Lateral Gas Migration Research Articles

The effect of H2S content on acid gas migration and storage in sandstone reservoir via numerical simulation

Acid gas injection, as one of the technologies for carbon dioxide capture, utilization and storage (CCUS) technologies, has been widely used in the development of sour oil and gas reservoirs. The current study numerically investigates the effect of hydrogen sulfide (H2S) content on the acid gas migration and storage in sandstone reservoir. The results suggest that the variations of acid gas density and viscosity induced by different H2S contents have little influence on the distribution of acid gas plume. However, the higher solubility of acid gas at higher H2S content results in a decrease of migration distance. The variation of relative permeability induced by different H2S contents has great influence on the acid gas migration. With the increase of H2S content, the horizontal migration distance decreases due to the decrease of gas relative permeability. Whereas, the water relative permeability increases and the gravity number increases with increasing H2S content. As a result, most of the horizontal flow occurs at the top of the reservoir and the lateral gas migration at the lower part of the reservoir is weakened. Furthermore, the variation of capillary pressure induced by different H2S contents has little influence on the distribution of acid gas plume during the injection period. However, capillary pressure hinders the migration of acid gas during post-injection period. Thus, for the sandstone reservoir with surface contamination, the horizontal and vertical migration distance decrease with increasing H2S content. For the sandstone reservoir with clean surface, the horizontal migration distance first increases and then remains unchanged with the increase of H2S content. After 50 years, the residual and mobile trapping of acid gas decrease with the increase of H2S content, and the solubility trapping of acid gas increases with the increase of H2S content.

Investigation of methane fluxes from temporary cover of Xi’an Jiangcungou landfill, China

Methane emissions from the Jiangcungou municipal solid waste landfill in China were examined using a static chamber. The monitoring results were compared firstly with those obtained from the California Landfill Methane Inventory Model (Calmim). The monitoring results show that average methane emission flux from the temporary cover is one to two orders of magnitude greater than those from the final cover and the working face. This is because the methane generation rate reaches its maximum at less than 1 year after the wastes were landfilled. The observed methane fluxes are comparable with those obtained by Calmim. The observed results are lower than those predicted by Calmim, which were 74.98 and 98.64 (g/m2)/day, respectively. The difference is reasonable because the results obtained by field monitoring are transient, while those simulated by Calmim are the average daily flux. Moreover, Calmim considers only one-dimensional gas transport through the cover and neglects lateral gas migration. The model results indicate that there is a strong seasonal variability due to the variation in rainfall amount in different months. The methane emission amount was greatly reduced with the increase in vegetation coverage due to the increase in the volumetric water content of the soil induced by the water retention capability of vegetation.

Flume Experiment Evaluation of Resistivity Probes as a New Tool for Monitoring Gas Migration in Multilayered Sediments

Gas leakage can cause serious safety risks to offshore engineering facilities and offshore engineering projects. Monitoring the migration of gas in sediments is of great significance to the safety of marine engineering. In this study, we analysed the ability of ring-electrode resistivity probes (REPs) and circular point-electrode resistivity probes (CPEPs) to monitor the migration, accumulation, and release of gas in multilayered sediments (when the particle size of the overlying soil is smaller) using two groups of gas injection experiments with different gas injection pressures. In addition, the ability of REPs to monitor lateral gas migration was analysed by numerical simulations. The experimental results show that the two probes have different monitoring capabilities for different gas migration stages in multilayered sediments. REPs can effectively reflect the different stages of gas migration within the monitoring range according to the variation characteristics of the monitoring results, and they can effectively monitor the pattern, direction, and rate of gas migration. CPEPs are more sensitive, that is, the range of their monitoring results is about 10 times that of the REPs. They can accurately estimate the changes in the gas saturation of the soil around the probe.

