5 - Salt Dome Drilling...What Are The Challenges?
Updated: Sep 6, 2018
Salt domes are important structures for Petroleum Geologists. Their unique features of ductility and impermeability made them ideal to trap Oil & Gas in relatively large hydrocarbon pools.
How Is A Salt Dome formed?
Salt rocks are the result of sea-water evaporation, followed by the formation of overlying sedimentary rocks after a sedimentation process.
While this article is focused on drilling challenges, we will briefly touch on a couple of hypotheses that explains how salt rocks are formed into a dome. There is agreement on the origins of salt rock creation, However; a debate remains on how salt rocks took the shape of a dome. Below is a couple of hypotheses:
Salt moves upward due to a difference in buoyancy. As overburden pressure increases from the overlying sedimentary rocks, this increase in pressure and compaction has little effect on salt density. This makes salt lighter in density compared to surrounding rocks which results in a significantly higher buoyancy, which pushes salt upward into forming a dome-shape structure.
Salt is pushed upward due to a downward force. As more sedimentary rocks are formed on top of salt formation, increased overburden pressure generates enough vertical stress (downward) which deforms salt into a dome-shape.
Despite the relatively low compressive strength of salt rocks, drilling remains challenging. We will touch on two aspects of drilling:
WBM or OBM, Which One To Choose?
Depending on the application and associated risks, both Water Based Mud (WBM) or Oil Based Mud (OBM) can be used. However, the following issues need to be taken into consideration:
Mud Losses: WBM is preferred if there is a risk of mud losses due to cost and ability to build mud system. Also, mud losses might result in loss of well control, which makes mud system selection crucial to safety and environment.
Wellbore Washout: WBM usually results in hole enlargement (depending on salt-saturation). If wellbore washout is a big concern, then OBM might be favored (considering an elevated Oil / Water ratio).
Regulations: especially in offshore applications, regulators are more inclined toward WBM usage. Regulatory bodies might impose: 1) restrictions on OBM usage or 2) excessive compliance-requirements, making OBM economically less attractive.
Salt Creeping: Since salt is ductile and relatively deforms easily, salt creeping (with varied severity from a field to another) might result in increased Torque and Drag during drilling and tripping operations. It is always easier to solve salt creeping with a Fresh Water Pill (FWP), but the chosen mud system and formation reaction to fresh water might impose restrictions.
Reactive Shale: embedded shale formations might favor OBM over WBM. this depends on shale reactivity and ability to inhibit WBM.
Desired Mud Weight: depending on the driver for MW increase (whether Well Control or Wellbore Stability), each mud system has limitations in MW increase. Therefore; the type of additives and their availability should be taken into consideration when choosing the mud system.
Directional Drilling, How Easy It Is To Execute?
Salt Dome might seem easy to drill due to its solubility in water. However; this solubility might results in issues when it comes to directional drilling.
Loss of bit steerabilty: Washouts are common in salt-formation if drilled with WBM. The level of Salt Saturation needs to be carefully chosen, and the flow rate needs to be controlled to minimize washout in front of the bit.
Angle Build-Up: kick-off is usually initiated in a competent formation (such as Anhydrite). Presence and thickness of such competent formation within the salt dome is essential for directional work.
Stuck Pipe & Tight Hole: A combination of mud weight and salt-saturation plays a role in overcoming salt creeping. In case of stuck-pipe in salt, a Fresh Water Pill (FWP) might release the stuck (with respect to all above considerations).
Any Other Risks To Consider?
Salt Water Kicks, and Reactive Shale below the salt dome are common problems that need to be accounted for.
To Conclude, there are several inter-related factors that need to be considered when designing a well that crosses a salt dome, especially when directional work is involved. A comprehensive risk assessment should be conducted to determine the best course of action and to minimize drilling risks.
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