Aquifer Pressure Research Articles

Inferring Fault Frictional and Reservoir Hydraulic Properties From Injection‐Induced Seismicity

AbstractCharacterizing the rheological properties of faults and the evolution of fault friction during seismic slip are fundamental problems in geology and seismology. Recent increases in the frequency of induced earthquakes have intensified the need for robust methods to estimate fault properties. Here we present a novel approach for estimation of aquifer and fault properties, which combines coupled multiphysics simulation of injection‐induced seismicity with adaptive surrogate‐based Bayesian inversion. In a synthetic 2‐D model, we use aquifer pressure, ground displacements, and fault slip measurements during fluid injection to estimate the dynamic fault friction, the critical slip distance, and the aquifer permeability. Our forward model allows us to observe nonmonotonic evolutions of shear traction and slip on the fault resulting from the interplay of several physical mechanisms, including injection‐induced aquifer expansion, stress transfer along the fault, and slip‐induced stress relaxation. This interplay provides the basis for a successful joint inversion of induced seismicity, yielding well‐informed Bayesian posterior distributions of dynamic friction and critical slip. We uncover an inverse relationship between dynamic friction and critical slip distance, which is in agreement with the small dynamic friction and large critical slip reported during seismicity on mature faults.

Ground deformation source model at Kuchinoerabu-jima volcano during 2006\u20132014 as revealed by campaign GPS observation

We analyzed campaign Global Positioning System observation data in Kuchinoerabu-jima during 2006–2014. Most benchmarks located around Shin-dake crater showed crater-centered radial horizontal displacements. Horizontal displacements at western rim of the Shin-dake crater were tended to be larger compared to those at eastern rim. In addition, benchmark KUC14 which locates near the cliff at Furu-dake showed westward horizontal displacement rather than crater-centered radial (southward) one. Meanwhile, small displacements were detected at the benchmarks located at the foot of Kuchinoerabu-jima. We modeled the observed displacements applying a finite element method. We set entire FE domain as 100 × 100 × 50 km3. We set top of the domain as a free surface, and sides and bottom to be fixed boundaries. Topography was introduced in the area within Kuchinoerabu-jima using digital elevation model data provided by Kagoshima prefecture and elevation information from Google earth, and elevation of the outside area was assumed to be sea level. We assumed a stratified structure based on a one-dimensional P-wave velocity structure. We applied a vertical spheroid source model and searched optimal values of horizontal location, depth, equatorial and polar radiuses, and internal pressure change of the source using the forward modeling method. A spherical source with a radius of 50 m was obtained beneath the Shin-dake crater at a depth of 400 m above sea level. The internal pressure increase of 361 MPa yields its volume increase of 31,700 m3. Taking effects of topography and heterogeneity of ground into account allowed reproduction of overall deformation in Kuchinoerabu-jima. The location of deformation source coincides with hypocenters of shallow volcano-tectonic (VT) earthquakes and the aquifer estimated from a two-dimensional resistivity model by audio-frequency magnetotellurics method. The obtained deformation source may be corresponding to the pressurized aquifer, and shallow VT earthquakes and demagnetization may be caused by pressure and strain accumulation in the rocks around the aquifer. Applying the obtained spherical source to the tilt change before August 3, 2014 eruption, we found that 520 m3 of volcanic materials were supplied toward shallower in 1.5 h before the eruption. The depth and volume change of deformation source before May 2015 eruption detected by precise leveling surveys is deeper and two orders of magnitude greater compared to that before August 2014 eruption.

Development of a new model tool for evaluating groundwater resources within the Floridan Aquifer System in Southern Florida, USA

The US Army Corps of Engineers (USACE) and the South Florida Water Management District (SFWMD) are partners in an ambitious plan to restore water flows throughout the Everglades ecosystem. An important component of the restoration plan involves storing excess stormwater deep underground in the Floridan Aquifer System using aquifer, storage and recovery (ASR) wells. In order to determine the optimal ASR system and to assess environmental impacts, USACE spent over 11 years and significant resources to develop a three-dimensional groundwater model of the Floridan Aquifer System covering a large portion of the Florida peninsula. This SEAWAT model is capable of evaluating changes in aquifer pressures and density-dependent flows in the entire study area. The model has been used to evaluate the Everglades ASR system already but could also be used by water managers for other important water resources studies in Florida including water supply estimates and adaptation to climate change. As part of an effort to make the model more readily available for other important studies, this study documents and summarizes the overall development of the SEAWAT model including a discussion regarding the intensive calibration and validation efforts undertaken during model development. The paper then demonstrates the use of the model using Everglades ASR project alternatives. Lastly, the paper outlines potential future uses of the model along with its overall limitations. Supplementary online resources are also included that provide researchers with further detail regarding the model development effort beyond the scope of this summary article as well as model development databases.

