scholarly journals Characterizing microseismicity at the Rotokawa and Ngatamariki geothermal fields

2021 ◽  
Author(s):  
◽  
Chet Hopp
Keyword(s):  
Matched Filter ◽  
Fluid Pressure ◽  
B Value ◽  
Injection Well ◽  
Taupo Volcanic Zone ◽  
Reservoir Properties ◽  
Injection Wells ◽  
Small Magnitude ◽  
Magnitude Distribution ◽  
Geothermal Fields

<p>In this thesis, we construct a four-year (2012–2015) catalog of microearthquakes for the Ngatamariki and Rotokawa geothermal fields in the Taupō Volcanic Zone of New Zealand, and use these data to improve the knowledge of reservoir behavior. These microearthquakes occur frequently, often every few seconds, and therefore provide a tool that we use to assess reservoir properties with dense spatial and temporal resolution as well as to illuminate the underlying processes of seismogenesis. Using a matched-filter detection technique we detect and precisely relocate nearly 9000 events, from which we calculate 982 focal mechanisms.  At Ngatamariki, these results constitute the first detailed analysis of seismicity at a newly-developed resource. It has been commonly assumed that induced shear on fractures increases reservoir permeability by offsetting asperities on either fracture wall, thereby propping the fracture open. During stimulation treatments of two boreholes (NM08 and NM09), borehole permeability experiences logarithmic growth. At NM08, this growth occurs for eight days in the absence of seismicity, while at NM09 only nine microearthquakes are observed during the one-month treatment. This suggests that hydro-shear, the process of inducing seismicity through increased pore pressure at critically-stressed fractures, is not the dominant mechanism of permeability increase at many geothermal wells. Instead, aseismic processes, likely thermal and overpressure induced fracture opening, dominate well stimulation in high-temperature geothermal settings.  At Rotokawa, the earthquake frequency-magnitude distribution (b-value) is positively correlated with both proximity to major injection wells and depth. In an inferred pressure compartment near injection well RK23, b is ~1.18, but is <1.0 elsewhere, suggesting a connection between increased pore-fluid pressure and small-magnitude events. In addition, throughout the reservoir b increases from a value of ~1.0 at injection depth to almost 1.5 two kilometers below the reservoir, consistent with observations at volcanic areas elsewhere, but opposing the conventional wisdom that b-value is inversely proportional to differential stress.  Finally, the 982 focal mechanism observations that we invert for stress show a normal faulting regime throughout both reservoirs. At Rotokawa, a lowering stress ratio, v, after reintroduction of injection well RK23 (v drops from 0.9 to 0.2 over six months) indicates that anisotropic reservoir cooling affects the reservoir stress state through a process of preferential stress reduction.</p>

scholarly journals Characterizing microseismicity at the Rotokawa and Ngatamariki geothermal fields

2021 ◽  
Author(s):  
◽  
Chet Hopp
Keyword(s):  
Matched Filter ◽  
Fluid Pressure ◽  
B Value ◽  
Injection Well ◽  
Taupo Volcanic Zone ◽  
Reservoir Properties ◽  
Injection Wells ◽  
Small Magnitude ◽  
Magnitude Distribution ◽  
Geothermal Fields

<p>In this thesis, we construct a four-year (2012–2015) catalog of microearthquakes for the Ngatamariki and Rotokawa geothermal fields in the Taupō Volcanic Zone of New Zealand, and use these data to improve the knowledge of reservoir behavior. These microearthquakes occur frequently, often every few seconds, and therefore provide a tool that we use to assess reservoir properties with dense spatial and temporal resolution as well as to illuminate the underlying processes of seismogenesis. Using a matched-filter detection technique we detect and precisely relocate nearly 9000 events, from which we calculate 982 focal mechanisms.  At Ngatamariki, these results constitute the first detailed analysis of seismicity at a newly-developed resource. It has been commonly assumed that induced shear on fractures increases reservoir permeability by offsetting asperities on either fracture wall, thereby propping the fracture open. During stimulation treatments of two boreholes (NM08 and NM09), borehole permeability experiences logarithmic growth. At NM08, this growth occurs for eight days in the absence of seismicity, while at NM09 only nine microearthquakes are observed during the one-month treatment. This suggests that hydro-shear, the process of inducing seismicity through increased pore pressure at critically-stressed fractures, is not the dominant mechanism of permeability increase at many geothermal wells. Instead, aseismic processes, likely thermal and overpressure induced fracture opening, dominate well stimulation in high-temperature geothermal settings.  At Rotokawa, the earthquake frequency-magnitude distribution (b-value) is positively correlated with both proximity to major injection wells and depth. In an inferred pressure compartment near injection well RK23, b is ~1.18, but is <1.0 elsewhere, suggesting a connection between increased pore-fluid pressure and small-magnitude events. In addition, throughout the reservoir b increases from a value of ~1.0 at injection depth to almost 1.5 two kilometers below the reservoir, consistent with observations at volcanic areas elsewhere, but opposing the conventional wisdom that b-value is inversely proportional to differential stress.  Finally, the 982 focal mechanism observations that we invert for stress show a normal faulting regime throughout both reservoirs. At Rotokawa, a lowering stress ratio, v, after reintroduction of injection well RK23 (v drops from 0.9 to 0.2 over six months) indicates that anisotropic reservoir cooling affects the reservoir stress state through a process of preferential stress reduction.</p>

