Novel Sandstone Stimulation Using Thermochemical Fluid: Successful Field Application

Sandstone acidizing is implemented to remove the damage from the near-wellbore region. Different techniques are used to remove the formation and damage and improve reservoir productivity. This paper presents a novel sandstone stimulation technique using thermochemical fluids. The used chemicals are not reactive at surface conditions and react only at the downhole conditions. The reservoir temperature or pH controller can be used to activate the chemical reaction. A successful field application of the proposed method is reported in this paper. Different measurements were conducted to assess the performance of the new technique. A compatibility study was conducted at different conditions to evaluate the generation of acid foam. Also, Coreflood experiments were performed by injecting the foam generating solutions into tight sandstone cores. The rock permeability and the pores network were evaluated before and after the chemical injection. Scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) and analyses were performed. Moreover, a field application of the in situ acid foam generation was conducted. The treatment was implemented by injecting the solutions to react at the downhole conditions and improve the well injectivity. The profiles of injection rate, circulation pressure, and total volume were monitored during the field treatment to assess the treatment performance. Results showed that the used solutions can generate foam in less time and the volume of the generated foam is around 30 folds of the original chemical volumes. The in situ generated foam can penetrate deeper in the reservoir due to the larger foam volume compared to original chemicals, leading to improve treatment efficiency. Also, the new technique increased the rock permeability from 0.6 to 420 mD due to the dissolution and removal of illite minerals as well as the generation of micro-fractures due to the pressure pulses. The field application showed a very successful performance and the well injectivity was increased by 18 times after the treatment. The proposed technique utilizes thermochemical fluids to generate acid foam at the reservoir conditions. This technique can eliminate all the risks associated with HSE concerns, in addition to the corrosion issues. Also, the proposed treatment showed a successful field application and increased the well injectivity up by 18 folds of the original injectivity.

Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

Cement / rock interaction in geothermal wells

<p>One of the main issues associated with the exploitation of geothermal energy is the durability of the cement that is used downhole to cement the steel casing to the formation. Cement durability can have a major impact on the lifetime of geothermal wells, which do not usually last as long as desirable. The cement formulations used in the construction of geothermal wells are designed to provide mechanical support to the metallic well casings and protect them against the downhole harsh environment, which often leads to corrosion. This research is focused on the way that these formulations interact with the surrounding rock formation in geothermal environments, and aims to understand whether these are likely to affect the cement durability and, consequently, the geothermal well lifetime. The experimental work in this thesis consists of examining the changes in the interfacial transition zone (ITZ) that forms between geothermal cements and the volcanic rocks, after hydrothermal treatment. Holes were drilled in blocks of volcanic rocks and cement slurries with distinct formulations were poured into the cavities. The assemblages were autoclaved under typical geothermal conditions. The main variables under study were the cement formulation, the temperature of curing (150°C and 290°C), the presence of drilling mud, CO₂ exposure and the type of rock. The results show that with all the Portland cement based systems a series of chemical reactions occur at the interface between the cement and the rock, the ITZ, where migration of Ca²⁺ and OH⁻ ions occurs from the cement into the rock pores. These reactions are ongoing, which occur faster during the first days/few weeks of curing, mostly driven by physical process of cement movement into the rock, followed by a slower second stage, controlled mostly by chemical driving forces. This work highlights the interdependence between the chemical and physical interactions between geothermal cements and volcanic rocks which are complex. Variables such as temperature and time of curing and silica addition affect the cement phases that form, while the amount of amorphous silica and rock permeability dictate the extent of rock interaction. The presence of carbon dioxide influences the extent of rock/cement interaction and this can be controlled by the rock permeability and cement formulation. Consequently, most of the above mentioned variables were found to have an impact on the geothermal cement durability, which depends on the way these factors are combined.</p>

The Geology, Mineralogy and Geothermometry of the Broadlands Geothermal Field, Taupo Volcanic Zone, New Zealand

<p>Following the commissioning of the Wairakei geothermal power station, several areas in the Rotorua-Taupo Volcanic Zone were investigated for their power-producing potential. One of these was the Broadlands district, where a resistivity survey had located a sizeable area of subsurface water. The first hole, east of the Waikato River (Fig. 2), was drilled in late 1965, but although the temperature at hole bottom is 278 degrees, rock permeability is low and the bore is a poor steam producer. However, further drillholes (Br 2, 3 and 4) in the Ohaki area tapped good supplies of high temperature water and prompted a fullscale scientific and exploratory drilling programme. This was completed in mid-1971 after 25 holes had been drilled and it was estimated that the field could produce about 150 megawatts of electricity.</p>

The Geology, Mineralogy and Geothermometry of the Broadlands Geothermal Field, Taupo Volcanic Zone, New Zealand

<p>Following the commissioning of the Wairakei geothermal power station, several areas in the Rotorua-Taupo Volcanic Zone were investigated for their power-producing potential. One of these was the Broadlands district, where a resistivity survey had located a sizeable area of subsurface water. The first hole, east of the Waikato River (Fig. 2), was drilled in late 1965, but although the temperature at hole bottom is 278 degrees, rock permeability is low and the bore is a poor steam producer. However, further drillholes (Br 2, 3 and 4) in the Ohaki area tapped good supplies of high temperature water and prompted a fullscale scientific and exploratory drilling programme. This was completed in mid-1971 after 25 holes had been drilled and it was estimated that the field could produce about 150 megawatts of electricity.</p>

The effect of rock permeability value on groundwater influx in underground coal gasification reactor

Rock permeability value is one of the most significant rock’s physical properties that affect groundwater influx processes in underground coal gasification (UCG). This value of rock permeability (K), namely the vertical permeability of flanking rocks (Kv) and horizontal permeability of coal (Kh). The purpose of this study was to determine the extent of the influence of the value of rock permeability on the potential of groundwater influx. The effect of rock permeability on groundwater influx into the UCG gasification reactor cavity in the presence of thermal loads and mineral composition content is large and significant to consider. Based on the resistance to heat loads, the type of sandstone lithology is relatively more resistant compared to siltstone and claystone lithology.

