Joint Industry Project Research Articles

A study on methods for fire load application with passive fire protection effects

The objective of this paper was to introduce more advanced and practical procedures for fire load application methods considering the effects of Passive fire protection (PFP) with the motivation to nonlinear structural consequence analysis of FPSO topsides structures under fire load. This paper is a part of phase III of the joint industry project on explosion and fire engineering of FPSOs (EFEF JIP). Temperature dependent material properties of steel and PFP materials were adopted to develop fire load application methods. But in case of PFP materials, only temperature dependent thermal properties were considered to focus on the thermal effects. Numerical simulations were performed and the modeling method of PFP materials were validated with published experimental results. The nonlinear finite element code LS-DYNA was used for numerical simulations. The results of this study will be useful for consequence structural analysis under fire load considering PFP effects.

An Improved Three-Phase Relative Permeability and Hysteresis Model for the Simulation of a Water-Alternating-Gas Injection

Summary Water-alternating-gas (WAG) injection in waterflooded reservoirs can increase oil recovery and extend the life of these reservoirs. Reliable reservoir simulations are needed to predict the performance of WAG injection before field implementation. This requires accurate sets of relative permeability (kr) and capillary pressure (Pc) functions for each fluid phase, in a three-phase-flow regime. The WAG process also involves another major complication, hysteresis, which is caused by flow reversal happening during WAG injection. Hysteresis is one of the most important phenomena manipulating the performance of WAG injection, and hence, it has to be carefully accounted for. In this study, we have benefited from the results of a series of coreflood experiments that we have been performing since 1997 as a part of the Characterization of Three-Phase Flow and WAG Injection JIP (joint industry project) at Heriot-Watt University. In particular, we focus on a WAG experiment carried out on a water-wet core to obtain three-phase relative permeability values for oil, water, and gas. The relative permeabilities exhibit significant and irreversible hysteresis for oil, water, and gas. The observed hysteresis, which is a result of the cyclic injection of water and gas during WAG injection, is not predicted by the existing hysteresis models. We present a new three-phase relative permeability model coupled with hysteresis effects for the modeling of the observed cycle-dependent relative permeabilities taking place during WAG injection. The approach has been successfully tested and verified with measured three-phase relative permeability values obtained from a WAG experiment. In line with our laboratory observations, the new model predicts the reduction of the gas relative permeability during consecutive water-and-gas-injection cycles as well as the increase in oil relative permeability happening in consecutive water-injection cycles.

Structural mechanics for design of grouted connections in monopile wind turbine structures

Over the last 30 years it has become standard practice to connect offshore oil and gas structures to their foundation piles using cylindrical shaped grouted connections with shear keys or weld beads. Circumferential shear keys, or weld beads, are provided around the outside of the piles and the inside of the pile sleeves in jacket structures to transfer forces through the grouted connection. The same methodology is also now being used by the wind energy industry to connect wind turbine support structures to their foundation piles. These structures are subjected to rather severe dynamic loading, it is therefore important to document the fatigue capacity of these grouted connections. As a direct result of this need, a joint industry project focusing on the capacity of cylindrical shaped grouted connections with shear keys was initiated by DNV in January 2011 and continued through until completion in May 2012. This project has involved fatigue testing of grouted test specimens in the laboratory, finite element analyses and assessment and development of a recommended design methodology. The design methodology includes the Ultimate Limit State and the Fatigue Limit State. Fatigue testing of full-scale specimens would require very large test setup and hydraulic actuators. Therefore special box specimens were designed with a representative radial stiffness similar to that of large diameter connections, with both full size grout thickness and geometry of the shear keys. An analytical approach for design of these specimens and for the design of grouted connections in monopiles is presented in this paper.

