Low-permeability Zones Research Articles

Bioclogging and Permeability Alteration by L. mesenteroides in a Sandstone Reservoir: A Reactive Transport Modeling Study

Selective bioclogging targets the biofilm growth in highly permeable zones of reservoirs or aquifers to divert water into low permeability zones. It alters the hydrodynamics of the subsurface flow systems to favorable performance conditions. Applications may include microbial-enhanced-hydrocarbon-recovery (MEHR) and bioremediation. Despite its success at the laboratory scale, application of bioclogging at the reservoir scale is hindered by the lack of understanding and advanced modeling and prediction tools. To understand controls of bioclogging processes at the reservoir scale, a Reactive Transport Model (RTM) has been developed in this work for in situ biostimulation of L. mesenteroides. This fermenting bacterium produces the biopolymer dextran in the presence of sucrose. As a first step, we considered the flow, transport, and bacterial growth and dextran production reactions in a single phase fluid (water) system, because most reactions occur either in the water phase or at the water–solid interface. Parameters for biomass growth and dextran production were obtained from column experimental data. The numerical experiments were carried out using the spatial distribution of porosity and permeability extracted from open-hole well logs collected at a characterization well near the King Island gas field in Southern Sacramento basin in California. The numerical experiments suggest that there exists an optimum range of injection rates (between 543 and 1,195 bbls/day). The volumetric injection rates need to be sufficiently fast to overcome microbial growth and clogging at the vicinity of the bore wells. They also need to be low enough to allow sufficiently long residence times for dextran production. Results show significant dextran formation and the associated porosity and permeability alterations to divert water into low permeability zones. The bioclogging effectiveness, measured by the percentage of the water diverted into the low permeability zones, varied between 10 to 75% depending on injection conditions. With the same total mass injection rates of sucrose, increasing flow rate is more effective in selectively bioclogging highly permeable zones than increasing sucrose concentration. Other processes, including the attachment of biomass to the solid surface without being washed out, are also important. The developed model offers a powerful tool to optimize injection conditions for effective bioclogging in naturally heterogeneous reservoirs.

Improving the Sweeping Efficiency of Permanganate into Low Permeable Zones To Treat TCE: Experimental Results and Model Development

The residual buildup and treatment of dissolved contaminants in low permeable zones (LPZs) is a particularly challenging issue for injection-based remedial treatments. Our objective was to improve the sweeping efficiency of permanganate into LPZs to treat dissolved-phase TCE. This was accomplished by conducting transport experiments that quantified the ability of xanthan-MnO4(-) solutions to penetrate and cover (i.e., sweep) an LPZ that was surrounded by transmissive sands. By incorporating the non-Newtonian fluid xanthan with MnO4(-), penetration of MnO4(-) into the LPZ improved dramatically and sweeping efficiency reached 100% in fewer pore volumes. To quantify how xanthan improved TCE removal, we spiked the LPZ and surrounding sands with (14)C-lableled TCE and used a multistep flooding procedure that quantified the mass of (14)C-TCE oxidized and bypassed during treatment. Results showed that TCE mass removal was 1.4 times greater in experiments where xanthan was employed. Combining xanthan with MnO4(-) also reduced the mass of TCE in the LPZ that was potentially available for rebound. By coupling a multiple species reactive transport model with the Brinkman equation for non-Newtonian flow, the simulated amount of (14)C-TCE oxidized during transport matched experimental results. These observations support the use of xanthan as a means of enhancing MnO4(-) delivery into LPZs for the treatment of dissolved-phase TCE.

Assessment of NPV Uncertainty on Heterogeneous Reservoirs during Polymer Flood

This study shows net present value (NPV) distribution by considering uncertainties in porosity, oil viscosity, water saturation, and permeability for polymer flood with Monte Carlo simulation. For high and low average permeability conditions, differences of NPV between polymer flooding and water flooding have been investigated. According to results both average NPV and range of NPV distribution tend to increase with porosity and permeability in all cases. Although water saturation and oil viscosity affect NPV, they are not important parameters that conclude uncertainty of NPV under the conditions considered in this study. For high permeability model which has Dykstra-Parsons coefficient (DP) as 0.72 and porosity as 0.3088, Monte Carol simulations for polymer flood show that 50th percentile (P50) of NPV is 352.81 M$. If porosity is decreased from 0.3088 to 0.1912, the P50 is also decreased 63.8 %. The reduction of NPV during polymer flooding in low permeability reservoirs are almost 40 % higher than that of water flood. These differences come from polymer adsorption and permeability reduction that easily occurs in low permeability zone. The procedure has proven to be useful tool to generate probability distribution of NPV when polymer flood is selected as a tertiary flood process.