Protection of groundwater dependent ecosystems in Canterbury, New Zealand: the Targeted Stream Augmentation Project

In Canterbury, New Zealand, native aquatic species habitat has significantly declined over the last century due primarily to land use change and climatic influences. The Canterbury Water Management Strategy (CWMS) aims to improve such habitat while also meeting other economic, environmental, social and cultural objectives. The focus of this paper will be on the Targeted Stream Augmentation (TSA) Project in the Selwyn-Waihora Zone in Canterbury. The semi-arid climate, significant demand for irrigation water and the up-gradient aquifer pressure provided by a coastal lake provide significant aquifer management challenges in this zone. In response to these challenges the local CWMS committee recommended that managed aquifer recharge trials to improve groundwater dependent ecosystems start in a highly targeted manner before gradually increasing the distance from recharge site to the target spring/s. A highly targeted pilot trial has shown the flow, temperature and nutrient concentration benefits for the target surface ecosystem. The implementation of two larger scale concepts combining surface and groundwater ecosystem benefits is now progressing. Lessons learned to date include the importance of keeping engagement processes ahead of technical assessments, and the benefits of high level scoping assessments before committing to detailed investigations and pilot projects.

An Approach to Estimation of Gas Reserve of Narshingdi Gas Field in Bangladesh with Dynamic Reservoir Simulation

Increasing demand of fuel globally formulates gas as one of the most valuable natural resources. There is lot of uncertainties in estimating hydrocarbon volume correctly from exploration to development stage of a gas field. The accuracy and reliability of data (reservoir geological model, fluid and rock properties) make the implement very hard-hitting. So estimating and updating the gas reserve has become vital issue, as it helps the planners for drawing mid-term and long-term development plan from field development level to national level. This paper presents the study of reserve estimation of a Narshingdi Gas Field in Bangladesh. In this paper, a dynamic reservoir simulation model has been used to perform a history match ?pressure and production? using commercial simulator for reserve estimation. The result of this study is expected to provide Gas Initially in Place (GIIP) and recoverable gas volume. Simultaneously three forecast scenarios have also been investigated. There is no strong aquifer pressure support in the producing gas zone, so gas production continues from the reservoir due to pressure depletion.Journal of Chemical Engineering, Vol. 29, No. 1, 2017: 19-23

A method of the determination of hydrogeological parameters of the aquifers on the results of the experimental constant-rate pumping tests

On the basis of the approximate solution of the problem of groundwater filtration during the experimental constant-rate pumping tests of the pressure aquifers, a method for the processing and interpreting the results of this operation has been developed in order to determine the filtration-volumetric parameters of the horizon. The method is combined as much as possible with the currently widely used graphic-analytical method. The proposed method has been shown to have substantially less restrictions on the duration of the experimental perturbation (pumping) than the currently used graphic-analytical one.

Pressure Management of CO2 Storage by Closed-loop Surface Dissolution

The closed-loop version of surface dissolution strategy is one of the safest techniques for CO2 storage as it greatly reduces the risks associated with buoyant CO2 leakage and aquifer pressurization, and eliminates the problem of brine disposal. However, large injection rates are crucial for the feasibility of closed-loop surface dissolution, and these rates could be constrained by the operation strategies and the buildup of average reservoir pressure. In this work, we compare two operating strategies to attain the optimal safe storage rates and find that the optimal operation is the multiple-rates injection. This provides the maximum injection rates and smallest well counts needed to satisfy a given storage target. The impact of elevated average reservoir pressure under infinite-acting boundary condition is quantified and evaluated for injection operation up to 30 years.

Perturbation with a constant drawdown of the underground water level in the perturbing well

On the basis of the method of integral relationships, an approximate solution of the problem of groundwater filtration in pressure aquifers has been obtained when they are tested with pumpings. The solution obtained is applicable for moments of time less than two orders than the approximate representation of the exact solution on which the existing technique for interpreting the results of the experimental filtration testing of horizons is based.

