Ground Subsidence Response Conditional On Large-Scale Geothermal Exploitation In A Karst Reservoir Area of North China

Carbonate karst geothermal resources are widely distributed and have large reserves in North China. Nowadays, the scale of exploitation and utilization of the carbonate karst geothermal resources is gradually increasing. In this work, a geothermal exploitation area where the karst geothermal reservoirs are exploited on a large scale, is selected as the study area, and methods including experiment and numerical simulation are used to study the exploitation-induced ground subsidence problems based on the long-term water level monitoring data of the geothermal reservoir. Through analyses of ground subsidence caused by water level change of the geothermal reservoir, the following conclusions were obtained. The water level drawdown of different types of geothermal reservoirs had different effects on ground subsidence. The maximum ground subsidence of the study area caused by the water level decline of the Jx w carbonate geothermal reservoir was only 0.29 mm/a from 1983 to 2019, which is generally insignificant. In contrast, the same water level change of the N m sandstone geothermal reservoir was predicted to cause 8.9 mm/a ground subsidence. To slow down or even prevent the ground subsidence, balanced production and reinjection are required. From the result of this work, the decline of the water level of the Jx w carbonate geothermal reservoir caused by current large-scale geothermal exploitation will not cause serious ground subsidence. However, attention should be paid to the N m sandstone type geothermal reservoirs as their structures are much more sensitive to the water pressure change.

Performance of Multi-Well Exploitation and Reinjection in a Small-Scale Shallow Geothermal Reservoir in Huailai County

The sustainable development of a shallow aquifer geothermal reservoir is strongly affected by the reinjection–production strategy. However, the reinjection–production strategy optimization of a small-scale exploitation unit with tens of meters of well spacing is site specific and has not yet been fulfilled. This study numerically investigates sustainable heat extraction based on various reinjection–production strategies which were conducted in a single-phase aquitard–aquifer geothermal system in Huailai County, Hebei Province, China. The response of the water level and production temperature is mainly discussed. The numerical results show that production without reinjection induces the highest production temperature and also the water level drawdown. Although reinjection in a single doublet well system is conducive to the control of water level drawdown, the introduction of the thermal breakthrough problem causes a decrease in the production temperature. The thermal breakthrough and sustainability of geothermal reservoirs highly depend on the well spacing between the production and reinjection wells, especially for the small-scale field. Therefore, a large well spacing is suggested. A multi-well system facilitates the control of water level drawdown while bringing intensive well interference and thermal breakthrough. Large spacing between the production and reinjection wells is also the basic principle for the design of the multi-well system. A decrease in openhole length leads to an increase in the production temperature and output thermal power. An increase in the production rate affects the thermal breakthrough highly and shortens the lifetime of the geothermal system. Furthermore, the extracted thermal energy is highly affected by the reduction in the reinjection temperature. The results in this study can provide references to achieve sustainable geothermal exploitation in small-scale geothermal reservoirs.

The Thermal Stability of the Naphthalene Sulfonic Acids Under Geothermal Conditions