Behaviour of grouted connections of monopile structures at ultimate static and cyclic limit states

In recent years, reports of offshore wind turbine topside structures settling on their monopile foundation structure have emerged, with the resulting force flow in the structures found to be different from that intended at the design stage. As a result, a joint industry project was carried out by Det Norske Veritas (DNV) between late 2009 and early 2011, to investigate the structural capacity of these connections. It was found that the axial capacity of the grouted connections is more sensitive to the diameter (capacity reducing more than predicted at larger diameters) and surface tolerances than had been accounted for in the existing design standards. This paper reviews the past and present industry practice relating to the design of grouted connections in monopile structures. The physical behaviour of the connections is explained and some of the most critical issues related to the design of large diameter grouted connections are assessed. Based on the experience with the settlements of cylindrical shaped grouted connections, a design procedure with conical shaped connections was developed. The recommended design methodology has been supported by a number of laboratory tests. The main findings from that joint industry project are presented in this paper.

Capacity of cylindrical shaped grouted connections with shear keys in offshore structures

Over the last 30 years it has become standard practice to connect offshore oil and gas structures to their foundation piles using cylindrical shaped grouted connections with shear keys or weld beads. Circumferential shear keys, or weld beads, are provided around the outside of the pile and the inside of the sleeve to give the required capacity. The same methodology is also now being used by the wind energy industry to connect wind turbine support structures to their foundation piles. These structures are subjected to rather severe dynamic loading. It is therefore important to document the fatigue capacity of these grouted connections. As a direct result of this need, a joint industry project focusing on the capacity of cylindrical shaped grouted connections with shear keys was initiated by Det Norske Veritas (DNV) in January 2011 and completed in May 2012. This project has involved fatigue testing of grouted test specimens in the laboratory, finite element analyses and assessment and development of a recommended design methodology. The design methodology includes the Ultimate Limit State and the Fatigue Limit State. Fatigue testing of full-scale specimens would require very large test setup and hydraulic actuators. Therefore, special box specimens were designed with a representative radial stiffness similar to that of large diameter connections, with both full-size grout thickness and geometry of the shear keys. The design and testing of these specimens is presented in this paper together with the main results from the project.

CUBE - A New Technology for the Containment of Subsea Blowouts

Young Technology Showcase After the Macondo incident, the oil industry started a significant review of its capabilities to respond to a subsea well blowout and launched some specific development projects. Eni decided to participate in the principal joint industry projects launched on this topic, and also started internal activities aimed at the development of new containment methods. Two years after Macondo, it is clear that the major containment method of a subsea blowout relies on the use of a capping stack. This can be deployed in water depths as deep as 3000 m and equipped with multiple outlets that allow liquid and gaseous hydrocarbons to be routed to surface. However, in some cases well capping cannot be applied when a) well-integrity issues arise, this may happen when the capping stack causes a substantial pressure increase at the wellhead, with a potential rupture of the casing and the lateral dispersion of well fluids; b) the blowout preventer (BOP) stack is not accessible because of mechanical problems (status of wellhead/BOP or irremovable debris); c) the blowout does not come directly from the well (i.e., from nearby sea lines or mud line). On the basis of these and similar considerations, it is quite clear that the containment of a blowout event cannot only rely upon the use of a capping stack and other methods must be available. Eni has been working in the last 2 years on a new concept with the support of TEASistemi, a research company of the University of Pisa, developing an alternative to the capping technology. This system is based on the use of an open-sea containment system, which is able to perform at least partial gas/liquid separation. After separation, most of the liquid hydrocarbons plus a minor amount of seawater are pumped to surface. The gas phase can be sent to surface through a separate riser or can be discharged at sea. In order to prevent hydrate formation within the separation system, there must be a low separator-residence time. The system conceived by the authors of this paper has been patented in June 2010 (De Ghetto and Andreussi, 2010) and the first tests were performed in September 2010 at the TEASistemi Laboratory in Pisa. Of course the concept of subsea phase separation is not new for the oil industry, but up to now it has not been adopted for the containment of subsea blowouts. The reason probably being that up to the Macondo incident, it was believed that subsea collection and transmission to surface of blowout fluids could be easily accomplished by some type of containment dome. Unfortunately, the Macondo incident showed that this is not always the case and that the use of a containment dome presents a number of potential problems which need to be faced, such as the formation of hydrates, the poor control of the system, and finally, issues associated with large fluxes of seawater into the dome.

