Low-permeability Layers Research Articles

Comparison of snowpack structure in gaps and under the canopy in a humid boreal forest

AbstractThe boreal forest covers a significant portion of the Northern Hemisphere and is snow‐covered for over half of the year. Understanding the interactions between the forest canopy and snow is essential in hydrological, meteorological, and climate modelling. However, this is challenging because the density of a forest can range from closed canopies to open gaps. In winter 2018–2019, we assessed differences in snowpack microstructure in small forest gaps and under the canopy of a humid boreal site in eastern Canada. Our experimental approach consisted of quasi‐continuous weekly observations of stratigraphy and measurements of density profiles and temperature in a series of snow pits in both environments. High‐resolution specific surface area (SSA) profiles were measured twice, allowing for an estimation of snow permeability and hydraulic conductivity. The shallower snowpack under the canopy displayed a stronger vertical temperature gradient and less compaction than in forest gaps. This resulted in the dominance of faceted snow crystals with a small SSA. In contrast, we observed that small, rounded grains with a larger SSA than that of faceted crystals prevailed in the gaps. Due to denser snow and higher SSA, snow permeability inside gaps was found to be lower than under the canopy. Implicitly, the estimated hydraulic conductivity was also lower in gaps. Following rain‐on‐snow events, snow under the canopy displayed layers of melt‐freeze polycrystals, while in the gaps, well‐defined ice layers were formed. The combination of low snow permeability and ice layers is likely to affect liquid water transport in the gap snowpack as compared to the canopy. Although observed at relatively small scales in our study, if these differences are confirmed at a catchment scale, they are likely to impact the hydrology of forested areas during snowmelt.

Aquifer depth determination of low permeability layers using geo-resistivity data: A case study of Enyigba mine area and environs, South Eastern Nigeria

Many geophysical methods have been applied to locate groundwater in Nigeria’s rural and urban villages. Locating groundwater in low permeability formations like shales and siltstones is even more challenging due to the difficulty of mapping fracture zones within these formations. The fracture zones serve as potential aquifers in low permeability formations and have been the object of groundwater search in shales, siltstones and other low permeability formations. The electrical resistivity method has proven helpful in fracture mapping within low permeability formations due to the existing resistivity contrast usually observed between the fractured and non-fractured sections in the Shales and Siltstones. Three vertical electrical geosounding datasets (VES 1, VES 2 and VES 3) were acquired in the Schlumberger configuration, using a maximum current electrode spacing of 200m to delineate the fracture zones based on their electrical resistivities. The acquired datasets were processed and modelled using IP12 Win software, while the processed datasets were correlated with local geology to estimate the depths of the fractured shales in the area. Results show five modelled geo-electric layers with depths to the fractured shales ranging from 17-25m, while aquifer thicknesses range from 7 to 12m. Aquifer resistivities range from 58 - 115 ohm-m. The curves are primarily of the QH type. One of the Vertical Electrical Sounding Data points (VES 2) encountered an anomalously low resistivity zone at a depth range of 5 to 8m which was interpreted as a galena lode. The low resistivity zone has been confirmed through exploratory drilling to tie with Lead-Zinc lodes at a depth of 8m.

A Fundamental Moving Boundary Problem of 1D Commingled Preferential Darcian Flow and Non-Darcian Flow Through Dual-Layered Porous Media

In consideration of vertical formation heterogeneity, a basic nonlinear model of 1D commingled preferential Darcian flow and non-Darcian flow with the threshold pressure gradient (TPG) in a dual-layered formation is presented. Non-Darcian flow in consideration of the TPG happens in the low-permeability tight layer, and the Darcian kinematic equation holds in the other high-permeability layer. The similarity transformation method is applied to analytically solve the model. Moreover, the existence and uniqueness of the analytical solution are proved strictly. Through analytical solution results, some significant conclusions are obtained. The existence of the TPG in the low-permeability tight layer can intensify the preferential Darcian flow in the high-permeability layer, and the intensity of the preferential Darcian flow is very sensitive to the dimensionless layer thickness ratio. The effect of the layer permeability ratio and layer elastic storage ratio on the production sub-rate is more sensitive than that of the layer thickness ratio. In addition, it is strictly demonstrated that moving boundary conditions caused by the TPG should be incorporated into the model. When the moving boundary is neglected, the preferential Darcian flow in the high-permeability layer will be exaggerated. Eventually, solid theoretical foundations are provided here, which are very significant for solving non-Darcian seepage flow problems in engineering by numerical simulation validation and physical experiment design. Furthermore, they are very helpful for better understanding the preferential flow behavior through the high-permeability paths (such as fractures) in the water flooding development of heterogeneous low-permeability reservoirs; then, the efficient profile control technology can be further developed to improve oil recovery.