scholarly journals Microseismicity during geothermal stimulation at the Ngatamariki geothermal field: New detections via a matched- filter method

2021 ◽  
Author(s):  
◽  
Gabriel Matson
Keyword(s):  
Thermal Stresses ◽  
Matched Filter ◽  
Time Lag ◽  
Fluid Pressure ◽  
Geothermal Field ◽  
Injection Well ◽  
Taupo Volcanic Zone ◽  
Double Difference ◽  
Travel Times ◽  
Microseismic Events

<p>The high-temperature, fluid-dominated Ngatamariki geothermal field is located in the central Taupo Volcanic Zone, North Island, New Zealand, and is used to generate electricity via an 82 MW power plant. Injection wells have been in operation since June 2012. During June and July 2012, injection well NM8 was injected with with cold water in order to improve reservoir permeability. Geothermal stimulation and production may trigger microearthquakes by fluid flow through the reservoir. Close clustering of microseismic events’ hypocentres relative to the source-receiver distance results in many events having similar waveforms. We capitalize on this relationship by using a matched-filter detection method in which high-quality seismograms corresponding to a well-recorded earthquake (“templates”) are cross-correlated against continuous data to reveal additional earthquakes with similar characteristics. Clustering of the detections’ hypocenters also implies that small variations in travel times between two events corresponds to small differences in hypocentral locations, which is the foundation of the double-difference relocation method.  Using an 11 station seismic network, we detect 863 events via cross-correlation of 110 matched-filter templates during the two months stimulation testing. We locate each of these detections using a double-difference relocation method by which events are relocated based on relative travel times. The locatable seismicity delineates: a northern Ngatamariki cluster, a southern Ngatamariki cluster, and a cluster to the south, at the neighboring Rotokawa field. Seismicity in the northern Ngatamariki cluster (522 events) is of greatest interest for this project due to its proximity to well NM8 and temporal signature relative to injection. The seismicity cluster centers around well NM8 at a depth of 2.1 km below sea level. Events in this cluster extend to up to 2.5 km from the injection well. An increase in seismicity near NM8 lags behind the onset of injection by 4–8 days. In contrast, a seismicity-rate decrease coincides with injection shut-in without any time lag. Local magnitudes in this cluster span the range −0.09 ≤ Ml ≤ 1.66 with a completeness magnitude of 0.25. Seismicity within 200 m of NM8 is induced by thermal stresses caused by the difference in temperature between the injectate and the reservoir. Seismicity further than 200 m, but still within this cluster, from NM8 is induced via pore fluid pressure increases from the injected fluid. The coupled mechanism acts on two different length scales and is known as a thermoporoelastic mechanism. The matched-filter detection of microseismic events allows interpretation of extent of injection well stimulation and the relationship between injection and seismicity.</p>

scholarly journals Microseismicity during geothermal stimulation at the Ngatamariki geothermal field: New detections via a matched- filter method

2021 ◽  
Author(s):  
◽  
Gabriel Matson
Keyword(s):  
Thermal Stresses ◽  
Matched Filter ◽  
Time Lag ◽  
Fluid Pressure ◽  
Geothermal Field ◽  
Injection Well ◽  
Taupo Volcanic Zone ◽  
Double Difference ◽  
Travel Times ◽  
Microseismic Events

