Snowie, Mike (Talisman Energy (UK) Ltd) | Malsolm, Clare (Talisman Energy (UK) Ltd) | Alexander, Gregg (Schlumberger) | Duncan, Roy Graeme (Smith Bits) | Alemi, Hassan (Schlumberger Oilfield UK Plc) | Jolly, Iain (Smith Bits, a Schlumberger Company) | Barlow, Adam (Smith Bits, a Schlumberger Company)
The operator requires a 12¼?? tangent borehole through interbedded shale, chalk/sandstone that contains hydratable montmorillonite clay, Auk North field, UK North Sea. The swelling clay and long open-hole section have proven troublesome creating hole cleaning issues, frequent wiper trips and stuck pipe events.
A comprehensive modeling project was initiated with the aim of reducing vibration whilst improving rate of penetration (ROP) by selecting the best drive system, bottom hole assembly (BHA) configuration and combination of operating parameters. A rock strength program was run and indicated unconfined compressive strength (UCS) of the Oligocene/Eocene formations at 2-10kpsi. Analogous formation samples were selected from the rock library and laboratory tests were conducted to replicate actual drilling conditions. The resulting data was then entered into the finite element analysis (FEA) based modeling system. A 12¼?? x 14?? concentric reamer equipped with three polycrystalline diamond compact (PDC) triple row cutter blocks, a pilot PDC bit and a rotary steerable system (RSS) were selected for the investigation. Dynamic simulations were preformed with various operating parameters to observe how changes influenced BHA performance.
The analysis provided valuable insight when the bit and reamer are drilling non-homogeneous formations. The weight-on-tool fluctuation creates BHA instability which can be mitigated by managing parameters. A driller's roadmap was produced showing optimum weight on bit (WOB)/revolutions per minute (RPM) combinations that would produce stable drilling for different formation scenarios.
The integrated BHA was run with the recommended parameters and drilled 5889ft in three runs at an average ROP of 77ft/hr. The BHA met all directional objectives with low lateral vibration and medium stick-slip levels. All cutting structures used showed minimal wear. The successful run enabled the operator to complete the section seven days ahead of authority for expenditure (AFE) plan.
The Auk field is located 168 miles east-southeast of Aberdeen, UK in water depth of 270ft. The field is situated on the western margin of the North Sea's Central Graben in Block 30/16 (Figure 1). The finder well 30/16-1 encountered a 400ft oil column (38API) within the Rotliegend sandstones, Zechstein dolomites and Lower Cretaceous breccias. The field was discovered in 1971 and oil production commenced in 1975.1,2 The reservoir is a complex tilted horst block sealed by Upper Cretaceous chalk, bounded to the west by a series of faults with throws of up to 1000ft and a dip closed to the North, East and South. Additional complexity is caused by cross flow between Rotliegend and Zechstein reservoirs and early water cut as a result of bottom water drive from a strong aquifer.
Full field reservoir static and dynamic modeling has improved the understanding of reservoir behavior. This coupled with the use of extended reach drilling (ERD) and horizontal well technology has increased accessible reserves in the Rotliegend reservoir.
Technology Focus - No abstract available.
While water-based fluids are generally preferred for drilling in environmentally sensitive locations, many are formulated with potassium chloride (KCl) to achieve good inhibition of reactive clays. The chloride ion however can be defined as a contaminant in land operations, with the potential to inhibit the growth of vegetation, and it can also be considered a potential pollutant to aquifers.
In the rain forest region of the Southern Highlands of Papua New Guinea (PNG), an alternative to the KCl high performance water based fluid (HPWBF) was needed to mitigate environmental concerns, affirm the operator's commitment to sustainable operations and provide continuous improvement as required by ISO 14001.
The fluid design team examined a range of alternatives and determined that potassium acetate (KAc) could replace KCl in the drilling fluid as a more environmentally acceptable inhibiting salt. Laboratory testing confirmed that the drilling performance properties of the fluid would not be impeded by substituting KAc for KCl. Experience using KAc fluids in other geographical areas was also examined to back up the laboratory testing.
The original KCl-based HPWBF had historically helped reduce issues related to the longer step-out wells, directional complexity, rock tectonics, and wellbore stability in the area. The new KAc system was used equally successfully to drill seven challenging wells. Both the technical and environmental objectives of the wells were achieved without sacrificing drilling performance. The change to KAc increased the fluid cost by approximately 10% when compared to the KCl-based system, but the operator concluded that this was a good investment, considering the distinct environmental benefits.