Stimulation of Tight Formation in Kazakhstan by Chelate Based Treatment Fluids

Stimulation systems have improved over past decades, yet challenges prevail in corrosion, unwanted precipitation and handling hazardous chemicals. The role of chelating agents in coping with such concerns, is undeniably positive: their limited corrosivity, effective metal control and outstanding HSE profile, make them effective acidizing alternatives. Particularly when seeking delayed reaction at high temperature or removing insoluble material like Barite, chelating agents like GLDA and DTPA respectively have been reported effective both at laboratory and field scale. Formulations based on abovementioned chelating agents were evaluated experimentally to assess potential stimulation of Kazakhstan formations. Core-plug samples used in this evaluation are predominantly carbonate rock originating from different wells. The coreflooding experiments were performed at HPHT conditions to assess performance of treatment fluids to a) create new flow-channels (wormholes) thus improving rock permeability, and b) remove BaSO4-based solids suspected to be affecting productivity in the field. In this work, five reservoir core plugs were stimulated by GLDA based formulation to assess wormholing mechanism, while two core-plugs were treated by DTPA based fluid to study the impact of matrix cleaning. The matrix cleaning properties of DTPA based fluid were investigated on the damaged core plugs which were artificially damaged by in-situ precipitation of BaSO4 scale. The coreflood study included injection of the preflush, the treatment fluid and the post-flush system at reservoir temperature of 270 °F and low injection rates to accommodate the low permeability of the formation. It was shown that GLDA based fluid can effectively stimulate the reservoir core samples. The effective mechanism was observed to be wormholing thus increasing rock permeability by over a thousand times. No signs of face dissolution were observed despite slow injection rate at such high temperature; something that was not possible when a fast reacting acid (i.e. HCl) was used under the same conditions. In addition, it was shown that the DTPA based fluid can efficiently improve the rock permeability through matrix cleaning by both Barium and Calcium chelation. In the first treatment test by this fluid system, around 45% of the damaged permeability was recovered. While in the second test, not only BaSO4 scale was dissolved but also the CaCO3 minerals were partly dissolved and the core permeability was significantly increased (Kf/Ki &gt;200). Experimental results bring promising prognosis for field implementation despite expected low injectivity at high downhole temperature. GLDA treatments avoid premature acid spending and face dissolution - common outcomes of HCl- which translate into deeper extent of stimulation. Additionally, in barite damaged wells, DTPA treatment represents an attractive solution for damage reduction and by-passing. Finally, intrinsic properties of chelating agents reduce asset integrity risks, improve operation HSE and simplify flow-back handling.

Fault Rock Property Prediction On Jurassic Clastics Of The Barents Sea/Norway

We analysed the fault rocks of a compartmentalized field in the Barents Sea, in an area with several tectonic elements, which formed at different tectonic events. Standard Fault Seal Analysis (FSA) was conducted to predict the shale content of the fault rock (SGR). A static cellular model based on well data, seismic data and geological concepts served as input. The fault rock calibration workflow required various data acquired by different methods. We analysed the Mid-Triassic to Upper Jurassic clastic deposits to reconstruct the tectonic history. Apatite fission track and (U-Th)/He thermochronology were used to determine the maximum burial depths and exhumation history. The results of high-resolution shale ductility analysis (BIB-SEM), a compaction trend study, kinematic analysis and structural modelling (section balancing) served as additional input constraints for fault rock calibration. The evaluation of the results helped to reconstruct the following tectonic evolution: The orthogonal faults of the analysed area developed at an early stage, during Late Triassic to Early Jurassic times at relatively shallow depth, below 1000 m. Ongoing subsidence created accommodation space for Upper Jurassic to Cenozoic deposits with a maximum burial depth of 2000 m for the analysed Mid-Jurassic rocks. Exhumation of the area started around 10 Ma and continued through to Quaternary times. The calculated values for fault rock permeability show a wide range when using poorly constrained input for fault rock calibration: 10 to 1000 mD for SGR values around 0.08 at reservoir/reservoir juxtaposition. Fault rock calibration using above described results concluded in reliable values for fault rock permeability and ultimately, for transmissibility multipliers. The reason for the sensitivity of the fault rock calibration is a combination of multiple factors: highly permeable reservoir sandstone, shallow depth of initial faulting, maximum burial depth and low shale content at the primary reservoir level. The study shows that an accurate reconstruction of the geohistory provides essential parameters for fault rock calibration and fault rock permeability calculation. The range of values can widely scatter if preconditions are not acknowledged. Well-constrained fault rock calibration reduces the uncertainty on possible flow scenarios, increases the reliability on production forecasts and helps to determine the most efficient drainage strategy.