Choosing the right fluid for your project

Generally, the greater the degree of complexity, the more difficult is the decision. In reality, most wells require several different systems since many different formations with varying challenges are to be drilled. Often, the drilling engineer will break the project into intervals starting with the casing design. He or she must design the casing program that will determine the intervals involved in mud selection.

The surface hole will usually be drilled with one system, usually a simple spud mud and may involve some lost circulation potential. An almost universal criteria is that the ground waters are to be protected from contaminants, particularly heavy metals and salts. Another important part of the surface hole is that it is adequately cemented. The way the mud prepares the hole for cementing is often overlooked and can play an important role in cementing success.

Below surface casing, whether the section is an intermediate or production interval, there is generally a section that is typically drilled with water. This is usually done to take advantage of fast penetration rates and to reduce fluid costs. Care must be taken that these advantages do not cost more later. One problem that typically occurs is that the water drilling weakens clay and shale beds which gradually deteriorate and enlarge. These zones can also become unstable over time so that sloughing and instability occur. These weak zones also become loss zones if gradually higher mud weights are employed thoughout the interval.

If there is an abnormal pressure interval, weighted fluid will be prescribed. Depending on the circumstances, barite is usually the weighting agent simply because it has a high density and is relatively inexpensive. Utilization of barite requires the fluid have the ability to suspend the barite particles. Alternate weighting agents are calcium carbonate, hematite, salts, brines, and formates.

Many production zones are now depleted and have very low pressure. This makes them susceptible to invasion and can be severley damaged by the drilling fluid. Careful planning of the payzone is required, especially when depleted or underpressured. May of these zones are susceptible to water invasion, usually because they contain clays which can swell on water contact and may block production. It is often the case that casing must be set above these payzones so that a specialized payzone fluid, or underbalanced fluid can be used to prevent the formation damage.

Some production zones, as well as shale and clay sections may require inhibitive fluids to prevent formation damage, or promote borehole stability. Inhibitive systems are oil muds, silicate systems, potassium systems, aphron systems, and other specialty fluids. Inhibitors include salts, such as KCl, polymers, methyl glucoside, asphalts, gilsonites, and filming barrier inhibitors.

Pnuematic fluids, such as foams, UBD, and aphron systems can enhance drilling performance and prevent formation damage. They can be used as stand-alone systems, or modified to meet the challenges of each drilling interval. For instance, foams can be modified for UBD, then modified as aphron fluids, aphrons can be used as weighted fluids. This allows pneumatics to become bridge technologies where the same fluid can bridge many different intervals yet meet very different challenges. Low pressure or depleted zones can be drilled together with normal or abnormal pressure zones, yet without losses or damage. This may allow the drilling engineer to modify the casing program since a string might be eliminated.
The thoughtful consideration of a fluids program will include environmental concerns. Each system and product must be evaluated for environmental impact. The evaluation should include specialty products that might be required such as biocides, corrosion inhibitors, lubricants, and other additives that might be required. Use of systems with enhanced properties may preclude the use of these specialties, for example, fluids with good lubricity and low corrosion tendencies may not need additional additives.

Waste minimization is another important part of the fluids program. The engineer should select fluids that minimize the volume generated in drilling. Bentonite based fluids, for example, require continual water addditions while drilling. This generates an excess volume for the hole drilled which must be treated and sent to disposal at some point. In order to provide a true waste minimization program, it is helpful to start with a fluid that does not require continual additions and creates no excess volume. Certain polymer systems, when properly managed, and the aphron systems have this feature since they are very efficient at preserving cuttings and bringing them to the surface for removal. They also hold the water phase effectively so that it is not lost and require very minimal water additions. These fluids are also generally environmentally neutral since they are organic and biodegradeable. With diligent care in maintaining these fluids, they can be stored and reused, and when the project is finished, they are candidates for landspreading. This represents the most complete waste minimization program which can be accomplished with good high speed shakers and a properly working centrifuge.

In summary, fluids planning is a crucial part of the effective drilling program. When properly done, the fluids specialist must have good understanding of the systems and products available, and efficiently blend and bridge the technologies so that drilling efficiency, formation protection, and economics are considered. The properly engineered fluids program will anticipate and prevent problems while maximizing the end game, that is, oil and gas production.