The principles of helium exploration

Commercial helium systems have been found to date as a serendipitous by-product of petroleum exploration. There are nevertheless significant differences in the source and migration properties of helium compared with petroleum. An understanding of these differences enables prospects for helium gas accumulations to be identified in regions where petroleum exploration would not be tenable. Here we show how the basic petroleum exploration playbook (source, primary migration from the source rock, secondary longer distance migration, trapping) can be modified to identify helium plays. Plays are the areas occupied by a prospective reservoir and overlying seal associated with a mature helium source. This is the first step in identifying the detail of helium prospects (discrete pools of trapped helium). We show how these principles, adapted for helium, can be applied using the Rukwa Basin in the Tanzanian section of the East African Rift as a case study. Thermal hiatus caused by rifting of the continental basement has resulted in a surface expression of deep crustal gas release in the form of high-nitrogen gas seeps containing up to 10% 4He. We calculate the total likely regional source rock helium generative capacity, identify the role of the Rungwe volcanic province in releasing the accumulated crustal helium, and show the spatial control of helium concentration dilution by the associated volcanic CO2. Nitrogen, both dissolved and as a free gas phase, plays a key role in the primary and secondary migration of crustal helium and its accumulation into what might become a commercially viable gas pool. This too is examined. We identify and discuss evidence that structures and seals suitable for trapping hydrocarbon and CO2 gases will likely also be efficient for helium accumulation on the timescale of the Rukwa basin activity.The Rukwa Basin prospective recoverable P50 resources of helium have been independently estimated to be about 138 billion standard cubic feet (2.78 x 109 m3 at STP). If this volume is confirmed it would represent about 25% of the current global helium reserve. Two exploration wells Tai 1 and Tai 2 completed by August 2021 have proved the presence of seal and reservoir horizons with the reservoirs containing significant helium shows.This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

Investigating the PS seismic imaging of faults using seismic modelling and data from the Snøhvit field, Barents Sea

PS seismic data from the Snøhvit field are compared with seismic modelling to understand the effect of azimuthal separation and incidence angle on the imaging of faults and associated horizon discontinuities. In addition, the frequency content of seismic waves backscattered from faults is analysed. The study area consists of a horst structure delimited by a northern fault dipping NW and oblique to the E-W survey orientation, and a southern fault dipping SSW and subparallel to the survey. Due to the raypath asymmetry of PS reflections, the northern fault is imaged better by azimuthally partitioned W data that include receivers downdip of the fault, relative to the sources, than by E data where the receivers are updip from the sources. Partial stack data show a systematic increase in the PS fault-reflected amplitude and therefore quality of fault imaging with increasing incidence angle. Fault images are dominated by internal low-medium frequency shadows surrounded by medium-high frequencies haloes. Synthetic experiments suggest that this is due to the interaction of specular waves and diffractions, and the spectral contribution from the fault signal, which increases with fault zone complexity. These results highlight the impact of survey geometry and processing workflows on fault imaging.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5727552

Impact of modelling decisions and rock typing schemes on oil in place estimates in a giant carbonate reservoir in the Middle East

We demonstrate how modelling decisions for a giant carbonate reservoir with a thick transition zone in the Middle East, most notably the approach to reservoir rock typing and modelling the initial fluid saturations, impact the hydrocarbon distributions and oil-in-place estimates in the reservoir. Rather than anchoring our model around a single base case with an upside and downside, we apply a comprehensive 3D multiple deterministic scenario workflow to compare-and-contrast how modelling decisions and geological uncertainties influence the volumetric estimates. We carry out a detailed analysis which shows that the variations in STOIIP estimates can be as high as 28% depending on the preferred modelling decision, which could potentially mask the impact of other geological uncertainties. These models were validated through repeated and randomised blind tests. We hence present a quantitative approach that helps us to assess if the static models are consistent in terms of the integration of geological and petrophysical data. Ultimately, the decision which of the different modelling options should be applied does not only influence STOIIP estimates, but also subsequent history matching & forecasts.