Experimental Characterization of Oil/Gas Interface Self-Adjustment in CO2-Assisted Gravity Drainage for Reverse Rhythm Reservoir

Worldwide practices have proven that gas-assisted gravity drainage can obviously enhance oil recovery, and this technology can be especially effective for reservoirs with a thick formation and large inclination angle. For the successful implementation of this process, a key technology is the stable control of gas–oil interface during gas injection. For a detailed exploration of this technique, a three-stage permeable visual model was designed and manufactured, with permeability decreasing from top to bottom, thus, a reverse rhythm reservoir was effectively modeled. Then the experiment concerning CO2-assisted gravity drainage was carried out with the adoption of a self-developed micro visual displacement device. This study mainly focused on the micro migration law of gas–oil interface and the development effects of CO2-assisted gravity drainage. According to the experiments, CO2 fingering somewhat happens in the same permeable layer from the beginning of gas injection. However, phenomena of “wait” and “gas–oil interface self-adjustment” occur instead of flowing into the next layer when the injected CO2 reaches the boundary of the next lower permeability layer through the dominant channel. By the “gas–oil interface self-adjustment”, the injected CO2 first enters into the pores of the relative higher permeability layer to the greatest extent, and thus expands the sweep volume. Futhermore, in the process of CO2 injection, obvious gas channeling occurs in the low permeability layer directly connected to the outlet, resulting in low sweep efficiency and poor development effect. After connecting the core with lower permeability at the outlet, the development indexes of the model, such as the producing degree of the low permeability layer, the oil recovery before and after gas breakthrough, are significantly improved, and the recovery degrees of the medium permeability layer and the high permeability layer are also improved, and the overall recovery factor is increased by 12.38%. This “gas–oil interface self-adjustment” phenomenon is explained reasonably from the two scales of macroscopic flow resistance and microscopic capillary force. Finally, the enlightenments of the new phenomenon are expounded on the application of gas-assisted gravity drainage on site and the treatment of producers with gas breakthrough in gas injection development.

Experimental Study on the Sweep Law of CO2 Miscible Flooding in Heterogeneous Reservoir in Jilin

It is very important to effectively describe the sweep characteristics of CO2 miscible flooding based on physical models for actual reservoir development. In this study, based on the geological characteristics of the Jilin ultra-low permeability reservoir, which has significant vertical heterogeneity, a two-dimensional double-layer heterogeneous visualization model with a permeability contrast of 10 and thickness contrast of 2 was designed to perform experimental research on the sweep law of CO2 miscible flooding with an injection-production mode of “united injection and single production”. With the goal of determining the obvious differences in the gas absorption capacity and displacement power of the two layers, the CO2 dynamic miscible flooding characteristics were comprehensively analyzed, and the sweep law of CO2 miscible flooding, including the oil and gas flow trend, migration direction of the oil–gas interface, and distribution characteristics of the miscible zone, was further studied in combination with the oil displacement effect. In this experiment, the gas absorption capacity was the key factor affecting the sweep efficiency of the CO2 miscible flooding. Under the combined influence of the internal and external control factors of the reservoir thickness, permeability, and injection-production mode, the gas absorption capacity of the high-permeability layer was much greater than that of the low-permeability layer, resulting in the retention of a large amount of remaining oil in the low-permeability layer, which effectively displaced and swept the oil in the high-permeability layer. The gas absorption capacity of the reservoir, gravitational differentiation, and miscible mass transfer were key factors affecting the migration of the oil–gas interface and distribution of the miscible zone. The entire displacement process could be divided into three stages: ① The gas-free rapid oil production stage, which was dominated by the displacement; ② the low gas–oil ratio stable oil production stage, which was jointly affected by the displacement and miscible mass transfer; and ③ the high gas–oil ratio slow oil production stage, which was dominated by the effect of CO2 carrying.

