When approaching the task of drilling a well, the driller has a number of things to consider. First would be the choice of drilling equipment. There are a number of choices and here are a few:
- Mud rotary
- Cable tool
- Reverse circulation
- Driven casing
- Auger drilling
The decision here will be based on the type of formation to be drilled, the dimensions of the well and the main use of the well. Available equipment and the lay of the land may also be factors that determine the equipment to be used. Knowledge of the area to be drilled is a key to a successful operation. This information can be obtained from:
- Personal experience
- Well logs
- Other drillers familiar with the area
- Geological information
- Well owners in the area
The type of drilling chosen will determine the needs to accomplish the job. If mud rotary drilling is the chosen method, drilling fluids come in to the picture. Here the driller’s choices really become varied. Some of the choices include:
- Water source for the drilling fluids
- Natural clays in the formation
- Bentonite and polymer
- Bentonite, polymer and other additives
One of the most important choices for drilling fluids is the water used to mix with the drilling material. If contractors and drilling consultants could take only one point from this article, this would be it. The quality of the make-up water is among the most important, if not the most important, factor.
There could be one of several problems with the choice of make-up water, but the main one we see is the pH. Depending on the source, the pH often is 7.0 or below. Bentonite drilling fluids, polymers and other additives perform best when the pH of the water is between 8.0 and 9.0. When lower pH is encountered, the addition of -pound to -pound of soda ash per 100 gallons of water will raise the pH to the proper level for drilling fluids.
What happens if the pH is not raised? The bentonite and polymer will not yield as well, mixing will be more difficult and some settling of solids in the slurry may occur. More bentonite or polymer will be needed to get the same result. This drives the drilling fluid cost up unnecessarily.
The cost of soda ash is minor when compared to the waste of bentonite or polymer and the cost in production and equipment wear. The recommended dosage of soda ash may cost as little as $0.05 to $0.10 per 100 gallons of drilling fluid, depending on the area and price of soda ash.
In our contacts with drillers, we find that some drillers are aware of the benefits derived from treating the make-up water. However, many others are not and we cannot stress enough the importance of having the proper pH. It is like the “icing on the cake,” in that it can cut costs and improve efficiency of the drilling operation.
Another choice the driller faces is the method used for mixing the drilling fluid. Bentonite is composed of many tiny platelets that need to be hydrated to get the best yield. An inch of bentonite can contain between 35,000 and 40,000 layers. The platelets in these layers need to be exposed to water in order to hydrate. This is why good mixing is so important. If the bentonite is not mixed properly it will not yield to its ultimate potential and more material will be required to do the same job. The same is true of polymers. A venturi hopper is often recommended for good mixing. The manufacturing and placement of this hopper is very important so that good mixing occurs when the bentonite comes in contact with the water.
In addition to good mixing of the fluid, there are a number of additives that can be added to bentonite drilling fluids to improve performance when necessary. Swelling or sticky clays and unconsolidated formations are a couple of examples of drilling problems that could be minimized by the use of polymers or additives. Listed below are some possible drilling scenarios and products that can make a difference in overcoming them.
The final choice for drillers that we want to consider here is the choice of protecting the drilling fluid during the drilling process. During the drilling operation, the drill fluid is designed to suspend and transport the drilled cuttings. Another key to successful drilling is the removal of these cuttings when the fluid returns to the surface and before it enters the borehole again. If the pit used for containing the drilling fluid is designed properly, it will both slow the flow of fluid and change its direction in order for the cuttings to drop out in the pit. A series of baffles in the mud pit can accomplish this.
If the mud pit does not clean the fluid well enough, desilters or desanders can be used to mechanically remove the cuttings. The importance of using this type of equipment cannot be over-emphasized here. The weight of the drill fluid should be maintained between 8.6 and 9 pounds per gallon. If the weight starts to climb over 9 pounds, action needs to be taken to lower the weight.
In summary, there are many choices facing the driller. These choices can make a difference as to how successful the drilling will be. We encourage anyone involved in drilling to take advantage of as much of the available industry education as possible. There are many technical articles, bulletins and books written about drilling fluids. Classes are also routinely offered for education on this subject. If you have questions on the points discussed in this article, please contact your drilling fluids distributor or manufacturer, or contact me through National Driller.
Bob Oliver is regional manager for CETCO Drilling Products.