Surface system design depends on numerous parameters and is relative to individual well situations.
Drilling in the new environmental era is pressuring operators into major dec-isions in regard to drilled cuttings disposal. With the new North Sea zero-discharge guidelines due from DPR in January 2001, the time for a decision is fast-approaching. Technological solutions in the North Sea cannot emulate other areas because of the higher operating cost.
Due to the harsh North Sea environmental conditions, drilled cuttings containment using conventional cuttings boxes inflates an already costly budget for zero discharge operations. The database gained by the operators has shown costly rig downtime and raised serious health and safety concerns associated with offloading these cuttings boxes during harsh North sea conditions.
Cuttings re-injection (CRI) and bulk shipment are two zero discharge solutions currently used in oil based mud operations. CRI is cost effective and not weather dependent, as containment either with cuttings boxes or bulk boxes results in no rig downtime. With CRI, the waste is permanently disposed of in the host formation with no future liabilities or added cost.
Environmental regulations are based upon the cradle-to-grave concept, thus the operator never relinquishes responsibility for the drill cuttings and the chemicals left on them.
Major disposal cost
The major cost for drilled cuttings disposal is handling and disposition, in addition to shipment to onshore locations, plus the cost to clean and move the drill cuttings several times to comply with ever tightening environmental regulations. Operators try to minimize cost by handling drill cuttings waste only once. The CRI process is an option to minimize handling. The process is:
- A permanent on-site disposal method that can fully comply with zero discharge to the surface environment
- Not reliant on land farming, treatment, solidification, encapsulation, or moving cuttings to another location, relieving the operator of future environmental concerns
- A method that returns cuttings to their native environment
- A method that does not discharge hydrocarbon waste into the air, unlike thermal operations, which add to the greenhouse effect
- Inexpensive, relative to many environmental solutions which are not permanent.
Complex modeling techniques have been created to establish fracturing parameters for increased hydrocarbon production in tight and porous, brittle and ductile formations. These models differ significantly from models required to simulate cuttings injection.
The CRI process consists of a different set of parameters other than typical fracture models. The fracture models for hydrocarbon stimulation are designed as follows:
- High rates of injection to prevent sand out
- Injection with specific brittle particles that are large, when compared to cuttings slurry particles
- No distribution of particle size
- High fluid horsepower at the formation face
- Short duration pumping
- Slurry rheologies that have low fluid loss and are ultimately designed to create the maximum fracture that can be obtained.
Disposal of cuttings slurries are exactly the opposite. Cuttings slurry particles are small in size, soft/ductile in nature, pumped at low rates for long periods of time, are purposely designed to keep the fluid horsepower low, and are generally high in fluid loss. The desired impact to the formation is minimized with slurry injection. The intent is not to create large fractures.
New monitoring techniques allow CRI technicians to read the real time formation effects from cuttings slurry injection and to plot this effect over time. Continuous data logging currently is used to record and analyze the formation reaction over time. This same equipment is also being used to control parts of the injection process and provide remote controlled, automated capabilities.
Quality control and site-specific rheological/physical property adjustments are crucial for maintaining zonal isolation and for completing the cuttings injection project successfully. It is much easier to start a CRI project than to successfully complete one. Most formations change over time, after cuttings particles have been injected into the formation. The resulting formation is not the same as when the project started, and experience/knowledge developed over time becomes crucial to continued injection success.
Grind size is critical to success. Large particles, 200-300 microns in size, typically fill up the near wellbore area because they cannot be forced into the formation. When this occurs, formations require higher pressures to inject that may cause cement shoe integrity problems. Fine particles do not fill close to the wellbore and injection pressures are significantly lower.
Also, the finer the grind, the less chemical volumes needed, and smaller quantities of slurries are required per hole drilled. For example, an Apollo design generates cuttings slurries at real-time drill rates to much less than 100 microns. After a homogeneous slurry is prepared and conditioned to design parameters for each site specific formation, the cuttings slurry is then injected through a dedicated conduit, such as the annular space between two strings of casing (annular injection) into the exposed formation. Cuttings slurries are pumped at planned rates into the formation. Typically, these rates are never higher than 2-3 bbl/min.
Cuttings transfer systems are important to the system. A cuttings transfer system that cannot perform at real time drill rates creates significant problems for drilling operations, whether cuttings are being boxed, bulk shipped, dried and discharged, or injected. As an example, an Apollo system now being used in the North Sea consists of a continuous discharge vacuum pod and solids transfer pumps. The continuous discharge vacuum pods push the cuttings out of the bottom of the pod.