Geological controls on the occurrence of recently formed highly concentrated gas hydrate accumulations in the Shenhu area, South China Sea

Abstract The Shenhu area is an important gas hydrate exploration and production test zone in the northern slope of South China Sea. A sediment layer with high gas hydrate saturations and abnormally high pore-water chlorinity concentrations has been identified from recently acquired logging-while-drilling (LWD) data and core samples. Data from four sites (W19, W18, SC-01 and SC-02) indicates that most gas hydrate-bearing sediments (GHBSs) form preferentially in low gamma ray log inferred coarser-grained units, except at SC-02. At this site, a striking increase in pore-water chlorinity values, with a maximum value exceeding 816 mM at a depth of 158 mbsf, indicates an active or recently-active system. A one-dimensional (1D) diffusion model is used to estimate the time when the gas hydrate formed based on the saturation, thickness, and porosity of gas hydrate-bearing units at each site. The results show that gas hydrates at sites SC-02 and W18 respectively formed 19–29 thousand and 26–28 thousand years ago (assuming a closed system). To further investigate the apparently young age of these highly saturated gas hydrate accumulations, three-dimensional (3D) seismic variance and frequency attributes extracted along different sedimentary layers are shown to provide evidence for vertical and lateral gas migration along normal faults and gas chimneys. The high pore-water chlorinities and evidence for fluid migration from deeper sedimentary sections suggest that upward migration of methane gas into the hydrate stability zone may have contributed to the recent formation of the highly concentrated gas hydrate occurrences identified in the Shenhu area.

Control of magmatism on gas accumulation in Linxing area, Ordos Basin, NW China: Evidence from fluid inclusions

This paper takes the Linxing area of the eastern margin of Ordos Basin as an example. Based on the data of reservoir fluid inclusions (FI) and combined with the history of buried thermal history in Linxing area, the main effects of magmatism on the gas accumulation time, accumulation period, gas accumulation dynamics and accumulation pattern are studied. The vitrinite reflectance (Ro) in source rocks of Zijinshan tectonic area is higher than that in gentle tectonic area. The closer to Zijinshan pluton, the more it tends to single period natural gas charging, the earlier time of gas charging. The characteristics of natural gas accumulation are evidently regional and zonal. Gas accumulation in gentle tectonic area conforms to the law of “gas accumulation in near source reservoir is earlier than remote source reservoir”. The gas generated from the source rocks in the lower formation of the Zijinshan tectonic area reached the upper formation rapidly through the faults and fractures caused by the uplift of the Zijinshan pluton, and the accumulation interval time between the near source and remote source formation is relatively short. The Zijinshan tectonic area is the long-term high-pressure area and high-fluid potential area. The fluid potential in gentle tectonic area is low, and the fluid potential difference between the north and south is the motive force to promote the long distance lateral migration of gas. Zijinshan tectonic area is the hydrocarbon generation center and driving force source area, and gas accumulated rapidly from bottom to top. The features of gas accumulation in gentle tectonic area are dominated by vertical accumulation, supplemented by lateral supplementation. Since the Late Cretaceous, a large number of deep faults have been caused by the strong uplift of Zijinshan pluton, and a large number of natural gas are scattered through the faults and the reservoirs are destroyed. The faults in gentle tectonic area are not developed, and the natural gas preservation conditions are good, with good exploration prospects.

Origin and migration model of natural gas in L gas field, eastern slope of Yinggehai Sag, China

