Drawdown Rate Research Articles

Impact of Cropping Pattern and Climatic Parameters in Lower Chenab Canal System—Case Study from Punjab Pakistan

In Pakistan, groundwater resources are depleting at an alarming rate due to intensive pumping, shifting of cropping patterns, and climate change vulnerability. The present study is aimed at investigating groundwater stress in the command area of Lower Chenab Canal (LCC) and associated branch canals. Groundwater stress is determined by considering the cropping patterns, surface water availability, groundwater levels, climatic variation, and crop water requirement (CWR) in the LCC command area. The climatic data is obtained from the Pakistan Meteorological Department (PMD) from 1990 to 2020. The records of temporal variation in cropping patterns are obtained from the Crop Reporting Service (CRS), Directorate of Agriculture, Lahore for the 1995–2020 period and classified according to Rabi season (November to April) and Kharif season (May to October). The LCC surface water flows data and groundwater levels are collected from the Punjab Irrigation Department (PID) Lahore from 2003 to 2018 and from 1995 to 2016, respectively. The CWR is estimated using the Cropwat 8.0 model and groundwater levels are estimated using the Inverse Distance Weighted (IDW) tool of ArcGIS software. It has been determined that Faisalabad, Sheikhupura, and Toba Tek Singh are highly groundwater stress cities having an average drawdown rate of 0.58 m/year. The surface water availability is also decreased from 7.75 to 4.81 billion cubic meters (Bm3) for the Kharif season whilst 4.17 to 2.63 Bm3 for the Rabi season. This study concluded that due to severe conditions in highly stressed areas, policy planners, decision-makers, and stakeholders should sincerely take some steps for maintaining groundwater levels either by capacity building workshops for the farmers or limiting the number of tubewells.

Evaluating Gas-Oil Ratio Behavior of Unconventional Wells in the Uinta Basin

On account of low matrix permeability and porosity, the fluid phase behavior in tight oil reservoirs significantly differs from the conventional plays, which increases the challenges of evaluating the well performance and providing reliable production prediction. One of the most important efforts is to investigate the behavior of unconventional well gas-to-oil ratio (GOR). The growth trend of GOR is always considered as a principal reference to modify the production strategies and enhance the well productivity. In this paper, long-term GOR (20 years) behavior of an unconventional well model was evaluated by a set of comprehensive sensitivity studies. This model’s dimensions and reservoir properties are based on information of the Altamont-Bluebell field in the Uinta Basin. The simulation results established a general framework for interpreting the GOR behavior. Key drivers such as completion designs, reservoir, and fracture properties are investigated. This study provides some key insights into the GOR behaviors, such as the high sensitivity of the hydraulic fracture properties, including fracture conductivity and effective fracture half-length. The GOR behavior could be considerably different on various conductivity values or fracture lengths, resulting from the conductivity impact of phase movement and the pressure transition along the fracture. The fracture design factors like drawdown scenario and cluster spacing also play indispensable roles in impacting the GOR response, where a less aggressive drawdown rate can help to delay the GOR increase at an early stage; a widening spacing also can mitigate the rise of GOR and keep it at low value during the whole production period. The impact from natural fractures is also examined and discussed for the first time, including the effect of natural fracture numbers and conductivity, where the latter variable is considered as a more effective contributor to the GOR prediction. The findings presented in this work promote a better understanding of the GOR behaviors in tight oil reservoirs. Results and discussion in this paper allow a more accurate forecasting production to improve the reservoir management efficiency and provide multiple valuable insights to guide the field development in future projects.

Coupled Numerical and Analytical Stability Analysis Charts for an Earth-Fill Dam under Rapid Drawdown Conditions

The presented article provides a comprehensive study on the stability analysis of earth-fill dams under rapid drawdown and transient flow conditions used to prepare stability analysis charts by conducting coupled finite-element numerical and analytical limit equilibrium procedures. In this regard, the impacts of different rapid drawdown conditions on the safety factor of the Alavian earth-fill dam are determined. The slope stability charts present for both shallow and deep slip surfaces with various permeabilities are verified by ground information obtained with extensive instrumentation on the dam’s site. The results showed that by decreasing the permeability of the core’s material, despite preventing seepage, the instability risk of the upstream slope as a result of rapid drawdown intensifies. Also, as stability charts can be stated, with increasing the slip surface’s depth and decreasing the hydraulic hydration, the reliability decreases, and the sliding surfaces’ sensitivity increases based on the drawdown rates, which have been revealed to be from 0.2 to 0.6, the most critical state for safety factors, showing significant declines.