<p>The high-temperature, fluid-dominated Ngatamariki geothermal field is located in the central Taupo Volcanic Zone, North Island, New Zealand, and is used to generate electricity via an 82 MW power plant. Injection wells have been in operation since June 2012. During June and July 2012, injection well NM8 was injected with with cold water in order to improve reservoir permeability. Geothermal stimulation and production may trigger microearthquakes by fluid flow through the reservoir. Close clustering of microseismic events’ hypocentres relative to the source-receiver distance results in many events having similar waveforms. We capitalize on this relationship by using a matched-filter detection method in which high-quality seismograms corresponding to a well-recorded earthquake (“templates”) are cross-correlated against continuous data to reveal additional earthquakes with similar characteristics. Clustering of the detections’ hypocenters also implies that small variations in travel times between two events corresponds to small differences in hypocentral locations, which is the foundation of the double-difference relocation method.  Using an 11 station seismic network, we detect 863 events via cross-correlation of 110 matched-filter templates during the two months stimulation testing. We locate each of these detections using a double-difference relocation method by which events are relocated based on relative travel times. The locatable seismicity delineates: a northern Ngatamariki cluster, a southern Ngatamariki cluster, and a cluster to the south, at the neighboring Rotokawa field. Seismicity in the northern Ngatamariki cluster (522 events) is of greatest interest for this project due to its proximity to well NM8 and temporal signature relative to injection. The seismicity cluster centers around well NM8 at a depth of 2.1 km below sea level. Events in this cluster extend to up to 2.5 km from the injection well. An increase in seismicity near NM8 lags behind the onset of injection by 4–8 days. In contrast, a seismicity-rate decrease coincides with injection shut-in without any time lag. Local magnitudes in this cluster span the range −0.09 ≤ Ml ≤ 1.66 with a completeness magnitude of 0.25. Seismicity within 200 m of NM8 is induced by thermal stresses caused by the difference in temperature between the injectate and the reservoir. Seismicity further than 200 m, but still within this cluster, from NM8 is induced via pore fluid pressure increases from the injected fluid. The coupled mechanism acts on two different length scales and is known as a thermoporoelastic mechanism. The matched-filter detection of microseismic events allows interpretation of extent of injection well stimulation and the relationship between injection and seismicity.</p>

scholarly journals Anisotropy as an indicator for reservoir changes: example from the Rotokawa and Ngatamariki geothermal fields, New Zealand

2019 ◽  
Vol 220 (1) ◽  
pp. 1-17 ◽  
Author(s):  
Stefan Mroczek ◽  
Martha K Savage ◽  
Chet Hopp ◽  
Steven M Sewell
Keyword(s):  
New Zealand ◽  
Shear Wave ◽  
Fluid Pressure ◽  
Geothermal Field ◽  
Taupo Volcanic Zone ◽  
Fracture Density ◽  
Splitting Technique ◽  
Geothermal Fluids ◽  
Geothermal Fields ◽  
Field Production

SUMMARY We investigate the relation between geothermal field production and fracture density and orientation in the Ngatamariki and Rotokawa geothermal fields, located in the Taupo Volcanic Zone, New Zealand using shear wave splitting (SWS). We determine the SWS parameters for 17 702 microseismic events across 38 stations spanning close to 4 yr from 2012 to 2015. We compare the strength of anisotropy to changes in field production and injection. We also compare the orientation of the anisotropy to in situ and regional measurements of maximum horizontal stress orientation. ($S_{\mathrm{ H}_{\mathrm{ max}}}$). Due to the volume of unique events (approximately 160 000), shear wave phases are picked automatically. We carry out automatic SWS measurements using the Multiple Filter Automatic Splitting Technique (MFAST). The SWS measurements are interpreted in the context of stress aligned microcracks. Outside both fields and within Ngatamariki, fast polarizations align with the NE–SW regional orientation of $S_{\mathrm{ H}_{\max}}$. Within Rotokawa a greater complexity is observed, with polarizations tending toward N–S. We observe increases in per cent anisotropy coinciding with the start of production/injection in Ngatamariki and then a later correlated drop in per cent anisotropy and vP/vS ratios in southern Ngatamariki as injection is shifted to the north. This relationship is consistent with pore fluid pressure within the reservoir being affected by local changes in production and injection of geothermal fluids causing cracks to open and close in response.