Abstract Based on the chemical and stable carbon isotopic composition of natural gas and light hydrocarbons, along with regional geological data, the genetic type, origin and migration of natural gases in the L lithologic gas field, the eastern slope of Yinggehai Sag were investigated. The results show that these gases have a considerable variation in chemical composition, with 33.6%–91.5% hydrocarbon, 0.5%–62.2% CO2, and dryness coefficients ranging from 0.94 to 0.99. The alkane gases are characterized by δ13C1 values of –40.71‰––27.40‰, δ13C2 values of –27.27‰––20.26‰, and the isoparaffin contents accounting for 55%–73% of the total C5–C7 light hydrocarbons. These data indicate that the natural gases belong to the coal-type gas and are mainly derived from the Miocene terrigenous organic-rich source rocks. When the CO2 contents are greater than 10%, the δ13CCO2 values are –9.04‰ to – 0.95‰ and the associated helium has a 3He/4He value of 7.78×10–8, suggesting that the CO2 here is crustal origin and inorganic and mainly sourced from the thermal decomposition of calcareous mudstone and carbonate in deep strata. The gas migrated in three ways, i.e., migration of gas from the Miocene source rock to the reservoirs nearby; vertical migration of highly mature gas from deeper Meishan and Sanya Formations source rock through concealed faults; and lateral migration along permeable sandbodies. The relatively large pressure difference between the “source” and “reservoir” is the key driving force for the vertical and lateral migration of gas. Short-distance migration and effective “source – reservoir” match control the gas distribution.

Mobility and persistence of methane in groundwater in a controlled-release field experiment

Expansion of shale gas extraction has fuelled global concern about the potential impact of fugitive methane on groundwater and climate. Although methane leakage from wells is well documented, the consequences on groundwater remain sparsely studied and are thought by some to be minor. Here we present the results of a 72-day methane gas injection experiment into a shallow, flat-lying sand aquifer. In our experiment, although a significant fraction of methane vented to the atmosphere, an equal portion remained in the groundwater. We find that methane migration in the aquifer was governed by subtle grain-scale bedding that impeded buoyant free-phase gas flow and led to episodic releases of free-phase gas. The result was lateral migration of gas beyond that expected by groundwater advection alone. Methane persisted in the groundwater zone despite active growth of methanotrophic bacteria, although much of the methane that vented into the vadose zone was oxidized. Our findings demonstrate that even small-volume releases of methane gas can cause extensive and persistent free phase and solute plumes emanating from leaks that are detectable only by contaminant hydrogeology monitoring at high resolution. Most monitoring of methane well leakage focuses on emissions of methane gas to the atmosphere. In a controlled-release field experiment, significant methane also persisted in aquifer groundwater due to lateral migration along bedding planes.

Geochemistry, origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin, South China Sea

The Qiongdongnan Basin, South China Sea has received huge thickness (>12 km) of Tertiary-Quaternary sediments in the deepwater area to which great attention has been paid due to the recent discoveries of the SS22-1 and the SS17-2 commercial gas fields in the Pliocene-Upper Miocene submarine canyon system with water depth over 1300 m. In this study, the geochemistry, origin and accumulation models of these gases were investigated. The results reveal that the gases are predominated by hydrocarbon gases (98%–99% by volume), with the ratio of C1/C1-5 ranging from 0.92 to 0.94, and they are characterized by relatively heavy δ13C1 (−36.8‰ to −39.4‰) and δDCH4 values (−144‰ to −147‰), similar to the thermogenic gases discovered in the shallow water area of the basin. The C5-7 light hydrocarbons associated with these gases are dominated by isoparaffins (35%–65%), implying an origin from higher plants. For the associated condensates, carbon isotopic compositions and high abundance of oleanane and presence of bicadinanes show close affinity with those from the YC13-1 gas field in the shallow water area. All these geochemical characteristics correlate well with those found in the shales of the Oligocene Yacheng Formation in the Qiongdongnan Basin. The Yacheng Formation in the deepwater area has TOC values in the range of 0.4–21% and contains type IIb–III gas-prone kerogens, indicating an excellent gas source rock. The kinetic modeling results show that the δ13C1 values of the gas generated from the Yacheng source rock since 3 or 4 Ma are well matched with those of the reservoir gases, indicating that the gas pool is young and likely formed after 4 Ma. The geologic and geochemical data show that the mud diapirs and faults provide the main pathways for the upward migration of gases from the deep gas kitchen into the shallow, normally pressured reservoirs, and that the deep overpressure is the key driving force for the vertical and lateral migration of gas. This gas migration pattern implies that the South Low Uplift and the No.2 Fault zone near the deepwater area are also favorable for gas accumulation because they are located in the pathway of gas migration, and therefore more attention should be paid to them in the future.

