Artform or science article 20140120145459 Resized WEBIn-situ chemical injection

Remedial schemes may look simple, but they’re not. An increasing number of subsurface clean-ups are based around fluid injection into the affected aquifer. This includes using conventional wells to delineate and quantify the target contaminants, installation of injection wells for fluid delivery and monitoring for excursions or plume drift. Simple? Perhaps, had it not been for nature’s fantastic heterogeneity of the sub-surface adding to its complexity.

Planning for a successful environmental injection project involves the consideration of a number of project-specific elements. Each element is interdependent on the other and developing a work plan can be challenging. There are many variables and it’s important that you understand them, how they interconnect and how they affect the outcome of your project.

What do you want to inject?

There are a lot of choices. Obviously the product you choose must be effective at treating the contaminant of concern. When choosing a product, you must be aware of the impact on project duration, the level of effort and safety considerations required when using in the field.

Duration and level of effort are functions of product-specific handling considerations and they take three basic forms:

  • Chemical compatibility – The equipment you select should be based on the type of product to be utilised.
  • Miscibility - Almost all existing products on the market are concentrated and are mixed with water on site. Naturally, some are easier to mix than others.
  • Stoichiometry - Different products will have different application concentrations and volumes. The site specific recipe will critically affect your project’s execution, with consideration required for the type and volume of storage, mixing and batching tanks, appropriate pumping equipment selection, and the expected time required to execute the project efficiently.

Most of these are the desk-top components, with predictable weights and rates. Based on these factors, the field application must then be considered if an injection project is to be successful.

How much do I need and where do I want it to go?

Site parameters, such as target contaminant concentrations, subsurface geochemical conditions and remediation amendment characteristics will determine the appropriate concentrations and volumes. But the story doesn't stop there. For any general chemical reaction, A + B » C, the amount of A, B and C are all interdependent. Lab chemists will adjust the volume of their mixing and reaction vessel to accommodate the actual quantities needed for the desired result. If the reaction turns ugly in the lab, they’ll use a smaller beaker. Environmental remediation project managers do not have this luxury. The volume of their vessel is already pre-determined by the extent of the plume and the porosity of the formation. The characteristics of the geologic formation dictate how much substrate you can actually put in the ground and where it will wind up going within a certain time frame.

Uniformity of distribution is a different matter. General assumptions about stoichiometry and radius of influence are null and void if heterogeneity preferentially loads everything into a single sand stringer (or utility trench). There are two key concepts for dealing with these issues:

1. Baby steps - Shorter vertical target intervals and more spatially distributed points will reduce the number of opportunities the product has to follow preferred pathways. It also provides flexibility to scrap uncooperative intervals without drastically overloading others.
2. Slow and steady - Adjusting the rate of injection, either through slower flow rates, pulsed injection methodology or dividing the application into multiple events is a good way to get more from less.

The second concept introduces another important factor – time.

How long is it going to take?

We've already discussed how the geologic formation will affect how quickly a product can be injected. While you're not going to get the liquid in any faster than the subsurface will accept it, inefficient processes and equipment can certainly add to the overall project duration. Performing the injection safely with proper training and safety gear will inevitably add to the duration. Factors that are within our control that affect project duration include:

1. Batching efficiency - If the product isn't ready to go in the ground, everything comes to a grinding halt. Efficient batch processing and equipment are different depending on what product is handled, the volume needed per interval and how many intervals there are.
2. Non-injection steps - Both the number of injections within a given vertical interval and the spatial distribution of the injection points each add steps to the process. Moving delivery lines between points can interrupt flow of product to the subsurface. Shutting disconnects can break hose connections or create line purges. If these non-injection steps can be performed at the same time as injection is taking place at other intervals, time can be saved. This requires an adequate amount of equipment and containment that is compatible with the product, and a batching sequence that can keep up with a varied approach.

The scientific community has found answers for many of the individual issues which can be faced during an injection project, yet when these various issues are combined, success may appear to be as much of an art as a science. For this reason, experience is often the key to project success!

If you’d like to learn more about in-situ chemical injection, please contact our remediation team at remediation@coffey.com