Dynamics of Embankment Slope Stability under Combination of Operating Water Levels and Drawdown Conditions

This study investigated the potential influence of operating water levels and loading conditions on the slope stability of an embankment dam. Four different operating reservoir levels (normal, reduced, embankment height, and overflow) were considered in the study. Numerical modeling was used to investigate the problem in the case of the Chardara dam within the Syrdarya catchment in Kazakhstan. Based on the drawdown rates and operating conditions, minimum factor of safety values ranging from 0.56 (total failure) to 2.5 were retrieved. Furthermore, a very high correlation was observed between drawdown days, the minimum factor of safety values, the maximum factor of safety values, and pore-water pressures, with correlation coefficients ranging from 0.561 to 0.997 (strong to very strong correlation). On the other hand, the highest negative correlation of 0.997 was observed between the minimum factor of safety values and pore-water pressures. Additionally, based on the results from the analysis of variance, three reservoir operating levels (normal, embankment height, and overflow) resulted in p-values less than 0.05, indicating that the variations in the factor of safety values from the drawdown rates were statistically significant. The findings of this study demonstrated that, not only may the drawdown rate be detrimental to the embankments, but that different operating levels can also affect slope stability in different ways.

What are the Three Paradigms of Retirement Income Planning?

There exists a semi-hidden and little researched connection between planning and software programming that supports that planning, as well as little research into the connection to planning and programming related to the paradigm that exists underlying those connections. This paper discusses the paradigms of retirement income planning from past, to present, and observations to advance the planning profession and research into the future. The profession, researchers, and software programmers have transitioned from the first to the second paradigm and has yet to visualize the third paradigm to make that paradigm shift. Retirees continue to age year by year, but each cohort has a different distribution period due to the ever-decreasing time remaining in longevity tables as they age. There is not a universal retiree (as implied by current research using a single distribution time period with a single allocation; or at best two or three time periods and allocations). Instead, there exist many retirees with many different allocations and many remaining distributions time periods. Thus, one rule of thumb approach e.g., the 4% rule (or any drawdown rate) with inflation applied thereafter, or one Monte Carlo simulation, can't apply to all allocations plus all time periods at the same time because of those different ages with correlating shorter or longer remaining longevity for income drawdown. Therefore, research to date has been narrowly focused, which unintentionally leads to a narrowly applied approach for practitioners as well. Frank 2022a introduced how to model retiree drawdowns through a method that applies to any retiree at any age and subsequently over each retiree's remaining lifetime by cohort ages.

Intelligent Optimization Based Multi-Factor Deep Learning Stock Selection Model and Quantitative Trading Strategy

With the rapid development of financial research theory and artificial intelligence technology, quantitative investment has gradually entered people’s attention. Compared with traditional investment, the advantage of quantitative investment lies in quantification and refinement. In quantitative investment technology, quantitative stock selection is the foundation. Without good stock selection ability, the effect of quantitative investment will be greatly reduced. Therefore, this paper builds an effective multi-factor stock selection model based on intelligent optimization algorithms and deep learning and proposes corresponding trading strategies based on this. First of all, this paper selects 26 effective factors of financial indicators, technical indicators and public opinion to construct the factor database. Secondly, a Gated Recurrent Unit (GRU) neural network based on the Cuckoo Search (CS) optimization algorithm is used to build a stock selection model. Finally, a quantitative investment strategy is designed, and the proposed multi-factor deep learning stock selection model based on intelligent optimization is applied to practice to test its effectiveness. The results show that the quantitative trading strategy based on this model achieved a Sharpe ratio of 127.08%, an annualized rate of return of 40.66%, an excess return of 13.13% and a maximum drawdown rate of −17.38% during the back test period. Compared with other benchmark models, the proposed stock selection model achieved better back test performance.

