Introduction to Sepiolite Clays in Potassium Brine HPHT Drilling Fluids

Drilling fluids play a vital role in oil and gas drilling operations, especially in high-pressure, high-temperature (HPHT) environments. These fluids, often brine-based, are used for various functions, including well pressure control, cuttings removal, and wellbore stabilization. However, maintaining fluid performance at elevated temperatures and pressures is a constant challenge.

Sepiolite, a magnesium silicate clay with a unique fibrous structure, has emerged as a promising additive for HPHT drilling fluids. When dispersed in drilling fluids, sepiolite exhibits thixotropic properties, meaning its viscosity decreases under stress and recovers once the stress is removed. This property is crucial for suspending weighting materials and cuttings during circulation while allowing the fluid to gel and prevent settling when circulation stops.

Sepiolite offers several advantages over traditional viscosifiers in potassium brine-based drilling fluids:

• High-temperature stability: Sepiolite maintains its stability and rheological performance at high temperatures, making it suitable for HPHT applications. Other viscosifiers, such as polymers, can degrade at elevated temperatures, compromising fluid properties.
• Salt tolerance: Sepiolite is compatible with high salt concentrations, an essential characteristic for brine-based drilling fluids. Unlike some polymers that can flocculate or lose viscosity in the presence of salt, sepiolite retains its rheological properties.
• Minimal formation damage potential: Sepiolite is considered a non-damaging material, making it an attractive option for use in sensitive formations. Its fibrous structure and low density minimize the risk of pore clogging, ensuring optimal wellbore flow.

Sepiolite clays can be used in combination with potassium formate (KFo) brine to create stable, high-performance drilling and completion fluids for HPHT applications. KFo is a popular completion brine due to its non-damaging nature and its ability to inhibit clay hydration. However, the maximum density of saturated KFo brine is 13 pounds per gallon, limiting its use in high-density wells.

To overcome this limitation, weighting materials such as manganese tetroxide (Mn3O4) can be added to KFo brine. Mn3O4 is an acid-soluble weighting material compatible with KFo brine and has a high density, allowing the formulation of high-density fluids. Sepiolite acts as a viscosifier to suspend Mn3O4 in KFo brine, preventing sedimentation and maintaining the desired rheological properties.

Example of a completion fluid: This describes a simple completion fluid that uses treated sepiolite as a viscosifier in KFo brine with Mn3O4 as a densifying agent. The components and their concentrations are listed below:

• Mn3O4: 184 lbm/bbl
• Treated sepiolite: 2.1 lbm/bbl
• HPHT dispersant: 2.5 lbm/bbl
• KFo brine (1.57 SG): 306 ml
• KFo powder: 179.8 lbm/bbl
• CaCO3 (medium, 20 μm): 7.9 lbm/bbl

Considerations when using sepiolite:

• Dispersion: Sepiolite must be properly dispersed in the brine to achieve optimal rheological performance. Using treated, easy-dispersing grades like Rheoplus FSC20 can significantly enhance dispersion efficiency and overall fluid performance:

• API fluid loss: Sepiolite suspensions can have very high API fluid loss, so it's important to control the rheological and fluid loss properties when using it in drilling operations.
• Thermal transformation: Sepiolite can partially transform into smectite at high temperatures, which may cause a slight increase in rheology. This effect can be mitigated by reducing the initial concentration of sepiolite.

In summary, sepiolite has proven to be a valuable additive for potassium brine-based HPHT drilling fluids, offering high-temperature stability, salt tolerance, and minimal formation damage potential. Its use in combination with KFo brine and Mn3O4 allows for the formulation of high-density, high-performance fluids that address the challenges of demanding HPHT drilling operations.