Subglacial blowouts in western Canada: insights into extreme meltwater pressures and hydrofracturing

This study interprets deformations of indurated 10s of metres thick bedrock strata by subglacial meltwater pressures at maximum levels, resulting in types of hydrodynamic structures not previously recognized. The structures provide insights into the range of extreme pressures possible with the backup of meltwaters sufficient to deform multi‐metre thick indurated beds, unlike elsewhere. Subglacial meltwater flows into the subsurface below the 50–100 m thick bitumen platform aquiclude of the Cretaceous Athabasca Oil Sands deposit were driven by the hydraulic head of the 1.5‐km‐thick Laurentide Ice Sheet. These meltwaters over‐pressured the regional Devonian aquifer waters, but the low permeability of the aquifer below the site of the Muskeg River Mine was insufficient to accommodate the voluminous influxes of subglacial meltwater. The resulting meltwater flows backed‐up, resulting in elevating aquifer pressures to maximum levels along the margins of the underlying Devonian Keg River reef mound and within water‐saturated Cretaceous sand beds offset to the east. The meltwater pressure build to extreme levels hydrofractured strata at sites along the margins of the Keg River mound preconditioned by dissolution‐induced subsidence in underlying salt beds. Release of the confining pressure upon withdrawal of the Laurentide Ice Sheet resulted in 45‐m‐high open blowout structures that punctured the bitumen platform at sites above the northern margin of the Keg River mound. Other blowout chimney sites above the southern margin of the mound were plugged because of insufficient pressure build to clear vents of all ejecta. Concurrently, pressured meltwater flows along the eastern margin resulted in dykes that cross‐cut and buckled Devonian limestone beds. Some dykes extended into overlying water‐saturated unconsolidated Cretaceous sand beds, channelling pressured water and hydroplastic mudflows that dissipated the extreme meltwater flow pressures and prevented puncturing of the overlying bitumen platform. These unusual deformation structures in western Canada resulted from catastrophic failure of hydrofractured rock zones responding to extreme meltwater pressures, in contrast to unconsolidated sediment deformations resulting from only moderately elevated meltwater pressures commonly observed elsewhere.

An Improved Vulnerability Assessment Model for Floor Water Bursting from a Confined Aquifer Based on the Water Inrush Coefficient Method

Pressurized confined water below coal seams are serious threats to mining. The conventional water inrush coefficient method fails to accurately assess the risk of floor water inrush under some specific conditions, such as high water pressure and low water yield in the source aquifers. Large amounts of water inrush data including water inrush flow rate, water inrush coefficient (T s ), floor aquiclude thickness (M), and water abundance, were collected and statistically analyzed. The results indicated that inrushes mostly occurred when M was less than 30 m and that the critical T s increased linearly with M. The occurrence of a water inrush and water inrush yield amount (Q in L/s) were related to both the values of T s and the unit water inflow (q in L/(s m)). In addition, 97.7% of the large- and medium-sized inrush events occurred when q > 2 L/(s m) and only a small proportion (3.2%) of the small-sized inrushes happened when q < 0.1 L (s m). T s , M and q were comprehensively analyzed and used to evaluate vulnerability to floor water inrush. By analyzing the distribution of water inrush points and the scale of water inrush events, the vulnerability was divided into four levels (safe, moderately safe, potentially dangerous, and highly risky) based on T s –M and T s –q models. Successful application of these models in the Huaibei mining area proved that they are feasible in practice. The T s –M and T s –q charts can be used independently or jointly. These new methods should improve the accuracy of predictions and evaluations during deep exploitation where the aquifers are often characterized with high pressure but low water abundance. The results could also help reduce the amount spent on mine water prevention and control.

Simulation of the mulltizones clastic reservoir: A case study of Upper Qishn Clastic Member, Masila Basin–Yemen

The Upper Qishn Clastic Member is one of the main oil-bearing reservoirs that are located at Masila Basin–Yemen. It produces oil from many zones with different reservoir properties. The aim of this study is to simulate and model the Qishn sandstone reservoir to provide more understanding of its properties. The available, core plugs, petrophysical, PVT, pressure and production datasets, as well as the seismic structural and geologic information, are all integrated and used in the simulation process. Eclipse simulator was used as a powerful tool for reservoir modeling. A simplified approach based on a pseudo steady-state productivity index and a material balance relationship between the aquifer pressure and the cumulative influx, is applied.The petrophysical properties of the Qishn sandstone reservoir are mainly investigated based on the well logging and core plug analyses. Three reservoir zones of good hydrocarbon potentiality are indicated and named from above to below as S1A, S1C and S2. Among of these zones, the S1A zone attains the best petrophysical and reservoir quality properties. It has an average hydrocarbon saturation of more than 65%, high effective porosity up to 20% and good permeability record (66 mD). The reservoir structure is represented by faulted anticline at the middle of the study with a down going decrease in geometry from S1A zone to S2 zone. It is limited by NE-SW and E-W bounding faults, with a weak aquifer connection from the east.The analysis of pressure and PVT data has revealed that the reservoir fluid type is dead oil with very low gas liquid ratio (GLR). The simulation results indicate heterogeneous reservoir associated with weak aquifer, supported by high initial water saturation and high water cut. Initial oil in place is estimated to be around 628 MM BBL, however, the oil recovery during the period of production is very low (<10%) because of the high water cut due to the fractures associated with many faults. Hence, secondary and tertiary methods are needed to enhance the oil recovery. Water flooding is recommended as the first step of oil recovery enhancement by changing some of high water cut wells to injectors.