Carbon stable isotopes as indicators of the origin and evolution of CO2 and CH4 in urban solid waste disposal sites and nearby areas

Isotopic compositions of CH4 and CO2 surface and subsurface gases and groundwater from an urban solid waste disposal site from Gualeguaychu city (Argentina) were measured to detect origin, depth distribution, migration lateral, CH4 oxidation, and dissolution in groundwater. The highest CH4 concentrations (60–88 %) with δ13C-CH4 (between −60 and −45 ‰) and δ2H-CH4 (between −350 and −260 ‰) were attributed to CH4 originated by microbial sources, v. gr. acetate fermentation. The δ13C-CO2 related to this CH4 (between −15 and −5.9 ‰) were compatible with this process. Also, the increase of DIC associated to an increase in δ13C-DIC values (−12.4, −6.4, −5.8, −1.5, +0.1 and +4 ‰) indicated the transport of dissolved gases (from methanogenesis) in the groundwater flow system. High excess deuterium in 3 piezometers suggests that there were hydrogen isotope exchange between CH4 and water too. Evidences for CH4 oxidation were decrease in CH4 concentrations, shift in C and H isotope ratios of CH4 to more enriched in the remaining CH4 (in both, δ13C-CH4 >−50 ‰ and δ2H >−260 ‰) and depletion in 13C of the associated CO2 (<−20 ‰). Since surface CH4 and CO2 concentrations over the covering layer were very low, and the major CH4 concentrations were found between 60m and 90 cm depth, it is very probable USW has been compacted with low permeability materials (e.g., clay) avoiding large emissions to the atmosphere and creating horizontal barriers within the waste that enable lateral gas migration. Horizontally, it can be seen that these gases migrated outside the disposal site following the topography and that CH4 disappeared leading to anomalous concentrations of CO2 whose values are greater than those produced by normal soil respiration. The isotopic value (δ13C) of this CO2 was also different from that of normal soil respiration (~−25 ‰) and reflected effects that generated isotopic depletion by diffusion (~−25 to −32 ‰). The analysis of C isotopes as the main tracer, together H isotopes, has allowed the effective detection of the origin and those secondary processes.

3D BASIN MODELLING IN THE CENTRAL PERSIAN GULF, OFFSHORE IRAN

In the Central Persian Gulf, super‐giant natural gas accumulations in Permo‐Triassic reservoirs are assumed to be derived from “hot shale” source rocks in the lower Llandoverian (base‐Silurian) Sarchahan Formation, whereas oil in Mesozoic reservoirs is derived from Mesozoic source rocks. A 3D basin model has been established for a study area located in the Iranian part of the Central Persian Gulf in order to help understand the petroleum systems there. Sensitivity analyses considered different heat flow scenarios, and differences in the timing of Cenozoic uplift and erosion. For the Palaeozoic petroleum system, different thicknesses and distributions of the Silurian source rocks were considered.From current temperature profiles measured in five wells, present‐day heat flow was found to be in the order of 65 mW/m2, while palaeo heat flow was probably between 60 and 68 mW/m2 during Cenozoic maximum burial. For Llandoverian source rocks, oil and gas generation commenced during Jurassic and Late Cretaceous time respectively, and gas generation continued until the Neogene. Sensitivity analyses show that different assumptions on the timing of Cenozoic erosion do not have significant effects on the calculated timing of hydrocarbon generation or on the volume of generated hydrocarbons. As expected however, different heat flow scenarios (e.g. time‐constant heat flow of 65 mW/m2 in the entire study area) had a significant influence.With an assumed 50 m thick Sarchahan “hot shale” succession developed uniformly in the study area (8 % TOC; 470 mg HC / g TOC HI), the model calculated gas accumulations which are of the same order of magnitude as those which have been discovered in this region (e.g. South Pars, Golshan and Balal fields). By contrast, scenarios with thinner “hot shales” and models without the Sarchahan Formation along the Qatar‐South Fars Arch do not predict the known accumulations. These scenarios suggest that prolific Silurian source rocks must be present on both sides of the South Pars / North Dome field, or that lateral gas migration from the south may have supplied the Permo‐Triassic reservoirs.This study shows that the Jurassic (mainly Hanifa / Tuwaiq Mountain Formation) and Cretaceous (Shilaif Formation) source units are not sufficiently mature in the study area to have generated significant volumes of oil. This result supports previous suggestions which envisaged lateral migration from the south of the oil present in Mesozoic reservoirs in the study area.

