Geological Salt Research Articles

Sparse‐node acquisition for data fitting velocity model building

AbstractThe salt interpretation can be quite time‐intensive and challenging. Full‐waveform inversion, as a data‐driven optimization algorithm with full wavefield modelling, has become one of the essential tools for earth model building. However, full‐waveform inversion application in the complex salt geology, especially with streamer data collection, is limited, whereas the ocean‐bottom node with ultra‐long offsets and low frequencies unleashes the power of full‐waveform inversion. Sparse ocean‐bottom node acquisition has become a standard approach to improve the earth model building by the addition of ultra‐long offsets and possibly low frequencies. The new survey designs with ocean‐bottom node coupled with simultaneous shooting can be deployed on a regional basis covering thousands of square kilometres in a cost‐effective manner. In complex geological settings, including irregular salt geometry, salt interpretation has a direct impact on subsalt imaging. The recent development of robust objective functions allows full‐waveform inversion work with sparse ocean‐bottom node surveys in the deep‐water environment as the Gulf of Mexico to build and refine the salt geometries and correct the background velocity error in subsalt, uncovering the structural configuration of the basin that has not been seen before. Although full‐waveform inversion mostly employs the acoustic wave equation, we know that the high velocity boundaries in the earth model may require elastic wavefield propagation. Increased physics in full‐waveform inversion allows us to interrogate the power of elastic full‐waveform inversion versus acoustic full‐waveform inversion, and here, we demonstrate that elastic full‐waveform inversion has an advantage over acoustic full‐waveform inversion despite the extra cost associated with implementing the more complete physics.

Applied 3D salt body reconstruction using shape optimization with level sets

As oil and gas extraction becomes more advanced, deep-water exploration becomes increasingly focused on imaging near or under complex salt geology, which necessitates detailed velocity models with strong contrast interfaces. These interfaces can be elegantly tracked using the level sets of an implicit surface. One can invert for the velocity model that best fits the recorded data in a full-waveform inversion (FWI) style objective function by reparameterizing the model in terms of an implicit surface representation of the salt interface. With this parameterization of the FWI objective function, we find the Hessian and solve a conjugate gradient system for the Newton step at every nonlinear iteration. We sparsify the representation of the implicit surface using radial basis functions, which can hasten convergence of the inner inversion by reducing the number of model parameters. We have developed a guided inversion approach that embeds information about the certainty of different salt boundary regions by the initialization of the implicit surface slope at the salt interface. This can help guide the inversion away from perceived local minima. The results of testing this inversion workflow on a 3D Gulf of Mexico data set show that it can be a useful tool for refining salt models because the initial and final seismic images show clearer and more consistent features below the updated salt area.

Coupling strategies in multiparameter geophysical joint inversion

SUMMARYA big potential lies in the quantitative integration of multiple geophysical measurements for what concerns more unique and robust inversion results, complementary sensitivity to geological features and enhanced resolution. The mechanisms that enable such integration typically rely on statistics where the multiphysics parameter values are related to each other through regression functions (rock physics) or through the shape of the parameter spatial distributions (structure). Such coupling operators are jointly minimized with the data misfit to obtain coupled parameter distributions. We explore the application of various coupling mechanisms to synthetic and real data comprising seismic and electromagnetic measurements acquired in complex geological conditions such as salt geology and complex near surface in desert environment. Two types of structure operators, consisting of the standard cross-gradient and a newly introduced summative gradient, together with rock-physics operators are tested and combined for velocity model reconstruction of salt overburden in a marine environment and in a complex near surface case. Results of seismic-EM joint minimization suggest that the summative gradient operator with the introduction of the sign of the gradient correlation provides a strong coupling mechanism that can become advantageous in the presence of noise-affected seismic data. The rock-physics coupling mechanism is extremely strong and its effectiveness depends primarily on the reliability of the rock-physics relation. A Bayesian approach for the rock-physics operator is introduced to balance the related uncertainties and it is successfully tested in a joint minimization scheme. The combined use of rock-physics and structure coupling operators provides the best results in synthetic and real data applications. The use and combination of various regularization operators, as described, provide a formidable toolbox for solving a wide variety of ill-posed and non-unique geophysical inverse problems.

