Well-Bore Stability 3/4

AFTER A HOLE IS DRILLED.

Hoop Stress

May be defined as  a normal stress in the tangential (azimuth) direction; Axial stress, a normal stress parallel to the axis of cylindrical symmetry; Radial stress, a stress in directions coplanar with but perpendicular to the symmetry axis.

Hoop stress is dependent upon wellbore pressure, stress magnitude and orientation, pore pressure, hole inclination and direction.

Wellbore pressure is directly related to mud weight/ECD.

For a vertical wellbore with equal horizontal stresses, hoop stress is dependent upon the mud weight and the magnitude of the horizontal stresses and is equally distributed around the wellbore.

A deviated well creates of hoop stress around the wellbore due to the redistribution of the horizontal and vertical stresses.

Hoop stress acting on a cross-section of the wellbore is maximum at the sides of the wellbore perpendicular to the maximum stress.

The same is true when drilling a vertical well in an environment of unequal horizontal stress. Hoop stress is maximum at the side of the wellbore perpendicular to the maximum horizontal stress.

Axial Stress

Axial stress is oriented along the wellbore path and can be unequally distributed around the wellbore. Axial stress is dependent upon; stress magnitude and orientation, pore pressure, hole inclination and direction. Axial stress is not directly affected by mud weight.

For a vertical well with equal horizontal stress, axial and vertical stress are the same. Axial stress in a deviated well is the resolution of the overburden and horizontal stresses.

Radial Stress

Radial stress is the difference in wellbore pressure and pore pressure and acts along the radius of the wellbore.

Since wellbore and pore pressures both stem from fluid pressure acting equally in all directions, this pressure difference is acting perpendicular to the wellbore wall, along the hole radius.

Mechanical Stability

Hoop, radial, and axial stress describe the near wellbore stress-state of the rock. Mechanical stability is the management of these stresses in an effort to prevent shear or tensile rock failure.

Normally the stresses are compressive and create shear stress within the rock. The more equal these stresses, the more stable the rock.

Shear Stress

Radial stress resists shear caused by the hoop stress.

Hoop, axial, and radial stress can be calculated and the greatest and least of the three indicated by a stress-state semicircle on the stability chart. Shear failure occurs if the stress-state falls outside of the stability envelop. Tensile failure occurs if the stress-state falls to the left of the shear stress axis and exceeds the tensile strength of the rock.

Whenever hoop or radial stress becomes tensile (negative), the rock is prone to fail in tension. Many unscheduled rig events are due to loss of circulation caused by tensile failure.

Time

Also an important consideration. The longer the formation is exposed to the drilling mud, the more near-wellbore pore pressure increases. The rock loses support provided by the mud weight.

Effect of Mud Weight/ECD

Mud weight, ECD, and pressure surges on the wellbore directly affect hoop and radial stress. An increase in MW decreases hoop stress and increases radial stress. Similarly, a decrease in MW increases hoop stress and decreases radial stress. The result on wellbore stability is therefore dependent upon the magnitude of the mud weight increase/decrease.

Mud Filter Cake and Permeable Formations

The filter cake plays an important role in stabilizing permeable formations.

An ideal filter cake isolates the wellbore fluids from the pore fluids next to the wellbore. This is important for hole stability and helps to assist in preventing differential sticking.

Ideal Filter Cake

If there is no filter cake, the pore pressure near the wellbore increases to the hydrostatic pressure; the effective radial stress is zero. The simultaneous decrease in effective hoop stress causes the stress-state to move left in the stability envelope; decreasing the stability of the formation. An ideal filter cake helps provide for a stable wellbore. The chemical composition of the mud and also the permeability of the formation control the filter cake quality and the time it takes to form.

Hole Inclination and Direction

The inclination and direction of the wellbore greatly impacts the stability of the well. Unequal distribution of hoop and axial stress around the circumference of the well tends to make the wellbore less stable.

Drilling a horizontal well causes the hoop and axial stress distribution around the wellbore to change.

Before drilling from vertical, the hoop stress is equally distributed. As angle increases to horizontal, the hoop stress on the high and low side of the wellbore decreases, but the hoop increases greatly on the perpendicular sides.

The radial stress remains fixed but the increasing hoop stress increases the stress-state.

Bottom-hole Temperature

High bottom-hole-temperature wells can experience stability problems as hoop stress changes because of temperature differences between the mud and formation.

If the mud is cooler than the formation, it reduces the hoop stress as the formation is cooled. This reduction in hoop stress can prevent shear failure and stabilize the hole, if the hoop stress were high due to low mud weight.

On the other hand, if the mud weight is too high and close to the fracture gradient, excessive cooling can lower the hoop stress and make it tensile.

This could cause tensile failure or fracturing as it effectively lowers the fracture gradient.

