The Pohokura Gas Field lies off the West Coast of New Zealand in the Taranaki Basin. To develop this field, a total of nine wells were drilled, from both onshore and offshore locations. The extended-reach wells drilled from land set new records for New Zealand, drilling to a measured depth of 7,409 m (24,309 ft) on POW 03, the longest well. The five wells drilled from the offshore drilling unit featured tortuous well paths through the reactive and dispersive shales of the Taranaki Basin. The Pohokura reservoir sands are low-porosity "tight" sandstones, notoriously susceptible to fluid-related production impairment. In addition to these challenges, the onshore and offshore environmental regulations applied both by the local regulator and self-imposed by the operator are among the most strict in Asia. The fluids-specific challenges associated with this project included the design of a synthetic-based fluid and cuttings handling system that could maintain the fluid-oncuttings discharge at the lowest practicable level, design of a highly inhibitive and lubricious water-based drilling fluid for the offshore drilling campaign, plus the development and testing of non-damaging drilling fluids for the reservoir sections of all wells. To achieve these goals, extensive laboratory testing and comprehensive reviews of field experience from other operations were undertaken. Key Performance Indicators were agreed between the operator and fluids provider. These benchmarks were reviewed on a well-by-well basis and used to facilitate ongoing continuous performance improvement. Introduction The Pohokura Gas Field is located just off the West Coast of the North Island of New Zealand in the Taranaki Basin. The field has estimated reserves of approximately 750 petajoules of gas- approximately 50% of the known gas reserves in New Zealand. Successful development of this field was considered vital to securing New Zealand's electricity supply for future generations. To develop this tight gas payzone, a total of nine wells were drilled (one injector well and three production wells) from an onshore location and five production wells, including one dual lateral drilled from a jackup rig. Due to the location and nature of the field, all development wells were characterized by tortuous well paths through the reactive and dispersive shales of the Taranaki Basin. The target reservoir sands were low-permeability sandstones, believed to be prone to fluid-related damage. The decision to drill the initial production wells from land ensured that this development would include some of the longest extended-reach wells ever to be drilled in Southeast Asia. With drilled depths in excess of 7,000 m (23,000 ft) planned, the choice of drilling fluid would be critical to the ultimate success of the project. To successfully meet the criteria of the land drilling program, it was clear that the properties of an invert emulsion, synthetic-based drilling fluid would be required to drill all intermediate and lower hole sections. The potential volume and nature of the drilling waste generated by this process created the need for an integrated drilling fluids and waste management program, thereby ensuring the volume of waste created could be minimized, and that the drill cuttings generated could be treated to reduce environmental impact.
Recent investigative work for cementing highly deviated liners offshore New Zealand (NZ) employed several types of laboratory tests and numerical models-with many developed specifically for this project-to determine the best way to prepare the hole and then complete the cement job. Well-established test methods were used along with tests designed specifically to understand certain formation, mud, and cement interactions. The newly developed laboratory testing methods include: mud film testing using mud, spacer, and cement; ultra-low shear rheology (ULSR) testing under downhole conditions to determine mud and cement static gel strength development, as well as erodibility and removal of the mud by spacer and cement; Dynamic High Angle Sag Testing (DHAST) for both mud and spacer; and dewatering and filter cake build-up testing of mud, spacer, and cement, using a Fann 90 test apparatus. Both standard and newly-developed numerical models aided lab test data analysis in making decisions on materials and equipment and ensuring successful cement placement. One new model used DHAST data to check for effects on cement placement by calculating settled solids volumes dynamically deposited ahead of the cement slurry. A second model analyzed ULSR data to predict mud displacement results with different fluid properties designed for the mud, spacer, and cement. A third new model predicted displacement pressure effects by mud and cement interfacial mixing associated with certain cementing conditions and job procedures. The model also qualified the risk of multifluid vs. single fluid annular flow which determined procedures to minimize bypassed mud and cement slurry channeling. The results of these efforts greatly aided the understanding of the cement, mud, and spacer interactions with the formation. This paper aims to highlight the developed testing and modeling methods, provide information on the results of the methods, and demonstrate how the methods enabled the successful cementation of several highly deviated offshore liners in NZ.
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