As much as the oil and gas industry is fun, there are some problems that exist in the field. One area that experiences problems regularly is the drilling section. Some drilling problems include hole instability, lost circulation, doglegs, key seats, excessive bottom-hole temperatures, etc.
When encountered formations slough, swell or flow, it is said to have hole instability. Shales and salt beds are some of the most unstable holes. Some causes of hole instability are pore pressure, earth’s movement forces, overburden pressure, and water absorption.
Hole instability occurs when there is a change in the relief of the overburden pressure and it exceeds the yield strength of the drill formation. This then results in the flow of the formation. Blowouts can occur when there is a really high pore pressure in the highly permeable formation. Structural stress can also cause hole instability. By using the proper drilling fluid, hole instability can be avoided.
No, we don’t mean actual doglegs. A dogleg is referred to as any deviation greater than 3” per 100ft in a wellbore and this happens when there’s a sharp change in direction in the wellbore. It is also caused by a change in the dip of the formation or when there’s a change in the weight applied to the bit. There are many results to a severe dogleg happening, they include drill pipe failure, stuck casing, and inability to run casing to total depth. However, there are ways to minimize the formation of doglegs such as large diameter drill collars, proper weight applied to the bit and by using properly placed stabilizers.
These form as a result of doglegs. When there is a channel or groove cut at the side of a hole which is parallel to the axis of the hole, a key seat is formed. One major cause of this groove is the dragging of a drill pipe through the sharp bend in a dogleg. To prevent key seats, doglegs should not be created at all.
High Bottom-hole Temperature
In deep wellbores or places with really high geothermal gradient, extremely high bottom-hole temperatures can occur. These high temperatures can cause drilling problems due to the accelerated thickening of water-based drilling fluids. An increase in viscosity and density of the drilling may cause tools to get stuck, lost circulation, and a reduction in penetration rates. Extremely high bottom-hole temperatures can be avoided by using oil-based muds.