Injection modeling, contingency planning critical to risk reduction
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- Generic pore pressure extimate for cuttings injection [107,800 bytes]
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Tightening of the allowable discharge limits, revocation of many once-accepted disposal methods of the past and the resulting major clean-up operations, and the increasing cost of land fills and farms (event on the seabed), have forced operators into switching from short term planning to long range planning for drilling waste.
The once short-sighted approach of just keeping up with regulatory law has cost operators a fortune. The reality is that the real cost for drill cuttings disposal is the cost to handle/dispose the drill cuttings today at the cheapest method known, plus the cost to clean the drill cuttings again and again to keep up with ever tightening environmental regulations.
Many operators are finally acknowledging that the cheapest way to handle their drill cuttings waste is to handle it only once, once and for all. Operators who have acknowledged the waste paradigm and have aggressively attacked the engineering concerns of CRI, are presently disposing of their waste in a routine manner. CRI represents the following:
- The only permanent on-site disposal method available that can fully comply with zero discharge to the surface environment
- Not reliant on land farming or treatment or solidification or encapsulation or moving cuttings to another location, thus creating a short sighted, future environmental concern for the operator
- A method that disposes of cuttings in their native environment
- Does not discharge hydrocarbon waste into the air, unlike thermal operations, which add to the greenhouse problem.
- Is an inexpensive process, relative to many environmental solutions which are not permanent
A set of guidelines are presented to assist operators and regulators when evaluating the merits of potential downhole cuttings injection applications. The guidelines are useful not only from an engineering perspective, but also from a regulator viewpoint since the guidelines address the regulatory concerns. Case history highlights from over 10 years of successful CRI operations/downhole designs are set forth to reference the understanding and new methods are detailed to further enable the use of CRI technology.
The industry acknowledges that there has been a learning curve associated with the new CRI technology and that the cost of reinventing the wheel, as regards to CRI processes, is no longer cost effective. We have learned what to do, and perhaps more importantly, what not to do.
Injection theoryComplex modeling techniques have been created to establish fracturing paramaters for increased hydrocarbon production in tight and porous, brittle and ductile formations. These models have proven to work well as a guide CRI. Models have been compared to what has actually occurred and are conservative.
To utilize the fracture models, an experienced CRI subsurface engineer must temper the fracture design with cuttings injection experience to adequately judge how the formations are impacted from injection operations. The reason for this is that CRI consists of a different set of parameters than what the fracture models were designed for. The fracture models for hydrocarbon stimulation are designed for:
- High rates of injection to prevent sand out
- Injection with specific brittle particles that are large when compared to cuttings slurry particles
- Having no distribution of particle size
- Are high in fluid horsepower at the formation face
- Are short in duration
- Feature slurry rheologies that have low fluid loss are ultimately designed to create the maximum fracture that can be obtained.
Having identified the differences, we have compared what has actually happened on over 200 CRI disposal projects to commercial fracture modeling program analysis. We find that the modeling techniques, coupled with cuttings injection experience, do design for worst case scenarios and are conservative for cuttings injection programs. Therefore, we have an analytical method for designing the injection subsurface program and for proving conservatively, that we can contain the cuttings slurries in a specified zone.
New monitoring techniques allow us to read how real time formation affects from cuttings slurry injection and to plot this affect over time. Quality control and experienced, site specific rheological/physical property control 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.
Injection processCuttings generated by drilling operations are removed from the drilling fluid using conventional solids control equipment, and then transported to the cuttings slurrification system using slides, vacuums or screw conveyors. When the cuttings reach the system, they are transformed into a pumpable slurry by mixing water/chemicals with the drilled cuttings at approximately a two/three to one ratio.
The finer the grind, the less chemical needed and the smaller quantities of slurries required per hole drilled. While the cuttings/water/ chemicals are blended, the cuttings are reduced to an acceptable particle size distribution/rheology by grinding/shearing them with specially modified centrifugal pumps and mills, into a homogeneous mixture.
After a homogeneous slurry is prepared and conditioned to site specific properties, the cuttings slurry is 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.
When the pressure increase resulting from the pumping operation exceeds the strength of the exposed formation rock and the natural pore pressure, the formation allows the cuttings slurries to flow into the formation. If the rheology/physical properties and pumping methods are correct, the formation will safely hold large amounts of cuttings.
Apollo has safely entombed over 178,000 bbl of cuttings slurries in impermeable, non-porous formations. The pumping operation continues, batch or batch/continuous, until all slurry is injected into the formation.
Operating considerationsA variety of operational details must be dealt with to properly plan the project. Efficient operations dictate that the majority of the work is done in the planning stage. Some of the details include:
- Identifying suitable cuttings disposal/sealing formations
- Selecting surface equipment
- Designing the casing program
- Design of the injection program/contingency planning
- Plug prevention in the annulus and the formation
- Preventing cuttings slurries from breaching to the surface or drinking water formations
- The impact on existing producing wells or future wells to drill
- Quality control/monitoring of injection procedures
- Abandonment of waste disposed to permanently entomb the waste
- Obtaining regulatory approval.