CSEM-regularized seismic velocity inversion: A multiscale, hierarchical workflow for subsalt imaging

Seismic imaging in salt geology is complicated by highly contrasted velocity fields and irregular salt geometries, which cause complex seismic wavefield scattering. Although the imaging challenges can be addressed by advanced imaging algorithms, a fundamental problem remains in the determination of robust velocity fields in high-noise conditions. Conventional migration velocity analysis is often ineffective, and even the most advanced methods for depth-domain velocity analysis, such as full-waveform inversion, require starting from a good initial estimate of the velocity model to converge to a correct result. Nonseismic methods, such as electromagnetics, can help guide the generation of robust velocity models to be used for further processing. Using the multiphysics data acquired in the deepwater section of the Red Sea, we apply a controlled-source electromagnetic (CSEM) resistivity-regularized seismic velocity inversion for enhancing the velocity model in a complex area dominated by nappe-style salt tectonics. The integration is achieved by a rigorous approach of multiscaled inversions looping over model dimensions (1D first, followed by 3D), variable offsets and increasing frequencies, data-driven and interpretation-supported approaches, leading to a hierarchical inversion guided by a parameter sensitivity analysis. The final step of the integration consists of the inversion of seismic traveltimes subject to CSEM model constraints in which a common-structure coupling mechanism is used. Minimization is performed over the seismic data residuals and cross-gradient objective functions without inverting for the resistivity model, which is used as a reference for the seismic inversion (hierarchical approach). Results are demonstrated through depth imaging in which the velocity model derived through CSEM-regularized hierarchical inversion outperforms the results of a seismic-only derived velocity model.

Micromechanical processes in consolidated granular salt

Granular salt is likely to be used as backfill material and a seal system component within geologic salt formations serving as a repository for long-term isolation of nuclear waste. Pressure from closure of the surrounding salt formation will promote consolidation of granular salt, eventually resulting in properties comparable to native salt. Understanding dependence of consolidation processes on stress state, moisture availability, temperature, and time is important for demonstrating sealing functions and long-term repository performance. This study characterizes laboratory-consolidated granular salt by means of microstructural observations. Granular salt material from mining operations was obtained from the bedded Salado Formation hosting the Waste Isolation Pilot Plant and the Avery Island salt dome. Laboratory test conditions included hydrostatic consolidation of jacketed granular salt with varying conditions of confining isochoric stress to 38 MPa, temperature to 250 °C, moisture additions of 1% by weight, time duration, and vented and non-vented states. Resultant porosities ranged between 1% and 22%. Optical and scanning electron microscopic techniques were used to ascertain consolidation mechanisms. From these investigations, samples with 1% added moisture or unvented during consolidation, exhibit clear pressure solution processes with tightly cohered grain boundaries and occluded fluid pores. Samples with only natural moisture content consolidated by a combination of brittle, cataclastic, and crystal plastic deformation. Recrystallization at 250 °C irrespective of moisture conditions was also observed. The range and variability of conditions applied in this study, combined with the techniques used to display microstructural features, are unique, and provide insight into an important area of governing deformation mechanism(s) occurring within salt repository applications.

SALT IN 21ST CENTURY – CURRENT PROBLEMS OF SALT GEOLOGY IN POLAND

This article provides an overview on the current state of knowledge on salt geology in Poland. Salt occurs in Poland in two ­salt-bearing formations: Zechstein (Permian) and Miocene (Neogene). The review briefly presents information on the various aspects of salt mining as well as description of results of different research topics discussed by scientists since the beginning of the 21st ­century: ­stratigraphy, sedimentology, geochemistry, ­mineralogy, geophysics, tectonics, hydrogeology as well as reserves and resources of salt ­deposits. Various forms of management of salt occurrences (crushed salt and brine production, storages of media, and waste disposals) and the post-mining activity of salt mines are commented. This review is based on extensive literature and ­proposes main directions of future studies in salt geology of Poland.

The Garden Banks model experience

To better understand deepwater imaging challenges in the Gulf of Mexico, we constructed a large 3D model based on the complex salt geology of the Garden Banks protraction area. We simulated a regional wide-azimuth (WAZ) streamer survey with offsets of ±8 km inline and ±4 km crossline over this model. Using the true velocity model, reverse time migration of this data set produced a usable subsalt image nearly everywhere. The same synthetic seismic data set was passed to interpreters and geophysicists to process and image as if it were real field data. They did not get to see the true velocity model. Instead they performed conventional “migrate, pick, and flood” top-down velocity-model building, followed by final imaging through their interpreted velocity model. Where the salt was relatively simple, we found that the interpreted velocity model was reasonably accurate. Reverse time migration of the seismic data through these parts of the velocity model produced an image that was imperfect, but still usable for exploration-scale, structural interpretation. Where the salt structure was complex, however, it was sometimes grossly misinterpreted. The ensuing large-scale errors in the interpreted velocity model resulted in an unusable, shattered subsalt image. We next simulated a low-frequency, ocean-bottom-node (OBN) acquisition, with offsets up to 30 km in all azimuths. We started with the imperfect model produced by the interpreters and investigated what it would take for full-waveform inversion (FWI) to successfully find and fix the gross interpretation errors. Initial results suggest this “interpretation followed by FWI” methodology could produce a velocity model capable of generating an adequate subsalt image, but it may require ultrawide offsets (30 km), ultralow frequencies (below 2 Hz), and improved FWI algorithms to do so.

