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.
Sidetracking a cased wellbore presents numerous challenges because the operators have to plan ahead to select the sidetracking depth and then ensure that all the objectives are met from a well authority for expenditure and geological target perspective. The quality of the geological target window is of great concern to operators because it ensures that the subsequent bottomhole assembly (BHA) will pass through the window without any problems. Sidetracking the wellbore when the milling assembly has to cut two strings of casing is an additional unique challenge. The centralizer and casing collar locator along the length of the wellbore at and in the region of the kickoff point are significant because they can be additional risks, which can lead to costly multiple trips to ensure that the window is in good-quality form. An operator was faced with potential geological losses at the kickoff point in a wellbore while attempting to sidetrack an existing wellbore containing 9.625-in., 43.5-lbm/ft L80 buttress thread casing, and 13.375-in., 68-lbm/ft K55 buttress thread casing, dual strings at the kickoff point. The BHA for this challenging application was modeled using a finite-element analysis (FEA)-based modeling system to select the optimum BHA to sidetrack the wellbore with the least amount of vibrations. The feasibility of using a bi-mill vs. tri-mill BHA was evaluated. As a result, a parameter road map, taking into consideration the dimensions of the whipstock slide and mill position during the milling operations, was finalized. The placement of the centralizers and casing collars along the length of the casing at the kickoff point was considered. The exact locations where the lead mill would initiate the cut at each casing string were analyzed to determine the whipstock setting depths. A corrosion and collar locator tool was used to identify the collar location along the 13.375-in. casing because standard casing collar locator logging tools were not be able to identify the location of the casing collars along the length of the 13.375-in. casing string. Whipstock simulation software was used to check the bending moments and stresses for the pass-through BHA because the dogleg through the window can create additional issues and challenges. The program calculates the dogleg severity for a liner or BHA pass-through in addition to forces and stresses on the liner or BHA. The total quantity of cuttings that would be generated from the milling operation while cutting two strings of casing was also analyzed. This methodical planning resulted in a successful dual-casing exit operation. The success is the result of a proactive planned initiative to mitigate BHA shock loading, which included real-time monitoring using a predictive compressive-strength analysis system. The proactive plan also increased confidence in the FEA-based modeling system's ability to accurately identify the root cause of damaging vibrations while sidetracking through carbonates in a dual-casing Kuwait well.
The complexity of drilling highly deviated wells in Kuwait drives the need for step changing in the well construction mindset, where severe to complete loss of circulation in Shuaiba formation significantly deteriorate the shale layers in Wara and Burgan formations leading to uncontrolled wellbore stability events. Casing while drilling (CWD) and two-stage cementing with a light density cement slurry were introduced as a technology system to drill the highly deviated complex wells through unstable and highly fractured formations. Fit for purpose engineering processes, advanced software solutions, a tailored bit and a bottom hole assembly dynamically simulated for drilling stability and directional tendency behavior were designed. A special light density cement slurry with high compressive strength was also designed to tackle the lost circulation issues when cementing the casing string. The paper will describe how the technologies can work as one system to solve complicated wellbore problems and address the problematic challenges of drilling unstable shales and fractured formations in the same section of the wellbore. This strategy enabled a significant time saving compared to drilling the section conventionally, removing Non-Productive Time (NPT) resulting from additional trips, cement plugs, stuck pipe, and subsequent sidetracks.