Retrograde condensation in natural porous media: An in situ experimental investigation

Complex in situ behavior of fluids during a retrograde condensation process is experimentally investigated in a miniature sandstone core sample. Two depletion experiments were conducted with various pressure decline rates using a three-component synthetic gas mixture with a dew point of 3610 psi. A state-of-the-art miniature core-flooding system integrated with a high-resolution micro-computed tomography scanner was employed to acquire pore-scale evidence of condensate nucleation, growth, accumulation, and mobilization in a natural porous medium under different depletion conditions. Analysis of pore-scale fluid occupancy maps demonstrates the formation of discrete nuclei of the condensate in pore throats and crevices as the pressure drops slightly below the dew point. The in situ fluid configurations show that a greater pressure drawdown rate significantly increases the condensate growth and accumulation. The results also illustrate the occurrence of condensate-to-gas imbibition displacements, i.e., snap-off and piston-like events, and the consequent trapping of the gas phase in the pore space. As the pore pressure is reduced, the condensate droplets are found to connect to each other through wetting layers, whereas the large gas clusters are continuously fragmented into smaller globules with reduced hydraulic connectivities. This effect was more pronounced in the case of the high depletion rate experiment. Furthermore, the condensate banking was not completely eliminated (through evaporation) by re-injecting the gas phase. This implies that in the development of a gas condensate reservoir, condensate dropout and banking should be minimized in the first place by, for instance, producing at lower pressure drawdown rates.

The Influence of Shell Permeability on Stability of Upstream Slope during Rapid Drawdown – Khassa Chai Earth Dam as a Case Study

Several factors affect the stability of earth dam during sudden drawdowns such as permeability and mechanical properties of soil, upstream side slope, drawdown ratio, and drawdown rate. This paper investigates the influence of shell permeability on earth dam upstream slope stability and its role in the change of pore water pressure at different locations of the embankment during the sudden drawdown, using different limit equilibrium methods. To accomplish the objective of this study, (Geo Studio 2012 Software) as one of the powerful geotechnical programs was used for the modeling and numerical analysis. The study shows that decreasing in the shell permeability resulted in the reduction of pore water pressure dissipation and variation of shell hydraulic conductivity plays a vital role in the overall stability of the upstream slope under rapid drawdown conditions.

Numerical History-Matching of Modeling and Actual Gas Production Behavior and Causes of the Discrepancy of the Nankai Trough Gas-Hydrate Production Test Cases

Comprehensive data sets were obtained during the gas production tests from a gas-hydrate reservoir in Eastern Nankai Trough (2013 and 2017), including the gas and water production rates, downhole pressure, temperature-monitoring data, log- and core-derived reservoir property information, and seismic volume. Numerical simulations using a compositional and thermal-coupling numerical model (MH21-HYDRES) had been conducted earlier and after each production test for production behavior prediction purposes and history-matching to reevaluate the reservoir property information. The actual production behavior demonstrated some deviations from the prediction by the numerical model. For example, the model predicted a gradual increase in the gas production rates under the constant pressure drawdown but real gas rates were almost constant or slightly decreasing, some inconsistencies in the temperature data and derived vertical and temporal production profiles were observed, and differences in the production behavior exist among the wells that were drilled within a 60 m radius area. To reduce the gap between the model and actual results, understand the nature of the gas-hydrate reservoir, and understand the physical processes during the gas-hydrate dissociation, comparison of the modeling results and obtained data was repeated, along with reexamination of the geological and geophysical data. The study results revealed two features, namely, vertical and horizontal heterogeneities of the formation and complexly disturbed situation in the near-wellbore region due to the unconsolidated nature of the sediments, which were determined as the main causes for the discrepancy in the model and actual production behavior.

