Stress Caging – An effective Wellbore Strengthening Approach
What is Stress Caging?
Stress Caging is the technique that strengthens the wellbore so that the hole can be drilled without inducing downhole losses. Essentially, the process increases the fracture resistance of the formation. Stress caging not only helps avoid downhole complications by avoiding loss of circulation but could also reduce the number of casing strings required for drilling the well to the planned target depth. The technique achieves the objective of formation strengthening by treating weak formations with drilling fluids containing engineered particulate lost circulation materials. The process of stress caging aims to increase hoop stresses in near wellbore regions and seal shallow fractures at the wellbore formation interface continuously while drilling.
Why Do we need Stress Caging?
Wellbore integrity is very important to avoid downhole complications and associated non-productive time while drilling. As part of the well design process, casing points are selected based on estimated formation and fracture pressures. Each hole section is designed such that the pressures while drilling a hole section will not exceed the fracture pressure of open formations. This objective is achieved by appropriately selecting casing points to cover weak formations behind the pipe before drilling to any higher-pressure formation. This avoids fracturing the weak formation while maintaining the mud weight to have the required overbalance for drilling through any higher-pressure formation. Despite these efforts, lost circulation is one of the most common problems in drilling. Wellbore strengthening is an effective technique to prevent and mitigate lost circulation.
There could be several situations where a weaker formation needs to be drilled with a higher-pressure formation:
In certain cases, a reservoir could deplete over time weakening the rock matrix. However, a low permeability shale layer adjacent to this weakened formation will still have a higher pressure. Drilling both these formations together will require strengthening the weaker layer to avoid downhole complications.
Deepwater drilling encounters a narrow window between fracture and formation pressure, which could cause downhole losses. Even if narrow window drilling is successfully achieved, the downhole pressures generated while running and cementing casing could break down weak formations. Increasing the upper limit of the pressure window through formation strengthening is a viable approach for such scenarios.
In case of encountering a kick, a breach in well integrity due to the breakdown of a weak formation either during shutting in the well or while circulating out the kick, would cause an underground blowout and further complicate the well control situation.
Concept of Stress Caging
The stress caging approach began with a better understanding of the phenomenon commonly known as ‘Wellbore Ballooning’. Wellbore ballooning refers to the instances where the wellbore seems to lose drilling fluid to the formation initially but regains it later. Many times, these gains are considered a well-flow event, prompting the crew to introduce well-control measures, but turn out to be a wellbore ballooning effect. Although the term ballooning creates an impression that the wellbore is elastic that moves in and out like a balloon but that is not the case, it can better be termed as a ‘Supercharge Phenomenon’. It is caused due to the opening and closing of microfractures while drilling. If the hydrostatic head of the drilling fluid is less than but very close to the fracture initiation pressure, the ECD effect in the dynamic conditions while drilling could be enough to open the fractures. When these fractures open, they take mud, giving the indication of losses. This lost mud would return to the well as the fractures close again when the circulation is stopped. The stress caging technique increases the stresses in the near wellbore region by wedging the particles into the fractures and effectively increasing the fracture initiation pressure of the formation.
How are the objectives of Stress Caging Achieved?
While drilling, the dynamic effects of ‘Equivalent Circulation Density (ECD)’, surge, and swab pressures are felt by the wellbore. If no treatment is done, microfractures in the wellbore open and close with pressure variations in the wellbore resulting in intermittent loss and gain indications.
In the ‘Stress Caging’ technique, the Wellbore Strengthening Materials (WSM) with engineered particle size distribution are added to the mud system while drilling.
As the microfractures open with higher wellbore pressures while drilling, the bridging material gets lodged into the near wellbore periphery and creates a plug while the drilling continues. Using the ultra-low fluid loss mud system, the filter cake is formed and prevents further fluid invasion through the bridge formed at the mouths of the fractures.
When the dynamic effects wane, the particles wedged into the fractures don’t allow the fractures to close. This creates compression in the formation to form a ‘Stress Cage’ that effectively strengthens the wellbore.
If the increased hoop stress in the near wellbore area balances the fluid column pressure in the wellbore or exceeds it, the fractures will not open while drilling. This prevents loss of circulation while drilling even with mud weight higher than the original fracture strength of the formation.
Selection criteria for bridging particles
Materials like deformable graphite, fiber, calcium carbonate, nut plug, etc are commonly used as bridging material. The bridging material should be strong enough to resist the fracture closing stress. The particle size distribution is also important to ensure that the bridging particles create the stress cage at the fracture mouth.
The four important properties that are considered while selecting bridging material for developing an effective stress cage are Elastic Deformation, Compactness, Strength, and Friction Coefficient.
Deformation is a change in the shape as a reaction to applied stress. Elastic deformation is temporary and is defined as the deformation that is fully reversed when the load is removed. Ideally, the bridging material should have a 5% - 20% elastic deformation rate.
Compactness is a measure of how well the fracture is filled with the bridging material. It is defined as the ratio of the volume of bridging material and the volume of the fracture channel plugged. A compactness of 90% or more is desirable in the bridging material.
Bridging effectiveness also depends on the strength of the bridging material. However, the strength of the material tends to degrade under load. Hence the strength of the material is measured by the degradation rate termed as d90 under 15 MPa. The higher the degradation rate lower the LCM strength. For achieving effective bridging, a degradation rate d90 of less than 5% is desirable.
Higher friction coefficient increases the effectiveness of bridging by making the dislodging of material difficult. A friction coefficient higher than 0.1 is desirable. Elastic LCMs manifest higher friction coefficients compared to rigid LCMs as they achieve a larger contact area.
The addition of fiber materials into LCM increases the compactness and friction coefficient. Whereas a more resilient material will have a lower degradation rate and higher strength. Hence the best bridging effectiveness is achieved with a combination of rigid particles, fibers, and resilient particles in an engineered proportion.
How is a wellbore strengthening different from measures for curing lost circulation?
Wellbore strengthening techniques aim at increasing the stresses in the near wellbore areas whereas lost circulation mitigation measures help seal hydraulically conductive fractures causing loss of circulation.
Wellbore strengthening is carried out proactively to avoid the well getting into a lost circulation situation, whereas most lost circulation measures are adopted on encountering losses.
Wellbore strengthening cannot provide a solution for large fractures, faults, or caverns causing massive losses.
Sealing hydraulically connected fractures through lost circulation mitigation measures can restore and improve the wellbore pressure containment to and beyond its original value. However, this improvement is limited to the maximum fracture pressure. The improvements through wellbore strengthening can go substantially beyond this limitation.