Aquifer Storage and Recovery in Layered Saline Aquifers: Importance of Layer-Arrangements

Aquifer storage and recovery (ASR) refers to injecting freshwater into an aquifer and later withdrawing it. In brackish-to-saline aquifers, density-driven convection and fresh-saline water mixing lead to a reduced recovery efficiency (RE, i.e., the volumetric ratio between recovered potable water and injected freshwater) of ASR. For a layered aquifer, previous studies assume a constant hydraulic conductivity ratio between neighboring layers. In order to reflect the realistic formation of layered aquifers, we systematically investigate 120 layered heterogeneous scenarios with different layer arrangements on multiple-cycle ASR using numerical simulations. Results show that the convection (as is reflected by the tilt of the fresh-saline interface) and mixing phenomena of the ASR system vary significantly among scenarios with different layer arrangements. In particular, the lower permeable layer underlying the higher permeable layer restricts the free convection and leads to the spreading of salinity at the bottom of the higher permeable layer and early salt breakthrough to the well. Correspondingly, the RE values are different among the heterogeneous scenarios, with a maximum absolute RE difference of 22% for the first cycle and 9% for the tenth cycle. Even though the difference in RE decreases with more ASR cycles, it is still non-negligible and needs to be considered after ten ASR cycles. The method to homogenize the layered heterogeneity by simply taking the arithmetic and geometric means of the hydraulic conductivities among different layers as the horizontal and vertical hydraulic conductivities is shown to overestimate the RE for multiple-cycle ASR. The outcomes of this research illustrate the importance of considering the geometric arrangement of layers in assessing the feasibility of multiple-cycle ASR operations in brackish-to-saline layered aquifers.

Mechanisms on the Injection Conformance Reversion during Polymer Flooding of Offshore Heterogeneous Heavy-Oil Reservoirs

Injection conformance reversion commonly observed during polymer flooding in offshore heterogeneous heavy-oil reservoirs weakens the volumetric sweep of polymer solution and compromises its EOR results. To investigate its mechanisms and impact factors, one mathematical model to predicate injection conformance behavior is constructed for heterogeneous reservoirs based on the Buckley-Leverett function. The different suction capability of each layer to polymer solution results in distinct change law of the flow resistance force, which in turn reacts upon the suction capability and creates dynamic redistribution of injection between layers. Conformance reversion takes place when the variation ratio of flow resistance force of different layers tends to be the same. The peak value and scope of conformance reversion decrease and reversion timing is advanced as oil viscosity or permeability contrast increases, or polymer concentration or relative thickness of low permeable layer decreases, which compromises the ability of polymer flooding to improve the volumetric sweep and lower suction of the low permeable layer. The features of offshore polymer flooding tend to make the injection conformance V-type and create low-efficiency circulation of polymer in a high permeable layer more easily. These results can provide guidance to improve the production performance of polymer flooding in offshore heterogeneous heavy-oil reservoirs.

QUANTITATIVE ASSESSMENT OF GROUNDWATER PROTECTION IN THE ZONE OF INFLUENCED OF THE GOMEL CHEMICAL PLANT

The formation of dumps from the production of phosphorous fertilizers at the JSC "Gomel Chemical Plant" is a significant factor affecting the geological environment, which leads primarily to contamination of soil and ground-water. The insignificant thickness of the aeration zone (0.5 to 5.0 m), the absence of low permeable layers in its section, and the downward aquifer recharge make it possible for sulfate, phosphate, and ammonium migrant components to enter groundwater from the surface. Based on the analysis of actual geological and hydrogeological data, a quantitative assessment of the protection of pressure water within the zone influenced by JSC "Gomel Chemical Plant" was given. It is established that their protection depends on the thickness of low permeable sediments, their material composition, and the ratio of aquifer levels. The time of migration of pollutants between groundwater and pressure water was determined. Categories of groundwater protection were identified and their territorial differentiation was established. The protective properties of low permeable sediments were evaluated, taking into account their potential to reduce groundwater contamination with sulfates, phosphates, and ammonium. It was found that the moraine deposits of the first low permeable layer most effectively reduce phosphate and ammonium contamination, while not interfering with the migration of sulfate ions. The second low permeable layer, represented by siltstones, leads to a significant reduction in the flow of pollutants into the Paleogene aquifer. Analysis of the dynamics of sulfate ion concentrations was given, which generally indicated a downward trend in the studied aquifers.

Technical note: The beneficial role of a natural permeable layer in slope stabilization by drainage trenches

Slope stabilization through drainage trenches is a classic approach in geotechnical engineering. Considering the low hydraulic conductivity of the soils in which this measure is usually adopted, a major constraint to the use of trenches is the time required to obtain a significant pore pressure decrease, here called “time lag”. In fact, especially when the slope safety factor is small, the use of drainage trenches may be a risky approach due to the probability that slope deformations will damage the system well before it will become fully operative. However, this paper shows that the presence of persistent permeable natural soil layers can provide a significant benefit by increasing drainage efficiency and reducing time lag. As a matter of fact, any permeable layer that is intercepted by trenches may operate as part of the global hydraulic system, reducing the drainage paths. A simplified approach to designing a drainage system that accounts for the presence of a persistent permeable layer is proposed. This approach, which can exploit solutions available in the literature for parallel drainage trenches, has been validated by numerical analyses.

Analisis Penurunan Parameter Pencemar Limbah Cair Laundry dengan Multi Soil Layering (MSL)

Laundry wastewater is grey water, which is part of domestic wastewater, which is not through the toilet. Laundry wastewater contains COD, BOD, TSS, Total Phosphate, pH, MBAS which includes derivatives of organic substances so that the accumulation causes an increase in the organic content of water or groundwater. The purpose of this study was to observe the effects of the hydraulic loading rate (HLR) and permeable layer (zeolite and gravel) on the efficiency of decreasing laundry wastewater pollutants. Variations in HLR consist of 500 l / m2 day, 750 l / m2 day and 1000 l / m2 day. In this study, laundry wastewater was introduced gravitationally into the MSL reactor with dimensions of 15x50x50 cm. The MSL reactor consists of impermeable layers and permeable layers. The impermeable layer is a layer consisting of a mixture of soil and charcoal with a ratio of 2: 1, and a permeable layer consisting of layers of gravel (MSL 1) and zeolite (MSL 2) with a diameter of 0.25 - 0.5cm. Based on the results of the study, the HLR and permeable media have an effect on the decrease of laundry wastewater pollutant parameters. The highest decrease in pollutants for COD, BOD, TSS, Total Phosphate, pH, and MBAS parameters was obtained at HLR 1000 l/m2.d with the highest percentage reduction is 95% at MSL 1 and 92% at MSL 2. Decreasing pollutant concentration of laundry wastewater is higher in MSL 2 (permeable layer of zeolite) compared to MSL 1 (permeable layer of gravel).