NaCl crystallization in apolar nanometer-sized confinement studied by atomistic simulations

The structure and growth of molecular NaCl crystals in bulk and in a narrow, nanometer-sized apolar confinement are examined by explicit-water molecular dynamics computer simulations. It is demonstrated that fast crystallization and subsequent diffusion-controlled cluster growth in bulk is triggered by supersaturations that exceed a certain threshold value. In confinement, simulated in a pseudo grand canonical setup, salt is shown to be expelled from the narrow apolar slab region, and the effective ion concentration inside the nanoconfinement is always considerably lower than the reservoir salt concentration so that no fast crystallization takes place. For very small slab widths (d<1.5 nm) salt is almost entirely expelled while water remains in the slab, indicating a capillary evaporation phenomenon for the polar ions. If forced into the apolar confinement by simulating in a strictly canonical setup, we find stable crystals only if at least three crystalline planes fit into the slab, which happens above a 2-nm slab width. In this case the (100) plane of the bulk crystal is oriented parallel to the apolar surface delimited by a subnanometer thin hydration layer. This work presents molecular-level insight of salt crystallization in apolar confinements of a nanometer scale with possible implications in double-layer supercapacitor physics and geological salt weathering.

Material Source Analysis of the Potash Deposit in Mengyejing, Yunnan

The Mengyejing potash deposit is the only solid chloride type potash deposit in our country. There has been divergence about the source material of this deposit. Some popular sayings in the old days were the Monophyletic Theory, the Marine Source Theory, and the Terrestrial Source Theory. With the further exploration of acknowledge in recent years, many geological scholars hold the point of the Polygenesis Theory. This thesis will put forward new metallogenic modes through the regional geological research and salt structure analysis, combining with its geochemical characteristics. That is, the potash layer is formed by the extrusion in the deep and plastic flows to the surface.

Degradation of Organic Complexing Agents by Halophilic Microorganisms in Brines

The potential use of geologic salt beds as terminal repositories for nuclear waste has necessitated research on the interaction of the waste with indigenous microbiota. Microorganisms may affect actinide solubility by degrading organic complexing agents present in the waste. A halophilic bacterium and Archaea indigenous to a salt formation in New Mexico were examined for their ability to degrade acetate, oxalate, citrate, and ethylenediamine tetraacetate under aerobic conditions in low and high-magnesium brines. All complexing agents, except EDTA, were utilized, suggesting that microorganisms indigenous to such repositories can potentially play a beneficial role in mitigating actinide mobility.

A New Idea of NREGS and its Application to Oil Exploration, Case Study: Detecting Geological Salt Structures in Northern Coasts of Persian Gulf

If Correlation of Gravitational Data with Topography is minimized correctly and coherently, Interpretation of Land-Based Gravitational Data will be a valuable clue leading us to detecting Lateral Variations of underground Mass-Density Anomalies particularly for Oil Exploration Purposes. For this reason, the author recommends a New Idea based on new mathematical definition of ?Nearest Reference Equi-Gravitational Surface (NREGS)? for making uncorrelated gravitational data with topography. NREGS is Reference Equi-Gravitational Surface in exterior of earth where the gravitational data will be upward-continued to and interpreted, too. The Mathematical Theory of the Idea explains that gravitational data on NREGS should be the most uncorrelated with topography. Hence, gravitational data on it can easily illustrate lateral density variation of geological substructure with a lot of clear contrasts being so useful for horizontally detecting underground density anomalies e.g. salt structures. In a case study based on 6350 gravitational observations in Coastal Fars of Iran, the author applied NREGS Idea for exploration of salt structures. There, NREGS idea detects all outcropped salt structures and also, it shows some other Low Anomalies which can be underground hidden salt structures. In addition, the case study shows that the gravitational data on NREGS is statistically more uncorrelated with topography than other data e.g. Bouguer Anomaly, Free-Air Anomaly, etc. It means that the case study practically proves the theory of the New Idea, too.

