Corrosion-resistant alloy (CRA) casing, especially high-chrome steel casing, has been widely used for deepwater wells because of the severely corrosive environment. Although sidetracking through standard low-alloy casings or liners is a common practice, milling high-chrome steel casing has proven to be challenging due to its higher hardness and significant work-hardening. Operators around the globe are looking for an effective solution to sidetrack through high-chrome casing. Sidetracking from CRA casing presents two major challenges: achieving the initial cutout through the casing wall and mitigating the cutter wear while completing the window. Traditionally, the crushed-carbide on the follow mills has proven to be ineffective due to the poor cutting ability, and the high heat from welding on the body makes it prone to twistoff. A pioneering milling solution—an integral bi-mill manufactured from a step-forged body with cylindrical carbide cutters brazed into discrete pockets of the mills to form a robust cutting structure—was designed to solve this challenge. Optimizing the cutter location to enable a uniform distribution of casing volume removal ensures that the workload and cutter wear are distributed equally among cutters, thereby maximizing the performance of each cutter in the mill. The design team optimized the cutter back rake angle to increase cutting aggressiveness to initiate the cutout process. Further, a protective physical vapor deposition multilayer coating was added to increase the hardness and lower the coefficient of friction at the cutting edge, thereby reducing the amount of abrasive wear on the cutters and casing work-hardening. Full-scale laboratory tests were conducted using two different carbide cutter grades to compare their durability, wear, and impact resistance. During both laboratory tests, the integral bi-mill assembly successfully milled the CRA casing for the next bottom hole assembly (BHA) to pass through. Based on successful laboratory tests, the field run was conducted on 7.00-in., 29-lbm/ft (28% Cr) liner in an offshore well environment in the Arabian Gulf. Two integral bi-mill assemblies were used to successfully mill 10.57 ft of window and drill 14.03 ft of hard limestone formation (20,000 to 25,000psi) with an acceptable mill dull grade. The subsequent 6-in. motor directional BHA passed through the window without any hole-drag. The breakthrough milling solution incorporating an integral bi-mill with robust cutting structure eliminated the high potential risk of getting a milling tool stuck and avoided subsequent fishing, redrilling, and casing operations that would have resulted in an additional cost of about USD 8 million. The successful milling job brought the new reservoir well into production within a short turnaround time. The success of this technology has opened new avenues for sidetracking applications with CRA casings in the North Sea, Middle East, and Brazil.
A typical method to sidetrack from a cased well is to use a milling assembly to traverse an anchored whipstock to create a window, and then continue to drill a rathole deviating from the original well. A sidetrack can also be conducted in an open, uncased wellbore. The operation usually utilizes a directional BHA to traverse anchored or cemented whipstock and drill ahead while steering away from the original wellbore. To achieve a successful wellbore departure, it is critical to minimize shock/vibration to avoid failure of the milling assembly and drillstring components. The milled window should allow sufficient clearance for the subsequent BHA to pass through without issues. The rathole also needs to deviate away to avoid collision into the original wellbore. Modeling with the capability to evaluate wellbore departure is valuable to plan the operation and avoid unexpected malfunctioning. The authors describe how a time-based modeling system was used to simulate drillstring dynamics and the cutting action of casing mills. Extensive laboratory tests were done to capture the interaction between the cutting elements/casing, whipstock, cement and formation. The data and the parameters from the specific application were then fed into the model for simulation. The results include milled window profile, rathole wellpath, BHA shock/vibration, and drillstring bending moment/stresses. By studying different scenarios, the milling assembly, BHA and drilling parameters were optimized to reduce the potential for vibration and chance of breakage, and to ensure appropriate window size and rathole length. Gantry milling tests were conducted in the laboratory. Milling assemblies were used to cut windows in casings cemented in steel containers. Drilling dynamics data including applied weight, RPM and flow rate were collected and analyzed. The geometry of the windows and the profiles of the whipstock were measured after the tests. This information was used to validate the model and good agreement was observed between laboratory tests and modeling results. Validation was also conducted with results from actual field milling runs/operations. The authors will present actual field examples to show the effectiveness of the model to help diagnose and prevent component failure and provide operating parameters recommendations for optimal results.
Conventional wellbore departure and drilling systems generally require the operator to make multiple downhole trips to achieve a specific objective. For example, a window milling bottom hole assembly (BHA) is run in hole to create an exit path in the existing casing and drill sufficient rathole for the next drilling assembly. In the subsequent trip, a directional drilling BHA is run to extend the rathole and drill laterally to the target. The industry requires an economical solution to accomplish the above objective in a single trip with good downhole dynamics control and overall BHA drillability. The re-entry system should be able to mill a window in the existing casing sufficiently large enough for obstruction free entry of the BHA and new liners. Once the BHA has exited the casing, the new system would be required to drill a full gauge lateral wellbore to the target with good directional control and minimal vibration. To solve the challenge engineers analyzed several key technological/operational issues including: Dynamic simulation of the BHA to study the nature and magnitude of the vibrationsControlling vibrations and feed rate to reduce premature cutter damage during window milling operationsUtilizing the latest force balance software for selection of shapes, sizes, number and location of cutters for maximizing on-bottom timeExplored hybrid cutting structures on bit/mill to maintain gaugeCombined knowledge and experience of subject matter experts (SME) from different engineering and operational groups within the organization including directional/geosteering personnel, fishing/remedial group and drill bit design team The evolution of the system and the associated technology occurred in three phases over several years. In all phases a window was milled in the existing casing and a lateral was drilled to various depths. During the first phase the system was run on a conventional rotary BHA in soft and medium formations. The second phase included testing on a positive displacement motor with bent sub or bent housing. The third phase, which is currently under extensive full-scale yard testing, includes a push-the-bit type rotary steerable system. The results obtained from all three phases indicate that a functional single-trip system for milling a window and drilling a lateral borehole is commercially feasible. The versatile system will contribute substantially to the technology required to efficiently and economically mill a window in the existing casing and then drill an extended length lateral wellbore to the target formation without tripping for equipment/bit change out. The wellbore departure and drilling system, which will be optimized with a sophisticated dynamic analysis software program, will incorporate the rate of penetration and footage benefits of polycrystalline diamond compacts (PDC) bit technology combined with the latest directional drilling tools. The paper will include field run details of phase one/two as wells as results of ongoing full-scale yard tests along with key observations and conclusions. The authors will also include an overview of the software used for analysis and plans/capabilities of the final version of the system.
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