Selection Criteria of Drilling Fluids


The most important criterion when designing for drilling fluid program is minimum overall well cost. Exploration and production issues, environmental impact, performance and logistics are others factors which are involved in drilling fluid program. The drilling fluid program is designed according to many factors; these factors are discussed below:

1- Environmental Impact 

Environmental issues are still the important considerations when planning and implementing for the drilling fluid. For example, in offshore operations, synthetic or mineral oil based systems are used rather than diesel oil based system. 


Fig 01- Deep-Water Horizon oil spill in The Gulf of Mexico

2- Well Type

Drilling can be generally classified into two categories: development operations and exploratory operations. In development drilling, the geological conditions are well known which can help to design effectively the drilling fluid program and optimize the other programs which are related to it like hydraulic design. In exploratory drilling, the information about the region are not enough to design mud properties. The designed mud system has to contribute in obtaining the geological information easily and safely. The mud composition has to allow for quick change in case of unexpected changes or problems.

3- Problems Related to Formation Types

Anhydrite

When using bentonitic drilling fluid through anhydritic formation, the released calcium ions into the mud system can retard the bentonite hydration which can affect the mud viscosity and the fluid loss. There are two solutions to overcome the presence of ions. They can be treated out of the mud system by adding sodium carbonate (Na2Co3) or converting the drilling fluid to an inhibitive system. 

High Temperature Environment

In the case when drilling in high temperature environment, the effectiveness of chemical additives is generally reduced and can lead to changes in viscosity and fluid loss. Drilling fluids which can resist to such environment can be used like the oil based mud systems.

Abnormal Pressure formation

When drilling through abnormal pressure formations, fluids can flow into the mud system leading to mud contamination or undesirable kicks. An adequate mud weight has to be planned for such formations to avoid these types of problems.

Loss Circulation Intervals

The drilling fluids can be lost into the formation. This problem can classified as partial loss circulation and total loss circulation. In the first case, the flow rate out of the hole is less than the inlet flow rate. In the second case, no mud return is occurring at the surface. The lost circulation can occur if the fracture pressure is exceeded when circulating the drilling fluids or the formation is naturally fractured where the mud can flow in it.

Shale

Many problems can be encountered when drilling through the shale intervals, it can be listed as follows:
- Mechanical pipe sticking
- High torque and drag
- Hole cleaning difficulties
- Drilling fluid contamination

4- Drilling Rig

The selection of the drilling fluids can be limited by the mud control system (solid control system and chemical treatment) and the available hydraulic horse power in the circulating system. Achieving to the planned targets can be related directly to the rig selection and its layout.

5- Casing Program

Casing setting depth is planned according to many points such as changes in pore pressure trend, lithology and isolation of troublesome zones. These requirements are also used to design the drilling fluid program. The optimization of the drilling fluid program can allow for drilling deeper section which reduces the overall well cost.

6- Water Requirement

The availability of water used in making the drilling fluid has to be considered when designing for the mud program. If the making water is available on site, then diluting can be the most economical treatment. In the other case when the water is transported to the rig site, optimizing the solid control treatment is the best choice. 

7- Corrosion

The drill string can fail by the combination of the corrosion and the mechanical cyclic loading. The dissolved gases like Co2 and H2S in drilling fluids reduce the life of the drill pipe significantly. 


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