For many years, the first line of defense in the maintaining of drilling fluids and the separation of rock particles in drilling operations on both onshore and offshore rigs has been solids control systems.
Shale shakers are the primary method of solids control today and consist of vibrating screens where the drilling fluids and drilled solids—that return from the well—flow onto the screens. Through vibrating G-forces, the solids phase is then filtered out for overboard discharge or for treatment on the rig or onshore.
The cleaned mud is then incorporated back into the active fluid system and reused to drill the well. It is this mud that cools and lubricates the drillbit, carries drill cuttings to the surface, controls pressure at the bottom of the well and ensures that the well remains stable.
Yet despite recent technology advances in vibration patterns, more efficient screens and more powerful motors, primary solids control remains a major issue when drilling many wells and particularly when it comes to deeper and extended reach wells where the control of low-gravity solids content is so vital.
For example, the high G-forces used by standard vibrating type shakers tend to break down the drilled solids into fine particles. This reduces the amount of solids that are removed and increases the solids content in the drilling fluid—in particular low-gravity solids. This leads to a decline in drilling fluid efficiency, a negative impact on penetration rates, and increased wear and tear on both the surface and downhole equipment.
Another drawback of solids control technologies today is that high volumes of mud are often lost with an increase in drilling waste. This has implications from both a cost and operational standpoint.
Finally, many solids control technologies can lead to a poor working environment, with personnel exposed to high noise levels and vibrations as well as the emission of oil and other vapors. In areas such as the North Sea—a region well known for its stringent HSE standards related to waste and employee conditions—this can be a significant drawback.
It’s time to look to other technologies for separating and treating drilling fluids on offshore facilities and to look at alternatives to the types of equipment currently being used to control drilled solids, which have been prevalent in the industry since the 1930s.
One such alternative is the MudCube from Norwegian-based company Cubility. The MudCube is being applied on offshore installations and offers benefits over the traditional types of equipment widely used today.
Vacuum-based filtration system
The MudCube is essentially a vacuum-based filtration system that removes solids from the drilling fluid. The system is now field-proven and being used on a global basis by many of the major operators.
Rather than noisy vibration-based shale shakers and rather than relying upon high G-forces to separate mud and the drilled solids, drilling fluids are vacuumed through a rotating filter belt using high airflow to separate the cuttings from the fluid more effectively. The rotating filter belt carries drilling fluid and drilled solids forward while air—at 20,000 l/min (5,283 gal/min)—is pulled through this filter belt, taking with it the drilling fluid.
In this way, the cleaned drilling fluids are returned to the active mud system, and the drilled solids are discharged either directly overboard (if they meet environmental discharge regulations) or to a cuttings handling system.
In addition, water-knives are installed on the inside of the vertical part of the filter belt to remove any cuttings or sticky clay that may have stuck to the belt, and pneumatic micro-vibrators are installed underneath the filter belt to create resonance and improve conductance. The solids removal efficiency of the MudCube is often higher than 90%.
There are a number of immediate benefits to drilling operations from using the MudCube when compared to the vibrating types of treatment processes used by the industry for many decades.
First, the improved separation capabilities of the MudCube lead to better quality mud, fewer chemicals required to maintain its properties and more mud recycled back to the mud tanks to be reused for drilling.
With the cost of oil-based mud on the Norwegian Continental Shelf around $1,300 per cubic meter, it is estimated that the reduced cost of drilling fluid per well can be as much as $150,000. An operator and mud company, for example, recently reported the reduced use of premix chemicals as bringing savings of up to $270,000 when using the MudCube as compared to similar operations with standard type shakers.
As well as cost savings, there also are significant operational benefits of high-quality drilling fluids. The stable mud properties with low equivalent circulating density and solids content result in improved drilling efficiencies on the rig and a decrease in nonproductive time. Effective solids control also results in higher ROP, reduced stuck-pipe incidents and improved wellbore stability.
Furthermore, as mud properties have been field-proven to be stable throughout the entire well when using the MudCube, there are corresponding low maintenance requirements for rig circulating equipment. For drilling rigs costing millions of dollars a day, the financial benefits of this are clear.
In addition to improved mud properties, there also is reduced and cleaner waste to dispose of. Due to less drilling fluid being lost, the MudCube generates substantially drier cuttings with lower oil content and substantially cheaper disposal. In the Gulf of Mexico, for example, the regulatory limit for oil on cuttings to allow for the disposal of cuttings directly to the sea is 6.9 %—something that the MudCube can deliver.
Alongside improved drilling fluids and reduced and cheaper waste, the MudCube has a lower deckload than traditional shakers and, when taking into account the potential to replace other equipment, can save up to 25 tons on existing facilities and much more on newbuilds. Much of the operation of the MudCube also takes place remotely, leading to improved efficiencies and cost savings.
Finally, the use of vacuum and airflow as opposed to high G-forces leads to a much-improved HSE environment with significantly less noise and vibrating and very limited exposure to oil vapor and mist.
From reduced waste and less fluid lost to fewer chemicals, reduced maintenance, and personnel and HSE savings, it is estimated that the savings from a MudCube on a typical rig can be as much as $8 million per annum.
Since its 2012 introduction to market, the MudCube has been adopted on a number of offshore rigs in the North Sea, Middle East, Far East and North and South America.
In one such installation on the Maersk Giant rig—a jackup drilling rig based in the North Sea—three MudCubes replaced four traditional shale shakers. Since the installation, the Maersk Giant has embarked on an ambitious drilling program in the North Sea with the MudCubes used in the drilling of 13 wells to date.
Benefits to the operators include improved working conditions and no costly HVAC upgrades to the shaker room, improved drilling efficiencies with less drilling fluid being lost and more returned to the mud tanks for reuse, and the cuttings having a low mud content for easier and cheaper disposal.
A new alternative
With the intensity of drilling operations, the need to monitor the bottom line and stringent environmental and HSE controls, solids control and waste management technologies are a critical element of optimizing drilling fluid performance and improving drilling efficiencies.
Operators now have an alternative solution when making their decision.