Common Causes of Wellbore Instability:
The failure mechanism for wellbore instability can broadly be attributed to Mechanical or Chemical reasons. In most cases, it will be due to a combination of the two factors. The mechanical failure is related to stress, in-situ pressure, or poor consolidation of the rock matrix. The chemical failure mode is related to the chemical composition of the formation and its reaction with the chemical composition of mud used for drilling. The main causes of wellbore instability are described below.
Unconsolidated formations: A common reason encountered in shallow depths is unconsolidated formation. These are shallow depositions of pebbles, boulders, gravel, rubble, or sand with very little or no bonding. They are primarily supported by adjoining rocks. As soon as that support is removed by drilling, the unconsolidated formation falls into the well. These could pack off around the drill string and get it stuck in the hole.
Naturally Fractured Formations: Tectonic activities create folding and faulting of the formation layers. Naturally fractured rocks can be found in a faulted formation due to rupture caused by the tectonic movement of formation layers. These naturally fractured rocks can be loosely bonded. They tend to fall into the wellbore as soon as the bit drills through this layer.
Stressed Formations: Some formations could exhibit high levels of stress. The stress could be due to high tectonic activities in that areas or high in-situ stress due to nearby faults or folds. At times the hydrostatic pressure required to stabilize the wellbore in such a high level of localized stressed zones may be much higher than the fracture pressure of the exposed formation. If there is a significant difference between the near-wellbore stress and the restraining pressure provided by the drilling fluid density, the formation particles fall in the well in the form of splintery caving.
Mobile Formations: Mobile formations are plastic and squeeze into the well due to overburden pressure above them. This is also called creeping. Salts are commonly known as mobile formations. There are different types of salts, which exhibit different creep rates. Mobility of formations depends on depth, temperature, water content, and chemical composition for formation. Mobile formations result in reduced wellbore diameter causing difficulty in running bottom hole assemblies, logging tools, and casing. If not restrained, downhole tools could get stuck due to mobile formation packing around them. Casing selection and cementation are also very important as over time, the formation can keep creeping and exert excessive pressure to collapse the casing.
Over Pressured Shale: Shale could be naturally over-pressured or induced over pressured. Naturally over-pressured shales have pore pressure greater than the normal hydrostatic pressure. This happens due to geological phenomena like under compaction, naturally removed overburden, or uplifting. If the mud weight is not increased to the level required for balancing the pore pressure in the shale, the hole becomes unstable and collapses.
In the case of induced over-pressured shale, initially, it is not over-pressured but assumes the hydrostatic pressure of the wellbore fluids after several days of exposure to that pressure. If the mud weight is not changed for a long time, the long exposure with higher assumed internal pressure, the shale becomes unstable and collapses. This mechanism normally occurs in water-based muds, after a reduction in mud weight or after a long exposure time during which the drilling fluid was unchanged like in casing rat holes.
Reactive Shale: Reactive shales absorb water and tend to swell into the wellbore. Swelling (hydration) depends on the amount and type of clays within the structure of shale. Two types of hydrations are ‘surface hydration’ and ‘osmotic hydration’. Surface hydration causes only slight expansion on the surface of the clay and does not cause any major issues. Osmotic hydration however is a primary concern and could cause significant expansion and could lead to the closure of the wellbore. Only a few clays such as montmorillonite can swell through osmotic hydration. Other types of shales are brittle shales that do not swell but become weak due to the hydration of their microfracture surfaces and tend to collapse into the well in pieces. Brittle shales usually contain a high percentage of kaolinite, illite, and chlorite.