Direct measurement of CO2 drawdown in mine wastes and rock powders: Implications for enhanced rock weathering

Enhanced rock weathering (ERW) sequesters CO2 via solubility and mineral trapping and can be implemented by the mining industry to reduce their net greenhouse gas emissions. Kimberlite residues from Venetia Diamond Mine in South Africa, as well as powdered forsterite, serpentinite, wollastonite skarn, and 10 wt.% brucite mixed with quartz sand, were tested as potential feedstocks for ERW. A CO2 flux system directly measured CO2 removal rates and sensors tracked laboratory conditions and pore water saturation during a series of 2-week experiments. With respect to kimberlites, unweathered residues achieved the greatest drawdown rate of -870 g CO2/m2/yr at 48% saturation. In contrast, fine and coarse residues previously exposed to mine process water achieved fluxes of -150 and -160 g CO2/m2/yr at 60% saturation. Brucite reached -2940 g CO2/m2/yr at 14% saturation compared to forsterite, serpentinite, and wollastonite that achieved fluxes of -500, -260, and -190 g CO2/m2/yr, respectively, at higher saturations of 53–60%. Mineralogical composition had the greatest effect on CO2 fluxes, followed by water content which drives carbonation reactions and affects permeability. Solid inorganic carbon increased in the brucite, wollastonite, and unweathered kimberlite, indicating that CO2 was stored via mineral trapping as opposed to solubility trapping in the other experiments. Increasing the exposure of unweathered residues, expanding dispersal area, and optimizing water saturation would lead to greater CO2 removal at mine sites.

SOME CLARITY ON THE ACCRUAL OF LIVING ANNUITIES AT DEATH OR DIVORCE CM v EM (1086/2018) [2020] ZASCA 48; [2020] 3 All SA 1 (SCA); 2020 (5) SA 49 (SCA) (5 May 2020)

A conventional life annuity is a contract in terms whereof an annuity underwriter guarantees a periodical payment to an insured in exchange for an initial non-refundable premium. The insurer pools all the annuity premiums together and assumes both the investment performance and the mortality risk by way of actuarial comparisons. The annuitant’s income is guaranteed for life or for a minimum period.Living annuities on the other hand are regulated by the Long-Term Insurance Act 52 of 1998 and are market-linked investments (with no income guarantee) in respect of which the annuitant annually chooses the drawdown rate – currently between 2.5 and 17.5 per cent per annum (compare Regulations in terms of s 36 of the Pensions Act 24 of 1956 and s 106(1)(a) read with s 108(1) of the Financial Sector Regulation Act 9 of 2017.) When an annuitant dies, the death benefit is payable to a nominated beneficiary or the estate of the insured. A pension-interest benefit is an asset for the purposes of the division of an estate at divorce, and includes both pension and provident funds. Living annuities, however, do not fall within the definition of “pension interest” as defined in s 1 of the Divorce Act.In CM v EM ((1086/2018) [2020] ZASCA 48; [2020] 3 All SA 1 (SCA); 2020 (5) SA 49 (SCA) (5 May 2020)), the Supreme Court of Appeal, in an appeal from the full court of the Gauteng Division of the High Court, sitting as court of appeal, had the opportunity to determine where the ownership of capital invested in the form of a living annuity vests, as well as whether the value of an annuitant spouse’s right to future annuity payments is an asset in his or her estate and therefore subject to accrual. Accrual in respect of an estate is the amount by which the net value of the estate at the dissolution of a marriage exceeds the net value of that estate at the commencement of the marriage. At the dissolution of a marriage owing to death and subject to the accrual system, the spouse whose estate shows no accrual, or a smaller accrual than the estate of the other spouse, has a claim against the other spouse or his or her deceased estate.It is submitted that some implications of the accrual dispensation, particularly within the context of certain pension and financial products, are still in their discovery phase, nearly 40 years after their introduction. In the absence of any reference to a living annuity in an antenuptial contract, the question was always whether such an investment is subject to the accrual system at divorce or death. In the context of a life assurance policy, the surrender value of the policy was taken into account in the event of divorce, but in the event of death, the question was whether, for accrual purposes, the factor taken into account should be the surrender value or the policy proceeds. As only assets that form part of the estate of a spouse can be considered for accrual purposes, the very nature of a living annuity had to be investigated in the matter of CM v EM (supra). This case was an application for special leave to appeal from the full court in the matter of Emilio Pietro Valfredo Montanari v Charmaine Helen Montanari (Montanari v Montanari).