The Potential to Preserve Relic Hydrates below the Stability Zone in Low-Permeability Layers of the Berezovo Formation of the North of Western Siberia

The Potential to Preserve Relic Hydrates below the Stability Zone in Low-Permeability Layers of the Berezovo Formation of the North of Western Siberia

Effects of morphology and sediment permeability on coastal lagoons’ hydrological patterns

Mediterranean coastal lagoons are influenced from a wide variety of external factors such as surface and groundwater flows and climate dynamics. They are also vulnerable to human activities, which have caused a significant loss of these types of habitats. As a result, the EU habitat management have now prioritized restoration of natural wetlands. However, there is a lack of systemic studies on the mechanisms of coastal wetland degradation and ecohydrological processes that determine restored ecological functioning as an end goal, as well as a lack of reference sites to make comparisons. Furthermore, while lagoon morphometry and underlying sediment permeability have been studied extensively, combining these disciplines to evaluate lagoon hydrology and salinity dynamics is yet to be fully explored. The aim of this study was to analyze the hydrological dynamics of 4 newly constructed lagoons and compare them with 2 natural lagoons in the La Pletera salt marshes and evaluate the restoration and conservation efforts. We use the General Lake Model (GLM) to assess water volume fluctuations, salinity variability and lagoon water circulation (groundwater and surface water inflows, outflows and evaporation). We also combine data of the underlying lithological characteristics and lagoon morphometry, to compare and better understand the interplay of these parameters on the hydrological behavior of each lagoon. Results indicated that the older and natural lagoons exhibited more consistent patterns of confinement; with deeper morphologies, lower evaporation effect, lower water circulation, and more annual patterns of salinity fluctuation. The presence of low-permeability layers also resulted in less fluctuation of higher salinity levels. Conversely, three of the four new lagoons had similar, shallower morphologies and higher evaporation fluxes, but exhibited different water circulation patterns due to the presence or absence of low permeability layers. Also, their salinity fluctuations were more influenced by seasonal mixing than by evaporation, indicating more susceptibility to climatic influence in their annual hydrological pattern than in the natural lagoons. This could prove important when constructing and restoring lagoons according to predetermined morphology and underlying sediment patterns, as it could ultimately limit or enhance the success of set objectives and overall ecological functioning in a flooding – confinement driven lagoon ecosystem conditioned by irregular and unpredictable climatic events.

Study on migration characteristics of oil gas water three-phase interface and distribution law of remaining oil in gas cap edge water reservoir

ABSTRACT Reservoirs with gas cap and edge water grow rapidly in the early stage of development, the oil well has no water, the recovery period is short, the three phases of oil, gas, and water are easy to cross each other, the production decreases rapidly, and the development is difficult. Therefore, it is necessary to clarify the migration law of oil, gas, and water three-phase interface, find out the distribution law of remaining oil, and put forward the direction of EOR. In order to study the migration law of oil, gas, and water three-phase interface in the development of this kind of reservoir, taking JZ25-1S gas cap edge water reservoir as an example, a three-dimensional physical model is established based on the similarity criterion. The characteristics of fluid interface migration and the distribution of remaining oil under depletion development and barrier waterflood development are studied through experiments. The research results show that gas channeling occurs after 30 minutes of depletion development, and the migration speed of the oil-gas interface is faster than that of the oil-water interface, and the migration speed of the high-permeability layer is faster. The remaining oil in the low-permeability layer is mainly distributed on both sides of the oil and gas and oil-water channels, and the remaining oil in the high-permeability layer is mainly composed of rock particles, pore walls, and surfaces of interstitial materials. The ultimate recovery rate of depletion development is 25%. At the barrier waterflooding development for 60 minutes, gas began to appear, the migration rate of oil-gas and oil-water interface was stable, and the shape of fluid interface was gentle. A small amount of remaining oil in each permeability layer is distributed in the oil ring and gas cap area, and the distribution of remaining oil is mainly affected by injection timing and injection-production ratio. The final recovery rate of barrier waterflooding development is 51%, which is 26% higher than that of depletion development. The findings of this study can help for better understanding of great significance to the formulation of production parameters for depleted development of gas cap and edge water reservoirs and the implementation of later production stimulation measures.

