Dual-Gradient Drilling

Cohen, J.H.; Stave, Roger; Schubert, Jerome; Elieff, Brandee A.

doi:10.1016/b978-1-933762-24-1.50014-0

Cited by 3 publications

(1 citation statement)

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“…The seawater manages the borehole, and the later prevents the borehole from collapsing and removing cuttings from the borehole. Mud return does not reach the surface through a conventional large diameter riser; instead, a separate mud lift system uses a return line powered by subsea pumps. , Figure shows different pressure profiles of a dual gradient drilling system.…”

Section: Drilling Techniques For a Gas Hydrate Reservoirmentioning

confidence: 99%

Comprehensive Review on Exploration and Drilling Techniques for Natural Gas Hydrate Reservoirs

2020

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With the depletion of conventional oil reservoirs and concerns about the increasing carbon footprint of coal, natural gas is poised to play a much bigger role in sustainable economic growth. Gas hydrates being an abundant source of clean methane have created great interest among academia and industry. Its demographic occurrence in shallow depths has motivated industry and academia to explore its abundance and exploitation potential. Despite being shallow, offshore gas hydrate exploration and drilling is a deepwater endeavor, which was considered expensive and risky as a result of its narrow thermodynamic stability and lack of exploitation experience. However, with recent advancement in technology and long-term field trials, it has been universally accepted that methane production from marine hydrate reservoirs is achievable with innovative drilling techniques and proper well design pertaining to the appropriate location and layout of wells, efficient well orientation, competent casing and tubing design, and completion design requirements for long operating life and reliability of the well. Much has been discussed about exploration and production, but very little has been addressed about the drilling technologies that have the potential for safe and effective exploitation of this resource in commercial quantities. The objective of this review is to enlighten the readers with exploration techniques used to locate gas hydrate reservoirs and discuss drilling tools and techniques that have the potential to avoid premature hydrate dissociation around the wellbore and enhance production life of the well with minimum interventions. The techniques discussed are mostly the learnings from the international drilling projects on different types of gas hydrate reservoirs. This review also focuses on different drilling fluids, pressurized coring systems, and well logs suitable for hydrate reservoirs. We believe that this review will provide a scientific reference material to engineers for safe exploration and drilling and stimulate upcoming exploration and drilling activities in the field of gas hydrates.

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Section: Drilling Techniques For a Gas Hydrate Reservoirmentioning

confidence: 99%

Comprehensive Review on Exploration and Drilling Techniques for Natural Gas Hydrate Reservoirs

2020

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Reliability Informed Drilling: Analysis for a Dual-Gradient Drilling System

Das

Samuel

2014

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Traditionally, reliability analysis is performed on a system design to estimate a reliability number with a given confidence level. Such a conventional method analyzes the failure rates of the components with an exponential distribution assuming a constant failure rate over time. However, in reality, for a drilling system reliability changes over the drilling period owing to component degradation, varied processes, and the drilling problems involved. Hence reliability becomes a function of not only component failure rates but also of the drilling operations, problems encountered, and depth of drilling or time. In this paper an example of Dual-Gradient Drilling (DGD) technology is evaluated. The subsea equipment studied in the analysis includes the drill string, drill string valve, blow-out preventer, subsea rotating diverter, and the subsea mud-lift pump. A dynamic reliability analysis is performed on the DGD system to account for multiple states. These states are analyzed using the Markov process. However, using only Markov chains does not analyze the failure mechanisms and their effects on other components. Hence to overcome this limitation, a Failure Modes and Effects Analysis (FMEA) is performed to better understand how a failure affects the system. Then the probabilities for the Markov process are based on these system interactions. During the FMEA analysis it was recognized that the change in failure rate of an equipment failure mode over time is a function of different factors and hence should be assessed using different failure density distributions. A limited drilling scenario is created with drilling rates and trip times calculated to provide the necessary data. There were two types of perturbations or disturbances modelled in the simulation that occur during the drilling scenarios. These are (1) the influx of formation fluids into the drilling mud and (2) equipment failures. A Monte Carlo simulation for the entire system is developed to assess the reliability of the drilling system at every 30' depth interval. The Monte Carlo method used different probability distribution models for different failure modes. Thus, the dynamics of system behavior and how equipment failures interact and change over time and damage accumulations and reversals (repairs/replacements) are incorporated into the model. Ten simulation runs were carried out for the scenarios and perturbations which gave results for a total of 70000' of drilling data over approximately 109,174 hours. This paper presents the observed influxes and equipment failures. The dynamic reliability results show reliability varying with depth which is a realistic case as compared to the system's conventional reliability results. The results database can be updated with new field data as drilling progresses. This will allow the analyst to compare anticipated reliability with the revised anticipated reliability going forward in the drilling operation. Thus, the paper introduces a methodology for applying the concept of ‘reliability-informed drilling’.

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