Adaptive responses of freshwater pearl mussels, Margaritifera margaritifera, to managed drawdowns

Abstract Alterations to water management practices, in response to a growing demand to maximize energy production from renewable sources, threaten to exacerbate anticipated future water shortages caused by severe drought episodes, brought on by climate change. Across Scotland, many highly managed systems are inhabited by some of the last remaining reproductively viable populations of the freshwater pearl mussel, Margaritifera margaritifera. A lack of empirical evidence concerning mussel responses to alterations in flow is inhibiting the development of effective conservation management practices to prolonged drought. This study addressed this knowledge gap by examining the response of M. margaritifera to controlled water level drawdowns in laboratory and field settings. Using a laboratory flume, the responses of M. margaritifera from two different populations (a regulated system versus an unregulated system) were compared across rates of drawdown and density treatments. An analogous field trial was undertaken to examine the responses of M. margaritifera in a regulated system, with a hydroelectric dam facilitating a controlled drawdown. The study showed that M. margaritifera can detect alterations in flow depth, which culminate in the emersion of mussel beds, and respond by undertaking vertical and horizontal movements to mitigate the risk of prolonged aerial exposure. Results from the field trial corroborated findings from the flume experiments, with 80% of mussels shown to avoid emersion successfully by tracking receding water levels. Findings from this study support the role of controlled drawdowns in regulated rivers to reduce mortalities associated with receding water levels during prolonged low‐flow episodes. Differences between populations in response highlight a need to adopt a context‐dependent approach to conservation efforts.

Stillorgan reservoir: maintaining stability during decommissioning and for new construction

Operated by Dublin City Council on behalf of Irish Water, Stillorgan reservoir comprises three open cells (Gray, Upper and Lower reservoirs), created and separated by earth embankments, which store drinking water to supply the Greater Dublin area. To maintain a secure and sustainable supply of safe drinking water into the long-term, Irish Water plans to replace the open reservoirs with a single covered reservoir constructed within the Gray reservoir basin. This paper will discuss the analysis and methodology used to safely complete the drawdown of Gray reservoir while both Upper and Lower reservoirs remained operational. Stability modelling of the shared embankment between Gray and Upper reservoirs was undertaken to establish initial safe drawdown rates and to manage embankment stability until drawdown was completed. This included the interpretation and validation of piezometric data obtained during the drawdown period. Once the reservoir was emptied, vibration monitors and surface markers were installed to monitor the embankment during the preparation of the basin for construction of the service reservoir.

Summertime productivity and carbon export potential in the Weddell Sea, with a focus on the waters adjacent to Larsen C Ice Shelf

Abstract. The Weddell Sea represents a point of origin in the Southern Ocean where globally important water masses form. Biological activities in Weddell Sea surface waters thus affect large-scale ocean biogeochemistry. During January–February 2019, we measured net primary production (NPP), nitrogen (nitrate, ammonium, urea) uptake, and nitrification in the western Weddell Sea at the Antarctic Peninsula (AP) and Larsen C Ice Shelf (LCIS), in the southwestern Weddell Gyre (WG), and at Fimbul Ice Shelf (FIS) in the south-eastern Weddell Sea. The highest average rates of NPP and greatest nutrient drawdown occurred at LCIS. Here, the phytoplankton community was dominated by colonial Phaeocystis antarctica, with diatoms increasing in abundance later in the season as sea ice melted. At the other stations, NPP was variable, and diatoms known to enhance carbon export (e.g. Thalassiosira spp.) were dominant. Euphotic zone nitrification was always below detection, such that nitrate uptake could be used as a proxy for carbon export potential, which was highest in absolute terms at LCIS and the AP. Surprisingly, the highest f ratios occurred near FIS rather than LCIS (average of 0.73±0.09 versus 0.47±0.08). We attribute this unexpected result to partial ammonium inhibition of nitrate uptake at LCIS (where ammonium concentrations were 0.6±0.4 µM, versus 0.05±0.1 µM at FIS), with elevated ammonium resulting from increased heterotrophy following the accumulation of nitrate-fuelled phytoplankton biomass in early summer. Across the Weddell Sea, carbon export appears to be controlled by a combination of physical, chemical, and biological factors, with the highest potential export flux occurring at the ice shelves and lowest in the central WG.