Research and Successful Application of the Diverted Fracturing Technology of Spontaneously Selecting Geologic Sweet Spots in Thin Interbedded Formation in Baikouquan Field

Baikouquan oil field is composed of multiple interbedded, thin, low-permeability layers, which are required to vertically fracture multiple layers and to create complex fractures for economic development. However, conventional fracture technologies create a single, simple fracture, having poor feasibility for this field. Therefore, we conducted research on fracturing technology by spontaneously selecting geologic sweet spots based on diversion. This technology can vertically fracture the thin layers one by one and horizontally divert the fracture to non-depleted areas. Firstly, a triaxial diverted fracturing experiment approach was setup, and then diverted fracturing experiments were carried out to verify the feasibility of diverted fracturing and to study the fracture geometry and the law of diversion. Next, experiments were carried out to evaluate the performance of the diversion agents. The valuated properties comprise the diversion pressure, stability time, and degradation based on which to optimize the selection of the diversion agents. Finally, the fracturing technology was applied to well b21004 of Baikouquan oil field, and post-frac performance was evaluated. The experimental results show that multiple and complex fractures are realized through temporary plugging. Diversion performance evaluation tests show that a 4 wt% concentration of 1–5 mm granules + 20/60 mesh powder and a 3 wt% concentration of 1–7 mm granules + 20/60 mesh powder + fiber can hold up enough pressure to force the fracture to divert. The field treating pressure curve shows that there is a 3–10 MPa pressure increase when there are pump diversion agents, which is a clear sign of fracture diversion. Plugging the fracture mouth gives a faster and a higher incremental pressure. Before this fracturing, the well had almost stopped oil production. After the stimulation, the initial oil production rate became 20 + t/d, which shows the effectiveness of this fracturing technology for Baikouquan oil field.

Impact of a Low-Permeability Layer on the Pumping Efficiency Under Threats of Saltwater Up-Coning

Previous studies on saltwater up-coning are mostly based on the assumption of a homogeneous aquifer and neglect the effect of heterogeneity. A numerical study was carried out to explore the impact of a low-permeability (k) layer on the pumping efficiency under threats of saltwater up-coning. It is found that the presence of the low-k layer could impede up-coning of saltwater, thereby improving significantly pumping efficiency, as expected. For the cases adopted, when the permeability of the 2 m thick low-k layer is three orders of magnitude less than that of the corresponding homogeneous aquifer, the pumping duration (i.e., the time before the salinity of pumped water reaches 2%) can be up to about 10 times longer than that of the homogeneous aquifer case. Additionally, a sound linear relationship could be developed between the normalized pumping duration of the layered case (E) (defined as the difference of the pumping duration between layered and homogeneous cases relative to the pumping duration in the homogeneous case) and the permeability of the homogeneous aquifer relative to that of the low-k layer (β). However, a short low-k layer may lead to the failure of this linear relationship. Moreover, a thicker low-k layer leads to a larger E when all other conditions are unchanged. When varying the location of the low-k layer, the large E occurs for the low-k layer located below and near the well bottom. The results obtained in the current study offer significant implications for preventing saltwater up-coning and improving the pumping efficiency through natural and artificial barriers.

Effects of spent viscoelastic-surfactant acid flow on wormholes propagation and diverting performance in heterogeneous carbonate reservoir

