WELL DEVELOPMENT – THE KEY TO OPTIMIZING AQUIFER STORAGE
AND RECOVERY (ASR) WELL OPERATIONS
Written by: Gary M. Gin, RG
There are no widely-accepted development standards or methods for Aquifer Storage and Recovery (ASR) wells, like there are for conventional water supply wells. If proven well development methods have been implemented during the construction of a well:
- Why is additional well development needed for an ASR well?
- What valuable information is derived from developing an ASR well?
- How does the operator know when the well needs to be redeveloped?
The construction of high-capacity water supply wells typically includes rigorous development steps ranging from swab and air-lift development to pump and surge development (Driscoll, 1986). The common thread between these methods is creating high flow velocities within the well coupled with vigorous reversal of flow (surging) to prevent sand grains from bridging and blocking flow (Schreurs, 1988). Movement in only one direction, such as pumping, is not adequate for well development. Instead, agitation from pumping and surging (turning the pump on and off) is needed to break down sand grain bridges, remove drilling mud wall cake, and purge fine sands from the well/borehole interface.
One of the best measures of well development effectiveness and well efficiency is specific capacity, which is defined by the well’s pumping rate (gallons per minute) divided by drawdown (feet). Collecting and analyzing specific capacity data when the well is constructed (baseline) and during development and operations allows the operator to monitor well efficiency over time and to determine when the well needs to be redeveloped.
Why is supplemental ASR well development necessary?
Conventional well development methods for production wells create multiple pathways for groundwater to flow towards the well to maximize well yield. ASR wells completed in unconsolidated alluvial basin-fill aquifers that do not undergo supplemental ASR well development will progressively clog because the fine-grained sediments in and around the well borehole will be pushed further into the formation (Figures 1 and 2).
Figure 1: ASR well development is focused on removing particles in pore space voids within the proximal and intermediate envelope outside the well casing, which ranges from inches to several feet. Distal clogs can form at distances (several feet) beyond the borehole wall and into the aquifer matrix. The removal of these distal clogs can be challenging, if the baseline mounding profile is not well established during cycle testing and cannot distinguish outlier mounding profiles (Figure 2).
With successive recharge cycles, fine-grained sediments in unconsolidated aquifers will continue to accumulate and be pushed further beyond the well borehole, forming distal clogs (Figure 2).
Figure 2: Recharge mounding trends for 3-day and 4-day injection testing cycles. This figure illustrates sustained recharge rates after ASR well development is implemented in comparison to the unsustainable trend typically seen when ASR well development is not conducted. Fine-grained particles in the void spaces at the borehole interface are developed out as part of ASR well development, improving recharge rates and efficiency.
Like conventional well development methods, the objective of ASR well development is to remove the fine-grained sediments at the interface between the filter pack envelope and formation (Figure 1). Once these particulates are removed from pore space voids, multiple flow pathways are created so that recharge supplies can flow into the aquifer. The creation of flow pathways optimizes bi-directional flow within the aquifer and, most importantly, maintains sustainable recharge rates and recharge mounding over time (Figure 2).
Creating multiple flow pathways is a step-wise process similar to pump and surge development. For pump and surge development, the development pump (recommended line-shaft turbine pump) operates in incremental time phases at increasing pumping rates. The pump is shut-off to allow the water column from surface to fall back to static water level and flush through the bowl assembly (no check valve is associated with pump assembly), through the well screen, and into the formation, which stirs up the fine-grained sediments. Once the pump is at rest, the pump is turned back on, creating agitation and liberating the fine-grained sediments to be pumped out. A Rossum sand tester is used to measure sand content to determine the effectiveness of each surge event (Figure 3). The pumping rate is increased and the pump and surge sequence is repeated until sand content is sufficiently reduced and the well’s specific capacity stabilizes.