Developing, modeling and in-situ-testing a long-term wellbore seal in the framework of the CO2 -EGR research project CLEAN in the natural gas field Altmark

Abstract In this paper we present a new concept developed to improve the abandonment procedure of wellbores in depleted hydrocarbon fields, amongst others in Enhanced Gas Recovery (EGR) operations. The sealing concept is designed for the long-term containment of CO 2 and considers the creep ability of deep Permian Zechstein salt formations in about 3,000 m depth. It is designed to amend the current standard abandonment procedure and consists of (a) an expansive gel filling in the reservoir formation, (b) a sealing element with cement extending to the impermeable caprock and (c) a salt plug as the major long-term sealing component, compatible in its composition and property to the surrounding caprock (halite). The focus of the project work described here is to develop and to model the last-mentioned long-term salt sealing element. Numerical results show that the creep ability of the mentioned salt formations can close a formerly reamed well section and hence recover the geological salt barrier with or even without using a crushed salt backfill. A field test is conducted in the North German Altmark Rotliegend gas field (Sachsen Anhalt) in order to study the technical feasibility for recovering the initial caprock integrity, to verify numerical models and to proof the sealing capability and therefore the sealing concept.

Salt geology and hydrocarbon plays in the northeastern Gulf of Mexico

Abu Chowdhury, TGS, Houston, describes the Crimson Tide depth migrated survey project initiated to provide a better understanding of salt tectonics and the associated accumulation of hydrocarbons in the northeastern Gulf of Mexico.

Salt geology and mining traditions: Kalush and Stebnyk mines (Fore-Carpathian region, Ukraine)

Development of industrial underground salt mining in western Ukraine has started in the mid-19th century with the discovery of large potash-magnesium salt deposits in Kalush (1854) and Stebnyk (1901). Potash salts concentrations occur within the Miocene sedimentary complexes of Ukrainian Carpathian Foredeep (correlated with the successions observed in the Polish part of Carpathian Foredeep) as isolated lenses or compressed folded layers of varied extent. Thickness of exploited potash seams varies from 4 m up to 150 m and the main mineral is sylvine, accompanied also by carnallite, kainite and langbeinite. Both Kalush and Stebnyk mines, now closed, belong to the first places in the world, where exploitation of potash salts were realized. Their unique historical character, documenting the salt exploitation traditions in this region and evolution of mining techniques, ought to be widely advertised as the valid points of Ukrainian-Polish cross-border geotouristic route, named “Traces of large extinct mammals, earth wax, oil and salt”.

Geologic and shallow salt tectonic setting of the Mad Dog and Atlantis fields: Relationship between salt, faults, and seafloor geomorphology

Many of today's active hydrocarbon provinces are in areas with mobile substrate (either salt or mud). Interpreting the style and activity of the substrate, and its impact on the seafloor and near-surface environment, is central to understanding the geohazards of such field areas.

Airborne 3D FTG data proves utility for both Minerals and Oil &amp; Gas Exploration/Exploitation

Moving platform 3D Full Tensor Gradiometry (FTG) is now available in both airborne and marine system versions. Current marine systems recently modified to go airborne are proving the adaptability/utility of FTG in transitional and land environments for both petroleum and minerals objectives. Two such surveys in North America, one targeting mineral deposits and the second, a hydrocarbon accumulation at a salt dome illustrate some current applications of this innovative technology.The minerals deposit data was flown using/comparing both constant altitude and drape survey acquisition methods over an area of rugged topography (7.5x15.5km). Two drilled/sampled ore body targets (250-350m in diameter) were successfully detected with the Tzz tensor and compared/integrated to a dense grid of existing land gravity, Tz. Additionally, the horizontal tensors Txz and Tyz were utilized to determine lineament geometries from grid peak analysis, linking local tectonic elements to prospective subsurface geology and providing insight to ore body formation and hence detection. Frequency-filtering techniques are also employed to further enhance and separate geologic features.The oil field data was gathered at constant altitude (300m AMSL) and covers an area of a shallow piercement salt diapir (9.7x10.3 km). The primary survey objectives are to image complex salt geology and detect areas of complex overhang geometries where 3D seismic data are inherently ambiguous. In this manner 3D FTG analysis is utilized to reduce risk and geologic uncertainty with respect to ongoing development/directional drilling programs. Further, it is anticipated that the FTG derived salt isopach will be used to reduce valuable iteration time during pre-stack depth migration (PSDM) of the 3D seismic.These two case studies clearly demonstrate the diverse utility of this exciting new technology.