The success of viscoelastic-surfactant (VES) acid in heterogeneous carbonate reservoirs depends on the wormholes propagation and diverting performance of acid. However, with the continuous propagation of wormholes in the high-permeability zone, the highly viscous spent acid enters the low-permeability zone under the pressure difference, thus affecting the wormholes propagation and diverting performance of acid in the low-permeability zone, which has been neglected in recent studies. Therefore, in this paper, the rheological model of acid is modified by considering the velocity of VES acid viscosity increase, and a two-scale continuous model was established. Based on this model, the diverting process is described, and reservoir connectivity, permeability contrast, and velocity of acid viscosity increase are used to investigate the influence of spent acid on wormholes propagation and diverting performance of acid in the low-permeability zone. The results show that the formation of wormholes in the high-permeability zone makes a pressure difference between the high-permeability zone and the low-permeability zone. The high viscous spent acid flows from the high permeability layer to the low permeability layer under the pressure difference, thus inhibiting the wormholes propagation and diverting performance of acid in the low-permeability zone of heterogeneous carbonate reservoirs. The influence degree of spent acid is 30.6% when the permeability contrast is 3, and the influence degree decreases with the enhancement of heterogeneity. At permeability contrasts greater than 10, there is almost no effect. The velocity of acid viscosity increase affects the diverting performance of acid, and the faster the viscosity increase, the farther the highly viscous zone formed in a short time, the better its diverting performance. In the VES acid system commonly used in carbonate acidizing, the maximum difference of diverting performance is 4.25 times. The parallel coreflood experiments overestimate the diverting performance of VES acid by about 20%. When selecting the diverting technology for acidizing treatment, the diverting acid system with higher diverting performance in the laboratory experiments or diverting technology with stronger diverting performance can be appropriately selected. In addition, the velocity of spent VES acid viscosity increase is a significant parameter affecting the acid diverting performance. In the low calcite content reservoir or dolomite reservoir, the spent acid viscosity increases relatively slowly due to the slower reaction rate, which will reduce the diverting performance in the reservoir.

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination.

Multilateral Jetting Technology Results in Increased Production in Offshore Field

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 21959, “Multilateral Jetting Technology Results in a 150% Uplift in Production During a Second Offshore Application in Abu Dhabi Offshore Field,” by Salman Farhan Nofal, SPE, Fazeel Ahmad, SPE, and Ahmad Shmakhy, SPE, ADNOC, et al. The paper has not been peer reviewed. Copyright 2022 International Petroleum Technology Conference. Reproduced by permission. A well offshore Abu Dhabi was deemed a success despite deployment challenges during the lower completion phase. Opportunities to address these challenges existed for a succeeding well, and in the search for improvement to the productivity index (PI), multilateral acid-jetting technology was adopted as a more-effective approach than typical drainage methods. Following successful adoption of this technology, the well has been producing for 1 year at the time of writing with positive results. Introduction The subject reservoir consists of a carbonate multilayer formation with differing porosities and permeabilities. The conventional method to inject or produce from these layers is by drilling slanted or horizontal holes followed by acidizing using conventional 15 or 28% hydrochloric acid (HCl). These holes sometimes do not meet the target because of low productivity (high drawdown) caused by tight reservoirs with less surface contact even after acidizing because the acid tends to propagate more in the highly permeable streaks than in the low-permeability ones. The new approach aims at addressing this problem by increasing the surface contact in the low-permeability streaks and connecting different thin separated layers to improve productivity using expanding metal needles. Challenges This reservoir consists of multiple thin, low-permeability layers separated by dense layers. Completion of wells in all sublayers with slanted holes did not lead to efficient production, and high drawdown led to low bottomhole flowing pressure. Completion of wells in one or two sublayers using horizontal stepdown was considered, but some wells were still not able to produce against the sea line, leading to the conclusion that the reservoirs suffered from a low ratio of vertical to horizontal permeability (kV/kH).

Numerical Simulation of the Enrichment of Chemotactic Bacteria in Oil-Water Two-Phase Transfer Fields of Heterogeneous Porous Media

Oil pollution in soil-groundwater systems is difficult to remove, and a large amount of residual oil is trapped in the low permeable layer of the heterogeneous aquifer. Aromatic hydrocarbons in oil have high toxicity and low solubility in water, which are harmful to the ecological environment. Chemotactic degrading bacteria can perceive the concentration gradient of non-aqueous phase liquid (NAPL) pollutants in the groundwater environment, and enrich and proliferate around the pollutants, thus achieving a more efficient and thorough remediation effect. However, the existing theoretical models are relatively simple. The physical fields of oil–water two-phase flow and oil-phase solute convection and diffusion in water are not coupled, which further restricts the accuracy of studies on bacterial chemotaxis to NAPL. In this study, geometric models based on the actual microfluidic experimental study were constructed. Based on the phase field model, diffusion convection equation and chemotaxis velocity equation, the effects of heterogeneity of porous media, wall wettability and groundwater flow rate on the residual oil and the concentration distribution of chemotaxis bacteria were studied. Under all of the simulation conditions, the residual oil in the high permeable area was significantly lower than that in the low permeable area, and the wall hydrophilicity enhanced the water flooding effect. Chemotactic bacteria could react to the concentration gradient of pollutants dissolved into water in the oil phase, and enrich near the oil–water interface with high concentration of NAPL, and the density of chemotactic bacteria at the oil–water interface can be up to 1.8–2 times higher than that in the water phase at flow rates from 1.13 to 6.78 m/d.