Sea-level change and free gas occurrence influencing a submarine landslide and pockmark formation and distribution in deepwater Nigeria

A series of pockmarks observed at the seabed matches well the perimeter of a large submarine landslide, called NG1, located on the outer shelf and continental slope of the Eastern Gulf of Guinea. NG1 extends over 200km2, is covered by a 120-m thick sedimentary layer which tapers downslope, and has an internal structure clearly identified in 3D seismic data consisting of three adjacent units on the upper continental slope. The pockmarks above NG1 have a diameter of several tens of meters and reveal distinct origins: (1) linked to >500m deep fluid reservoirs, (2) rooted in NG1 internal discontinuities between NG1 units, and (3) well above NG1, superficially rooted in a regional conformity (D40), which marks the lowest sea level of the Marine Isotope Stage 6.The regional stratigraphic pattern of the study area is composed of muddy sedimentary sequences separated by correlative conformities and transgressive condensed units of coarser grain size. Mud-confined coarser-grained units constitute transient gas reservoirs favoring lateral gas migration and formation of pockmarks rooted in the condensed units. The buried NG1 landslide modifies the layered structure of the sedimentary column providing (1) overall, a barrier to fluid migration, and (2) localized pathways for fluid migration. The triggering factor for the formation of pockmarks above NG1 can be the variation of hydrostatic pressure driven by relative sea-level fall during Marine Isotopic Stages 6 and 2 and consequent gas exsolution and fluid flow. We anticipate our result to be a starting point for understanding the role of gas seeps on climate change worldwide. Furthermore, gas release intensifies during lowstands with relevant implication on global warming after ice ages.

Seismic monitoring of an old underground blowout – 20 years later

When Saga Petroleum drilled a deep exploration well in the southern part of the North Sea in 1989, it encountered a high pressure zone, and the well developed into an underground blowout that lasted for 326 days. A relief well was spudded 11 days after the blowout, and the underground blowout was successfully killed by pumping drilling mud of high density, 2.25 g/cc, into the blowing well. During this period, Saga Petroleum decided to use shallow seismic data to monitor the underground blowout. Ten monitor surveys were acquired during and after the blowout, and this campaign is probably the first successful time-lapse seismic acquisition done in the Norwegian part of the North Sea. In 2009 some of these 2D lines were repeated, using approximately the same acquisition parameters as in 1989 and 1990. Comparing the seismic data from 2009 with the data from 1990 and 1989, most of the gas appears still to be in the same layers as in 1990. There has been some lateral migration of gas, and only a minor amount of vertical gas migration.

Formation of gas hydrate deposits in the Siberian Arctic shelf

Natural gas hydrate deposits have been estimated to store about 10% of gas in hydrate form (even with regard to a higher concentration of gas in hydrates), proceeding from the known ratio of dissolved-to-deposited gas. This high percentage is largely due to the fact that the buffer factor in natural gas hydrate deposits is lower than that for free gas because of less diverse structural conditions for gas accumulation. Therefore, the available appraisal of world resources of hydrated gas needs a revision. Hydrates in rocks are either syngenetic or epigenetic. Syngenetic hydrates originate from free or dissolved gas which was present in rocks in situ at the time when PT-conditions became favorable for gas hydrate formation. Epigenetic hydrates are derived from gas which came by migration into rocks with their PT-conditions corresponding to formation of gas hydrates. In addition to the optimum PT-conditions and water salinity, economic gas hydrate accumulation requires sustained supply of natural gas into a specific zone of gas hydrate formation. This condition is feasible only in the case of vertical migration of natural gas along faults, fractured zones, and lithologic windows, or, less often, as a result of lateral migration. Of practical importance are only the gas hydrate deposits produced by vertical or lateral gas migration.