Risk based Fatigue Inspection Planning – State of the Art

The present paper presents the methodology and the practical calculations for risk based inspection planning of fatigue cracks in welded offshore steel structures. Due to the uncertainty in the variables involved in the problem the planning has to be carried out by stochastic modeling and risk based assessments. Scatter in potential crack growth has to be analyzed by applied probabilistic facture mechanics and the uncertainty in the performance of the actual inspection technique has to be determined. With given risk acceptance criteria the practical outcome of the analyses is recommended inspection techniques and associated planned inspection time intervals. The classical theory is briefly outlined and the latest recommendations from a Joint Industry Project recently completed in Norway are included. A practical case study for life extension of an oil loading system in the North Sea is presented.

A Novel Technique for Monitoring Hydrate Safety Margin

Summary A novel technique has been developed for optimizing hydrate-inhibitor dose rates by monitoring the actual hydrate safety margin. The technique is based on measuring acoustic velocity and electrical conductivity in an aqueous sample taken from the pipeline/separator. The developed system then determines salt and organic inhibitor concentration (e.g., methanol, monoethylene glycol, and kinetic hydrate inhibitors). In the case of thermodynamic inhibitors, the information is then fed to a thermodynamic model determining the hydrate stability zone (HSZ). Superimposing the pipeline operating conditions (e.g., pressure and temperature profile), the developed system can determine the hydrate safety margin. The system can provide warning if the operating conditions are inside the HSZ or if too much inhibitor is being injected. The technique can be used in optimizing inhibitor-injection rates, reducing the impact on the environment and operational costs. It can also improve production-system reliability by constant monitoring the hydrate safety margin and protecting the system against pump malfunction and/or changes in process variables (e.g., water cut). This technique has been developed through a joint-industry project, and its performance has been intensively evaluated using synthetic samples and real produced-water samples by the authors and the project sponsors in field trials. The results demonstrate that this system can be used for different inhibitor systems, including methanol (MeOH)/NaCl, monoethylene glycol (MEG)/NaCl, and kinetic-hydrate-inhibitor/salt systems, with good accuracy. The developed system can have a major impact on reducing the inhibitor-injection rates and environmental impact and improving the reliability of production systems against risks associated with hydrate formation. The developed system can be laboratory-based, a mobile unit, or online, with near-real-time samples taken from live pipelines/separators.

Fatigue Strength Of Thin- Plated Block Joints With Typical Shipbuilding Imperfections

During design of block joints in ship structures it is assumed that the permissible tolerances for axial and angular misalignment given in shipbuilding standards are not exceeded. This is particularly important for fatigue assessments based on the nominal stress approach which implicitly incorporates stress magnification due to imperfections. Preliminary investigations of thin-plated ship structures (t = 4 mm) showed that (a) it is very difficult to keep the thickness-dependent tolerances for thin plates, and (b) even if the tolerances are kept, the fatigue strength might not be sufficient as the stress magnification incorporated in the nominal stress approach is far exceeded. The present paper highlights the problem in the beginning. The main part describes fatigue tests of components performed in a joint industry project. Stiffened panels were fabricated by different shipyards under realistic shipbuilding constraints. The partly large imperfections (buckles, axial and angular misalignment) were extensively recorded using photogrammetric technique in order to obtain the characteristics of typical pre-deformations. The component tests establishing the link to the fatigue strength were carried out with a stress ratio R = −1 which is typical for naval vessels. Concurrent nonlinear finite element calculations containing the whole test set-up were carried out to obtain relations between pre-deformation and stress increase. After description of the test results, a procedure for the assessment of imperfections is discussed which are measured in the usual way in shipbuilding.