Experimental and Numerical Investigation on Oil Displacement Mechanism of Weak Gel in Waterflood Reservoirs.

The production performance of waterflood reservoirs with years of production is severely challenged by high water cuts and extensive water channels. Among IOR/EOR methods, weak gel injection is particularly effective in improving the water displacement efficiency and oil recovery. The visualized microscopic oil displacement experiments were designed to comprehensively investigate the weak gel mechanisms in porous media and the numerical simulations coupling equations characterizing weak gel viscosity induced dynamics were implemented to understand its planar and vertical block and movement behaviors at the field scale. From experiments, the residual oil of initial water flooding mainly exists in the form of cluster, column, dead end, and membranous, and it mainly exists in the form of cluster and dead end in subsequent water flooding stage following weak gel injection. The porous flow mechanism of weak gel includes the preferential plugging of large channels, the integral and staged transport of weak gel, and the residual oil flow along pore walls in weak gel displacement. The profile-control mechanism of weak gel is as follows: weak gel selectively enters the large channels, weak gel blocks large channels and forces subsequent water flow to change direction, weak gel uses viscoelastic bulk motion to form negative pressure oil absorption, and the oil droplets converge to form an oil stream, respectively. The numerical simulation indicates that weak gel can effectively reduce the water-oil mobility ratio, preferentially block the high permeability layer and the large pore channels, divert the subsequent water to flood the low permeability layer, and improve the water injection swept efficiency. It is found numerically that a weak gel system is able to flow forward under high-pressure differences in the subsequent water flooding, which can further improve oil displacement efficiency. Unlike the conventional profile-control methods, weak gels make it possible to displace the bypassed oil in the deep inter-well regions with significant potential to enhance oil recovery.

Evolution of convection in a layered porous medium

The effect of a series of thin, horizontal, low-permeability layers on convective motion from a distributed dense source along an upper boundary in an otherwise homogeneous, two-dimensional porous medium is considered. This set-up provides an idealised version of a relatively common form of heterogeneity in geological formations. The thickness and permeability of the thin layers are assumed to be small relative to the distance between them and the bulk permeability, respectively. As such, the layers can be parameterised by their impedance $\varOmega$ – a dimensionless ratio of the effective layer thickness and permeability – while the strength of convection is controlled by the dimensionless distance $H \gg 1$ between layers, which can also be interpreted as an effective Rayleigh number for the flow. The role of $\varOmega$ is explored with the aid of high-resolution numerical simulations, and simple analytical models are developed for the evolution of the mean concentration and the flux in the limits of small and large $\varOmega$ . For intermediate values of $\varOmega$ , the flow undergoes a transition from predominantly diffusive transfer across the layers to predominantly advective transfer, and the lateral scale of the flow can become very large. This transition is characterised and a simple model is developed.

Theoretical Study on the Micro-Flow Mechanism of Polymer Flooding in a Double Heterogeneous Oil Layer

Critical issues in the development of oil fields include the differences in the layer properties as well as serious interlayer conflicts and disturbances that can lead to the formation of a preferential flow pathway. In order to understand the interlayer disturbance mechanism between the heterogeneous oil layers, mathematical models of the polymer, and oil two-phase micro-flow in porous media are established based on the Navier-Stokes equation. The phase-field method is used to track the two-phase interface during the displacement process. Then, the influences of wettability, injection modes, and permeability contrasts on the front length coefficient and the displacement efficiency are studied. The results showed that when the rock surface is water-wet (oil-wet), the polymer displaced the low (high) permeability layer first, and the interlayer breakthrough is obvious in the early stages of displacement. After the front broke through, the water-wet (oil-wet) rocks began to displace the high (low) permeability layer, and the preferential flow pathway is formed, which slowed the subsequent polymer flooding. When the rock surface is oil-wet, the perforation degree of the inlet had a greater effect on the micro-oil displacement efficiency. The micro-oil displacement efficiency of the full perforation and commingling production model is 26.21% and 37.75% higher than that of the separate-layer injection and commingling production, as well as the partial perforation and commingling production-injection models, respectively. The larger the permeability contrast, the more obvious the interlayer breakthrough. This study reveals the influence of different wettability characteristics, injection modes, and permeability contrasts on the front length coefficient and the displacement efficiency in a micro-heterogeneous model and provides an important theoretical basis for the formulation of enhanced oil recovery schemes for heterogeneous oil layers.

