February 2022 TAGD Business Meeting Study Results: Delineating Injection Well Buffer Zones in Brackish Aquifers Juan Acevedo, BRACS Hydrologist, TWDB Jack Sharp, Professor Emeritus in Geology, UT- Austin

1. Study Results: Delineating Areas Designated or
Used for Class II Well Wastewater Injectate
TAGD Winter Business Meeting
Juan P. Acevedo
TWDB – BRACS
February 08, 2022

2. Background: Texas needs Water

3. Background: Texas needs Water

4. HB 30: Requirements
— TWDB-BRACS Identifies and designates Brackish
Groundwater Production Zones (BGPZs)
— BRACS = Brackish Resources Aquifer Characterization System
— Determine the amount of brackish groundwater
production from BGPZs over:
— a 30-year period,
— a 50-year period; and
— without causing significant impact to water availability or
water quality
— Make recommendations regarding reasonable
monitoring
— to observe the effects of brackish groundwater production
within the BGPZs

5. Statutory Requirements for BGPZ Designation
Must have brackish water In areas of the state with moderate to high availability and
productivity
Must have hydrogeologic
barriers
Sufficient to prevent significant impacts to freshwater
availability or quality
Cannot be within these
boundaries
Edwards Aquifer within the Edwards Aquifer Authority, Barton
Springs-Edwards Aquifer Conservation District, Harris-
Galveston Subsidence District, Fort Bend Subsidence District,
and Dockum Aquifer
Cannot be already in use Brackish water already serving as a significant source of water
supply for municipal, domestic or agricultural
Cannot be used for
wastewater injection
Permitted under Title 2 of Texas Water Code, Chapter 27
[Class II Underground Injection Control (UIC) Wells used for
saltwater disposal (SWD)]

6. Statutory Requirements & Criteria for BGPZ
Class II wells are
potentially
injecting
wastewater
into the
brackish
portions of
Texas Aquifers

7. Aquifer BGPZ Example
- Previous BRACS studies
applied 15-mile buffer to
all class II wells injecting
into aquifer study areas.
- Stakeholders pushed back
on the 15-mile buffer
designation
- too conservative
- scientifically
defensible?

8. Aquifer Assessment

9. Aquifer Assessment Maps

10. Aquifer Assessment Maps

11. Aquifer Assessment
Master Table – default aquifer parameters

12. Potential Mapping Techniques
1. Analytical solutions (Preferred method):
— Stable
— Easy-to-use
— Simplifying assumptions but exact solutions
— EPA (1994), Bear & Jacobs (1965)
— Applicable on a regional scale
2. Numerical solutions :
— Accommodate complex systems
— Intensive data requirements
— potentially unstable, require advanced users
— Modflow 6
— Applicable on a small/local scale
Current EPA procedure is to
calculate
“Zone of Endangering Influence”

13. — Tier 1 Analysis (no flow direction)
— EPA (1994)
— Bear and Jacobs (1965)
— Compute maximum migration extent
— Tier 2 Analysis (flow direction
available)
— Both gradient and direction of flow
— Bear and Jacobs (1965)
Proposed Methods:
Two-tiered – Analytical Solutions
Injection Well
Potential
Injectate
Migration without
gradient
Potential
Injectate
Migration extent
with gradient
Injection Well
Potential
Extent of
Injectate
Migration
Tier 1
Tier 2
x
x
Ambient Flow Direction

14. — Underestimates
injectate migration
when compared to
numerical solution
Limitation of Analytical Solutions
— Analytical solutions
solve on a well-well
basis

15. Final Workflow
RRC
Datasets
Download
UIC
Database
Download
O&G Full
Wellbore
Database
Data
Processing
Tool
Well
Intersection
Tool
BRACS
Aquifer
boundaries,
surfaces &
TX DEM
Processed
Injection
Statistics
table
Processed
Injection
well data
table Processed
Injection
rates and
Aquifer
location
table
Tool Tool
Injectate
Mapping
Tool
Tool
BRACS Aquifer Studies
Injection
Transport Model
Output/Input
Tables
Output/Input
Table
INPUT
OUTPUT
BRACS Aquifer Studies

16. FME Tools

17. Injectate Mapping Tool

18. Tool Testing: Nacatoch Aquifer
— Original designation (2019)
— 525 Class II wells
—84 SWD wells
—441 EOR wells
— Updated Method (2021 Study)
— 435 Class II wells
—60 SWD wells
—375 EOR wells
— Largest injectate radius
—6 miles

19. Next Steps
BRACS team to apply tools and analyze outputs of injectate
mapping tool and complete QA/QC
1. Apply tools and procedures for Class II injection wells to aquifers
ready for zone evaluation.
2. Provide list of Class II injection wells injecting to formation of
interest to RRC for review and feedback.
3. Conduct the whole aquifer zone evaluation for all other statute
requirements
4. Select buffer distances for Class II injection wells
— Injection mapping tool provides model for injectate migration but
not buffer distance.
— BRACS is developing guidance and will seek stakeholder feedback.
Please contact us to be added to the stakeholder list.