Research and Development at Universities: Worldwide Change in Petroleum Engineering

The fundamental properties of hydrocarbons and the basics of drilling will always remain the same. Nevertheless, innovation in petroleum engineering by researchers at universities around the world is changing the face of oil and gas drilling and production in the 21st century. In addition to traditional approaches, technologies for enhanced production from mature fields; production methods for unconventional resources such as tight gas, methane hydrate, and heavy oil; and experimental nanotechnology applications today are critical and increasingly are a part of petroleum engineering research and technology development. Schulich School of Engineering, University of Calgary, Canada The upstream research at Schulich’s Department of Chemical and Petroleum Engineering concentrates on improving the exploitation of Canadian unconventional oil and gas resources such as heavy oil and bitumen, and tight gas formations. The department has a dozen research teams, said professor Brij Maini. The In Situ Combustion Research Group specializes in laboratory and numerical simulation studies of the in-situ combustion of heavy oils, and high-pressure air injection in light oil reservoirs. The faculty’s EnCana/Paleontological Society of Canadian Institute of Mining, Metallurgy, and Petroleum Endowed Chair heads the department’s joint industry project studying the geomechanics of shale gas. The CMG Foundation Chair in Reservoir Simulation is developing fast and reliable reservoir simulators for the recovery of oil, gas, and coal. The NSERC Industrial Research Chair in Heavy Oil Properties and Processing is researching the properties and phase behavior of heavy oil and solvents. Schulich’s SHARP (Solvent/Heat Assisted Recovery Processes) consortium is supported by 11 local and international companies. The improved heavy oil science and technology group is studying various aspects of heavy oil recovery technology, including cold heavy oil production with sand, and solvent-based recovery processes and steam additives for steam injection-based processes.

Two-phase jet releases, droplet dispersion and rainout, II. Rainout experiments

This paper describes the results of the first experimental stage of Phase IV of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of this stage of the JIP was to generate experimental rainout data for non-flashing water and xylene experiments. See the overview companion paper I for a wider overview of the problem, model implementation and associated model validation.A range of orifice sizes (2.5 and 5 mm) and stagnation pressures (4–16 barg) were applied. Measurements included flow rate, initial droplet size, plume concentrations/temperatures for a range of downstream locations, and distributed rainout.Instead of the Phase Doppler Anemometry method used for droplet size measurements earlier in the JIP, a photographic technique was applied in an attempt to include measurement of the larger (non-spherical) droplets. This enabled a more accurate evaluation of the initial droplet size distribution and a much clearer understanding of the droplet morphology. The results showed that the droplet behaviour in the jet is more complex than had been anticipated with the mass distribution dominated by a very small number of large non-spherical droplets. Consequently a large number of spray images were required to evaluate an accurate size distribution.Distributed rainout was measured by weighing the amount of rainout in trays positioned along the jet direction. The rainout results showed a good degree of repeatability and internal consistency. They indicated that an increasing proportion of the released material did not rainout for increasing pressure. Rainout distance also increased with increasing pressure. Evaporation of the liquid was confirmed by temperature measurements, which showed the effect of evaporative cooling.Xylene concentration measurements (up to 1%) were carried out using a direct reading photoionization detector calibrated for xylene (measuring vapour only). For a limited dataset, the accuracy of these measurements was estimated by means of comparison against an alternative more time-consuming concentration method (xylene absorption onto a charcoal filter; measuring both vapour and liquid). The concentration measurements displayed several consistent qualitative features. For example, at a given downstream distance, the peak concentration increases with increasing pressure and nozzle diameter and the vertical height at which the peak is achieved increases. The cross-stream profiles displayed a consistent tendency to increased concentration at the edge of the jet, and the reason for this has not been established.Finally recommendations are provided for potential future work.