Design and Testing of Recharge Wells in a Coastal Aquifer: Summary of Field Scale Pilot Tests

Surplus water from seawater desalination plants along the Israeli Coast can be injected underground for seasonal storage. Two pilot projects were established to simulate the movement of air bubbles and changes in the well hydraulic parameters during pumping and recharging. The study showed that it is impossible to remove the smaller air bubbles (dissolved air) that are created during the injection process, even when the injection pipe is fully saturated. The pumping tests showed that there were large differences in the well hydraulic parameters between the pumping and the recharge tests despite that they were conducted at the same well. Two mechanisms are responsible for the reduction in the aquifer coefficients during the recharge event. The first mechanism is the pressures that the injected water needs to overcome; the aquifer pressure and the pore water pressure it is supposed to replace at the time of the injection. The second mechanism is the pressure that the injected water needs to overcome the clogging process. It is expressed as the high water level inside the recharge well in comparison to the small rising of the water level in the observation wells. This research gives good insight into the injection mechanism through wells and is essential for any further development of injection facilities and for the operation and management protocols.

Critical Break Length of Floor Aquiclude During Longwall Mining Above Confined Aquifer

Floor water inrushes not only cause heavy casualties and property losses, but seriously restrict the production of coal mines. The aquiclude was simplified as the fixed - fixed beam under the uniform load, and critical break length formula of floor aquiclude in three failure modes including bending failure, shear failure and buckling of pressure bar were established. Then the critical lengths of aquiclude in three failure modes were calculated according to the geological and mining conditions of 15031 panel in Jiulishan coalmine. The results show that the critical break length in bending failure is the smallest, while the critical length in shear failure mode is the biggest. And failures of fixed – fixed beam in three failure modes all occur at two ends so that water inrush from floor may occur at open-off cut or working face. Additionally, three critical break lengths increase with aquiclude thickness but decrease with confined aquifer pressure. According to the calculated results, water inrush may occur at the working face or open-off cut when mining advance reaches 12.47 m, and thus three control measures of lowering aquifer pressure, increasing aquiclude thickness and shortening the distances of first and interval weighting of main roof were discussed. The conclusions can provide references to control water inrush from floor aquifer in coal mines.

Modeling the Movement of Groundwater from the Pits, Surrounded with Tongues of Zhukovsky

In the hydrodynamic statement the filtration low from ditches, walled tongues of Zhukovsky is considered. The fluid moves through the layer of soil, underlain by a well-permeable pressure aquifer, which is contained waterproof area on the roof. For study the infiltration to the free surface of groundwater is formulated a mixed multi-parameter boundary value problem of the theory of analytic function, which is solved by the Polubarinova-Kochina's method and ways the conformal mapping areas of a special kind, which are characteristic for tasks of an underground hydromechanics.

Modeling the Movement of Groundwater from the Pits, Surrounded with Piles of Zhukovsky

In the hydrodynamic statement the filtration low from ditches, walled tongues of Zhukovsky is considered. The fluid moves through the layer of soil, underlain by a well-permeable pressure aquifer, which is contained waterproof area on the roof. For study the infiltration to the free surface of groundwater is formulated a mixed multi-parameter boundary value problem of the theory of analytic function, which is solved by the Polubarinova-Kochina's method and ways the conformal mapping areas of a special kind, which are characteristic for tasks of an underground hydromechanics.