Air and water flows induced by pumping tests in unconfined aquifers with low‐permeability zones

AbstractAir flows from the atmosphere into an unconfined aquifer when the water table falls during pumping tests. Pumping test results in unconfined aquifers may be significantly affected by low‐permeability zones (LPZs) near the initial water table position, because they restrict the downward movement of air. A transient, three‐dimensional air–water two‐phase flow model is employed to investigate numerically the effects of local heterogeneity on pumping test results in unconfined aquifers. Two cases of local heterogeneities are considered herein: a LPZ around the pumping well and on one side of the pumping well. Results show that the drawdown with the LPZ is significantly greater than that of the homogeneous aquifer. The differences in drawdown are the most significant at intermediate times and gradually diminish at later times. The LPZ significantly reduces air flow from the atmosphere to the aquifer. The pore air velocity in the LPZ is very low. The air pressure at the observation point under the LPZ when air begins to enter is significantly lower than the air pressure of the homogeneous aquifer at the same point. After that, the air pressure increases quickly and then increases slowly. The time for the air pressure to reach the atmospheric pressure is significantly longer. Copyright © 2013 John Wiley & Sons, Ltd.

Groundwater Risk Assessment Model (GRAM): Groundwater Risk Assessment Model for Wellfield Protection

A groundwater risk assessment was carried out for 30 potable water supply systems under a framework of protecting drinking water quality across South Australia. A semi-quantitative Groundwater Risk Assessment Model (GRAM) was developed based on a “multi-barrier” approach using likelihood of release, contaminant pathway and consequence equation. Groundwater vulnerability and well integrity have been incorporated to the pathway component of the risk equation. The land use of the study basins varies from protected water reserves to heavily stocked grazing lands. Based on the risk assessment, 15 systems were considered as low risk, four as medium and 11 systems as at high risk. The GRAM risk levels were comparable with indicator bacteria—total coliform—detection. Most high risk systems were the result of poor well construction and casing corrosion rather than the land use. We carried out risk management actions, including changes to well designs and well operational practices, design to increase time of residence and setting the production zone below identified low permeable zones to provide additional barriers to contaminants. The highlight of the risk management element is the well integrity testing using down hole geophysical methods and camera views of the casing condition.

Permeability impairment of a limestone reservoir triggered by heterogeneous dissolution and particles migration during CO2‐rich injection

AbstractA CO2‐rich brine core‐flood experiment in calcite limestone for conditions representative of underground storage (P = 12 MPa and T = 100°C) was performed in order to explore the dissolution mechanisms arising at moderate CO2 partial pressure (0.3 MPa). An increase of the total porosity (φT) accompanied by a persistent permeability (k) decrease was measured. The mechanisms controlling this atypical anticorrelated k − φT relationship were investigated from the analysis of high‐resolution X‐ray microtomography images of the sample acquired before and after the experiment. All the evidences converge to the conclusion that the ubiquitous decrease of permeability measured during the 44 h of dissolution is due to the clogging of a fraction of the macroporosity by microporous material triggered by the rearrangement of the detached undissolved particles. This mechanism results in the development of low permeability zones bridging the macroporosity.

Evolution of geomechanical and electro-hydrodynamic fields in deep well drilling in rocks

The authors have developed and numerically implemented 3D model of evolution of geomechanical and hydrodynamic fields during deep well drilling. The modeling exercises showed low-permeable zones of irreversible deformations that appear under definitely interrelated rock strength properties, drilling mud pressure and in situ horizontal stresses in the borehole environment. These low-permeable irreversible-deformation zones give rise to angular anisotropy of distribution of water content and drilling mud resistivity in the invaded zone, which greatly affects geophysical logging data and must be taken into account in the logging data inversion.

