Groudwater - How it works

VALLEY GROUNDWATER – HOW IT WORKS (page 2)

The Development of Groundwater in Unconsolidated Alluvial Formations.

There are many different methods by which water wells can be constructed; these are the most common. Generally speaking, most other drilling methods employ one or more of these fundamental drilling methods discussed.

Hansen Well-Do Service Inc. excels in the design, drilling and construction of large diameter, high capacity water wells completed in unconsolidated formations. Common design features will include the installation of casing to the bottom of the well to the surface, gravel pack material over the length of the production intervals, installation of cement seals between the out side of the casing and the inside of the bore hole for ground water protection, and an observation tube to monitor static and pumping water levels.

Bucket Rigs

Bucket rigs are used for large diameter, shallow holes. As drilling proceeds, material is excavated by a cylindrical bucket fitted with auger-type cutting blades on the bottom. When the bucket is full, it is raised above the drive table and swung off to the side by the dumping arm.

Bucket method is commonly used to install a surface conductor casing for water wells. Surface conductor casings are often used to stabilize the upper portion of a bore hole when completing a water well by either the direct mud rotary method or reverse circulation method.

Drilling in Unconsolidated Formations Direct Circulation Mud Rotary

The direct circulation mud rotary method utilizes a rotating bit placed on the bottom of a drill pipe. Cuttings are removed by continuous circulation of drilling fluid as the bit penetrates the formations. In the direct mud rotary system, drilling fluid is pumped down though the drill pipe and out through jets in the bit; the fluid then flows upwards in the annular space between the bore hole and drill pipe, carrying the cuttings in suspension to the surface. At the surface, the drilling fluid is picked up by a mud cleaning system were all solid material is removed. The drilling fluid is then re-circulated by the use of high pressure pumps Borehole stability is maintained by the use of a bentonite based drilling fluid and hydrostatic head pressure. The weight of the column of fluid above the static water level maintains sufficient force to hold the walls of the borehole open.

A direct mud rotary rig must be equipped with a mud pump capable of sufficient pressure and volume for the diameter of the hole being drilled to maintain the required up-hole velocity for drill cutting removal from the borehole.

A successful direct circulation rotary program is often dependant on the informed use of drilling fluid.

An essential part of an efficient direct mud rotary operation is drilling fluids control. The viscosity (degree to which a fluid resists flow under applied force) of the drilling fluid and the up-hole velocity required to remove cuttings will depend on a number of factors. Many drilling fluids develop gel strength, that is the ability to suspend cuttings, when flow slows or stops.

Direct Rotary Mud Cleaning/Cleaning

Key Element In Good Direct Rotary Practices

> Aids in representative samples
> Lessens fluid invasion of production interval
> Necessary for gravel pack installation in completed wells

The test hole should be drilled with mud cleaning equipment that insures that clean mud is used to recover samples of water bearing formation. Highly productive water bearing formations can contain high amounts of clean sand, that sand should be removed from the drilling fluid to assure that representative samples have been recovered from each of the different water bearing zones that are penetrated in the exploratory test hole.

The drill cuttings on the left were removed by de-silting equipment operated by the mud cleaning system. Had this material not been removed, it is fine enough that it would have been re-circulated in the bore hole. It would not be known where this fine grain material originated in the bore hole and would compromise the water well construction phases of the project. Also this material is abrasive and increases wear on mud lines, pumps and drill pipe.

Also removed by the mud cleaner is the material on the right. This coarse grain material was removed from the drilling fluid by a coarse vibrating screen before reaching the de-silting equipment

Large Hole Mud Cleaning Equipment

The fabrication of drilling equipment can be made to order for the size of a project. Bigger, deeper holes require large volumes of drilling fluid.

Drilling large diameter deep wells with the direct mud rotary method requires large equipment. The fluid circulation and mud cleaning equipment capacities must be large enough to maintain sufficient up hole velocities and low mud weights.

Drilling In Unconsolidated Formations – Reverse Circulations

In the reverse circulation method of drilling, drill cuttings are removed from the borehole with the use of high velocity water moving through the inside of a large diameter rotating drill string. In a borehole flooded with water, air is injected inside the drill pipe below the water line. As the air bubbles come to the surface, water inside the drill string is forced up and is discharged into a settling pit. The re-circulated drilling fluid travels back to the bit via the inside of the borehole and outside of the drill pipe where the process is repeated. Solid drill cuttings are removed from the pit with a backhoe and the drill fluid is returned to the borehole to repeat the process.

Reverse circulation method usually does not involve the use of benonite based fluids. The drilling fluid is more commonly water where the native clay material will be carried in suspension. This method of drilling can require large amounts of water to maintain hole stability. The borehole must be kept full of water to maintain a positive hydrostatic head pressure to hold formations open. A 7 to 10 foot column over the static ground water level is required.

A pit is excavated with a return trench to the borehole next to the drill rig in the reverse circulation method. As the bore hole is advanced, drilling fluid carrying solid drill cuttings is discharge into the pit. The solid material is dug out of the pit with a back hoe.

By properly maintaining the capacities and configuration of the pit, finer particles of drill cutting and sand are allowed to settle out. It is also necessary to manage the introduction of clear water into the drilling fluid. If care is not used, it is common to get conditions of fluid imbalance in the borehole.

Reverse Rotary Fluid Circulation Tubs

Some drilling locations require modification to normal drilling procedures. The fabrication of special equipment can sometimes be essential for the timely completion of a project.