Challenges and Mitigations for Salt Drilling
Mechanism and Characteristics of Salt Formations and Domes Your Comments
Salt domes primarily comprise rock salt (halite), often mixed with minor amounts of other evaporite minerals like anhydrite, gypsum, or potash.
Salt has a lower density (about 2.2 g/cm³) than surrounding sedimentary rocks (2.5–2.7 g/cm³). Due to its lower density, salt is buoyant relative to the denser overlying sedimentary rocks and moves upward, forming structures known as salt domes or diapirs. As salt moves, it can push and warp the overlying sedimentary layers, creating folds, faults, and other complex structures.
The density contrast between salt and surrounding rocks results in strong seismic reflections, making salt bodies relatively easy to identify in seismic surveys. However, salt bodies can also distort seismic waves, creating challenges in subsurface imaging.
Above salt domes, a top layer of minerals like limestone, anhydrite, or gypsum forms the cap rock.
Salt is highly impermeable and soluble in water,
The salt is ductile, which allows it to flow under pressure. This behaviour is known as ‘Creep.’
Salt creep highly depends on the overburden pressure, temperature, moisture content, and salt composition. The creep tendency is more likely and severe at the bottom of the thick salt bed.
Salt formations are considered to have high fracture resistance and lack natural porosity and permeability.
Operational Implications for Drilling Your Comments
Salt is ductile and flows over geologic time, forming overhangs, cavities, and unpredictable structures.
When drilled, they undergo plastic deformation due to deviatoric stress (creep), so they can deform and close around the borehole, leading to stuck pipe, wellbore collapse, or bit sticking.
Salt is water solubility makes it susceptible to washouts or dissolution in aqueous mud, creating massive washouts and poor hole cleaning.
Salt exerts additional stress on the adjoining formations. The altered stress field can cause the formations to become unstable.
Salt can create over-pressurized zones in adjacent rocks, complicating drilling operations.
There could be a shale sheath that may also be encased within salt, called a lens. Such lenses may exhibit trapped pressure when penetrated.
Although salt formation lacks porosity and permeability, the possibility of encountering a flow while drilling a thick salt bed cannot be ruled out. High-pressure sand lenses embedded within a salt formation or fissures connected to a high-pressure zone can flow on penetration.
Mitigation Strategies for Salt Drilling Challenges Your Comments
Employ advanced technologies and strategies:
Seismic Mapping: High-resolution seismic surveys help map the dome’s structure, identifying faults, caprock, and salt boundaries before drilling begins.
Specialized Drilling Fluids: Non-aqueous or high-salinity drilling fluids minimize salt dissolution.
Robust Casing Design: Stronger casing materials and designs are employed to withstand salt creep and pressure.
Use 3D seismic imaging and velocity models to detect salt boundaries and bodies.
Choose a salt entry location with the flattest salt formation profile. The seam at the salt diapers has more stress, so entering it at an angle poses higher formation instability.
It is recommended to isolate the shallow, weaker formation before drilling the salt zone and set the casing well into the salt. This ensures the casing covers "dirty salt," inclusions, and other troublesome geologic features behind the pipe.
Real-Time Monitoring: Continuous monitoring of pressure and stress during drilling helps detect and manage anomalies.
Use appropriate mud weight based on salt balance load computations to avoid salt creep during drilling.
Drill with minimal stoppage to avoid time-dependent creep.
Logging while drilling (LWD) tools detect when the salt is exiting and prepare it for casing setting.
Conventionally, the salt section is cased before drilling the remaining part of the well in separate hole sections. However, it is not uncommon to encounter inclusions that could be misread as a rubble zone and the end of the salt section. Evaluate carefully to ensure that there is no more salt to be drilled before running casing.
Corrective Actions to Handle Salt Drilling Challenges Your Comments
Due to its plastic nature, salt tends to close around the drill string, especially under high-pressure and high-temperature conditions. If the string gets stuck while drilling salt, circulate the freshwater pill around the BHA to dissolve the salt around the stuck section.
Increase mud weight to hold the salt back while drilling the section.
If washouts occur, evaluate whether the deviation profile is maintainable, and consider sidetracking if the wellbore geometry cannot be recovered.
Run casing as soon as the salt zone is drilled to TD.
Monitor for signs of washouts and torque/drag increases.
If a high creep rate is encountered, drill a slightly larger hole (oversized) to provide a buffer against creep. This extra space allows more time to run casing and properly cement it before the salt closes.
Contingency Measures for Drilling Salt Your Comments
Saturated brine systems (sodium chloride or calcium chloride) can be used for drilling salt. These help if saturated with the same salinity as the salt formation. However, keep premixed oil-based mud to quickly replace it if hole washouts are seen with the saturated brine system.
Keep a sufficient volume of freshwater on standby for emergency spotting pills.
Plan for underreamers or expandable reamers as a backup option if creeping salt reduces the hole size.
Keep high-strength casing string available on standby.
Caliper logging tools to evaluate hole geometry across salt.
Prepare and keep ready an alternate well path with a suitable kick-off point in case the original becomes unusable. Maintain sidetrack equipment and tools (whipstock, motors) on standby.
Set the salt section's acceptable creep rate threshold (e.g., mm/hour). Use real-time data (torque, drag, caliper logs) to monitor deformation. If the creep rate exceeds the threshold, initiate corrective actions (e.g., ream, enlarge the hole, or run casing).