Effect of the Gravel Zone Thickness Created in the Deep Well Test Simulation on the Operating Characteristics of the Pump and Head Loss

This study was conducted in a deep well simulator used for typical irrigation studies. In this work, the changes in the pump flow rate, drawdown, noise level, and pump pressures were analyzed for three different gravel zone thicknesses used in the well. From the study, it was found that a high gravel zone thickness increased the well’s drawdown levels during pumping. For drawdown values of 40, 45, 50 and 55 m3 h–1, an increase in the gravel thickness by 10 cm increased these values by 2.92, 2.41, 2.38 and 2.37 times, respectively. When the gravel thickness was doubled (from 5 cm to 10 cm), the hydraulic conductivity decreased by about half and head loss doubled. As a result, gravel thickness directly affected the drawdown rate of the pump. It was shown that different drawdown values resulting due to different gravel thicknesses should be taken into consideration when placing the pump in a deep well.

A saturated–unsaturated coupling model for groundwater flowing into seepage wells: a modeling study for groundwater development in river basins

As a major and popular groundwater extraction structure, seepage wells are often used to transfer river water into aquifers for harvesting water resources. It can help ameliorate the imbalance between supply and demand, in particular, in areas of water scarcity. Large drawdowns due to pumping may cause the river to disconnect from the groundwater and form an unsaturated zone, which seriously affects the efficiency of seepage wells. However, most of the current models of extraction structures of non-tube wells account for only saturated flow and do not consider unsaturated conditions. To address this limitation, a saturated–unsaturated coupling model was developed using the exchange flow rate between the well pipe and the aquifer as the coupling point. Moreover, the model was evaluated with physical simulation test data. The statistical results indicated that the model could estimate the drawdown and pumping rate well with root-mean-square deviations of 0.0114 m and 0.0079 L s−1, respectively, for a river with strong leakage capacity; and 0.0129 m and 0.0099 L s−1, respectively, for a river with weak leakage capacity. The critical drawdown, where the river disconnects from the aquifer, as well as variations of the unsaturated zone, was also discussed. The present study provides important information for the design of seepage wells with reasonable drawdown, while being able to predict the potential water yield, and at the same time help protect the groundwater environment.

FABM-NflexPD 1.0: assessing an instantaneous acclimation approach for modeling phytoplankton growth

Abstract. Coupled physical–biogeochemical models can generally reproduce large-scale patterns of primary production and biogeochemistry, but they often underestimate observed variability and gradients. This is partially caused by insufficient representation of systematic variations in the elemental composition and pigment density of phytoplankton. Although progress has been made through approaches accounting for the dynamics of phytoplankton composition with additional state variables, formidable computational challenges arise when these are applied in spatially explicit setups. The instantaneous acclimation (IA) approach addresses these challenges by assuming that Chl:C:nutrient ratios are instantly optimized locally (within each modeled grid cell, at each time step), such that they can be resolved as diagnostic variables. Here, we present the first tests of IA in an idealized 1-D setup: we implemented the IA in the Framework for Aquatic Biogeochemical Models (FABM) and coupled it with the General Ocean Turbulence Model (GOTM) to simulate the spatiotemporal dynamics in a 1-D water column. We compare the IA model against a fully dynamic, otherwise equivalently acclimative (dynamic acclimation; DA) variant with an additional state variable and a third, non-acclimative and fixed-stoichiometry (FS) variant. We find that the IA and DA variants, which require the same parameter set, behave similarly in many respects, although some differences do emerge especially during the winter–spring and autumn–winter transitions. These differences however are relatively small in comparison to the differences between the DA and FS variants, suggesting that the IA approach can be used as a cost-effective improvement over a fixed-stoichiometry approach. Our analysis provides insights into the roles of acclimative flexibilities in simulated primary production and nutrient drawdown rates, seasonal and vertical distribution of phytoplankton biomass, formation of thin chlorophyll layers and stoichiometry of detrital material.