A New Method of Fall-Off Test in Water Injection Well in Tahe Oilfield

2013 ◽  
Vol 807-809 ◽  
pp. 2508-2513
Author(s):  
Qiang Wang ◽  
Wan Long Huang ◽  
Hai Min Xu
Keyword(s):  
Water Injection ◽  
Injection Well ◽  
Well Test ◽  
Well Bore ◽  
Residual Oil ◽  
Injection Wells ◽  
Oil Saturation ◽  
Tahe Oilfield ◽  
Fitting Method ◽  
Front Radius

In pressure drop well test of the clasolite water injection well of Tahe oilfield, through nonlinear automatic fitting method in the multi-complex reservoir mode for water injection wells, we got layer permeability, skin factor, well bore storage coefficient and flood front radius, and then we calculated the residual oil saturation distribution. Through the examples of the four wells of Tahe oilfield analyzed by our software, we found that the method is one of the most powerful analysis tools.

The New Generation of Outflow Control Devices Autonomously Controlling the Conformance of Water Injection Well- A Case Study with ADNOC Onshore

2021 ◽  
Author(s):  
Sultan Ibrahim Al Shemaili ◽  
Ahmed Mohamed Fawzy ◽  
Elamari Assreti ◽  
Mohamed El Maghraby ◽  
Mojtaba Moradi ◽  
...  
Keyword(s):  
Oil Recovery ◽  
Dynamic Properties ◽  
Short Circuit ◽  
Water Injection ◽  
Performance Outcomes ◽  
Injection Well ◽  
Well Performance ◽  
Injection Wells ◽  
Horizontal Section ◽  
Control Devices

Abstract Several techniques have been applied to improve the water conformance of injection wells to eventually improve field oil recovery. Standalone Passive flow control devices or these devices combined with Sliding sleeves have been successful to improve the conformance in the wells, however, they may fail to provide the required performance in the reservoirs with complex/dynamic properties including propagating/dilating fractures or faults and may also require intervention. This is mainly because the continuously increasing contrast in the injectivity of a section with the feature compared to the rest of the well causes diverting a great portion of the injected fluid into the thief zone which ultimately creates short-circuit to the nearby producer wells. The new autonomous injection device overcomes this issue by selectively choking the injection of fluid into the growing fractures crossing the well. Once a predefined upper flowrate limit is reached at the zone, the valves autonomously close. Well A has been injecting water into reservoir B for several years. It has been recognised from the surveys that the well passes through two major faults and the other two features/fractures with huge uncertainty around their properties. The use of the autonomous valve was considered the best solution to control the water conformance in this well. The device initially operates as a normal passive outflow control valve, and if the injected flowrate flowing through the valve exceeds a designed limit, the device will automatically shut off. This provides the advantage of controlling the faults and fractures in case they were highly conductive as compared to other sections of the well and also once these zones are closed, the device enables the fluid to be distributed to other sections of the well, thereby improving the overall injection conformance. A comprehensive study was performed to change the existing dual completion to a single completion and determine the optimum completion design for delivering the targeted rate for the well while taking into account the huge uncertainty around the faults and features properties. The retrofitted completion including 9 joints with Autonomous valves and 5 joints with Bypass ICD valves were installed in the horizontal section of the well in six compartments separated with five swell packers. The completion was installed in mid-2020 and the well has been on the injection since September 2020. The well performance outcomes show that new completion has successfully delivered the target rate. Also, the data from a PLT survey performed in Feb 2021 shows that the valves have successfully minimised the outflow toward the faults and fractures. This allows achieving the optimised well performance autonomously as the impacts of thief zones on the injected fluid conformance is mitigated and a balanced-prescribed injection distribution is maintained. This paper presents the results from one of the early installations of the valves in a water injection well in the Middle East for ADNOC onshore. The paper discusses the applied completion design workflow as well as some field performance and PLT data.

Successful Development and Deployment of a Novel Chemical Package for Stimulation of Injection wells, Offshore UK

2021 ◽  
Author(s):  
Dennis Alexis ◽  
Gayani Pinnawala ◽  
Do Hoon Kim ◽  
Varadarajan Dwarakanath ◽  
Ruth Hahn ◽  
...  
Keyword(s):  
Product Quality ◽  
Injection Well ◽  
Concentrated Solutions ◽  
Successful Development ◽  
Injection Wells ◽  
Polymer Injection ◽  
Remediation Process ◽  
Field Location ◽  
Stimulation Of ◽  
Injection Rates