20. —Contact us with comments/questions or to be
added to the stakeholder list
— Juan P. Acevedo, Juan.Acevedo@twdb.texas.gov
—Or visit the study website:
www.twdb.texas.gov/innovativewater/bracs/projects/Injection/index.asp
-
Thank you!
Caverns of Sonora, TX

21. CONCLUSIONS
• Brackish groundwater desalination is being and will continue to be
implemented to meet Texas’ growing water needs.
• There are abundant Class II injection wells that could be used to dispose
of the residual concentrates from the desalination plants – 30,000-100,000
mg/L.
• We propose tier I and tier II analytical models to assess the areas of
review (zones of endangering influence).
• In some cases, numerical models may have to be used.
• Class II wells offer a viable potential solution to disposal of concentrates.
Suggestion
• Underpressured, depleted petroleum reservoirs offer another attractive
potential for disposal of desalination waste fluids.
(Class II EOR wells become Class I wells) JMS

22. Desalination - One Future Solution
• Texas has abundant brackish and saline water.
• Desalination technology is improving; costs of
desalination are decreasing. Reverse osmosis currently
the preferred/most promising process.
• Concerns:
• Location of waters for desalination;
• Effects of fluid production; &
• Disposal of residual fluids
JMS

23. JMS

24. Texas has
numerous brackish
“aquifers.”
TWDB, 2021 JMS

25. Texas has
numerous
brackish water
wells.
TWDB, 2021
TWDB, 2021
JMS

26. Class II Wells
• There are ~ 180,000 Class II wells in the USA.
• Most are in Texas, California, Oklahoma, and
Kansas.
• Texas has the most (over 53,000, although only about
31,000 are active).
• Produced waters are generally “brines” with a TDS of
~45,000 ppm (DOE, 2006). This can vary from ~30,000
to >100,000 ppm depending upon the source of waters being
desalinated.
JMS

27. Analytical Solutions (Drawdown/Mounding)
Abundant literature on
EPA’s “Area of review” or
“Zone of endangering
influence”
This estimates head
change, not migration
Injection
well
Injection well

28. Injectate Mapping Techniques
– EPA (1994) assumes no ambient hydraulic gradient
– Bear and Jacobs (1965) includes an ambient hydraulic gradient
Tier 1
Screening level
evaluation
Tier 2
Used when
refining model

29. • No dispersion
Effect of Dispersion
Analytical Solution
Numerical Solution

30. Effect of Dispersion
• Longitudinal dispersion = 40 feet
• Transverse dispersion = 1/10 LD
Analytical Solution
Numerical Solution
Analytical Solution
Numerical Solution

31. Effect of Dispersion
• Longitudinal dispersion = 400
feet
• Transverse dispersion = 1/10 LD
Analytical Solution
Numerical Solution

32. Summary: Dispersion
• 50% isocontour represents average injectate migration
• Analytical solutions match average injectate migration
• Analytical solutions must consider radial flow from the
injection well and regional flow.

33. Fully penetrating well -
Variable density fluids
Comparing Bear and Jacobs (1965) with numerical model
Denser into lighter Lighter into denser
Same densities

34. Partially penetrating well –
Variable density fluids
Comparing Bear and Jacobs (1965) with numerical model
Denser into lighter Lighter into denser
Same densities

35. • Density may vertically stratify injectate
• Anisotropy plays an important role in the spread of
injectate, needs site-specific information
• Migration underestimated in fully penetrating wells
• Migration overestimated in partially penetrating wells
• More Class II wells are partially penetrating
• Modeling variable-density is complex and is not suitable
for regional-scale studies
• Suitable numerical solutions include:
SEAWAT; USG-Transport; MODFLOW 6, FEFLOW,
SUTRA, HST3D
Summary: Partial Penetration & Density

36. CONCLUSIONS
• Brackish groundwater desalination is being and will continue to
be implemented to meet Texas’ growing water needs.
• There are abundant Class II injection wells that could be used to
dispose of the residual concentrates from the desalination plants
– 30,000-100,000 mg/L.
• We propose tier I and tier II analytical models to assess the
areas of review (zones of endangering influence).
• In some cases, numerical models may have to be used.
• Class II wells offer a viable potential solution to disposal of
concentrates.
Suggestion
• Underpressured, depleted petroleum reservoirs offer another
attractive potential for disposal of desalination waste fluids.
(Class II EOR wells become Class I wells)
JMS

37. The end