This allows the second continuous discharge pod to provide 100% backup. Gravity feed pods cannot discharge and vacuum simultaneously with the gravity feed pod. If one of the gravity feed pods plugs up or breaks down, then the rate of transfer is cut to 50%. Cuttings solids transfer pumps are providing an even more elegant solution where they are applicable because of the low horsepower required to drive the cuttings to the cuttings re-injection unit.
Characteristics of the subsurface environment, sealing formations, injection zone, slurry properties, drilling plans, subsurface slurry disposal dimensions, and other elements directly impact all operational considerations. Of the various technical details that must be evaluated, the least understood but equally important are those questions associated with downhole considerations:
- Into what formation can the cuttings slurry be injected?
- How will the cuttings slurry be contained?
- In what direction will the cuttings slurry propagate? And how far?
- How significant of an impact will the cuttings slurry have on nearby well bores and formations?
- How will the cuttings slurry affect existing wells and future drilling plans?
- What volume of cuttings slurry can be safely disposed of?
- What forces will be put on the well casing?
- How do we inject the cuttings slurries to minimize formation impact?
- How do we protect the annulus and formation?
- When the formation changes, what does the CRI operator do next?
Accurate description of the various lithologies and the transition depths from one lithology to another is integral to determining where injection of the cuttings slurry should take place. The targeted formation should not contain natural fractures or faults that might communicate slurry to the surface or to formations containing potable water. Additionally, the disposal formation must be associated with some type of seal mechanism that will adequately restrict the slurry to the specified formation interval. This sealing mechanism can be reinforced by slurry design.
Review of mechanical property logs, cores, leak off tests, pore pressures, mud logs, and other data from offset wells can be used as a tool when addressing these issues. Fracture modeling, although currently designed for hydrocarbon stimulation operations, have proven useful in estimating the size and shape of the disposal plumes.
Seismic data can be utilized for identification of natural vertical fracturing that could make the project fail and can be utilized to define the formation properties, such as, fracture rock strengths, pore pressures, and other elements crucial to CRI.
The type of surface equipment required to process the drilled cuttings is based on a number of parameters established after addressing down-hole considerations. The properties of the drill cuttings dictate the type of grinding equipment required. Modified centrifugal pumps designed to reduce the size of cuttings using high shear rates are most effective when processing cuttings from soft, hydratable shale formations.
In those instances where a sizable quantity of hard cuttings will be processed, the use of a mechanical grinder should be used. Maximum particle size of 100 microns has worked well in over 300 Apollo projects, where cuttings slurries of differing properties have been successfully injected into many types of porous/permeable and non-porous/impermeable formations.
In zero-discharge operations the rig cannot drill if the CRI surface equipment is not adequately designed and installed to stay ahead of the drill rate/surge conditions. The cost for CRI equipment/related costs skyrockets when the drilling progress is negatively impacted.
The casing program is developed after the injection zone and sealing formations have been identified. The cement integrity of the surface casing defines the upper sealing boundary of the injection zone and the top of cement for the intermediate casing string provides the lower boundary.
Recall that the injection plume takes the path of least resistance, will likely be initiated at the casing shoe, and could grow vertically and outward from that point, depending on a variety of conditions. For this reason, the casing shoe needs to be set at an adequate depth below the top of the specified injection zone.
In theory, the height of the top of the cement for the intermediate casing string is set to make certain that the length of exposed formation will allow for desired downward fracture growth. Experience in designing the subsurface injection profile and related casing/cement programs are paramount to successfully implementing the technology.
Based on potential injection pressures, the casing strings used to conduct the cuttings slurries and any other existing/future casings, which may see injection pressures, are selected to provide adequate burst and collapse safety factors during the injection operations.
Although properly designed cuttings slurries and injection programs do not cause wear to the wellhead equipment, steps are usually taken to prevent any possibility of erosion at the well head slurry injection point. Process parameters to be monitored include:
- Pressure impact on nearby wells
- Disposal plumes, direction, and location
- Injection rate, total volume, and pressure
- Disposal slurry properties, density, viscosity, rheology, and particle size
- Equipment condition
- Experience level of operators/management.
Cuttings dryers are being tested to impact bulk shipment systems and to reduce synthetic fluids on cuttings quantities to less than 5%. These machines are new in the industry and are achieving amazing results. Apollo's system uses the cuttings dryer to cut the disposal volume to about 50% of typical disposal volumes. Where synthetics are dischargeable, the system takes all of the free synthetic fluids off the cuttings, and discharges cuttings to the sea that are powder dry.