Abstract The work described in this paper details the development of a single stimulation package that was successfully used for treating an offshore horizontal polymer injection well to improve near wellbore injectivity in the Captain field, offshore UK. The practice was to pump these concentrated surfactant streams using multiple pumps from a stimulation vessel which is diluted with the polymer injection stream in the platform to be injected downhole. The operational challenges were maintaining steady injection rates of the different liquid streams which was exacerbated by the viscous nature of the concentrated surfactants that would require pre-dilution using cosolvent or heating the concentrated solutions before pumping to make them flowable. We have developed a single, concentrated liquid blend of surfactant, polymer and cosolvent that was used in near-wellbore remediation. This approach significantly simplifies the chemical remediation process in the field while also ensuring consistent product quality and efficiency. The developed single package is multiphase, multicomponent in nature that can be readily pumped. This blend was formulated based on the previous stimulation experience where concentrated surfactant packages were confirmed to work. Commercial blending of the single package was carried out based on lab scale to yard scale blending and dilution studies. About 420 MT of the blend was manufactured, stored, and transported by rail, road and offshore stimulation vessel to the field location and successfully injected.

scholarly journals Capillary processes increase salt precipitation during CO2 injection in saline formations

2018 ◽  
Vol 852 ◽  
pp. 398-421
Author(s):  
Helena L. Kelly ◽  
Simon A. Mathias
Keyword(s):  
High Salinity ◽  
Volume Fraction ◽  
Injection Rate ◽  
Injection Well ◽  
Co2 Injection ◽  
Salt Precipitation ◽  
Injection Wells ◽  
Tenfold Increase ◽  
Entry Pressure ◽  
Injection Rates

An important attraction of saline formations for CO2 storage is that their high salinity renders their associated brine unlikely to be identified as a potential water resource in the future. However, high salinity can lead to dissolved salt precipitating around injection wells, resulting in loss of injectivity and well deterioration. Earlier numerical simulations have revealed that salt precipitation becomes more problematic at lower injection rates. This article presents a new similarity solution, which is used to study the relationship between capillary pressure and salt precipitation around CO2 injection wells in saline formations. Mathematical analysis reveals that the process is strongly controlled by a dimensionless capillary number, which represents the ratio of the CO2 injection rate to the product of the CO2 mobility and air-entry pressure of the porous medium. Low injection rates lead to low capillary numbers, which in turn are found to lead to large volume fractions of precipitated salt around the injection well. For one example studied, reducing the CO2 injection rate by 94 % led to a tenfold increase in the volume fraction of precipitated salt around the injection well.

A Retrospective Analysis of b-Value Changes Preceding Strong Earthquakes

2021 ◽  
Author(s):  
Nicolas D. DeSalvio ◽  
Maxwell L. Rudolph
Keyword(s):  
B Value ◽  
Earthquake Precursors ◽  
Earthquake Catalog ◽  
Traffic Light ◽  
Magnitude Distribution ◽  
Parameter Combination ◽  
Definition Of ◽  
Value Changes ◽  
Frequency Magnitude Distribution ◽  
Optimal Set

Abstract Earthquake precursors have long been sought as a means to predict earthquakes with very limited success. Recently, it has been suggested that a decrease in the Gutenberg–Richter b-value after a magnitude 6 earthquake is predictive of an imminent mainshock of larger magnitude, and a three-level traffic-light system has been proposed. However, this method is dependent on parameters that must be chosen by an expert. We systematically explore the parameter space to find an optimal set of parameters based on the Matthews correlation coefficient. For each parameter combination, we analyze the temporal changes in the frequency–magnitude distribution for every M ≥ 6 earthquake sequence in the U.S. Geological Survey Comprehensive Earthquake Catalog for western North America. We then consider smaller events, those with a foreshock magnitude as small as 5, and repeat the analysis to assess its performance for events that modify stresses over smaller spatial regions. We analyze 25 M ≥ 6 events and 88 M 5–6 events. We find that no perfect parameter combination exists. Although the method generates correct retrodictions for some M 5 events, the predictions are dependent on the retrospectively selected parameters. About 80%–95% of magnitude 5–6 events have too little data to generate a result. Predictions are time dependent and have large uncertainties. Without a precise definition of precursory b-value changes, this and similar prediction schemes are incompatible with the IASPEI criteria for evaluating earthquake precursors. If limitations on measuring precursory changes in seismicity and relating them to the state of stress in the crust can be overcome, real-time forecasting of mainshocks could reduce the loss of lives.

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