A wellbore strengthening treatment initiative incorporating particulate or chemical-based lost-circulation material (LCM) and comprehensive pre-drill planning addresses the hazards associated with hole instability.
|In the wellbore strengthening concept, shallow fractures are opened and filled with LCM to prevent the fracture from closing again. (Images courtesy of M-I SWACO)|
Historically, the loss of large volumes of whole drilling fluid to the formation — lost circulation — has been a root cause of wellbore stability problems and high mud costs.
Lost circulation has been attributed to a host of drilling hazards such as hole collapse, stuck pipe, and even blowouts. Losses can occur naturally in fractured formations or in those with high permeability and porosity, or they can be the result of induced pressure. The latter is of particular concern when drilling into depleted zones where reservoir production has reduced the pore pressure and fracture pressure.
Proper pre-drill planning should allow for the identification of risk zones, optimization of drilling practices, and the establishment of both preventive and remedial treatments. Here the term “preventive treatment” also refers to the philosophy of wellbore strengthening.
When a hole is drilled, it disturbs a pre-existing, uniformly distributed stress field in the rock. In response to this disequilibrium of the stress field, the rock attempts to close the hole by collapsing. It is this closure effect that gives rise to the elevated stresses in the vicinity of the hole — the “near-wellbore stress field” or the “hoop stress.”
One type of wellbore strengthening involves increasing the near-wellbore stress further by opening a shallow fracture at the well bore and filling it with material that prevents the fracture from closing. This process places the rock in the near well bore under additional compression, thereby increasing the near wellbore stress and the mud weight required to initiate a new fracture. Various wellbore strengthening processes of this type are the same fundamentally but differ slightly in the way the LCM treatment is designed and implemented. Theoretically, the width of the fracture that is formed and plugged with LCM determines the increase in the fracture pressure.
Wellbore strengthening process
Strengthening the well bore can be effected either by using particulate-based or chemical-based LCM. Particulate-based solutions include large, granular, and tough materials such as sized marble and carbon-based products. Chemical-based solutions include some new experimental systems using resins and cross-linked polymers. M-I SWACO is developing and field- testing these for both non-aqueous and water-based fluid systems.
Particulate-based LCMs normally are used for permeable formations. The LCM is transported into the fracture via convection, and normal filtration generates a tight filter cake on the wellbore side of the LCM bridge. Fluid and excess pressure within the fracture behind the seal leak off into the formation. Here, the particle size distribution relative to the fracture and the uniformity of the LCM are important. Broad particle size distributions (PSD) containing a high percentage of extra-coarse LCM promote plugging and sealing near the mouth of the fracture. Alternatively, finer and more uniform blends promote filling and sealing of the interior of the fracture through high fluid loss and dehydration of the LCM. Both methods are used for lost circulation prevention and wellbore strengthening treatments, although the broad and coarse PSD approach is thought to generate greater wellbore stress enhancement.
In contrast to permeable formations, impermeable rocks do not permit leak-off, so there is significant risk that anything placed in the fracture will be pushed out again by the trapped fluid pressure as the fracture closes after treatment pressure is removed. Consequently, chemical-based treatments have been developed that set or cure in situ to form immobile, impermeable fracture plugs.
For particulate-based LCM, particle size distribution can be optimized to the fracture aperture by blending various products. M-I SWACO has developed two unique fracture-sealing testing devices for studying the sealing and wellbore strengthening properties of LCM formulations. These devices, which are housed in the Houston Technology Center, have been used to tailor LCM blends to specific drilling fluid and predicted wellbore conditions.
These devices have been at the heart of two large operator-supported projects focused on fracture sealing and wellbore strengthening. The first project, which focused on fracture sealing, was completed in 2006. The second project, focusing on wellbore strengthening, is ongoing. Through these customer-focused projects, the plugging and sealing behavior of a large number of materials in various drilling fluids has been evaluated, resulting in a better understanding of the interplay among LCM, fluid loss, and formation permeability.
Software package aids planning
Another aid to designing wellbore strengthening treatments is now available with the launch of a new proprietary software tool, Opti-Stress. This software is used to calculate the desired fracture size and matching LCM particle size distribution to achieve a desired wellbore strengthening effect from simple rock mechanical properties and borehole geometry. As with all wellbore stability modeling, there is significant uncertainty in the input values, and this carries through and influences the results. A unique feature to Opti-Stress is its ability to calculate the impact of the input uncertainties on the final LCM design. This allows for a much more robust design, which improves
the utility and success of the wellbore strengthening application.
Applying the process
Two main methods exist for applying wellbore strengthening: pills and continuous particle additions to the circulating drilling fluid. The former can be used for both chemical- and particulate-based products. The principal technique is called hesitation squeeze. The wellbore strengthening product is spotted over the interval to be treated, and the wellbore pressure is increased in steps to the desired level above fracture pressure — all the while monitoring fluid losses and pressure.
The alternative method is to continuously add LCM particles to the drilling fluid while drilling ahead with an equivalent circulation density (ECD) that exceeds the fracture pressure. In theory, fractures are formed continuously by excessive ECD and are immediately plugged by the circulating LCM. The logistics of such an operation are critical to its success. The main challenge is to maintain the required PSD and concentration in the drilling fluid. This means taking note of particle attrition and particle removal at the shaker screens. Currently, specially formulated LCM slurry blends are being field-tested. These blends can be matched to the shaker screens and also allow for variable concentrations of LCM to be added at any one time. The use of such slurries provides greater operational flexibility and reduces the logistics associated with sack-based products.
Among other recent developments is an approach that combines continuous particle additions for wellbore strengthening with the Managed Particle Size Recovery System, which maximizes re-use of the LCM and real-time, in-line monitoring of the PSD of the LCM in the drilling fluid to ensure success of the wellbore strengthening treatment.
Wellbore strengthening is rapidly becoming a mainstream operational practice for constructing stable well bores, particularly for expensive, high-risk drilling operations.
Wellbore strengthening has allowed holes previously thought to be “undrillable” to be drilled. In some wells, as many as five intervals have been drilled using wellbore strengthening techniques, resulting in stable well bores with minimal drilling fluid losses.