Desalination Concentrate Disposal Using Injection Wells: Technical Challenges

Transcription

1 Desalination Concentrate Disposal Using Injection Wells: Technical Challenges Robert G. Maliva, Ph.D., and Scott Manahan, P.E. Schlumberger GWPC UIC Conference Sarasota, Florida (January 22-24, 2013)

2 Concentrate Disposal Concentrate disposal is often the critical factor in the feasibility of desalination projects Concentrate disposal option must: - Be permittable. All required regulatory approvals can be obtained (environmental and water quality issues) - Be reliable over desalination system life (20+ years) - Be economically viable Desalination projects have died because there was no economical option for concentrate disposal!

3 Concentrate Disposal Options Surface water (marine) outfalls - option in coastal areas, but difficult and expensive to permit (if possible) Discharge to wastewater treatment plants - may be viable for small systems; impacts to treatment and reuse systems Reuse - limited because of salinity Zero-liquid discharge - high costs (energy), salt disposal Evaporation ponds - arid areas, environmental concerns Injection wells

4 Concentrate Injection Well Types Shallow wells in coastal regions (Class I or V) Deep high-capacity wells (Class I or V) Deep high-pressure (Class I) Key technical issues Feasibility and system type depend upon local hydrogeology Optimization of design and operation

5 Shallow Coastal Injection Wells Avoids coastal construction and point discharge impacts Concentrate may eventually diffusely seep out Systems are used on some Caribbean islands

6 Shallow Class V Injection Wells Shallow unconfined aquifer containing saline water underlain by a well confined USDW aquifer (e.g., Little Gasparilla Island, FL)

7 Deep High-Capacity Injection Well Systems Depths => 500 m (1,500 ft) Capacities => 3,800 m 3 /d (1 Mgd) One well may dispose of entire concentrate flow Key technical issues Requires very high-transmissivity injection zone that can accept concentrate flows uncommon Upward migration of injected concentrate is retarded by density stratification

8 Deep High-Capacity Injection Well Systems Commonly used in South Florida for disposal of concentrate from brackish RO Injection zone is the Boulder Zone of Floridan Aquifer System Kay Bailey Hutchison Desalination Plant, El Paso

9 Deep High-Capacity Injection Well Systems Umm Er Radhuma Formation in Arabian Gulf Region

10 Deep High-Pressure Injection Wells Depths => 1,500 m (5,000 ft) Injection pressures => 7,000 Kpa (1,000 psi) Oil-field technology Low to moderate capacities (< 1,900 m 3 /d, 0.5 Mgd) Existing oil-gas injection wells may be used - oil and gas injection wells typically operate at relatively low flow rates

11 Deep High-Pressure Injection Wells Applications Inland RO facilities in which there are no other viable concentrate disposal options. Brackish RO systems with high recoveries and low concentrate flows (secondary RO or other treatment to reduce volume). - It may be much less expensive to reduce the volume of the concentrate flow than to install additional injection well capacity. - Injection may be less expensive than final volume reduction for ZLD.

12 Deep High-Pressure Injection Wells Technical Issues Deep high-pressure injection wells are far more complicated that other injection and production wells! Low transmissivity aquifer vulnerability to clogging Geochemical compatibility (scaling) high TDS and ion concentrations Temperature and salinity viscosity effects Management of pressure high pressures can induce fracturing and in some locations low-level seismicity Optimization of completion (perforated vs. liner or screened; hydraulic fracturing [fracking]) Improper design or geochemical incompatibility can cause permanent well damage

13 Deep High-Pressure Injection Wells Tools Quantitative log analysis reinterpretation of existing data Pressure transient test evaluate reservoir size and evaluate skin damage Advanced borehole geophysical logging (e.g., NMR, FMI) locate and characterized permeable intervals Perforation technology tailor to project specifics Screens and liners Formation sampling and testing technology e.g., Modular Formation Dynamic Tester (MDT) Advanced data workflow (Petrel) and modeling software (Eclipse)

14 Fundamental Injection Well Issues Location of an injection zone and designing injection well system that can provide the target disposal rate and volume over the life of the plant (20+ years) Avoiding adverse impacts to the environment or groundwater resources from the migration of injected water out of the injection zone Maintaining well performance (management of clogging) - Specific injectivity (injection rate/injection pressure) - O&M (workover/rehabilitation program needs to be effective and affordable) Regulatory issues - Obtaining project approval - Monitoring requirements

15 Technical Issues Injection wells are more challenging to design and operate than production wells because the flow of water is into the formation. The entire injected water flow and all entrained materials must pass through a relatively small surface (screen and borehole wall).

16 Technical Issues - Clogging Management of clogging is the critical injection well system design and operational issue Additional causes of clogging: 1) Clay mineral dispersion and swelling 2) Air entrapment 3) Particle rearrangement

17 Fate of Injected Water Requires detailed aquifer characterization and (density-dependent) solute-transport modeling. South Florida upward Migration study

18 Injection Well Issues and Opportunities Well design Optimization of design high efficiency completions are critical For example, - Maximization of screen open area - Use of open hole completion - Accurate identification of most permeable strata for perforation - Proper development material left in well or near borehole wall will be forced into the formation during injection

19 Injection Well Issues and Opportunities Clogging Management - characterization of injected fluids and analysis of its compatibility with injection zone water and rock (geochemical modeling) - pretreatment (e.g., filtration and chemical adjustments) - Assessment of causes of clogging - Pressure transient testing for formation damage - Borehole geophysics - Well rehabilitation (workover) part of normal operations and maintenance

20 Injection Well Issues and Opportunities Where does injected water go? - Aquifer characterization and groundwater modeling are fundamental tools - Traditional groundwater tools such as pump testing and basic geophysical logging - Advanced borehole geophysical logging - Pressure transient testing reservoir size

21 Concentrate Injection Well Design Issues Injection well systems are more complex than production wells and cannot be designed in the same manner! Water quality, geochemistry, and aquifer characterization are critical Improper design or geochemical incompatibility can cause permanent well damage Injection wells typically require more frequent rehabilitation than production wells Fate and transport of injected waters needs to be evaluated (groundwater modeling)

22 Conclusions Injection wells are a valuable tool for the sustainability of desalination by providing an environmentally safe means for concentrate disposal Injection well systems require favorable hydrogeologic conditions, which may not be locally present Technical challenge is the optimization of both the design and operation of the wells in order to ensure reliable longterm performance Key lesson is approaching projects with an understanding of their complexity