Common Drilling Troubles
Several downhole complications can occur while drilling, adversely affecting the time, cost, and quality of the well. Below is a simplistic explanation of possible drilling troubles that can be faced while drilling a well.
Pipe or Equipment Sticking:
What Happens: The pipe or downhole equipment gets stuck in the well and cannot be moved, rotated, or pulled out.
Why it happens: This can occur if mud pressure is too high (differential sticking) compared to the formation pressure and the string is stationary for a longer time, the pipe gets wedged in a narrow spot (mechanical sticking), excessive cuttings build up around the pipe, or the hole collapses while the drilling tools are in the hole.
How to prevent it: Use appropriate mud weight, ensure drilling fluid properties to reduce friction and avoid thick mud cake, maintain hole cleaning efficiency to avoid cuttings buildup, closely monitor the wellbore conditions all the time and maintain smooth well profile.
How to fix it: The strategy for freeing a stuck pipe largely depends on the mechanism and the reasons for the stuck. To free the stuck pipe, you can use jars (tools that deliver a sudden impact), back reaming, or lubricants to loosen it. The stuck point can be estimated by measuring stretch or using a free-point indicator.
Wellbore Instability:
What Happens: The wellbore walls collapse or crumble.
Why It Happens: Rubble or fractured formations break, water-sensitive shale could react to water in the drilling fluid and cave into the wellbore, or the drilling fluid column's hydrostatic pressure is insufficient to keep the wellbore stable.
How to Prevent It: Use a balanced mud weight to stabilize the well and use inhibitive mud to minimize shale reactivity.
How to Fix It: Optimize the mud system and adjust the drilling technique to reduce pressure fluctuations if instability occurs. You may also need to stabilize the zone with casing or liners.
Blowouts and Well Control:
What Happens: Uncontrolled oil, gas, or fluids flow from the well.
Why It Happens: Formation fluid enters the well because the formation pressure exceeds the drilling fluid’s hydrostatic pressure. The drilling fluid in the well is pushed out by the formation fluid, which can flow uncontrollably if the secondary controls on the surface fail to contain the flow. Indicators: Sudden increase in mud return flow, gas bubbling at the surface, or loss of mud weight.
How to Prevent It: Use appropriate mud weight. Closely monitor the well conditions and pressure to maintain primary control of the well. Test blowout preventers (BOPs) and conduct regular drills to ensure crew awareness.
How to Fix It: Close the BOP, circulate out the formation fluid that entered the wellbore, kill the well with heavier mud, and regain control by balancing pressures.
Environmental Impact:
What Happens: Drilling activities harm the environment.
Why It Happens: Spills, leaks, improper waste disposal, or poor operational practices.
How to Prevent It: Follow strict environmental guidelines, use containment systems for spills, and properly manage waste.
How to Fix It: In case of a spill, contain it immediately, clean up, and remediate the affected area.
Well Collision:
What Happens: One well unintentionally intersects another well.
Why It Happens: Poor planning, inaccurate information, or directional control.
How to Prevent It: Use anti-collision software and correctly map nearby wells.
How to Fix It: If it happens, stop drilling, evaluate the damage, plug back, and redesign the trajectory.
Water Influx:
What Happens: Water flows into the well unexpectedly.
Why It Happens: Weak formations or water-bearing zones.
How to Prevent It: Use mud with sufficient weight and plan to isolate water zones with casing.
How to Fix It: Plug the zone with cement or adjust mud properties.
Gas Influx:
What Happens: Formation gas enters the wellbore.
Why It Happens: Formation pressure exceeds the drilling fluid’s hydrostatic pressure in the well.
How to Prevent It: Keep mud weight balanced and monitor for gas kicks.
How to Fix It: Shut in the well and circulate out the gas and regain control of the well using the proper well control procedures.
H2S Zones:
What Happens: Hydrogen sulfide gas, a very toxic gas, enters the well.
Why It Happens: Drilling through H2S-rich formations.
How to Prevent It: Use H2S monitors, train the crew, and treat the mud with H2S scavengers.
How to Fix It: Evacuate the area if necessary, circulate out the gas, and treat mud with H2S scavengers.
Shallow Gas:
What Happens: Gas flows from shallow formations.
Why It Happens: Insufficient mud weight or no protective casing.
How to Prevent It: Identify shallow gas zones in pre-drill surveys and take precautions while drilling.
How to Fix It: Close and divert to circulate the gas out of the well. Cement the zone if needed.
Low Drilling Rate of Penetration (ROP):
What Happens: Drilling progress slows significantly.
Why It Happens: Dull drill bits, hard formation, balled-up bit, poor bit hydraulics, improper drilling parameters, improper drill bit selection, improper drill string design, or high vibrations.
How to Prevent It: Use suitable drill bits for formation type and characteristics. Optimize drilling parameters (weight, RPM, and flow rate), reduce downhole string vibrations, and maintain optimum bit hydraulics.
How to Fix It: Replace the bit, adjust weight or RPM, redesign the drilling assembly to reduce vibrations, and ensure efficient mud properties and circulation to remove cuttings.
Uncontrolled Hole Deviation:
What Happens: The wellbore deviates unintentionally from its planned trajectory.