ТӨМЕН ӨНІМДІ ҰҢҒЫМАЛАРДЫ ПАЙДАЛАНУ ӘДІСТЕМЕЛЕРІН ЖЕТІЛДІРУ

The current stage of development of the oil industry of Kazakhstan is characterized by its entry into the stage of declining production. Many large deposits are characterized by a rapid increase in product wetting. An unfavorable geological and technological structure of oil reserves has been formed at the oilfield, where the share of traditional (technologically developed) reserves is only 35%. At the same time, the share of hard-to-extract oil reserves (low-permeable layers, residual reserves, deeply loaded horizons, high-viscosity oil, gas subsurface areas) is 2/3 or 65%. Based on the estimates given in this paper, it is proposed to consider the development of low- permeable layers and residual oil resources as priority areas for the development of hard-toextract oil reserves. Their successful development will ensure the effective development of about half of the industrial oil reserves. Increasing the efficiency of development of low-permeable layers will attract several billion tons of industrial oil reserves for development.

Seismic monitoring for subsurface uncertainties at the Endurance CO2 store

The development of the Endurance structure in the southern North Sea as a CO2 store offers a potentially large decarbonization opportunity for industry along the east coast of England. The Bunter Sandstone Formation in the Endurance structure has been assessed as an excellent candidate for CO2 storage due to its position, depth, reservoir properties, and extensive seal. Monitoring CO2 during active injection and after closure is vital to confirming containment and conformance of the CO2 within the store. There are two key subsurface uncertainties: compartmentalization and vertical permeability. Current seismic resolution does not provide imaging of thin, low-permeability layers that may create low relative vertical permeability, and the high net to gross of the sandstone limits imaging of any minor faulting. However, the strong CO2 response on 4D seismic likely will illuminate baffles and barriers to flow along with variations in reservoir permeability. In 2020, four two-dimensional high-resolution (2DHR) seismic data test lines were acquired to assess the best technology for imaging and monitoring Endurance. High-resolution seismic was concluded to provide quality imaging from the base of the reservoir (approximately 1500 m true vertical depth subsea) up through the overburden to image the entire subsurface and to demonstrate safe storage of CO2.

Mechanisms for Overpressure Development in Marine Sediments

Overpressure is widely developed in marine sediments; it is not only a critical factor related to hydrocarbon accumulation, but also a serious safety issue for oil/gas exploration and exploitation. Although the mechanisms for overpressure development in sedimentary basins have been intensively studied, some new mechanisms are proposed for overpressure development with the advancements in marine geological investigation, e.g., natural gas hydrate formation and microbial activity. In this study, the mechanisms for overpressure development are reviewed and further classified as being related to associated physical, chemical, and biological processes. The physical overpressure mechanisms include disequilibrium compaction, hydrate formation sealing, degasification, buoyancy, hydrothermal pressuring, tectonic movement, overpressure transfer, etc. The chemical overpressure mechanisms are ascribed to hydrate decomposition, diagenesis, hydrocarbon generation, etc. The biological overpressure mechanisms are mainly induced by microbial gas production and microbial plugging. In gas hydrate-bearing sediments, overpressure is a critical factor affecting the formation and distribution of gas hydrate. The mechanisms for overpressure development in marine gas hydrate systems are associated with permeability deterioration due to hydrate formation and free gas accumulation below bottom-simulating reflectors (BSR). In marine sediments, overpressure developments are generally related to a sediment layer of low permeability above and natural gas accumulation below, and overpressure is mainly developed below a sulphate–methane interface (SMI), because methane will be consumed by anaerobic oxidation above SMI.