Toe drain size and slope stability of homogeneous embankment dam under rapid drawdown

The slope stability of an embankment dam has been always a serious concern of any design team. Unfortunately, the information on the potential influence of a toe drain size on the slope stability of an embankment dam under rapid drawdown conditions is still scarce. This study investigated the potential effect of a toe drain size on the slope stability of a homogeneous embankment dam under rapid drawdown conditions. Three different sizes (5m, 10m, and 15m) of the toe drain were investigated under instantaneous (worst scenario) and 5 days (more realistic) drawdown rates with the help of numerical modeling in GeoStudio. From the results, it was observed that the pore-water pressures at the upstream face of the embankment decreased with the increase in the toe drain size, while the pore-water pressures at the downstream toe were increasing with the increase in the toe drain size. The factor of safety values were also observed to be affected by the changes in the toe drain size.

Fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O

Driven by the large resource volume, extraction of CH4 from hydrate reservoirs has received worldwide attention. Depressurization is a feasible method and has been widely used in past field tests. However, fluid production behavior from water-saturated hydrate-bearing sediments below the quadruple point of CH4 + H2O has not been well understood and warrants investigation. In this study, we designed a series of experiments using ultra-deep depressurization with bottom hole pressure (BHP) of 1.0 MPa, 1.5 MPa and 2.1 MPa in water-saturated hydrate-bearing sediments to investigate the fluid production behavior, particularly in the region with potential ice or secondary hydrate formation. The effect of hydrate saturations and pressure drawdown rates on fluid production below the quadruple point were systematically examined. Our experimental results show that ice formation was likely based on the temperature response, however, the self-preservation effect by ice formation prevents a further drop in temperature to its equilibrium temperature at the corresponding BHP. The gas and water recovery ratios increased by 6.5% and 49.1%, respectively for BHP below the quadruple point. No significant benefit of water trapping due to ice formation was observed. Faster pressure drawdown rate results in a more substantial temperature drop, which could trigger ice formation. Still, interestingly gas production was continuous and not affected in the phase diagram of ice + CH4. Higher hydrate phase saturation improves gas production rate and final gas recovery ratio, yielding a lower water-gas ratio for more economic-viable CH4 recovery. Our results indicate that ultra-deep depressurization could be one technical feasible option for increasing both gas production rate and final recovery ratio. The findings provide valuable information for designing an ultra-deep depressurization scheme based on SH of interest in future production tests.

Study on Ground Collapse of Covered Karst Soil Caves by Sudden Drop of Groundwater

In this study, the ellipsoidal soil cave with vertical collapses in the covering karst area is studied. Based on certain assumptions, the mechanical model of karst collapse caused by groundwater drop was established. Then, based on the negative pressure calculation formula of soil cave cavity according to Boyle–Mariotte’s law, the expression of the stability coefficient of the soil cave was proposed. Subsequently, the feasibility of the theoretical formula was verified. The calculation example analyzed the relationship of groundwater parameters and overburden thickness. The results show that when the initial groundwater level is higher than the top of cave, the law between the stability coefficient of soil cave and groundwater drawdown shows the jumping horizontal broken line. Thus, soil cave tends to collapse when the falling groundwater level drops over the vault; when the initial groundwater level ranges from the bottom to the top of the cave body, the stability coefficient and groundwater drawdown show a negative correlation law, the curve is steep at the early stage and then becomes gentle at the latter stage, and the higher the initial groundwater level in the cave is, the greater stability coefficient of soil cave reduces; when the initial groundwater level is lower than the bottom of the cave, the effect of drawdown is limited. In addition, for the small drawdown or low initial groundwater level, the stability coefficient of soil cave first decreases and then increases with the increases in thickness of overburden, and the thinner the overburden is, the greater the drawdown rate is; when the drawdown or the initial groundwater level is higher, the stability coefficient of soil cave positively relates to the thickness of the overburden layer.