Formation of secondary phases after long-term corrosion of simulated HLW glass in brine solutions at 190 °C

SummaryThe behavior of rare earth elements (REE) as chemical analogues for actinides during glass corrosion was studied with static long-term batch experiments (7.5 years) at 190 °C. Corrosion tests were carried out using a simulated inactive high level waste (HLW) glass powder. Two different highly concentrated salt solutions (NaCl-rich and MgCl2-rich) have been used to simulate the potential corrosion by aqueous solutions in geological salt formations. Samples with surface-to-volume ratios (S/V) of 1000 m−1and 10000 m−1were investigated. The chemical composition of the aqueous solution and the formation of secondary phases at the glass/solution interface as well as their elemental composition, especially with respect to the behavior of REE was studied. The glass corrosion behavior and secondary phase formation was significantly different in the two brines. In the MgCl2-rich salt solution, with a final pH &lt; 3.5, hydrous Mg-silicates and (Ba,Sr)SO4solid solutions with considerable amounts of Sr (up to 15 wt.%) have been observed. Further, REE-sulfates and REE-molybdates with an indication of dissolution (etch pits) as secondary phases could be identified. In the NaCl-rich solution with a final high pH, REE-containing hydrous silicates (up to 35 wt.%) and (Ba,Sr)SO4solid solutions with small amounts of Ce (up to 5 wt.%) have been observed. Frequently observed Ca-molybdates (powellite) did not contain detectable amounts of REE.

Sectrophotometric Investigation of U(VI) Chloride Complexation in the Naci/NaClO4 system

AbstractPost closure radioactive release scenarios from geologic salt formation, such as the WIPP (Waste Isolation Pilot Plant) (USA) include hydrologic transport of radionuclides through a chloride saturated aquifer. Consequently, the understanding of actinide solution chemistry in brines is essential for modeling requiring accurate knowledge of the interaction between AnO22+ and chloride ions. Complexation constants of two U(VI) chloride species UO2Cl+ and UO2Cl20 have been intensively studied for about 40 years using different methods. However, large uncertainties reflect the general difficulty in determining accurate stability constants of weak complexes. In order to model the behavior of U(VI) in brines, we studied the formation of its chloride complexes by UV-Vis spectroscopy as a function of the NaCI concentration at 25 °C. The experiments were performed at constant ionic strength by varying the concentration ratio of NaCI and NaClO4. Deconvolution resulted in single component absorption sepectrum for UO2Cl+ and UO2Cl20. The apparent stability constants of UO2Cl+ and UO2Cl20 are determined at different ionic strengths. The experimental data are used to parameterize using the SIT approach.

Calculation of the radiation transport in rock salt using Monte Carlo methods

To improve the final concept for a high-level radioactive waste (HAW) disposal in geological salt formations, plans have been developed in Germany for a full scale testing of an underground repository. As a main part of this so-called HAW project, the testwise emplacement of 30 vitrified highly radioactive canisters was planned in test galleries at the 800 m level of the Asse salt mine near Braunschweig, Germany. An attempt should be made to correlate the changes induced to the salt with the amount of energy deposited there. This paper provides absorbed dose rate and photon fluence rate distributions in rock salt around these highly radioactive canisters, containing the radionuclides 137Cs and 90Sr, and around a single canister containing radioactive material of a lower activity level (INHAW experiment). The data given were calculated using a Monte Carlo method simulating photon transport in complex geometries of differently composed materials. The aim of these calculations was to enable determination of the dose absorbed in any arbitrary sample of salt 1 m around the canisters to be further examined in the future with sufficient reliability for safety considerations. The geometry of the test arrangement, the materials involved, the calculational method and selected results are shown.

French gas-storage project nearing completion

Geomethane, jointly formed by Gaz de France and Geostock, is currently converting 7 of 36 solution-mined salt cavities at Manosque in southeast France from liquid hydrocarbon storage to natural-gas storage. In view of the large diameter (13 3/8 in.) of the original production wells and safety requirements, a unique high-capacity well completion has been developed for this project. It will have two fail-safe valves and a flow crossover 30 m below ground to isolate the production well in the event of problems at the surface. The project lies in the wooded Luberon Nature Reserve and due consideration has been given to locating the surface plant and blending it with the surroundings. The production wellheads are extra-low designs, the main plant was located outside the sensitive area, and the pipeline routes were landscaped. The paper discusses the history of salt cavern storage of natural gas; site characteristics; Manosque salt geology; salt mining and early storage; siting; engineering and construction; completion and monitoring; nature reserve protection; and fire and earthquake hazard mitigation.