Why It Happens: High formation dips, drill string buckling, improper drilling parameters or drilling assembly configuration.
How to Prevent It: Use stabilizers and centralizers to guide the drill string and regularly check the trajectory using survey tools. Use rotary steerable systems for complex well trajectories.
How to Fix It: Adjust the drilling assembly, adjust to the appropriate weight on the bit, or use rotary steerable systems to correct the deviation.
Loss of Circulation:
What Happens: Drilling mud flows out of the well into the formation.
Why It Happens: Fractured, vugular, or porous formations, the high hydrostatic pressure of the drilling fluid induces fractures in the formation.
How to Prevent It: Use lost circulation materials (LCMs) in the mud and monitor mud losses closely.
How to Fix It: Stop drilling and pump LCM pills to seal the formation. If severe, use cement plugs to restore circulation.
Drill String Vibration Downhole:
What Happens: The drill string vibrates excessively, leading to equipment damage.
Why It Happens: Poor bit design, unstable drilling parameters, improper drill string design, or resonance with formation conditions.
How to Prevent It: Use vibration dampeners and balance drilling parameters like RPM and weight on the bit.
How to Fix It: Adjust drilling parameters and replace the bit or dampeners if needed. Evaluate and implement changes to the drilling assembly configuration.
Drill String Damage on Surface and Downhole:
What Happens: Drill string components suffer thread damage, over torque, wear, cracks, or breaks.
Why It Happens: Improper handling, overloading, or fatigue from downhole conditions.
How to Prevent It: Follow proper handling procedures, inspect equipment regularly, and optimize drilling parameters to avoid excessive vibrations.
How to Fix It: Replace damaged components immediately and analyze failures to prevent recurrence.
Drill String or Equipment Corrosion:
What Happens: Metal components corrode due to chemical reactions with drilling fluids or formation fluid.
Why It Happens: Exposure to corrosive substances like H2S or saltwater.
How to Prevent It: Use corrosion inhibitors in the mud and corrosion-resistant materials for the string.
How to Fix It: Replace corroded parts and adjust the mud chemistry to reduce further corrosion.
Mud Contamination:
What Happens: Drilling mud properties are compromised.
Why It Happens: Influx of formation fluids, salt, or other contaminants into the mud system.
How to Prevent It: Maintain good wellbore isolation and monitor mud properties constantly.
How to Fix It: Treat the mud to remove contaminants or replace it with fresh mud if necessary.
Hole Cleaning Issues:
What Happens: Cuttings accumulate in the wellbore, hindering drilling progress.
Why It Happens: Low mud flow rate, improper mud properties, or insufficient drill pipe rotation.
How to Prevent It: Optimize mud circulation, use high-viscosity sweeps, and maintain proper drill string rotation.
How to Fix It: Increase circulation rate, pump cleaning sweeps, or use specialized hole-cleaning tools.
Formation Damage:
What Happens: The reservoir near the wellbore is damaged, adversely affecting production.
Why It Happens: Invasion of drilling fluids, fines migration, or chemical reactions.
How to Prevent It: Use reservoir-friendly drilling fluids and minimize mud invasion.
How to Fix It: Perform acidizing or fracturing treatments to restore reservoir permeability.
Equipment Failure Downhole:
What Happens: Tools or equipment break or malfunction in the well.
Why It Happens: Excessive wear, improper handling, or extreme downhole conditions.
How to Prevent It: Regularly inspect and maintain equipment and ensure it is suitable for the wellbore conditions.
How to Fix It: Retrieve the failed equipment and replace or repair it.
What Happens: The wellbore path becomes overly curved or twisted.
Why It Happens: Poor directional control or geological factors.
How to Prevent It: Use proper directional tools and closely monitor trajectory.
How to Fix It: Smooth out the well trajectory using rotary steerable systems or adjust the drilling assembly.
Formation Integrity Test Failure:
What Happens: The formation doesn’t hold pressure during testing.
Why It Happens: Weak formations or poor cementing.
How to Prevent It: Optimize cementing and pre-drill assessments.
How to Fix It: Remedial cement job, stress caging to strengthen the formation, or redesign the well to avoid high-pressure zones.
Casing Collapse:
What Happens: The casing collapses under external pressure.
Why It Happens: Weak casing material or high external loads. Poor well design or inaccurate geological information.
How to Prevent It: Use casing with appropriate strength and proper cementing.
How to Fix It: Mill out the collapsed section and run a new casing string or an expandable liner, if feasible.
Bad Cement Behind Casing:
What Happens: Poor cement bonding leaves gaps or channels.
Why It Happens: Improper cementing techniques or poor mud removal before cementing.
How to Prevent It: Use pre-cementing washes to ensure good hole cleaning and use centralizers for good cement placement.
How to Fix It: Perform remedial cementing or squeeze cementing.
Financial Risks (Well Cost Overrun):
What Happens: The well ends up costing much more than planned.
Why It Happens: Poor planning, delays, unexpected problems, or inefficient operations.
How to Prevent It: Develop a detailed budget, plan for contingencies, and use efficient practices to minimize downtime.
How to Fix It: Analyze cost overruns, identify the root causes, and apply corrective measures for future wells.