The rejuvenations of hydrocarbon exploration in the Eastern Mediterranean

The exploration history of the large East Mediterranean Basin, which encompasses the Nile delta, Levantine, Herodotus and Eratosthenes provinces, has seen several phases of rejuvenation since exploration started in the 1950s, with new plays opened repeatedly after the basin was considered mature by the industry. The 584 exploration wells drilled to date have discovered more than 23 Bboe recoverable reserves/resources, mostly gas. The first discovery was the Abu Madi Field, in 1967, which opened the Messinian clastic play. Over time, other plays and sub plays were opened, including the Serravallian-Tortonian, the Plio-Pleistocene, the Oligo/Miocene in the Levantine, the intra Oligocene and the Cretaceous carbonates (Zohr discovery, 2015). The exceptional variety of plays, with different trapping styles, reservoir and seal facies patterns has few equivalents worldwide and makes the region a valuable training ground for explorers. The geological variety is not the only reason for such a complex and episodic exploration history: commercial (gas market) and geopolitical issues have also had an impact on the activity in parts of the basin. The largest discoveries have been made in the last 10 years (Tamar, Leviathan, Zohr) and, despite the intense exploration activity, parts of the basin remain underexplored. The company with the longest and most successful play opening history in the basin is Eni. Today, most major oil companies are active in the basin, which even after 70 years is still considered one of the world's exploration hotspots.

Negatively buoyant CO2 solution sequestration in synformal traps

Dissolving CO2 into water or brine produces a denser fluid than the CO2-free equivalent at all salinity, temperature and pressure conditions relevant to sedimentary basins. Negative buoyancy of CO2 solutions opens the possibility of utilizing negative relief trapping configurations for CO2 sequestration, as opposed to structural highs conventionally sought for positively buoyant fluids such as hydrocarbons or pure CO2. Exploring sedimentary basins for negative buoyancy traps can readily utilize hydrocarbon exploration datasets and techniques. Some major systemic differences when exploring for negative as opposed to positive buoyancy traps are examined here. Trap spatial scale is a consideration due to the inherent long-wavelength synformal geometry of basins. Antiforms are areally restricted relative to synforms, which may be embedded within larger-scale synformal closure at length scales right up to that of the basin itself. Multiscale synformal structure varies with basin type and may not be fully identified due to truncation effects arising from data coverage limitations. Similar to hydrocarbon exploration, CO2 trap exploration must consider potential sequestration volumes in an uncertainty and risk framework. Charge risk is unnecessary in sequestration projects, however, the multiscale nature of synformal traps should be considered when estimating range of storage volumes.This article is part of the Energy Geoscience Series available at https://www.lyellcollection.org/cc/energy-geoscience-series

3D modelling and capacity estimation of potential targets for CO2 storage in the Adriatic Sea, Italy

One of the most innovative and effective technologies developed in recent decades for reducing carbon dioxide emissions to the atmosphere is CCS (Carbon Capture & Storage). It consists of capture, transport and injection of CO2 produced by energy production plants or other industries. The injection takes place in deep geological formations with the suitable geometrical and petrophysical characteristics to permanently trap CO2 in the subsurface, which is called geological storage. In the development process of a potential geological storage site, correct capacity estimation of the injectable volumes of CO2 is one of the most important aspects. There are various approaches to estimate CO2 storage capacities for potential traps, including geometrical equations, dynamic modelling, numerical modelling, and 3D modelling. In this work, generation of three-dimensional petrophysical models and equations for calculation of the storage volumesare used to estimate the effective storage capacity of four potential saline aquifers in the Adriatic Sea offshore. The results show how different saline aquifers, with different lithologies at favourable depths, can host a fair amount of CO2, that will imply a further and more detailed feasibility studies for each of these structures. A detailed analysis is carried out for each saline aquifer identified, varying the parameters of each structure identified, and adapting them for a realistic estimate of potential geological storage capacity.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

CO2 injection and storage in porous rocks: coupled geomechanical yielding below failure threshold and permeability evolution

With the increasing demand for CO2 storage into the subsurface, it is important to recognize that candidate formations may present complex stress conditions and material characteristics. Consequently, modelling of CO2 injection requires the selection of the most appropriate constitutive material model for the best possible representation of the material response. The authors focus on modelling the geomechanical behaviour of the reservoir material, coupled with multi-phase flow solution of CO2 injection into a saline saturated medium. It is proposed to use the SR3 critical state material model which considers a direct link between strength-volume-permeability that evolves during the simulation; furthermore the material is considered to yield prior to reaching a peak strength in agreement with experimental observations. Verification of the material model against established laboratory tests is conducted, including multi-phase flow accounting for relative permeabilities and fluid densities. Multi-phase flow coupled to advanced geomechanics provides a holistic approach to modelling CO2 injection into sandstone reservoirs. The resulting injection pressures, CO2 migration extent and patterns, formation dilation and strength reduction are compared for a range of in-situ porosities and incremental material enhancements. This work aims to demonstrate a numerical modelling framework to aid in the understanding of geomechanical responses to CO2 injection for safe and efficient deployment and is particularly applicable to CO2 sequestration in less favourable aquifers with a relatively low permeability, receiving CO2 from a limited number of injection wells at high flow rates. The proposed framework can also enable additional features to be incorporated into the model such as faults and detailed overburden representation.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