Hydrogeological protection of the buchak-kaniv aquifer complex in the central part of Dnipro-Donetsk artesian basin from possible technological contamination

Formulation of the problem. Every year, the problem of studying the ecological state of drinking groundwater becomes more and more acute, as the reserves of ecologically clean groundwater is becoming more and more depleted. At the same time, at the time of significant influence of technogenic factors on the quality of drinking groundwater, there is an urgent need for a more detailed assessment of the protection index of aquifer complexes promising for water use purposes. The purpose of the article. The main purpose of the research is to show the degree of modern hydrogeological protection of one of the strategic reserves of drinking underground water in the region – Buchak-Kaniv aquifer complex – in the area of its greatest distribution and use – the central part of Dnipro-Donetsk artesian basin from possible contamination of water that lie above. Corresponding trends in protection changes during the long period of time (55 years) within the territory of work were also traced. Methods. Taking into account the hydrogeological and lithological features of aquifer complexes and waterproof rocks in the interior of the subsoil, as well as on the basis of previous research by scientists, a multi-factor, adapted to these conditions, methodology for carrying out the research was developed. The methodology includes the definition of both static and dynamic components of the protection index. Calculations and graphic constructions were made for 2 periods of time – 1960 and 2015. Results. The peculiarities of the hydrogeological protection of the aquifer complex within the territory of works during these periods were analyzed and relevant trends in changes in the indicator over the 55-year period have been traced. It has been determined that the territories of areas with unprotected and conditionally protected underground water expanded, and protected areas – on the contrary, decreased. The largest changes occurred in the areas affected by water intakes of large cities in the region. The reasons for these processes are presented, the main of which is an anthropogenic factor. Scientific novelty and practical significance. This methodology in assessing the protection in this region has been used for the first time. The obtained results of the research should be used to develop recommendations for the ecological protection of drinking groundwater in the region. This method of work can be used to determine a hydrogeological protection index of other pressure aquifer complexes in similar conditions.

The impact of reactive surface area on brine-rock-carbon dioxide reactions in CO2 sequestration

Injection of carbon dioxide into saline aquifers is one the most promising methods for mitigating the increase in atmospheric CO2 concentrations. Reactions of CO2 with brine and formation rocks are important in determining the ultimate fate of CO2. Rates of reactions of the different relevant minerals are dependent on rate constants and mineral surface area. It is important to quantify the effect of surface area on reaction rates when different types of rock samples are employed in experiments. Batch experiments with different forms of the Berea sandstone were set up and performed at high pressure (2000 psi) and reservoir temperature (60°C) for two-week under reservoir CO2 sequestration conditions. Experiments were performed with markedly different samples (cores, rock chips, and powdered) with progressively increased surface areas. Experiments with the different forms of sandstones showed similar trends in major minerals dissolution with increase in cation concentrations of iron, magnesium, and calcium in the effluent brine. Under realistic aquifer pressure and temperature conditions, iron chemistry plays an important role in the dissolution reactions in the Berea sandstone. Reactivities increased leading to larger cationic concentrations in brine (as determined using ICP-MS) as the surface areas increased. Morphology of the reacted volume was viewed using QEMSCAN and Micro-CT for core plug samples. These experiments helped quantity the effect of surface area (higher surface area leading to more dissolution) and revealed that reactions appear to be limited more to the surface in Berea sandstone. It was shown that the factor used to obtain geometric surface area from a core decreased when samples with higher surface areas were used. This indicated that some internal surface area became available for fragmented (fractured), and powdered samples. Mineral dissolution caused the growth and expansion of pores in Berea sandstone and surface area measurements (BET) showed that the new porosity generated was characterized by a smaller pore size distribution in comparison to the unreacted one.

Assessing aquitard hydraulic performance from hydrocarbon migration indicators: Surat and Bowen basins, Australia

Basin hydrodynamic assessment methods are being applied to Australia's developing coal seam gas (CSG) to liquefied natural gas (LNG) export developments in the Queensland's Bowen and Surat basins. Technical challenges include the need to predict the cumulative impacts of CSG production on formation pressure in adjacent Great Artesian Basin aquifers and assess to what degree, if any, there may be fugitive methane liberated from the reservoir but not captured by production wells. Both these challenges require an understanding of the distribution of aquitard hydraulic sealing performance. We use hydrocarbon systems analysis to illuminate basin scale geological seal performance of the low permeability strata and provide baseline data on hydrocarbons in groundwater.We collected indicators of hydrocarbons including: 1) drill stem, wireline and production test sample analysis, 2) staining, fluorescence, and streaming hydrocarbon occurrences from drill cuttings and core, and 3) mud log gas detector data. This data was allocated to stratigraphic horizons, mapped and compared to the geographic/stratigraphic location of source rocks, conventional and unconventional reservoired hydrocarbons, and aquitard/sealing strata and faults.We found asymmetric trapping of conventional hydrocarbon reservoirs with eastern basin leakage and western basin preservation. Indications of some thermogenic gas in the Walloon Coal Measures exist yet maturity indicators are sub-thermogenic, suggesting that although much of the Walloon Coal Measures gas is biogenic, thermogenic gas may have previously migrated into the Walloon Coal Measures from elsewhere. Stratigraphically below the Walloon Coal Measures, hydrocarbon migration indicators are often clustered along lineations such as the Moonie-Goondiwindi and Burunga-Leichhardt fault systems. At the top of the Walloon Coal Measures, hydrocarbon migration indicators are clustered along the northeastern subcrop edge of the basin where the muddy strata at the top are either thin or eroded suggesting vertical leakage occurred to the overlying Springbok Sandstone aquifer. Whilst we can see where seals are either more, or less effective, at this stage we cannot make a quantitative assessment of background methane flux to surface or potential future proportional allocation of methane flux to natural vs anthropogenic causes.

