Environmental liabilities and risk of contamination associated with the disposal of drill cuttings have given many contractors sleepless nights.
With strict regulations and hawkish monitoring of all construction activities, researchers are busy trying to find methods that will adhere to the regulations while minimizing environmental and financial liability for the contractor.
Summary of Benefits of Cuttings Reinjection (CRI) Technology
- Cost-effectiveness.
- Non-reliant on other disposal techniques.
- CRI returns cuttings to their original environment.
- A permanent on-site disposal method.
- Enables disposal of drill cuttings without disturbing the initial stress state of the formation.
- Free from environmental liability contamination risks.
Cuttings reinjection (CRI) technology is one such method that has proved to be a reliable option for managing drilling waste while complying with the most stringent regulations.
Research has shown that this method enables the disposal of drill cuttings without disturbing the initial stress state of the formation, and has minimal changes in horizontal stress and fracture propagation pressure, post-injection.
Why Use CRI Technology?
The CRI process involves the injection of drilling waste into the sub-surface using high pressure pumping above the injection formations fracture gradient.
Every CRI operation is unique and depends on factors such as drilling parameters, geologies, the operational environment, and local regulations.
CRI is a zero-discharge solution, cost-effective, weather independent, and leaves no future liability for the contractor. The zero-discharge solution helps diminish the risks associated with the transportation, treatment, and disposal of drilling waste especially concerning the environment.
CRI has been recognized as the best solution for drilling waste management in remote and environmentally sensitive areas where traditional disposal techniques are not feasible, such as the Arctic, forests, and offshore fields.
How Does CRI Work?
There are two methods to carry out CRI:
Annular injection
In the annular injection method, the slurry is injected into the annular space behind the intermediate or production casing above the top of the cement.
Dedicated disposal wells
In the dedicated disposal well method, the slurry is injected into an injection well dedicated to the purpose.
The CRI cycle begins by removing the drill cuttings from the drilling fluid using solids control equipment. The cuttings are then passed on via screw conveyors or vacuum to the slurrification system.
The CRI process involves the milling and shearing of slurried drill cuttings and oil field wastes with water into an appropriate size.
The resulting slurry is then disposed of by pumping it into dedicated disposal well or through the open annulus of a previous well into a fracture created near the casing shoe set in a suitable formation.
Important Considerations for CRI
- The targeted formation for CRI should not contain natural fractures or faults that might allow the slurry to return to the surface or migrate to formations containing potable water. The formation should also contain some form of seal mechanism to restrict the slurry to the specified formation interval.
Vertical fractures should not be present as it can lead to failure of the project if not detected. For this purpose, seismic data can be utilized to define the formation properties such as fracture strength of the rock and pore pressure.
- The trajectory of CRI wells should be drilled in such a way that the injection zone is well away from any producing wells.
It was found that dedicated disposal wells drilled with deviation in the middle section as high as 60° and inclination of 35° over the injection interval had no plugging issues.
- The most common complication experienced during CRI is solids settling and the resulting plugging of perforations. The selection of correct tubing size is an important factor to prevent the settling of solids.
It is necessary to avoid laminar flow by ensuring that the slurry has adequate flow velocity and flow regime inside the tubing, preferably turbulent or transitional. Tubing that is too small can amplify erosion and frictional pressure losses while too large tubing can lead to accelerated settling. A lab test of the slurry can help ensure adequate rheology to minimize settling.
- The location of the tubing tail and rat hole is another important factor for CRI well performance. It is advisable to place the tubing tail at an optimum depth above the perforations than below as it can lead to plugging of the well bottom.
The rat-hole is placed below the perforations to catch any settled solid so that it does not block the perforation immediately. It has been observed that CRI wells with no rat hole below 6 meters of the perforated interval, plugged in 3 months.
Benefits of CRI Technology
- Cost-effective compared to other environmental solutions.
- Not reliant on other disposal techniques such as land-farming, incineration, and thermal desorption which requires moving cuttings to another location. The contractor might face future environmental concerns related to the methods.
- CRI returns cuttings to their original environment.
- A permanent on-site disposal method that fully complies with stringent environmental regulations and is a zero-discharge solution.
- Enables disposal of drill cuttings without disturbing the initial stress state of the formation, and has minimal changes in horizontal stress and fracture propagation pressure, post-injection.
- Once successfully completed, the contractor is free from environmental liability and the risk of contamination of the ground surface.