Calibration of naturally fractured reservoir models using integrated well-test analysis – an illustration with field data from the Barents Sea

This paper successfully applied the geoengineering workflow for integrated well-test analysis to characterise fluid flow in a newly discovered fractured reservoir in the Barents Sea. A reservoir model containing fractures and matrix was built and calibrated using this workflow to match complex pressure transients measured in the field. We outline different geological scenarios that could potentially reproduce the pressure response observed in the field, highlighting the challenge of non-uniqueness when analysing well-test data. However, integrating other field data into the analysis allowed us to narrow the range of uncertainty, enabling the most plausible geological scenario to be taken forward for more detailed reservoir characterisation and history matching. The results provide new insights into the reservoir geology and the key flow processes that generate the pressure response observed in the field. This paper demonstrates that the geoengineering workflow used here can be applied to better characterise naturally fractured reservoirs. We also provide reference solutions for interpreting well-tests in fractured reservoirs where troughs in the pressure derivative are recognisable in the data.

The impact of heterogeneous mixed siliciclastic-carbonate systems on CO2 geological storage

Three different outcrops are selected in this study, each representing a shallow marine system with varying heterogeneity provided by siliciclastic-carbonate mixing that may form a small or large stratigraphic trap. The impact of these styles of mixed facies on CO2 storage is relatively poorly known. This study demonstrates the significance of these systems for safe CO2 geological storage, as stratigraphic traps are likely to be a significant feature of many future storage sites. The three 3D models are based on the: 1. Grayburg Formation (US), which displays spatial permeability linked to variations in the mixture of siliciclastic-carbonate sediments; 2. Lorca Basin outcrop (Spain), which demonstrates the interfingering of clastic and carbonate facies; and 3. Bridport Sand Formation outcrop (UK), an example of a layered reservoir, which has thin carbonate-cemented horizons.This study demonstrates that facies interplay and associated sediment heterogeneity have a varying effect on fluid flow, storage capacity and security. In the Grayburg Formation, storage security and capacity are not controlled by heterogeneity alone but influenced mainly by the permeability of each facies (i.e., permeability contrast), the degree of heterogeneity, and the relative permeability characteristic of the system. In the case of the Lorca Basin, heterogeneity through interfingering of the carbonate and clastic facies improved the storage security regardless of their permeability. For the Bridport Sand Formation, the existence of extended sheets of cemented carbonate contributed to storage security but not storage capacity, which depends on the continuity of the sheets. These mixed systems specially minimise the large buoyancy force that act on the top seal and reduce the reliance of the storage security on the overlying caprock. They also increase the contact area between injected CO2 and brine, thereby promoting the CO2 dissolution processes. Overall, mixed systems contribute to the safe storage of CO2.Thematic collection: This article is part of the Geoscience for CO2 storage collection available at: https://www.lyellcollection.org/cc/geoscience-for-co2-storage

Quantitative seismic interpretation of the Lower Cretaceous reservoirs in the Valdemar Field, Danish North Sea

A quantitative seismic interpretation study is presented for the Lower Cretaceous Tuxen reservoir in the Valdemar Field, which is associated with heterogeneous and complex geology. Our objective is to better outline the reservoir quality variations of the Tuxen reservoir across the Valdemar Field. Seismic pre-stack data and well logs from two appraisal wells forms the basis of this study. The workflow used includes seismic and rock physics forward modelling, attribute analysis, a coloured inversion and a Bayesian pre-stack inversion for litho-fluid classification. Based on log data, the rock physics properties of the Tuxen interval reveals that the seismic signal is more governed by porosity than water saturation changes at near-offset (or small-angle). The coloured and Bayesian inversion results were generally consistent with well-log observations at the reservoir level and conformed to interpreted horizons. Although the available data has some limitations and the geological setting is complex, the results implied more promising reservoir quality in some areas than others. Hence, the results may offer useful information for delineating the best reservoir zones for further field development and selecting appropriate production strategies.