In a comprehensive review of the background to, and the successful implementation of the KAc HPWBF in to PNG Southern Highlands drilling, this paper firstly discusses the purposes of a drilling fluid. It will show how the presence of the potassium ion is regarded as essential within the drilling fluid, but the chloride ion has no discernible benefit. It considers the unique PNG Highland rainforest environment were the drilling operations occur, the local communities who live and interact with the operation, and how the operator has successfully protected this area. The drilling fluid disposal and discharge solutions in use in the Highlands operating area are discussed, how appropriate disposal solutions are selected, and how drilling fluid discharge is controlled to minimise any environmental impact.
In the second half of the paper, the methods used to develop the new chlorides-free KAc HPWBF system will be discussed. Results will then be presented from the seven wells drilled with KAc HPWBF, with two previous KCl wells used as reference. It will demonstrate that along with the improved environmental acceptance, drilling fluid robustness was maintained, as was on bottom drilling performance. Finally a review of material usage and costs indicates the change to the new KAc fluid did not have as significant an impact as first assumed, as KAc concentrations within the HPWBF were optimized.
Technology Update - No abstract available.
Maximizing profitability in mature fields is dependent on reducing drilling and operational expenses to maintain optimized hydrocarbon production. As Forties field UK North Sea matures, drilling challenges are becoming increasingly more difficult and complex. Wellbore instability caused by the loss of reservoir pressure and anisotropic properties of overburden shale is a major issue as targets are pushed further away from the platform.
To continue successful development of Forties field, the operator is required to drill high-inclination deviated wellbores sidetracked from existing boreholes. The unstable shale outside of the sidetrack window requires a low-side casing exit. To optimize operations the sidetrack must be completed on the first attempt. When a first sidetrack fails, a second is often initiated approximately 10m (or interval thereof) further up in the wellbore with a higher mud weight. Unable to get more than a few feet away from the original wellbore within such a short distance, the new sidetrack can frequently re-enter the zone already damaged by the previous attempt and again runs into trouble. This broken formation becomes even more destabilized with increased mud weight.
To solve the operational / economic challenges, a unique wellbore departure system was developed to deliver fast, high-quality windows and sidetracks tailored specifically to meet operator?s low-side application objectives without compromising performance. The low-side exit requires a unique set of pre-job equipment modifications which is performed in the service provider?s workshop prior to shipping equipment to the well-site. The modification allows an upward force to be exerted at the tip of the whip face on setting the permanent packer / anchor thereby overcoming the natural gravitational forces. This upward force does not come into effect until the packer is energized, thus ensuring the whipstock assembly remains flexible enough to mitigate wellbore tortuosity encountered whilst running in the hole.
The system was successfully applied initially on three challenging uncemented whipstock sidetracks with single-trip window success (up to 77° inclination / 180° orientation). On all three jobs the anchoring and milling technology worked flawlessly with no issues when subsequently tripping directional BHA or liners through the window. Application engineers performed pre / post-job briefings with service provider?s rig site / offshore supervisors to ensure specific low-side exit guidelines were followed and that lessons learned or suggestions for improvement were captured and documented for prosperity.
The authors will present Forties field case studies that document procedural repeatability and how the tools and techniques could be used for any challenging low-side uncemented casing exits.
An efficient two-stage algebraic multiscale solver (TAMS) that converges to the fine-scale solution is described. The first (global) stage is a multiscale solution obtained algebraically for the given fine-scale problem. In the second stage, a local preconditioner, such as the Block ILU (BILU) or the Additive Schwarz (AS) method, is used. Spectral analysis shows that the multiscale solution step captures the low-frequency parts of the error spectrum quite well, while the local preconditioner represents the high-frequency components accurately. Combining the two stages in an iterative scheme results in efficient treatment of all the error components associated with the fine-scale problem. TAMS is shown to converge to the reference fine-scale solution. Moreover, the eigenvalues of the TAMS iteration matrix show significant clustering, which is favorable for Krylov-based methods. Accurate solution of the nonlinear saturation equations (i.e., transport problem) requires having locally conservative velocity fields. TAMS guarantees local mass conservation by concluding the iterations with a multiscale finite-volume step. We demonstrate the performance of TAMS using several test cases with strong permeability heterogeneity and large-grid aspect ratios. Different choices in the TAMS algorithm are investigated, including the Galerkin and finite-volume restriction operators, as well as the BILU and AS preconditioners for the second stage. TAMS for the elliptic flow problem is comparable to state-of-the-art algebraic multigrid methods, which are in wide use. Moreover, the computational time of TAMS grows nearly linearly with problem size.