Two-phase jet releases, droplet dispersion and rainout I. Overview and model validation

Many accidents involve two-phase releases of hazardous chemicals into the atmosphere. This paper describes the results of the fourth phase of a Joint Industry Project (JIP) on liquid jets and two-phase droplet dispersion. The objective of Phase IV of the JIP was to generate experimental rainout data for non-flashing experiments, and to develop recommendations for the best methodology to predict rainout [total rainout mass and its spatial distribution (‘distributed’ rainout)].Phase IV of the JIP first included rainout experiments by the UK Health and Safety Laboratory (HSL) for sub-cooled releases of water and xylene with a range of orifice sizes and stagnation pressures. See the companion paper II for further details. Secondly model validation was carried out by DNV Software for these experiments using different correlations for the initial droplet size (Sauter Mean Diameter, SMD), i.e. the CCPS SMD correlation and the Phase III JIP SMD correlation. The validation includes flow rates, droplet size, distributed rainout and cloud temperature drop. Subsequently validation was considered for a wider range of experiments from the literature (sub-cooled and superheated releases) for both SMD and total rainout. Adopted rainout methods comprised both methods including explicit modelling of the droplets (using an extended version of Phast dispersion model UDM), as well as more simple methods based on rainout correlations without droplet modelling. Recommendations are made for the most accurate droplet size and rainout modelling. A modified CCPS UDM droplet size correlation has been shown to agree best against experimental rainout data.

Software-Based Deepwater-Blowout- Preventer Testing

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 21688, ’Advances and Effects of Software-Based Deepwater-Blowout-Preventer Testing,’ by C. Mark Franklin, SPE, IPT Global; Tim Sargent, SPE, Noble Energy; Charles Brown, SPE, Hamilton Engineering; Gary W. Owen, SPE, Devon Energy; Steven Griffith, SPE, Shell; Jerry Osmond, SPE, Hess Corp.; Tore Gronas, Statoil; and Rick Cully, IPT Global, prepared for the 2011 Offshore Technology Conference, Houston, 2-5 May. The paper has not been peer reviewed. To advance the technology of blowout-preventer (BOP) leak detection through a software-development initiative, operators, service providers, and drilling contractors established an industry consortium in early 2009. The initial development focused on testing BOPs in deep water. Upon completion of the pilot program on five deepwater rigs, the results exceeded original expectations. The test results proved the software effective, efficient, and rig friendly. Introduction As drilling moves into deeper water and use of nonwater-based fluids increases, the challenges associated with obtaining effective pressure tests continue to grow. Prevailing interpretations of regulatory requirements to “hold pressure” have been to demonstrate a “flat line” on a pressure chart for a specified period of time. With the relatively low resolution of a circular-chart recorder (CCR), a flat line was produced, essentially, when the resolution of the device was not exceeded (typically 4–6 psi). Also, excessive damping and manipulation would not be recorded. Joint-Industry Project (JIP) During 2009 and 2010, four operators, with the support of several service providers and drilling contractors, participated in a JIP. As the main goal, the consortium wanted to develop state-of-the-art leak detection, with best practices for testing BOPs in deep water. The JIP was set up in two phases. Phase-1 gathered digital data from BOP tests performed in the Gulf of Mexico (GOM) and around the world. Archived data were used as test data, and a library of more than 100 data sets was built. Review of the test results showed that each data set had from 10 to 25 tests, 86% of which represented passing tests. The data archives provided insight into an automated approach for validating deepwater-BOP pressure tests. A software prototype was developed, along with workflow-automation features, and it was refined through multiple design-review meetings with the JIP members and the US Bureau of Ocean Energy Management, Regulation, and Enforcement. Phase-2 field-trial installations commenced on five deepwater rigs in the GOM. The software was also run in the background in a real-time operation center during BOP and manifold testing from multiple rig types (i.e., deepwater, tension-leg-platform, and jackup). The goal of Phase-2 was to validate the software and repair software issues.