The plug and abandon (P&A) challenges of each well are known to be different. This paper narrates unique challenges faced during the abandonment of a land well which intersected multiple over-pressured reservoirs containing high concentration of H2S and CO2. Because zonal isolation was paramount for this project, section milling was selected to enable a rock-to-rock cement plug to restore 5 critical caprocks. Remediation of annular cement was complex because all production and intermediate casings were cemented to surface with 2 or 3 casings across the caprocks. Conventional methods would entail pilot milling the 7 inch production casing, exposing the A-annulus to enable section milling of the 9.625 inch intermediate casing for a cement plug across the caprock. This technique is time consuming and uncertain, which adds to the cost and complexity of the P&A operations. In response to these challenges, the operation was optimized utilizing both a standard section mill and a new High-Ratio Section Milling (HRSM) technology, which allows for milling windows through two casing strings. The HRSM is a combination of a high-ratio hydraulic section mill, achieving a 180% expansion ratio, and an expandable stabilizer. The orientation of the stabilizer is set to enable 6-point contact stabilization in the outer casing and helps to reduce dynamic shocks and vibrations. The HRSM is deployed after the inner 7-inch casing window has been milled for a length of approximately 140 ft. The expandable stabilizer in the system ensures that the section milling assembly can efficiently mill a 110 ft casing window in the 9.625 inch casing, through a 7 inch casing window. A high-ratio underreamer is utilized to clean the formation and enlarge the diameter to 13.5 inch to enable a rock-to-rock seal through two casing strings, without pilot milling the inner string from surface. Milling two casing strings is done in 5 stages. The inner casing window is initiated with a dedicated run using rapid cutout knives. This allows for deployment of "flush knives" while milling the inner window, reducing the risk of skimming the outer casing and enabling a single run of 139 ft casing milled. Following a clean out run with an under reamer, the new HRSM technology was then run and completed a 111 ft of 9.625-inch casing in one run, which was followed by 100 ft of hole enlargement by a high ratio underreamer to open the hole to 13.5 inches. All of the above stages were carried out in a single run and with good ROP. The cement job was completed and the objective of restoring the cap rock seal across two strings of casing was achieved, saving rig time and cost for the plug and abandonment operation. The development and successful deployment of HRSM technology provides a reliable solution to achieve a rock-to-rock cement plug in a dual-casing environment. During the execution phase various lessons were learnt and implemented as best practice, this included design changes of the HRSM technology and the bottom hole assembly. The combination resulted in setting new benchmarks for HRSM technology and enabled savings of 30 days rig time when compared to the conventional method of pilot milling the inner casing.
Sidetracking a preexisting drilled and cased wellbore poses numerous challenges. When sidetracking in an openhole environment, additional verifications of hole conditions are needed, which contribute additional unique challenges. In this type of wellbore, the operators must plan by selecting the sidetracking depth and then ensure that all the objectives are met from a well authorization for expenditure standpoint and geological target perspective. The quality of the openhole window or rathole is of immense concern to operators because this quality ensures that the bottomhole assemblies (BHAs) will pass through the rathole without difficulty. The openhole gauge must be confirmed because it can pose additional risks that might lead to costly multiple trips into the well to ensure that the rathole is in good condition. This paper presents a unique case study in which the operator, Kuwait Oil Company (KOC), was faced with the possibility of geological losses at the kickoff in the wellbore while attempting to sidetrack an existing wellbore. The operator contacted an oilfield services company and requested a unique technical solution to precisely sidetrack the wellbore in the difficult formation containing fractured dolomites, which are known to cause severe to complete losses. While drilling the 12.25-in. section, the BHA became stuck in the fractured dolomitic limestone formation. In this section, the operator had previously experienced severe to complete losses. Because fishing attempts to free the stuck BHA were not successful, the operator decided to sidetrack the wellbore in the open hole using an openhole whipstock. A casedhole sidetrack option was ruled out because reactive swelling shales with producing sands were located above the sidetrack depth; therefore, combining these zones was not practical because of the low-mud-weight limit required for drilling the fractured dolomite below the target depth. In addition, because only 70 ft of open hole existed between the 13.375-in. casing shoe and the stuck BHA in the hole, sidetracking with a cement plug was nearly impossible. The openhole wellbore was logged with a caliper to confirm the wellbore gauge. Prejob planning consisted of understanding the compressive rock strength from the offset wells to identify the lithological challenges unique to this application. A hazard analysis risk-control method was adopted to identify the risks and apply appropriate mitigation measures. An operating parameters plan was formulated by the engineering team and discussed with the operator and service company personnel and followed throughout the job. The wellbore was successfully sidetracked in the 12.25-in. section in a single run using an openhole whipstock, avoiding the loss zone, and resulting in additional cost savings to the operator. The condition of the sidetracked rathole enabled smooth passage of the directional BHA to meet the directional objectives. Furthermore, the openhole whipstock operation eliminated the need for multiple cement plugs in the sidetrack (in view of severe loss zones below) as well as the time required for drilling with a dedicated motor BHA for openhole sidetracking operation, saving the operator a minimum of 6 days of rig time. This operation was the first successful 12.25-in. openhole sidetrack operation in the Middle East, Asia-Pacific, and sub-Sahara Africa regions. As a result of this successful operation, the operator is proactively recommending the new solution across the entire KOC organization for wells with similar scenarios. By applying this unique and reliable openhole whipstock technical solution, the drilling team was able to deliver a successful well based on the original casing plan without any need for further sidetracks or changes to the wellbore casing design.
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