The Slurry Rotary Method: What You Need to Know

There are a number of different ways to drill a domestic water well. One is what we call the "mud rotary" method. Whether this is the ideal and/or best method of drilling a well is explained more fully in this short summary.

Air and water are both fluids that can be used to remove rock chips from the bottom of the borehole when drilling water wells.

The mud rotary drilling method

One of the advantages of drilling with compressed air is that it tells you when groundwater is encountered and indicates the amount of water produced by the borehole. When water drilling with the mud rotary method, the superintendent must rely on his interpretation of the rock chips in the borehole and any changes in the recirculating fluid that he can observe. Mud rotary drilling rigs can also use borehole geophysical tools to interpret which areas may be sufficiently productive for your water well.

The mud rotary drilling method is considered to be a closed loop system. This means that the mud is removed of its cuttings and then recycled back into the borehole. Calling this method of drilling "mud" is a misnomer, but it has been popular in the industry for many years and most people understand what the term actually means.

Water is carefully mixed with a product that should not be called mud because it is a highly refined and formulated clay product - bentonite. It is added, mixed and carefully monitored throughout the drilling process.

The purpose of using bentonite additives in water is to form a film on the borehole wall to seal it and prevent water loss during drilling. The film also helps to support the borehole wall from caving in or collapsing as the hydraulic pressure of the bentonite mixture is applied to the wall. The purpose of the fluid mixture is to bring rock chips from the bottom of the borehole to the surface, where they are dislodged from the fluid or filtered out so they can be pumped back into the borehole again.

Shale shaker

When using the mud rotation method, the stoker must have an oil sump, a water tank or a small pool to hold several thousand gallons of circulating fluid. If they cannot dig a pit or small pond, they must have a mud treatment plant to mechanically screen and remove the sand and gravel from the mixture. This equipment is called a "shale shaker".

Rock chips coming off the shaker

The fluid mixture must have a gel strength sufficient to support the marble-sized gravels and sand to the surface as the fluid moves. Once the rock chips are brought to the surface and the fluid is allowed to slow down, the fluid is designed to allow the sand and gravel to be dislodged.

The Divisional Drill does not want to pump the fine sand through and back into the borehole. To avoid this, the shaker uses various sized shakers and desanding cones to remove the sand from the fluid as it flows through the shaker so that the fluid can be used again.

When the borehole has reached the required depth and there is evidence that the formation it penetrates will produce sufficient water, it is time to drill the borehole into a well.

The circulating fluid is slowly diluted by the addition of fresh water before the well casing and screen tubing is lowered into the borehole, as the fluid no longer needs to support sand and gravel. The stoker will usually circulate the borehole from the bottom upwards while adding fresh water to reduce the viscosity or thickness of the fluid. Once the fluid has been sufficiently diluted, the casing and screen pipe are installed and the annular space is filled with gravel.

Wave Screen

The gravel fill, installed between the borehole wall and the outside of the well casing, acts as a filter to keep sand out and maintain the borehole wall over time. The thin layer of bentonite that prevents the well wall from leaking drilling fluid water out of the recirculation system during the gravel filling of the well now prevents formation water from entering the well.

This is where the well is developed to remove the thin bentonite layer or 'wall cake' that is left behind. Various methods are used to remove the wall cake and develop the well to its maximum productivity.

Some drillers use compressed air to blow the well, starting with the first screening interval and slowly working down to the bottom - blowing all the water off the top of the drill pipe, letting it recover and repeating the operation until the water is clear of sand from the well and relatively clean. If, after repeated airlift pumping and recovery cycles, the stoker is unable to find any sand in the water, then it is time to install a drilling development pump.

Additional development of the well can be carried out using a development pump with a higher capacity than the final pump installed. Just like the well's gas lift pump cycle, the development pump will cycle at different flow rates until the maximum capacity of the well can be determined. If the development pump can run briefly at a flow rate 50% greater than the permanent pump, the well should not pump sand.

Before selecting a mud rotary drilling method you may need to consider the following economic factors.

1. the cost of bentonite products, water supply and mud mixing equipment

2. the need and price of geophysical logging

3. the possibility of digging underground mud pits to store rock chips

4. the ability to discharge water from the development well into the ground

5. post-completion disposal of drilling mud and rock chips

6. the time required to develop the well compared to other drilling methods.

Over the decades, mud rotary well drilling rigs have found ways to make this particular system work for drilling and constructing domestic water wells. In some areas it is the ideal method to use due to the geological formations there, while other areas of the country tend to use the air rotary method.

Some rigs are equipped to drill using either method, so the contractor must decide which method will work best in your area, your well and your point in time.