Review of progress in evaluating gas hydrate drilling hazards

Shallow drilling hazard assessment consists of geological/geophysical review of drill sites to enable the avoidance or effective mitigation of problems that can affect the safe drilling of the “top-hole” section of wells. Primary seafloor hazards include infrastructure, hardgrounds, chemosynthetic communities, and areas of high slope or potential seafloor instability. Shallow subsurface hazards include strata that may contain overpressured water, free gas, or gas hydrate. Among these issues, the potential hazards posed by gas hydrate have historically been the most difficult to quantify and constrain. This paper reviews the history of gas hydrate shallow hazard assessment, much of which has been framed by difficulties in remote detection and quantification of typical low-saturation, mud-hosted hydrate occurrences. Recent drilling results indicate that such accumulations can be safely drilled using existing industry protocols. However, assessment of drilling hazards associated with high saturation, sand-hosted hydrates warrant further investigation, and therefore was a key focus of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) program conducted in 2009. Pre-drill site hazard assessment for JIP Leg II utilized standard industry methods in which geohazards associated with gas hydrate and free gas were identified in a primarily-qualitative manner using seismic amplitude analysis, seismic stratigraphic interpretation, and geo-pressure analysis. While these methods can reliably detect shallow hydrocarbons, including highly-concentrated gas hydrates, they are not sufficient to determine if free gas may exist below gas hydrate, particularly where they occur in close vertical succession. Such undetected free gas occurrences can pose potential drilling hazards that can be quantitatively assessed using advanced seismic methods.

Horizontal stress contrast in the shallow marine sediments of the Gulf of Mexico sites Walker Ridge 313 and Atwater Valley 13 and 14 – Geological observations, effects on wellbore stability, and implications for drilling

Significant horizontal stress anisotropy was encountered in three blocks in the Gulf of Mexico drilled by the Gulf of Mexico gas hydrates Joint Industry Project, namely, Atwater Valley 13, Atwater Valley 14, and Walker Ridge 313. The geological factors responsible for this state of stress and the implications for wellbore stability and drilling operations are explored. In Atwater Valley 13 and 14, stresses at the well sites were perturbed by the underlying salt. In Walker Ridge 313, a combination of thrusting by adjacent salt and material anisotropy associated with the regional minibasin structure caused the maximum horizontal stress to rotate with depth. Horizontal stress anisotropy combined with a lack of heavy mud caused breakouts to form in the well Walker Ridge 313-G (WR313-G). A simple formula to predict the depth at which wells drilled with seawater are prone to breakouts is derived. Evidence is presented that the breakouts in WR313-G produced heavy cavings which were mainly responsible for the tight hole conditions encountered while drilling this well. However, it is shown that bottom hole assembly (BHA) design and drilling practices such as backreaming may have exacerbated these problems. Modifications to drilling practices based on the experience gained from drilling WR313-G helped to improve the quality of the neighboring borehole WR313-H.

Rock physics-based seismic trace analysis of unconsolidated sediments containing gas hydrate and free gas in Green Canyon 955, Northern Gulf of Mexico

The gas hydrate petroleum system at the 2009 Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) Green Canyon 955 (GC955) site shows a complex seismic amplitude and waveform response of highly negative and positive amplitudes with continuous and discontinuous character within inferred gas-hydrate- and gas-bearing sand reservoirs. Logging-while-drilling (LWD) data obtained during JIP Leg II and conventional 3-D seismic data allowed for the identification of thick highly concentrated hydrate layers by integrating rock physics modeling, amplitude and thin layer analysis, and spectral decomposition. Rock physics modeling with constraints from three JIP LWD holes allowed for the analysis of variations in acoustic amplitude characteristics as a product of hydrate saturation, gas saturation, and reservoir thickness. Using the well log-derived acoustic models, thick highly concentrated gas hydrate with and without underlying free gas accumulations have been identified. These results suggest that thick highly concentrated gas-hydrate-bearing sand units (with thicknesses greater than half of the seismic tuning thickness and gas hydrate saturations greater than 50%) underlain by gas can be differentiated from sands containing only gas, but thin gas-hydrate-bearing sand units with low gas hydrate concentrations (with thicknesses less than half of the seismic tuning thickness and gas hydrate saturations less than 50%) are difficult to identify from post-stack seismic amplitude data alone. Within GC955, we have identified six zones with seismic amplitude anomalies interpreted as being caused by gas hydrate deposits with variable lateral extent, thickness and saturation, and in some cases overlying free-gas-bearing intervals. Synthetic seismic images produced from well-log- and model-derived velocity and density distributions mimic similar reflection characteristics in the corresponding field seismic data.