- Into what formation can the cuttings slurry be injected?
- How will the cuttings slurry be contained?
- In what direction will the cuttings slurry propagate?
- How big is the impact will cuttings slurry have on nearby wellbores?
- How will the cuttings slurry affect existing wells and future drilling plans?
- How much cuttings slurry can be safely disposed of?
- What forces will be put on well casing?
- How do we inject the cuttings slurries?
- How do we protect the annulus and the formation?
Lithology concernsAccurate 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 disposal formation must be able to readily accept the cuttings slurry, and must also be massive enough to accommodate the volume of cuttings slurry. The targeted formation should not contain natural fractures or faults that might communicate slurry to the surface or to formations contained 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.
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.
Seizmic 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.
CRI feasibility plans need to be prepared by experienced CRI engineers, utilizing a host of data, to properly identify the limiting parameters essential for successful CRI operation.
Being successful with CRI begins with choosing a qualified contractor who has the ability to engineer and plan the disposal project, beginning at the subsurface disposal formation and working up. A track record in the type of cuttings injection operations planned is paramount.
Since CRI is still young, it is very important for the operator to go to locations where a contractor is doing similar CRI work, to interview CRI personnel and contractor management team, and to talk to the drillers to be certain of a complete understanding. Bidding the contractors generally requires a format which specifies in detail the equipment and personnel needs, with performance standards requiring an unqualified CRI bidder to pay for it's own mistakes, rather than the operator paying to teach the CRI contractor on the job.
Installing the equipment in a way that it can be easily removed allows the operator to change an unqualified contractor. Since, in zero discharge operations the driller cannot drill ahead without the cuttings being disposed of correctly, the CRI operation is a critical path component to the drilling operation. When the rig has switched to oil base fluid and the cuttings are coming over the shaker is too late to do homework.
Also, the iron is cheap and easy to change; qualified CRI personnel are not. An oilfield adage - "The iron is only as good as the people who are operating it," rings very true. Good operators can make poor iron work, but poor operators cannot make good iron work.
Surface requirementsThe type of surface equipment required to process the drilled cuttings is based on a number of parameters established after addressing downhole 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. All modified centrifugal pumps are not the same.
In those instances where a sizable quantity of hard cuttings will be processed the use of a mechanical grinder should be utilized. Maximum particle size of 100 micron, with the majority between 10-40 microns, has worked well in over 200 projects where cuttings slurries of differing properties have been successfully injected into many types of porous/impermeable and non porous/impermeable formations.
Equipment durability, manpower requirements, utilities, installation requirements and contingency plans must all be considered when designing the surface equipment system. Proper system design is important since any downtime for repairs or maintenance directly impacts on drilling progress.
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 and cannot keep the formation/annulus in good shape. Incurred cost for CRI equipment/related costs skyrocket when the drilling progress is negatively impacted.
Changing to high cost synthetic muds, switching to "skip and ship" cuttings, or shutting the rig down for days, all have occurred and will occur if the surface equipment contractor is not qualified. Also, operators must ensure that cuttings grinding power is sufficient to do the job. Performance guarantees to protect the operator are commonplace. Sensible contingency plans must be in place for handling cuttings, injection zones/conduits, and surface system/component failure. Problems can and do develop regardless of how carefully the downhole injection operation is designed.
Casing programThe casing program is developed after the injection zone and sealing formations have been identified. The casing shoe and 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, therefore, will likely be initiated at the casing shoe and could grow vertically upward somewhat 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 is paramount to successfully implementing the technology. To ignore the required engineering experience and judgement in this phase of the operation will result in failure.
Based on potential injection pressures, the casing strings used to conduct the cuttings slurry 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.
Experience is necessary to judge the condition of casing strings, after being drilled through or in place for long periods of time, to correctly downgrade new casing strength values. Although properly designed cuttings slurries and injection programs do not cause wear to the wellhead equipment, steps are usually taken when possible to prevent any possibility of erosion at the wellhead slurry injection point.
Monitoring proceduresNo matter how well the project is planned, what happens on the rig in the middle of the night is subject to a break down of procedure and this breakdown will result in disposal failure. Impact on the existing drilling program, impact on future wells, and impact to the environment is at risk if proper quality control of the slurry and the surface operation is not maintained. Quality control should monitor, at minimum, the following:
- Pressure impact on nearby wells
- Disposal plumes, direction, and location
- Injection rate, total volume, and pressure
- Disposal slurry properties, density, viscocity, rheology, and particle size
- Equipment condition and the experience level of operators.
- Involving the regulatory agencies at the beginning indicates to them that the operator wants to comply with pertinent regulations.
- It provides the operator with an opportunity to hear concerns of regulatory personnel so that special needs can be addressed and appropriate changes made to the work plan.
Copyright 1998 Oil & Gas Journal. All Rights Reserved.