In drilling industry, it is common that drill string will fail while drilling. Two main factors causing drillstring failure are stresses and corrosion.
Stress Affects on Drillstring
Drillstring is exposed to the following stresses:
Tension – Suspended weight of drillstring sometimes can be several thousand pounds. Additionally, overpull weight while pulling out can be over drill string limit resulting failure (see Figure 1).
Figure 1 – Drillstring in Tensile
Torsion (torque) – While drilling, the drillstring is being rotated therefore torsion is occurred in the opposite direction of rotation and torsion can damage anywhere of string (see Figure 2). There are several factors affecting torque as RPM, well depth, well trajectory, well bore condition, formation, etc.
Figure 2 – Drillstring in Torsion
Cyclic Stress – In directional wells, the drillstring is exposed to both compression and tension at the same time. Because the string is bent along the curvature of the well, one side is in compression however the opposite side is in tension. As the string is being rotated, each joint absorbs a cycle of compressive and tensile forces (see Figure 3).
Figure 3 – Cyclic Stress on Drillstring
Corrosion Affects on Drillstring
Corrosion is another factor which can silently damage the drillstring. Many people don’t realize this point and tend to focus on the mechanical way when they want to prevent failure. Chemicals dissolved in the drilling mud that can cause the issue are as follows;
Oxygen (O2) – Small amount of oxygen (less than 1 ppm) still can cause pitting and rusting. Eventually, it leads to twist off (parting the string) and wash out issue (small holes in the pipe). This gas can be removed from the mud by adding chemical to deactivate O2 (oxygen scavenger).
Carbon Dioxide (CO2) – Carbon dioxide forms carbonic acid which will corrode steel. There are several ways introducing CO2 as formation gas, surface water and bacteria). CO2 can be removed from the drilling fluids by adding excess lime (Ca(OH)2) into the system will control acid gases as CO2. CO2 and lime will chemically react as the following chemical reaction:
Ca(OH)2 + CO2 -> CaCO3 + H2O
Dissolved Salts – Dissolved salts can come from several sources as formation fluid, chemical additives, mixing water, etc. This chemical increases corrosion rate which will accelerate the failure. You need salts in the mud to control clay swelling, therefore, in order to minimize this problem; controlling salt content is the best way to do.
Hydrogen Sulphide (H2S) –Hydrogen is absorbed on to the surface of steel in the presence of sulphide. This process is called “hydrogen embrittlement” or “sulphide stress cracking”. This will lead a rapid brittle of drill string. Adding lime (Ca(OH)2 into drilling fluids is a typical procedure to remove H2S. Lime (Ca(OH)2) will control acid gases as H2S and CO2. These following chemical equations demonstrate how lime reacts with H2S and CO2, respectively.
Ca(OH)2 + H2S -> CaS + 2(H2O)
Ca(OH)2 + CO2 -> CaCO3 + H2O
Organic Acids – The organic acids will remove protective films therefore other dissolved chemical can chemically react with steel and cause the drillstring failure.
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