Subsurface gas hydrates in the northern Gulf of Mexico

The northern Gulf of Mexico (GoM) has long been a focus area for the study of gas hydrates. Throughout the 1980s and 1990s, work focused on massive gas hydrates deposits that were found to form at and near the seafloor in association with hydrocarbon seeps. However, as global scientific and industrial interest in assessment of the drilling hazards and resource implications of gas hydrate accelerated, focus shifted to understanding the nature and abundance of “buried” gas hydrates. Through 2005, despite the drilling of more than 1200 oil and gas industry wells through the gas hydrate stability zone, published evidence of significant sub-seafloor gas hydrate in the GoM was lacking. A 2005 drilling program by the GoM Gas Hydrate Joint Industry Project (the JIP) provided an initial confirmation of the occurrence of gas hydrates below the GoM seafloor. In 2006, release of data from a 2003 industry well in Alaminos Canyon 818 provided initial documentation of gas hydrate occurrence at high concentrations in sand reservoirs in the GoM. From 2006 to 2008, the JIP facilitated the integration of geophysical and geological data to identify sites prospective for gas hydrate-bearing sands, culminating in the recommendation of numerous drilling targets within four sites spanning a range of typical deepwater settings. Concurrent with, but independent of, the JIP prospecting effort, the Bureau of Ocean Energy Management (BOEM) conducted a preliminary assessment of the GoM gas hydrate petroleum system, resulting in an estimate of 607 trillion cubic meters (21,444 trillion cubic feet) gas-in-place of which roughly one-third occurs at expected high concentrations in sand reservoirs. In 2009, the JIP drilled seven wells at three sites, discovering gas hydrate at high saturation in sand reservoirs in four wells and suspected gas hydrate at low to moderate saturations in two other wells. These results provide an initial confirmation of the complex nature and occurrence of gas hydrate-bearing sands in the GoM, the efficacy of the integrated geological/geophysical prospecting approach used to identify the JIP drilling sites, and the relevance of the 2008 BOEM assessment.

Estimating saturation of gas hydrates using conventional 3D seismic data, Gulf of Mexico Joint Industry Project Leg II

We present a methodology for generating pre-drill estimations of hydrate saturations using conventional 3D seismic data. These seismic-based estimates will be compared with well log derived saturations from the subsequently drilled wells of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) expedition.Predicting saturation of gas hydrates (Sh-seismic) combines pre-stack seismic inversion, rock physics modeling and stratigraphic interpretation. Before the wells were drilled, no nearby sonic and density logs were available to define and calibrate the elastic property trends for the shallow target interval containing the gas hydrate stability zone. Therefore, rock property trends were established by applying principles of rock physics and shallow sediment compaction, constrained by known regional geological parameters. Sh-seismic volumes were generated by inverting pre-stack data to acoustic impedance (PI) and shear impedance (SI) volumes, and then analyzing deviations from modeled impedance trends. To enhance the quality of the inversion, the signal-to-noise ratio of the offset data was maximized by conditioning the seismic prior to inversion. Seismic stratigraphic interpretation plays an important role by identifying the more promising strata and structures for the presence of gas hydrates.The pre-drill Sh-seismic results are compared with saturations calculated from log data, Sh-log, of the wells drilled in the JIP Leg II campaign. Due to weaker seismic reflections, predictions may be less accurate for low concentrations, such as saturations less than 40%, and for thin intervals below the vertical resolution of the seismic data (about 15m). However, the integrated geophysical workflow is very effective for identifying and quantifying significant hydrate concentrations, making the method a promising prospecting technique.