<p>A primary goal of this thesis was to obtain kinetic data on the breakdown and isomerisation reactions of naphthalene disulfonate (NDS) and naphthalene sulfonate (NSA) compounds under geothermal conditions. A secondary aim of this study was to investigate NDS/NSA isomerisation transformations as well as to study their kinetics and identify products of thermal disproportionation. Because of their apparent thermal stability, naphthalene disulfonate solutions have been frequently injected into active geothermal reservoirs and their subsequent detection (“recovery”) in nearby wells/bore holes used as an indicator of well connectivity and local permeability. The results obtained in this thesis will enable a more insightful interpretation of field injection results and fluid flow in active geothermal reservoirs. The studies presented in this thesis were designed to determine the thermal stability of aqueous NDS and NSA at high temperatures from 100 to 400°C in pure water and different salt solutions (i.e. NaCl +/- Na2SO4 and Na2S) at saturated vapour pressure. The stabilities and isomerisation transformations of NDS and NSA were also studied in the presence of solid materials (i.e. quartz, greywacke, pumice) which may occur in the host geological environment of hydrothermal/geothermal reservoirs in the Earth’s crust. Dilute aqueous solutions of NDS and NSA were contained in sealed silica glass ampoules (purged of atmospheric oxygen) and placed in stainless steel pressure vessels and heated for varying times to the desired high temperatures. Additional experiments were also conducted in which dilute NDS and NSA solutions were pumped from a de-oxygenated reservoir container through a flow-through autoclave containing different rock and mineral phases at temperatures up 400°C. The resulting NDS and NSA isomers were then analysed using HPLC and GC-MS methodologies. The 1,5-naphthalene disulfonate isomer (1,5-NDS) was found to be the least stable at pHt = 3 - 8 and readily transformed to 1-naphthalene sulfonate (1-NSA) at t ≥ 200°C. The 2-NSA was found to be the most stable isomer but disappeared at t ≥ 300°. The experimental data indicated that the stabilities of all the NDS and NSA studied as a function of temperature, pH and salt (NaCl) concentration were in the sequence: 1,5-NDS < 1,6-NDS < 2,6-NDS ≈ 2,7-NDS < 2-NSA. The presence of dissolved salts was shown to slow down the decomposition rates. Results from flow-through autoclave experiments suggest that between 100 and 250°C, the stabilities of 2,6-NDS, 2,7-NDS, 1,5-NDS and 1,6-NDS are mainly controlled by solution pH, while at t ≥ 300°C, temperature is the main stability controlling factor. Additionally, no adsorption of NDS/NSA on the surface of minerals was observed. A new high-performance liquid chromatography (HPLC) method combined with solid-phase extraction (SPE) was developed to enable detection of NDS/NSA breakdown products at t ≥ 300°C. In hydrothermal solutions at temperatures greater than 300°C, all the naphthalene sulfonate isomers become unstable with the formation naphthalene (NAP) and the two naphthol isomers, 1-naphthol (1-NAP) and 2-naphthol (2-NAP), as confirmed by both the new HPLC/SPE method and GC-MS (gas chromatography–mass spectroscopy). In addition, 1-chloronaphthalene was also detected (using GC-MS) as a high temperature reaction product NDS/NSA disproportionation in 0.05 m NaCl solutions. The results of the experiments carried out during this thesis indicate that the stabilities the naphthalene mono- and disulfonates are a function of temperature, pH and salt concentration. The naphthalene sulfonates transform to different isomers and the kinetics of these isomerisation reactions have been determined. At temperatures ≥ 300°C, the NDS and NSA compounds disproportionate to the naphthalene “backbone” molecule as well as to the two stable naphthols and 1-chloronaphthalene (in chloride containing solutions). The application of naphthalene sulfonates to determine well connectivity and local permeabilities in active geothermal reservoirs is thus rather more complicated than previously appreciated. An understanding of the various isomer transformations and their kinetics is required. Furthermore, naphthalene sulfonates injected into high temperature geothermal reservoirs are unstable and breakdown to naphthalene, naphthols and probable halogenated naphthalene compounds, none of which have been considered in the interpretation of NDS/NSA recovery data in active geothermal reservoirs. The thermal stabilities of NAP, 1- and 2-NAP and 1-chloronaphthalene indicate that these compounds may also be employed as connectivity tracers in high temperature (t ≥ 300°C) systems.</p>

The Thermal Stability of the Naphthalene Sulfonic Acids Under Geothermal Conditions

<p>A primary goal of this thesis was to obtain kinetic data on the breakdown and isomerisation reactions of naphthalene disulfonate (NDS) and naphthalene sulfonate (NSA) compounds under geothermal conditions. A secondary aim of this study was to investigate NDS/NSA isomerisation transformations as well as to study their kinetics and identify products of thermal disproportionation. Because of their apparent thermal stability, naphthalene disulfonate solutions have been frequently injected into active geothermal reservoirs and their subsequent detection (“recovery”) in nearby wells/bore holes used as an indicator of well connectivity and local permeability. The results obtained in this thesis will enable a more insightful interpretation of field injection results and fluid flow in active geothermal reservoirs. The studies presented in this thesis were designed to determine the thermal stability of aqueous NDS and NSA at high temperatures from 100 to 400°C in pure water and different salt solutions (i.e. NaCl +/- Na2SO4 and Na2S) at saturated vapour pressure. The stabilities and isomerisation transformations of NDS and NSA were also studied in the presence of solid materials (i.e. quartz, greywacke, pumice) which may occur in the host geological environment of hydrothermal/geothermal reservoirs in the Earth’s crust. Dilute aqueous solutions of NDS and NSA were contained in sealed silica glass ampoules (purged of atmospheric oxygen) and placed in stainless steel pressure vessels and heated for varying times to the desired high temperatures. Additional experiments were also conducted in which dilute NDS and NSA solutions were pumped from a de-oxygenated reservoir container through a flow-through autoclave containing different rock and mineral phases at temperatures up 400°C. The resulting NDS and NSA isomers were then analysed using HPLC and GC-MS methodologies. The 1,5-naphthalene disulfonate isomer (1,5-NDS) was found to be the least stable at pHt = 3 - 8 and readily transformed to 1-naphthalene sulfonate (1-NSA) at t ≥ 200°C. The 2-NSA was found to be the most stable isomer but disappeared at t ≥ 300°. The experimental data indicated that the stabilities of all the NDS and NSA studied as a function of temperature, pH and salt (NaCl) concentration were in the sequence: 1,5-NDS < 1,6-NDS < 2,6-NDS ≈ 2,7-NDS < 2-NSA. The presence of dissolved salts was shown to slow down the decomposition rates. Results from flow-through autoclave experiments suggest that between 100 and 250°C, the stabilities of 2,6-NDS, 2,7-NDS, 1,5-NDS and 1,6-NDS are mainly controlled by solution pH, while at t ≥ 300°C, temperature is the main stability controlling factor. Additionally, no adsorption of NDS/NSA on the surface of minerals was observed. A new high-performance liquid chromatography (HPLC) method combined with solid-phase extraction (SPE) was developed to enable detection of NDS/NSA breakdown products at t ≥ 300°C. In hydrothermal solutions at temperatures greater than 300°C, all the naphthalene sulfonate isomers become unstable with the formation naphthalene (NAP) and the two naphthol isomers, 1-naphthol (1-NAP) and 2-naphthol (2-NAP), as confirmed by both the new HPLC/SPE method and GC-MS (gas chromatography–mass spectroscopy). In addition, 1-chloronaphthalene was also detected (using GC-MS) as a high temperature reaction product NDS/NSA disproportionation in 0.05 m NaCl solutions. The results of the experiments carried out during this thesis indicate that the stabilities the naphthalene mono- and disulfonates are a function of temperature, pH and salt concentration. The naphthalene sulfonates transform to different isomers and the kinetics of these isomerisation reactions have been determined. At temperatures ≥ 300°C, the NDS and NSA compounds disproportionate to the naphthalene “backbone” molecule as well as to the two stable naphthols and 1-chloronaphthalene (in chloride containing solutions). The application of naphthalene sulfonates to determine well connectivity and local permeabilities in active geothermal reservoirs is thus rather more complicated than previously appreciated. An understanding of the various isomer transformations and their kinetics is required. Furthermore, naphthalene sulfonates injected into high temperature geothermal reservoirs are unstable and breakdown to naphthalene, naphthols and probable halogenated naphthalene compounds, none of which have been considered in the interpretation of NDS/NSA recovery data in active geothermal reservoirs. The thermal stabilities of NAP, 1- and 2-NAP and 1-chloronaphthalene indicate that these compounds may also be employed as connectivity tracers in high temperature (t ≥ 300°C) systems.</p>