Implications of the change in confinement status of a heterogeneous aquifer for scale-dependent dispersion and mass-transfer processes

A series of experimental tracer tests were performed to explore the implications of the change in the pressure status of a heterogeneous bimodal aquifer for scale-dependent dispersion and mass-transfer processes. The sandbox was filled with sands and gravel channels and patches to form an alluvial-like bimodal aquifer. We performed multiple injections of a conservative tracer from 26 different locations of the sandbox and interpreted the resulting depth-integrated breakthrough curves (BTCs) at the central pumping well to obtain a scale-dependent distribution of local and field-integrated apparent longitudinal dispersivity (respectively, αLloc and αLapp). We repeated the experiments under confined (CS) and unconfined (UNS) pressure status, keeping the same heterogeneous configuration. Results showed that αLloc(associated with transport through gravel zones) was poorly influenced by the change in aquifer pressure and the presence of channels. Instead, αLapp(i.e. macrodispersion) strongly increased when changing from CS to UNS. In specific, we found αLapp≈0.03r for the CS and αLapp≈0.15r for the UNS (being r the distance from the well). Second-to-fourth-order temporal moments showed strong spatial dependence in the UNS and no spatial dependence in the CS. These results seem consistent with a “vadose-zone-driven” kinetic mass-transfer process occurring in the UNS but not in the CS. The vadose zone enhances vertical flow due to the presence of free surface and large contrasts in hydraulic conductivity triggered by the desaturation of gravel channels nearby the pumping well. The vadose zone enhances vertical mixing between gravel and sands and generates BTC tailing. In the CS vertical mixing is negligible and anomalous transport is not observed.

Numerical modelling of Pohutu geyser, Rotorua, New Zealand

The Rotorua geothermal field (RGF) in the North Island of New Zealand is renowned for an abundance of natural geothermal manifestations and contains one of New Zealand’s last remaining areas of major geyser activity at Whakarewarewa. Close proximity of the geothermal resource to a population centre and ease of access for end-users resulted in intensive drilling and fluid abstraction from shallow bores for domestic and commercial usage from the 1950s onwards. Increasing concern about the effect of geothermal fluid withdrawal on the activity of springs and geysers led to the establishment of the Rotorua Geothermal Monitoring Programme (RGMP) in 1982. By 1986, aquifer pressures declined to the lowest levels since the monitoring programme began, and to prevent further deterioration of spring and geyser activity, a Bore Closure Programme was enforced. By 1988, the programme contributed to a 75% decrease in net withdrawal and an immediate increase in reservoir pressures was observed. During the ensuing years recovery of some surface features has also been observed.Thus, although the geysers at Rotorua, New Zealand have shown some natural variation in their behaviour, they have also been significantly affected by human interference. The aim of this study is to quantify the past response of geyser activity to human-induced changes in the state of the Rotorua geothermal reservoir and to provide a tool for predicting the future behaviour of the geysers. The study is based on a simple computer model of a geyser that consists of a chamber linked to cold recharge and deeper hot recharge. The chamber also has an outlet to the surface through a narrow channel. The TOUGH2 simulator is used to carry out many numerical experiments to determine how parameters such as the size of chamber, cold recharge pressure, hot recharge pressure and permeability of the channel affect whether or not the model produces geysering behaviour and how they affect the period of the eruptions. The model successfully matches the observations that: (i) geysering ceases if hot recharge diminishes, (ii) the frequency of eruption increases if hot recharge increases and (iii) if hot recharge increases enough, then geysering turns into continual spouting.