Wellbore instability while drilling mechanically weak, unstable or vugular formations has been a problem for decades. The cost of wellbore instability is a major challenge in achieving safe and economical drilling operations. As drilling operations moved into challenging formations in Kuwait, the operator sought to drill the Burgan shale and Shuaiba limestone formations in one section as opposed to the traditional two sections required to isolate each formation separately. This paper focuses on a class of technology additives used to mitigate the challenges of drilling weak and unstable formations. One approach for drilling micro-fractured shale and weak sands with vugular limestone is to mitigate the invasion of drilling fluids into the formation. Other approaches include: stabilizing the reactive shale by preventing hydration and swelling, improving the filtercake texture and strength, and sealing natural micro-fractures. Drilling fluid invasion can change the pore pressure, which may trigger wellbore instability problems. Thus, using ultra-low invasion drilling fluids, sealing micro-fractures and maximizing shale inhibition are key components for mitigating wellbore instability. Field data for the wells using the ultra-low invasion additives and shale stabilizers is presented and compared with previous wells drilled across Burgan and Shuaiba formations in Kuwait. The field data demonstrates the successful application of these additives to meet challenging key performance indicators (KPI) when drilling the Burgan shale and the vugular Shuaiba limestone in the same hole section. Using the ultra-low invasion additives along with shale inhibitors and borehole stabilizers, resulted in successful drilling operations with no differential sticking, torque-and-drag issues, sloughing, or tight hole problems as compared with usual incidences of differential sticking, pack-offs, and tight hole in other wells within the area. Using those additives also eliminated the need for a higher density fluid to control micro-fractured and tectonically stressed shales. The addition of the additive combination did not affect the rheological profile of the drilling fluid. Meeting these goals through the use of chemical additives in the drilling fluid reduced both non-productive time and formation damage in a cost-effective manner. Data from this paper specifically addresses a chemical solution for drilling the Burgan shale formation together with vugular Shuaiba limestone in a major Middle East producing field. However, the technique of mitigating wellbore instability by using this combination of chemical additives is fundamental to safe and economical drilling operations for any depleted, weak or micro-fractured formations globally.
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.
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