Physical Property Characterization of the Waipapa Greywacke: An Important Geothermal Reservoir Basement Rock in New Zealand

Greywacke basement rocks in New Zealand host conventional geothermal reservoirs and may supply important hotter and deeper geothermal energy resources in the future. This work combines petrological analyses and physical property measurements of Waipapa greywacke, a basement unit hosting New Zealand geothermal reservoirs, in order to understand better how structurally controlled flow networks develop and channel geothermal fluids within it. Results show intact Waipapa greywacke has high tensile and triaxial compressive strengths, and low intrinsic permeability (~10-21 m2). Permeability of intact Waipapa greywacke does not increase significantly during triaxial loading to failure and is accompanied by minimal changes ultrasonic wave velocities. These data taken together suggest that microcrack development during brittle deformation is very limited. Upon failure, the permeability increases by two orders of magnitude and shows similar permeability to tests performed on synthetic, single, mode I fractures in intact Waipapa greywacke. Permeability persists in Waipapa greywacke fractures under confining pressures of at least 150 MPa. It is concluded that Waipapa greywacke rocks will not allow fluid flow through the matrix of the rock and that substantial geothermal fluid flow will only occur through macrofracture networks.

Investigation of Water Composition on Formation Damage and Related Energy Recovery from Geothermal Reservoirs: Geochemical and Geomechanics Insights

The main challenge in extracting geothermal energy is to overcome issues relating to geothermal reservoirs such as the formation damage and formation fracturing. The objective of this study is to develop an integrated framework that considers the geochemical and geomechanics aspects of a reservoir and characterizes various formation damages such as impairment of formation porosity and permeability, hydraulic fracturing, lowering of formation breakdown pressure, and the associated heat recovery. In this research study, various shallow, deep and high temperature geothermal reservoirs with different formation water compositions were simulated to predict the severity/challenges during water injection in hot geothermal reservoirs. The developed model solves various geochemical reactions and processes that take place during water injection in geothermal reservoirs. The results obtained were then used to investigate the geomechanics aspect of cold-water injection. Our findings presented that the formation temperature, injected water temperature, the concentration of sulfate in the injected water, and its dilution have a noticeable impact on rock dissolution and precipitation. In addition, anhydrite precipitation has a controlling effect on permeability impairment in the investigated case study. It was observed that the dilution of water could decrease formation of scale while the injection of sulfate rich water could intensify scale precipitation. Thus, the reservoir permeability could decrease to a critical level, where the production of hot water reduces and the generation of geothermal energy no longer remains economical. It evident that injection of incompatible water would decrease the formation porosity. Thus, the geomechanics investigation was performed to determine the effect of porosity decrease. It was found that for the 50% porosity reduction case, the initial formation breakdown pressure reduced from 2588 psi to 2586 psi, and for the 75% porosity reduction case it decreased to 2584 psi. Thus, geochemical based formation damage is significant but geomechanics based formation fracturing is insignificant in the selected case study. We propose that water composition should be designed to minimize damage and that high water injection pressures in shallow reservoirs should be avoided.