so. The 

converse is also presumably true, suggesting that equilibrium water quality models 

would not be accurate in characterizing early flowback. This is important because most 

of the flowback recovery happens before such equilibrium is approached (before day 

45). Moreover, if the lion’s share of flowback water does not come into equilibrium with 

the most soluble species (chloride) it certainly does not with species that are far less 

soluble. 

Figure 4 – Field-measured chloride concentration of recovered flowback water 

over time. 
7

The cumulative mass of chloride recovered (Figure 5) in flowback water is a function of 

both the total flowback volume (Figure 2) recovered as well as the flowback water 

chloride concentration (Figure 4). The total mass of recovered dissolved solids (TDS) 

is a parallel, linear function of recovered chlorides (Figures 6 and 7). Figure 7 thus 

indicates that approximately 2.5 million pounds (1,250 U.S. tons) of total dissolved 

solids have been recovered from this well over the course of three months. Given that 

the initial chloride concentration of the fracwater used to complete the well was less 

than 200 mg/kg it is clear that virtually all of this TDS mass is from the shale gas 

formation. 

Figure 5 – Cumulative chloride mass recovered in flowback water over time. 
8

Figure 6 – Relationship between laboratory measured total dissolved solids and 

chloride concentrations for four producing shale gas wells. 
9

Figure 7 – Estimated mass of total dissolved solids (TDS) recovered in flowback 

over time, calculated according to the linear, regression relationship given in 

Figure 6.

Summary

These data, taken together, indicate that Barnett shale gas flowback waters exhibit the 

following general trends and attributes:

1. Flowback begins at an initially high rate (greater than 2,000 bbls/d for the well 

presented here) and rapidly declines toward zero after about two months.

2. Most of the fracwater that is recovered is captured within the first thirty to forty-five

days of the onset of flowback. 

3. Dissolved inorganic constituents (as Cl- and TDS) increase logarithmically and 

asymptotically over time, indicating that early flowback waters (which represent most 

of the fracwater that is recovered) are not in chemical equilibrium with the shale 

formation.
10

4. The total cumulative mass of total dissolved solids (TDS) recovered are reflective of 

both the flowback water volume and its TDS concentration. Thus, although most 

flowback water is recovered early on, the relatively higher TDS concentration of later 

flowback causes the cumulative TDS recovery to continue to rise disproportionately 

to flowback water recovery; (compare Figure 7 to Figure 1).

Potential Application

As expected, a high correlation was observed between TDS and specific conductance. 

Using measurements on actual flowback waters from four well completion locations, a 

plot of this relationship is shown in Figure 8, where the correlation coefficient (as R

2

) is 

greater than 0.97

Figure 8 – Relationship between total dissolved solids (TDS) and specific 

conductance. 
11

Specific conductance can be readily measured, calibrated and automated. The close 

correlation between TDS and conductance indicates that operators should be able to 

measure, separate and segregate flowback waters in real time using in-line conductivity 

transducers more or less according to the scheme presented below in Figure 9. In 

simple terms, flowback water would be analyzed in real time using an electronic, 

recording electrical conductivity sensor. This would be coupled to an electronically 

operated valve that would shunt water to different storage facilities according to 

programmed levels of measured conductivity. The numeric criteria for determining 

number and levels of measured conductivity would be determined based upon 

operational needs and logistical constraints.

In the example shown in Figure 9, the lowest TDS waters would presumably be the 

most valuable to recover and recycle since these pose the least restraints and 

requirements with respect to handling, treatment and re-use. Yet, given the rapid rise in 

TDS (shown as chlorides in Figure 4) it may not be realistic to capture nearly fresh (less 

than 3,000 TDS mg/kg) from Barnett shale gas flowback waters. However, if more 

realistic thresholds are set (say 10,000 mg/kg for low TDS waters and 25,000 mg/kg for 

high TDS waters) then such a system may afford realistic potential to recover and 

recycle early flowback waters and thus significantly reduce the total quantity of “new” 

water used to fracture subsequent wells.
12

Figure 9 – Simple, schematic layout of a hypothetical flowback measurement, separation and segregation 

system.
13

Conclusions

Barnett Shale flowback waters generally exhibit an exponential decline in flowback 

quantity over time (Figure 1) and a logarithmic increase in chloride (and by correlation, 

total dissolved solids, or TDS) as flowback diminishes (Figure 4). Approximately twenty 

to thirty per cent of the fracwater used to fracture the gas-bearing shale is recovered as 

“flowback” during the first thirty to ninety days of fluids production (Figure 3). The 

recovered mass of total dissolved solids (TDS) is reflective of both the flowback rate 

and the TDS concentration. Although the rate of flowback declines rapidly over time 

the TDS concentration continues to rise. TDS recovery therefore continues to rise even 

as flowback tapers off; (compare Figure 7 to Figure 1). A high correlation (R2= 0.97) 

was observed between TDS and specific conductance (Figure 8). Specific conductance 

can be readily measured, calibrated and automated. 

The close correlation between TDS and specific conductance indicates that operators 

should be able to measure, separate and segregate flowback waters in real time using 

an automated, electronically mediated system. In simple terms, and in-line conductivity 

transducer (sensor) would measure the specific conductance of flowback water from the 

well. The electronic signal from this sensor would be interpreted by a logic controller 

(computer). The controller would operate solenoid valves to direct and segregate water 

into separate tankage based on its electrical conductivity. The cut points for the 

segregation of flowback water could be readily adjusted according to the performance of 

the well and to the needs and capabilities for handling flowback waters of varying quality 

parameters (using specific conductivity as the primary surrogate measure). 

The segregation of shale-gas flowback waters according to specific conductance 

implies significant potential for recycling flowback water according to its re-use potential 

for various purposes in well drilling and completion. This could serve to significantly 

reduce the quantity of fresh water that is presently used to drill and complete shale gas 

wells and to reduce the quantity of high TDS flowback water that needs to be disposed. 
14

This could lead to significant logistical efficiencies and economic savings with respect to 

water acquisition, handling, trucking and disposal. 

Acknowledgements

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. TestAmerica Pittsburgh 301 

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

www.testamericainc.com .