For ASR well development, a step-wise process is implemented where short duration injection slugs (e.g., 5 mins to 15 mins) are initially conducted to create the first disturbances between the filter pack envelope and formation. After each injection slug, a series of pump cycles are conducted to evaluate specific capacity of the well and sand content. Once sand content has been sufficiently removed around the borehole interface, longer injection cycles (1-hour, 24-hr, and etc.) are implemented to reach further beyond the well borehole to create multiple pathways for recharge supplies to flow into and through the formation. The goal for this type of development is to connect the filter pack envelope with the formation coupled with the removal of fine-grained sediments in and around the well borehole interface. As injection and pumping cycles are increased in duration, the well’s efficiency and performance should improve:
- Specific capacity for both injection and pumping will improve and stabilize; and
- Sand content will diminish over time.
Figure 3: Rossum Sand Tester
Ultimately, multiple radial flow-paths into the formation are created, allowing the recharge system to operate at its optimal injection rate and duration.
When should ASR well development be employed?
The ideal time to employ ASR well development is during the commissioning phase. This a critical period because the contractor is testing and transitioning the recharge system to the owner. The well might sit idle during the equipping and site construction phase and needs to be appropriately exercised to remove particles and debris within and around the well screen.
ASR well development is conducted during this phase to confirm programming stability, reliability, and also to identify necessary programming modifications to aid in operational flexibility for backwashing operations. Lastly, once the ASR well development phase is completed, the hydrogeologist will transition into cycle testing,
which consists of increasing injection cycles (1- to 4-day continuous injection periods) (Figure 2). During the cycle testing phase, operators and managers are trained on how to operate the system, interpret performance results, confirm geochemical compatibility between the recharge water and groundwater, and develop strategies on how to continue and maintain recharge performance over time.
What valuable information is derived from ASR well development?
Here are some common questions asked by owners before they take ownership of the recharge system:
- How long should this ASR well operate in recharge mode?
- What is the optimal recharge rate?
- What is the frequency and duration of backwash operations?
- How do we identify clogging in the well?
- If the ASR well is clogged, what strategies can be employed to unclog the well?
These are critical operational questions because the goal is to operate the ASR well system consistently, maximum recharge volumes, and confirm that the permanent pump assembly can be used to remove well clogs at the well screen interface. Managers are looking for a road map on how to keep ASR wells operating as efficiently as possible. To create that operational road map, the LRE Water team and others have spent the last 15 years collecting and analyzing extensive amounts of data on ASR wells throughout the United States to verify the efficacy of ASR well development methods. This body of knowledge and experience has allowed the LRE Water team to develop an ASR well development methodology that establishes the following:
- Duration of recharge operations,
- Optimize recharge rate,
- Estimating the backwashing frequency and duration, and
- Strategies on how to unclog ASR wells.
ASR wells are becoming a critical water resource management asset for water providers. It is very important that these recharge systems be properly developed for bi-directional flows and managed appropriately so that water can be efficiently recharged and that clogs can be removed. Proper development and routine maintenance of ASR wells allows for years of sustained operations and cost savings.
ASR well development analyses may be perceived as indirectly related to recharge and backwashing performance. However, we believe that these development data (specific capacity and sand content) can be directly related to the recharge performance metrics based on our extensive research, our hands-on industry experience, and the consistent performance of ASR wells we have worked on.
For those who manage ASR well recharge systems, our experience has shown tangible benefits in the form of both operational and maintenance costs and capital costs. If you would like a better understanding of how the technical methodology is implemented please contact me via email at Gary.Gin@LREWater.com.
Tom Morris created the concept of ASR well development also known as Radial Injection Surge Development. He initiated the initial research, spent endless hours collecting and analyzing data on many wells throughout the United States, and over the course of 15 years worked with me to refine the methodology and analysis.
Jackie Tappan (LRE Water- Hydrogeologist II) is a newer, yet critical member of the team. Jackie has already contributed 100+ hours of field work implementing these development techniques and continues to push the technical envelope on improving our data collection process and analyzing the results.
Driscoll, F., 1986, Groundwater and Wells, 2nd Edition, by Johnson Screen, St. Paul, Minnesota, p. 1089.
Schreurs, R. (1988) “Well Development is Critical”, Developing World Water, Hong Kong: Grosvenor Press Int’l.
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