Stress Caging – An Effective Wellbore Strengthening Approach
What is Stress Caging?
Stress Caging is a technique that strengthens the wellbore, enabling drilling without inducing downhole losses. Essentially, the process increases the formation's fracture resistance. Stress caging not only helps avoid downhole complications by preventing loss of circulation but could also reduce the number of casing strings required to drill 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 continuously seal shallow fractures at the wellbore formation interface while drilling.
Why Do We Need Stress Caging?
Wellbore integrity is crucial to avoid downhole complications and associated non-productive time while drilling. Casing points are selected based on estimated formation and fracture pressures during the well design process. Each hole section is designed to ensure that drilling pressures do not exceed the fracture pressure of the open formations. This objective is achieved by selecting casing points that cover weak formations behind the pipe before drilling into any higher-pressure formation. This avoids fracturing the weak formation while maintaining the mud weight to provide the required overbalance for drilling through higher-pressure formations. Despite these efforts, lost circulation is one of the most common problems in drilling. Wellbore strengthening is an effective technique for preventing and mitigating lost circulation.
There could be several situations where a weaker formation needs to be drilled with a higher-pressure formation:
Occasionally, 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 these formations together will require strengthening the weaker layer to avoid downhole complications.
Deepwater drilling operates within a narrow window between fracture and formation pressures, which can 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 by strengthening the formation is a viable approach for such scenarios.
In the event of a kick, a breach in well integrity due to the breakdown of a weak formation, either while 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 instances in which the wellbore initially loses drilling fluid to the formation but later regains it. These gains are often considered as 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, which moves in and out like a balloon, that is not the case; it can better be termed as a ‘Supercharge Phenomenon’. It is caused by 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 under dynamic drilling conditions could be sufficient to open the fractures. When these fractures open, they take mud, indicating losses. This lost mud would return to the well as the fractures close again when the circulation is stopped. The stress caging technique increases stresses in the near-wellbore region by wedging particles into fractures, thereby increasing the formation's fracture initiation pressure.
How are the objectives of Stress Caging Achieved?
While drilling, the wellbore feels the dynamic effects of ‘Equivalent Circulation Density (ECD),’ surge, and swab pressures. If no treatment is performed, microfractures in the wellbore open and close in response to pressure variations, 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 under higher wellbore pressures during drilling, the bridging material becomes lodged in the near-wellbore periphery, forming a plug as drilling continues. Using the ultra-low fluid-loss mud system, a filter cake forms, preventing further fluid invasion through the bridges at the mouths of the fractures.
When the dynamic effects wane, the particles wedged in the fractures prevent them from closing. This creates compression in the formation, forming a ‘Stress Cage’ that effectively strengthens the wellbore.
The fractures will not open while drilling if the increased hoop stress in the near-wellbore area balances or exceeds the fluid column pressure. This prevents circulation loss during drilling, even with mud weights exceeding the formation's original fracture strength.
Selection criteria for bridging particles
Materials such as deformable graphite, fiber, calcium carbonate, and nut plug are commonly used as bridging materials. The bridging material should be strong enough to resist the fracture closing stress. The particle size distribution is also essential to ensure that bridging particles form a stress cage at the fracture mouth.
The four crucial properties considered while selecting bridging material for developing an effective stress cage are Elastic Deformation, Compactness, Strength, and Friction Coefficient.
Deformation is a change in shape caused by applied stress. Elastic deformation is temporary and fully reversible when the load is removed. The bridging material should ideally have an elastic deformation rate of 5%—20%.
Compactness measures how well the fracture is filled with the bridging material. It is defined as the ratio of the volume of bridging material to the volume of the plugged fracture channel. 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 material's strength tends to degrade under load. Hence, the strength of the material is measured by the degradation rate, which is denoted by d90 at 15 MPa. The higher the degradation rate, the lower the LCM strength. A d90 degradation rate of less than 5% is desirable for effective bridging.
A higher friction coefficient increases the effectiveness of bridging by making material dislodgement difficult. A friction coefficient higher than 0.1 is desirable. Elastic LCMs manifest higher friction coefficients than rigid LCMs as they achieve a larger contact area.
The addition of fiber materials to LCM increases compactness and the friction coefficient. A more resilient material will have a lower degradation rate and higher strength. Hence, combining rigid particles, fibers, and resilient particles in an engineered proportion achieves the best bridging effectiveness.
How is wellbore strengthening different from measures for curing lost circulation?
Wellbore strengthening techniques aim to increase stresses in the near-wellbore area, whereas lost-circulation mitigation measures help seal hydraulically conductive fractures that cause circulation loss.
Wellbore strengthening is carried out proactively to prevent the well from encountering a lost-circulation situation, whereas most lost-circulation measures are adopted only after losses occur.
Wellbore strengthening cannot address large fractures, faults, or caverns, which can cause massive losses.
Sealing hydraulically connected fractures through lost-circulation mitigation measures can restore and improve 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.