Geochemistry and origin of the world's largest gas field from Persian Gulf, Iran

The South Pars field, which is the Iranian part of the world largest non-associated gas accumulation (taking into account its Qatari part, North Field, with 1342 tcf proved reserve), is hosted by Upper Permian Dalan and Lower Triassic Kangan carbonates. Carbon isotopic and molecular composition studies were undertaken to investigate the origin of non-associated gases from this field. In general, evaluation of the δ 13C values of methane, ethane and propane and gas wetness (C 2–C 5/C 1–C 5) indicates a thermogenic origin. On the other hand, results of the isotopic analyses showed no mixing of biogenic gases in all of the analyzed gas samples. A further evaluation using natural gas plot, carbon isotopic composition of methane to pentane vs. 1 / n, showed δ 13C 1 < δ 13C 2 < δ 13C 3 as normal trend. However, the carbon isotopic compositions of butane and pentane indicate a minor reverse trend in the normal progression of carbon isotopic compositions. This reversal in carbon isotope ratio among butane and pentane is interpreted as a consequence of mixing from multiple sources with different maturities. δ 13C CO 2 in South Pars field is similar with those of thermogenic origin. The diagram of δ 13C CO 2 vs. δ 13C CH 4 indicates that South Pars gases plot into the isotopic compositional field for methane derived from thermogenic origin. The H 2S concentration is much lower than the Khuff Formation and it seems it has originated from different processes. Apparently, the Lower Silurian organic-rich shales, which are the main source for hydrocarbons in the Arabian Plate and southern Iran, including South Pars field, generally contain kerogen type II with some contribution of type III in some areas. Burial history modeling indicates that at the end of the Cretaceous time pre-Permian sediments remained immature in the Qatar Arch, and their present day maturity is lower than values obtained based on isotopic data. Therefore, lateral migration of gas from the nearby Silurian source rock kitchens towards the north and south of the Qatar Arch is the most probable origin for the significant natural gas accumulations in South Pars field.

Fault detection using soil gas geochemistry

Abstract Soil gas surveys have been carried out at research sites in Great Britain and Italy to test soil gas geochemistry as a site investigation technique for the detection of faults and discontinuities. At a site on Oxford Clay in Gloucestershire, soil gas anomalies of high He, Rn and CO 2 and low O 2 were shown to correspond to the outcrop of a fault, identified by drilling and geophysics. Other apparently random anomalies remained unexplained and lateral migration of gas through superficial horizons complicated interpretation. Using three parallel sample lines at the same distance apart as the sample spacing it has proved not only possible statistically to remove spurious anomalies, but also to enhance and concentrate clusters of high values resulting from gas migration. This method was used to investigate a fault in Neogene clay at Narni in Italy where a gas pathway permeable to Rn and CO 2 was identified, corresponding to a geoelectrical anomaly indicating displaced strata. Further work was carried out over a fault revealed by trenching in the Caithness Flags in Scotland. For each three-line traverse an integrated gas anomaly map was produced to aid interpretation by allowing both sharp and diffuse anomalies to be more easily identified.

Landfill gas – a potential environmental hazard

Methane derived from the decomposition of organic material contained within a landfill may escape beyond the site boundary where it can pose an explosion or fire hazard. Methods are described to prevent die occurrence of such lateral gas migration. Problems due to the accumulation of gas in buildings, erected on landfill sites, have occurred and techniques are now available to overcome these at some sites. However, it is recognized that at other sites, redevelopment should not be allowed to take place on die grounds of safety.