Electrical anisotropy of gas hydrate-bearing sand reservoirs in the Gulf of Mexico

We present new results and interpretations of the electrical anisotropy and reservoir architecture in gas hydrate-bearing sands using logging data collected during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II. We focus specifically on sand reservoirs in Hole Alaminos Canyon 21 A (AC21-A), Hole Green Canyon 955 H (GC955-H) and Hole Walker Ridge 313 H (WR313-H). Using a new logging-while-drilling directional resistivity tool and a one-dimensional inversion developed by Schlumberger, we resolve the resistivity of the current flowing parallel to the bedding, R‖ and the resistivity of the current flowing perpendicular to the bedding, R⊥. We find the sand reservoir in Hole AC21-A to be relatively isotropic, with R‖ and R⊥ values close to 2Ωm. In contrast, the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic. In these reservoirs, R‖ is between 2 and 30Ωm, and R⊥ is generally an order of magnitude higher.Using Schlumberger’s WebMI models, we were able to replicate multiple resistivity measurements and determine the formation resistivity the gas hydrate-bearing sand reservoir in Hole WR313-H. The results showed that gas hydrate saturations within a single reservoir unit are highly variable. For example, the sand units in Hole WR313-H contain thin layers (on the order of 10–100cm) with varying gas hydrate saturations between 15 and 95%. Our combined modeling results clearly indicate that the gas hydrate-bearing sand reservoirs in Holes GC955-H and WR313-H are highly anisotropic due to varying saturations of gas hydrate forming in thin layers within larger sand units.

Numerical simulations of depressurization-induced gas production from gas hydrate reservoirs at the Walker Ridge 313 site, northern Gulf of Mexico

In 2009, the Gulf of Mexico (GOM) Gas Hydrates Joint-Industry-Project (JIP) Leg II drilling program confirmed that gas hydrate occurs at high saturations within reservoir-quality sands in the GOM. A comprehensive logging-while-drilling dataset was collected from seven wells at three sites, including two wells at the Walker Ridge 313 site. By constraining the saturations and thicknesses of hydrate-bearing sands using logging-while-drilling data, two-dimensional (2D), cylindrical, r-z and three-dimensional (3D) reservoir models were simulated. The gas hydrate occurrences inferred from seismic analysis are used to delineate the areal extent of the 3D reservoir models. Numerical simulations of gas production from the Walker Ridge reservoirs were conducted using the depressurization method at a constant bottomhole pressure. Results of these simulations indicate that these hydrate deposits are readily produced, owing to high intrinsic reservoir-quality and their proximity to the base of hydrate stability. The elevated in situ reservoir temperatures contribute to high (5–40 MMscf/day) predicted production rates. The production rates obtained from the 2D and 3D models are in close agreement. To evaluate the effect of spatial dimensions, the 2D reservoir domains were simulated at two outer radii. The results showed increased potential for formation of secondary hydrate and appearance of lag time for production rates as reservoir size increases. Similar phenomena were observed in the 3D reservoir models. The results also suggest that interbedded gas hydrate accumulations might be preferable targets for gas production in comparison with massive deposits. Hydrate in such accumulations can be readily dissociated due to heat supply from surrounding hydrate-free zones. Special cases were considered to evaluate the effect of overburden and underburden permeability on production. The obtained data show that production can be significantly degraded in comparison with a case using impermeable boundaries. The main reason for the reduced productivity is water influx from the surrounding strata; a secondary cause is gas escape into the overburden. The results dictate that in order to reliably estimate production potential, permeability of the surroundings has to be included in a model.