Movement along fault zones and slip planes may cause significant shear deformations of wellbores that appear as lateral displacement of the pipe. These often form an “S” shape with two distinct bends in opposite directions.
Faults may move due to:
- ongoing tectonic activity
- changes in the in-situ stresses caused by pore pressure depletion during production operations
- pore pressure increases that can occur due to well drilling, stimulation or disposal of produced water.
The width of the fault zone can vary from less than a metre to several tens of metres. This is dependent on the strength of the rock and the total amount of movement that has occurred on the fault over geologic time.
Faults along major plate boundaries may consist of a series of discrete shear displacements through formations that have been greatly remolded due to the ongoing movement. Determining the scale and direction of shear displacements in wellbores can be used to map fault movements.
Mapping can help Operators understand the in-situ stress regime. They can correlate fault movement events to earthquakes or oil and gas production or waste injection operations.
Shear deformations on slip planes associated with bedding planes, geological unconformities, facies changes and existing rock joints typically occur over intervals of less than 1 m.
In competent formations at depth, the shear may occur along a single discrete plane. There may be very little plastic deformation of the formation material as it impacts the pipe.
At shallow depths, formations or sediments are typically softer. The shear deformation is usually distributed over longer intervals. This is because the surrounding soil deforms instead of the pipe. In these cases, a discrete plane of weakness in the formation may not be apparent.
Well operations and reservoir processes affecting shear
Shear caused by water injection
Water injection into a depleted reservoir or disposal zone tends to cause localized increases in pore pressure in the injection zone. This causes a decrease in the effective stress acting on existing slip planes such as faults and fracture networks in the formation.
This stress reduction may upset the balance between the forces that are acting normal to the slip plane that keep it “locked” in place, and the forces that are acting parallel to the slip plane that cause movement along it.
Some picture this as a “lubricating” effect where the water changes the coefficient of friction on the slip plane. In fact, it is the change in the effective stresses that causes movement along the slip plane.
If the water is injected at high enough rates that a significant increase in pore pressure occurs, the formation can hydraulically fracture or “break down”. This also changes the in-situ stress distribution around the wellbore and around the fracture away from the wellbore. This can impact the stability of other fractures, faults and planes of weakness.
Properly managed water injection applied as a pressure maintenance technique in depleted reservoirs can minimize overburden subsidence and associated shear deformations in the overburden.
Shear caused by steam injection operations
In heavy oil operations, where steam injection is used to heat the reservoir, the combination of increased pore pressure and thermal expansion of the formations can cause shear deformations along planes of weakness such as bedding planes, changes in lithology and unconformities.
This is most pronounced when the steam is injected above formation fracture pressure such as in cyclic steam operations. In these cases, shear displacement can occur across the field, affecting numerous wells in one massive displacement event.
Operators in steam injection projects have developed various steaming strategies consisting of injecting steam into patterns of wells to limit these large catastrophic events. Caliper logs are run routinely to map shear displacements in wells so that the steaming strategies can be modified to minimize further shear.
Shear caused by overburden subsidence
Pore pressure decline in weak formations such as chalk and weak sands can cause compaction of the reservoir and subsidence of the overburden.
Generally, a subsidence bowl occurs with the most subsidence near the centre of the depleted reservoir, with less subsidence occurring on the margins of the reservoir. As the overburden formations move downward, relative shear movement occurs along horizontal planes of weakness in and between formations throughout the overburden sequence. Normal faulting may also occur, causing high angle shear displacements in the overburden materials.
Caliper logs can be used to map well deformations to indicate where pressure depletion and reservoir compaction is greatest. This information can be used to design water injection strategies to repressure critical areas of the reservoir and mitigate further well deformations.
Shear caused by mobile salt bodies
Salt deposits, often including both halite (rock salt) and sylvite (potash), can occur as horizontal beds or as sub-surface intrusions (domes and sills).
Bedded salt and domes are used to develop subsurface storage or disposal caverns where the salt is removed by solution mining. This changes the stresses in the salt surrounding and overlying the cavern.
Due to the plastic nature of salt, these stress changes can result in creep deformation of the salt. The cavern can close and cause instability in the cavern roof. These movements in the salt can cause shear displacements that intersect the wellbore above the cavern and can lead to subsidence of the overburden.
Salt intrusions can form “sub-salt” or “pre-salt” hydrocarbon traps. They may be targeted by oil and gas drilling operations. These salt intrusions are continuously moving relative to the surrounding rock, creating shear zones and local changes in pore pressure that destabilize the margins of the intrusion. These unstable zones can cause difficulties during drilling and may cause ongoing deformation of the well completion as the salt movement continues.