Best Practices and Guidelines for Drilling Below Salt Formation

Below the salt, the drilling challenges are mainly due to a narrow margin between pore and fracture pressure in the rubble zone. As per standard practice of conventional drilling, high enough mud density is maintained to ensure that the hydrostatic pressure exerted by the hydrostatic column is above the pore pressure. This margin varies according to company policies, the well type, drilled formations, and reservoir fluid. In a narrow-margin situation, the mud column hydrostatic pressure is highly likely to exceed the fracture gradient, even with minor pressure fluctuations resulting in lost circulation situations.

Salt has high fracture strength and can be drilled with high mud weight with minimal risk of losses. Mud loss could still occur if shear zones or sutures are present within the salt body, but a predominantly clean salt bed can be drilled with high mud weight to avoid any salt creep issues. However, a pressure regression occurs in the rubble zone below the salt. While exiting the salt, the well must have high mud weight to counter the salt creep, but the rubble zone may not be strong enough to sustain that hydrostatic head, giving way and causing losses.   

Below are recommended best practices for drilling out of and below salt.

1)       Planning for narrow pressure margin:

a)   Optimize mud weight to drill salt: Calculate the salt balance load and use enough mud weight to hold back the salt. Increase mud weight if sticking situations related to salt creep are observed.

b)   Asses the transition: Consider running Seismic While Drilling or run a wireline seismic before the expected salt exit to ascertain the transition and being prepared.

c)    Do not run casing too soon: 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. After drilling through this short section, there could be more salt beds. If the casing is run too soon, the ensuing salt bed might need high mud weight again and require an additional casing. Hence, manage to drill until the first porous formation is encountered and confirm that the salt bed has ended.

d)   Including a Pressure While Drilling (PWD) tool in the BHA helps avoid excessive cutting loading in the annulus.

e)    Monitor for flow and adjust the mud weight accordingly: High pressure can occur right below the salt. Maintain a baseline for drilling fluid flow back, set all alarms for parameter changes, and monitor closely. Perform a flow check at every drilling break and follow the well control practices. 

2)       Controlled Drilling Practices:

a)       Controlling ROP is crucial to avoid annulus loading and have more response time.

b)       Determine and maintain optimum RPM-WOB before and after salt exit while drilling.

c)       Perform a flow check at each drilling break and connection and watch for indications of a formation change, such as a torque increase/decrease or change in its character.

3)       Managing Drilling Losses

a)   Prepare and discuss a pre-drill loss mitigation road map with the team.

b)   Pre-plan formulations and preparation to mix pills for combating moderate to severe levels of losses.

c)    Plan to deploy downhole tools to pump light to heavy LCM pills.

d)   Be vigilant for pressure regression that may lead to severe losses.

e)    Upon the first indication of losses, reduce the pump rate and initiate picking up.

f)      Assess loss rates and pump the required formulation of the LCM pill.

g)    Pull back to the shoe and allow the hole to stabilize.

4)       Equivalent Circulation Density (ECD) management:

a)   Condition mud to minimize gel and frictional losses.

b)   Plan tripping practices carefully.

c)    When tripping at certain intervals, break circulation slowly.

d)   Calculate swab and surge pressures when tripping and benchmark with a PWD tool.

e)    If a PWD tool is not in the hole, use a calibrated hydraulic model to estimate the casing running speed to minimize surge and swab pressures.

f)      In some cases, the mud weight may be close to pore pressure, requiring pumping out of the hole.

g)    Rotate the pipe before bringing the pump online and gradually increase pump speed to break gels gently.

h)   Plan short trips to minimize cutting beds during hole cleaning and use a PWD log to monitor the cutting load in the annulus.

Observe for any fill on the bottom when tripping back in.