A 1.9 SG oil-based fluid for suspending open-hole pre-perforated liner completions in 1100 m long, horizontal openhole sections, under HPHT conditions (162°C, +800 bar reservoir pressure) has been engineered for the Morvin field development. The main challenge was to provide this fluid with sufficient stability so that it still was mobile when wells were backproduced after up to 4 weeks of subsequent well completion activities. The main drivers for the development were the constraints that alternative fluid concepts pose under such conditions. Standard oil-based fluids weighted with barite, under the prevailing conditions up to 1.4 tons per cubic-meter of fluid, showed to be highly instable under simulated reservoir conditions. Barite settled out and deposited thick immobile mud bodies that were potentially completion damaging. Other fluid alternatives such as Cesium-formate weighted water-based suspension fluids were suspected to be incompatible with the selected oil-based reservoir drilling fluid. Based on previous experiences with low solids (barite free) oil-based completion fluids (screen running and perforation fluids), a so-called "reduced solids oil-based completion fluid" was engineered in cooperation with the fluids contractor. The main features of the fluid were a close-to 50/50 oil-to-brine ratio, a Cesium-formate internal brine phase and fine barite to weigh up the fluid to approach the required weight. Several variations of this fluid were rigorously tested to evaluate stability and mobility of fluid sample samples. The fluid has so far successfully been used on three completions. This paper reports in detail on the fluid development process, operational experiences and well-clean-up observations.
Several improvements on fluids, equipment, completion design and practices have been successfully implemented in the first subsea well of Morvin HPHT field offshore Norwegian Continental Shelf. These improvements are:Using new heavy weight reduced solid oil-based completion fluid for running the lower completion (predrilled liner) and achieving the expected initial productivity ;Extra well cleaning prior to running the lower and upper completion;A designed predrilled liner combined with open hole swell packers for zonal isolation and HPHT tracer-subs for data acquisition;Downhole barrier being installed as an integral part of the lower completion, eliminating the middle completion ;New HPHT permanentretrievable production packer; andSuccessful installation of downhole P/T gauge with pig tail connection below the tubing hanger. Completion operation on the first well was completed in 40 days (i.e. 12 days less than the planned days) with no major operational set back, safety and environmental incidents. After the well clean up, the well has been flow-tested up to 1500 Sm3/d (limited by rig capacity) and the well testing evaluation shows that the well delivers productivity as expected. Tracer samples analyses also show good production contribution from both reservoirs being penetrated by 1000 m of 8-½?? horizontal hole section. The HPHT reservoir (819 bar / 162 oC) in Morvin wells requires a different approach to completion design and practices in order to avoid well control incidents, operational problems, and achieve full well integrity during its production lifecycle. It demands the use of non conventional materials and fluids, careful equipment selection which is seldom available off-the-shelf in today's market, equipment qualifications and new procedures1. This paper describes the original completion concept that was much influenced by well completion experiences from the Kristin field, the world's first HPHT field to be developed subsea from multi-well templates. The paper then discusses the improvements made from the original completion design, challenges and contingencies. Introduction Today, oil and gas E&P activities have been steadily expanded into handling more complex and challenging projects such as high pressure high temperature (HPHT) wells. Among the regulatory bodies, the Norwegian Petroleum Directorate (NPD) defines HPHT wells as deeper than 4000 mTVD and/or having an expected shut-in wellhead pressure (SIWHP) exceeding 690 bar and/or having a bottom hole temperature exceeding 150 oC2. The main factors affecting the completion design and well integrity in HPHT wells are the extreme surface pressure, very high temperature, and various load scenarios during their lifecycle combined with the corrosivity level of the well environment. Risk of failures in this HPHT completion design is usually associated with2:High stress environment, both tension and compression.High operating temperatures.Chemical activity of well fluid components enhanced by the high temperature.Narrow margin between the boundaries of load uncertainties and equipment's material rating. Longer planning process is sometimes required to anticipate long lead times of HPHT specific equipment. Longer time is also needed for the equipment qualification.
A major operator in the Norwegian continental shelf recently implemented an improvement program focused on increasing standardization and efficiency on subsea well operations. By including a Vertical Christmas Tree (VXT) rather than a Horizontal Christmas Tree (HXT) in their subsea completion design, several wells can be batch completed with a rig. VXT Installation can then be completed with an Installation, Maintenance, and Repair (IMR) vessel, eliminating the need for a Rig or Light Weight Intervention (LWI) vessel. In order to use an IMR vessel for VXT installation, a new method needed to be developed for the establishment/removal of a temporary second barrier between the well and subsea completion. The goal was to achieve a barrier valve solution that would be installed as part of the completion string. Furthermore, without requiring intervention via wireline or an LWI, it would accomplish three positional phases: open, closed, and then open again. The tool would be installed in the open position, allowing for all standard completion operations to be conducted. For example, lighter fluid could be pumped into the tubing prior to setting a packer, allowing for bull heading in a well control situation. By incorporating the valve into the completion string and ensuring the well design and operation is planned accordingly, wireline intervention can be entirely avoided The more common methods for suspension and initiation of subsea wells during BOP removal and VXT installation include installing shallow set bridge plugs in the tubing and/or a tubing hanger plug. The intervention plugs would then be removed using a riserless LWI vessel. This method is often associated with high cost and enhanced operational risk. A technology group was assembled in order to close the technical gaps inhibiting a fully intervention-free operation for completion installation. Technology personnel from both the operator and service company worked together closely to develop a glass plug solution that met all the necessary requirements needed to overcome this challenge while still minimizing associated risk. The proposed solution is a barrier valve to be integrated as part of the completion string; using glass plug technology, it will accomplish three positions: open, closed and then open again. The glass plug will first be installed with the bypass ports in open position, allowing for standard completion activities to be conducted. For example, an operator would still be able to accomplish bull heading in the event of a well control situation. The bypass is shifted to the closed position after a predefined number of tubing pressure cycles have acted on the glass plug system. Once the bypass is closed a ISO14310 V0Q1 well barrier is established. The glass plug barrier is only shattered after the tubing above the glass has been subjected to a predefined number of pressure cycles while applying a specific overbalance. After, the barrier element has shattered the valve it activates its final open state, resulting in full bore ID through the valve. This was developed and qualified according to ISO 14310 V0Q1 / ISO 28781 V1Q1 and installed in less than a year. By utilizing this glass plug the drilling rig can batch complete subsea wells and at a later stage batch install the VXT with an IMR vessel. The product can also be installed as part of conventional completion string were shallow barrier is required, eliminating the need of intervention. By implementing this technology, the operator saves time and cost addition to reduced HSE exposure associated with well intervention.
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