If the mud is hotter than the formation, exactly the opposite occurs as hoop stress is increased. This could promote spalling or shear failure.

Consider what happens during a typical round-trip on a deep high temperature well. During the trip, formation temperature returns to its ambient value. This causes the hoop stress to increase. When back on bottom and circulation resumes, the cooler mud traveling down the drillstring reduces the temperature of the nearby formation, causing hoop stress to decrease.

As the hot bottoms-up mud circulates past formations at shallower depths, hoop stress increases as the mud heats up the formations.

These variations in hoop stress have the same effect as pressure surges associated with swabbing and surging and can cause both tensile and shear failure downhole.

Impact of Mechanical Stability on the Wellbore

Mechanical stability problems directly account for many unscheduled rig events. Stability problems also effect overall drilling efficiency by altering the shape of the hole being drilled.

Severe hole deformation occurs when extreme stress environments are penetrated. Consider the path of a typical well, and consider this deformation over several thousand feet of open hole;

it is easy to see the impact of such a wellbore on operations.

Resulting Operational Problems Include: Stuck pipe, casing, logging tools, etc., Ineffective hole cleaning, Ledges and breakouts, High torque and severe slip-stick, leading to potential drillstring failures.

Chemical Stability

Is the control of the drilling fluid/rock interaction; usually most problematic when drilling shales.

Shales

One factor that distinguishes shale from other rock is it's sensitivity to the water component of drilling fluids. With time, shale/water interaction will decrease the strength of the shale; making it more prone to mechanical stability failure.

As shale is drilled, a sequence of events takes place that can lead to the stressing, weakening, and eventual failure of the shale. Several parameters contribute to the chemical stability of shale.

Advection

is the transport of fluid through shale due to pressure differential. Typically, wellbore hydrostatic pressure is greater than formation fluid pressure. When exposed to a permeable formation, the

liquid phase of the mud is "pushed" into the pore openings by the pressure differential.

In a highly permeable sand, the flow rate of fluid loss is sufficient to form a filter cake that controls fluid loss. With shales, however, the filter cake cannot develop, since the permeability of a typical shale is much less than that of any filter cake. Also, the particle size of a typical filter cake is too

large to plug the pore throats of shale.

Capillary Effects

Drilling fluid must overcome capillary pressure to enter the pore throats of shale. Capillary pressure, developed at the drilling fluid /pore fluid interface, is dependent on several factors; pore throat radius, interfacial tension, and contact angle. When drilling water-wet shales with water base mud; surface tension between the mud's water phase and the pore fluid is very low. Under favourable salinity conditions, the water phase enters the pore throat.

Osmosis

Osmosis is caused by the imbalance of salt concentration between the mud's water phase and the pore water. The salinity imbalance is separated by shale which acts as a semi-permeable membrane that allows the transport of water only. Water moves from low salinity to high salinity until the salinity difference (chemical activity) is balanced. If the mud salinity is too low, water moves into the shale increasing the pore pressure. As pore pressure increases, it has an adverse effect on stability. If the mud salinity is too high, pore water flows into the mud system dehydrating the shale. As pore pressure decreases, effective hoop stress increases also promoting shear failure

Pressure Diffusion

Is the change in near-wellbore pore pressure relative to time. This occurs as overbalance and osmotic pressures drive the pressure front through the pore throat, increasing pore fluid pressure away from the wall of the hole. This pore pressure penetration leads to a less stable condition at and near the wellbore wall. When drilling water-wet shales with oil base mud, the capillary pressure is very high (i.e., 8000 to 10,000 psi) due to the large interfacial tension and extremely small pore throat radius. The high capillary pressure prevents entry of the oil phase as overbalance pressures are very low in comparison.

However, if the salinity of the mud's water phase is not balanced with shale salinity, water transfer through osmosis can still occur. As pressure diffusion increases pore pressure near the wellbore, shear strength of the rock is reduced. The time for pressure diffusion to impact shale may result in failure of a shale section exposed for several days.

Time required for the pressure front to penetrate a given depth depends primarily on the permeability of the shale (connectivity of the pores) and the pressure differential between the wellbore and pore pressure

Swelling /Hydration

Over geologic time, mud/clay solidifies into shale as overburden stress drives off the water envelope (dehydration) and cements the platelets with the minerals left behind after dehydration. After drilling, water enters the shale by advection and osmosis. Negatively charged clay ions attract and hold the polar water. The increasing volume of attached water produces a swelling stress that "wedges" the clay platelets apart.

The swelling pressure and behaviour of shales are directly related to the type and amount of clay minerals contained in the shale. Shales with high concentrations of negatively charged ions can produce very high swelling pressure (50,000 psi plus).

Swelling pressure decreases the strength of the shale by destroying the natural cement bond between the clay platelets. Brittle shale becomes ductile and is pushed into the well-bore by the compressive hoop stress and the swelling stress.