It is not uncommon for the drillpipe to undergo tensile failure (twistoff) whilst drilling. When this happens, drilling has to stop and the drillstring must be pulled from the borehole. The part of the string below the point of failure will of course be left in the borehole when the upper part of the string is retrieved. The retrieval of the lower part of the string is a very difficult and time consuming operation.
The failure of a drillstring can be due to excessively high stresses and/or corrosion. Drillpipe is exposed to the following stresses:
1. Tension – the weight of the suspended drillstring exposes each joint of drillpipe to several thousand pounds of tensile load. Extra tension may be exerted due to overpull (drag caused by difficult hole conditions e.g. dog legs) when pulling out of hole.
2. Torque – during drilling, rotation is transmitted down the string. Again, poor hole conditions can increase the amount of torque or twisting force on each joint.
3. Cyclic Stress Fatigue – in deviated holes, the wall of the pipe is exposed to compressive and tensile forces at points of bending in the hole. As the string is rotated each joint sustains a cycle of compressive and tensile forces (Figure 3). This can result in fatigue in the wall of the pipe.
Stresses are also induced by vibration, abrasive friction and bouncing the bit off bottom.
Corrosion of a drillstring in a water based mud is primarily due to dissolved gases, dissolved salts and acids in the wellbore, such as:
1. Oxygen – present in all drilling fluids. It causes rusting and pitting. This may lead to washouts (small eroded hole in the pipe) and twist offs (parting of the drillstring). Oxygen can be removed from drilling fluids using a scavenger, such as sodium sulphate. Even small concentrations of oxygen (< 1 ppm) can be very damaging.
2. Carbon dioxide – can be introduced into the wellbore with the drilling fluid (makeup water, organic drilling fluid additives or bacterial action on additives in the drilling fluid) or from the formation. It forms carbonic acid which corrodes steel.
3. Dissolved Salts – increase the rates of corrosion due to the increased conductivity due to the presence of dissolved salts. Dissolved salts in drilling fluids may come from the makeup water, formation fluid inflow, drilled formations, or drilling fluid additives.
4. Hydrogen sulphide – may be present in the formations being drilled. It causes “hydrogen embrittlement” or “sulphide stress cracking”. Hydrogen is absorbed on to the surface of a steel in the presence of sulphide. If the local concentration of hydrogen is sufficient, cracks can be formed, leading rapidly to a brittle failure. Hydrogen embrittlement in itself does not cause a failure, but will accelerate failure of the pipe if it is already under stress or notched. Only small amounts of H2S need be present to induce fatigue (< 13 ppm). Special scavengers can be circulated in the mud to remove the H2S (e.g. filming amines).
5. Organic acids – These produce corrosion by lowering the pH, remove protective films and provide hydrogen to increase hydrogen embrittlement.
Although added chemicals can build up a layer of protection against corrosion, the fatigue stresses easily break this layer down, allowing corrosion to re-occur. It is this interaction of fatigue and corrosion which is difficult to combat.
Institute of Petroleum Engineering, Heriot-Watt University