Optimization of Injection/Extraction Rates for Surface-Dissolution Process

Summary Implementing geological carbon sequestration at a large scale to mitigate anthropogenic emissions involves the injection of carbon dioxide (CO2) into deep brine-filled structures. An alternative to injecting CO2 as a buoyant phase is to dissolve it into brine extracted from the storage formation, then inject the CO2-saturated brine into the storage formation. The CO2-concentration front shape, when it reaches the saturation-pressure contour, defines the maximal areal extent of CO2-saturated brine and thus the aquifer-utilization efficiency. The heterogeneity of the aquifer reduces the utilization efficiency significantly. We illustrate by comparing the utilization efficiency in a homogeneous-permeability field with that in uncorrelated and correlated heterogeneous fields under the same well control. The example cases yield significant reductions of the utilization efficiency. We develop an optimal-control strategy of the injection/extraction rates to maximize the utilization efficiency for heterogeneous aquifers. We propose two objective functions: One seeks to improve the areal sweep by minimizing the mismatch between the CO2-concentration front and the saturation-pressure contour; the other directly formulates the utilization efficiency while penalizing zones that contain gas-phase CO2. Both approaches have improved the aquifer-utilization efficiency by delaying the arrival of the dissolved CO2 front at the contour of saturation pressure. Heterogeneity plays an important role in determining the location of the saturation-pressure contour within the storage formation. For a simple example domain, we propose a well-pattern orientation strategy by placing line-drive injectors in a high-permeability zone and extractors in a low-permeability zone, so that the saturation-pressure contour is closer to the extractors and thus increases the aquifer utilization efficiency. Illustration of this concept in the correlated heterogeneous field shows an improvement of the utilization efficiency. When combined with the optimal control of injection/extraction rates, the increase in the utilization efficiency almost compensates the reduction because of the heterogeneity.

Study on Performance Evaluation of Dispersed Particle Gel for Improved Oil Recovery

This work investigates the performance of dispersed particle gel (DPG) by core flow tests including injectivity, selective plugging, thermal stability, and improved oil recovery (IOR). Results showed that the resistance factor is small when DPG was injected, but obviously became larger while turning into brine water flooding. Both the oil and water relative permeability were reduced and greater reduction appeared in water relative permeability. DPG could block water flow without affecting oil flow, and oil–water segregated flow mechanism was proposed to explain this selective plugging. The injection pressure increases, caused by strong plugging due to the DPG aggregation aging in high temperature, which was consistent with the observation of atomic force microscope (AFM) photos. The DPG could effectively block high permeability zone and produce oil from low permeability zone, which could provide a practical way to enhance hydrocarbon recovery while reducing water production for extremely heterogeneous reservoirs.

Using heat as a tracer to estimate spatially distributed mean residence times in the hyporheic zone of a riffle-pool sequence

[1] Biochemical reactions that occur in the hyporheic zone are highly dependent on the time solutes that are in contact with sediments of the riverbed. In this investigation, we developed a 2-D longitudinal flow and solute-transport model to estimate the spatial distribution of mean residence time in the hyporheic zone. The flow model was calibrated using observations of temperature and pressure, and the mean residence times were simulated using the age-mass approach for steady-state flow conditions. The approach used in this investigation includes the mixing of different ages and flow paths of water through advection and dispersion. Uncertainty of flow and transport parameters was evaluated using standard Monte Carlo and the generalized likelihood uncertainty estimation method. Results of parameter estimation support the presence of a low-permeable zone in the riffle area that induced horizontal flow at a shallow depth within the riffle area. This establishes shallow and localized flow paths and limits deep vertical exchange. For the optimal model, mean residence times were found to be relatively long (9–40.0 days). The uncertainty of hydraulic conductivity resulted in a mean interquartile range (IQR) of 13 days across all piezometers and was reduced by 24% with the inclusion of temperature and pressure observations. To a lesser extent, uncertainty in streambed porosity and dispersivity resulted in a mean IQR of 2.2 and 4.7 days, respectively. Alternative conceptual models demonstrate the importance of accounting for the spatial distribution of hydraulic conductivity in simulating mean residence times in a riffle-pool sequence.

Improvement of Water Flooding Efficiency Using Mixed Culture of Microorganisms in Heterogeneous Micro-models

One of the possible methods for reducing unwanted problem of reservoir heterogeneity in flooding processes is the application of microbial slugs for bioplugging of high permeable zones. Objective of this research is visualization of potential of mixed-culture microorganisms as bioplugging agents for improving the efficiency of water flooding in two different heterogeneous micro-models: a glass bead and an inventive etched one. In both micro-models, water flooding removed the oil in high permeable zones while it had little effect on the oil in low permeable areas. Injection of a slug, containing a mixed culture of oil degrading microorganisms was done in glass bead micro-model. The etched micro-model was used to investigate the effect of in situ microbial growth and subsequent water flooding. In both cases the microbial treatment improved the oil recovery in comparison with water flooding in low permeable zone considerably. The results of this study showed that mixed-culture microorganisms have an acceptable potential for bioplugging of high permeable layers of heterogeneous porous media.

Diverting mechanism of viscoelastic surfactant-based self-diverting acid and its simulation

Abstract In carbonate acidizing, acid selectively flows into the high permeability zones to dissolve the rock, leaving the low permeability zones untreated. Many techniques including mechanical ones and chemical ones are found to do their best to avoid the uneven distribution of acid. Among these techniques, injecting the viscoelastic surfactant-based self-diverting acid (VES acid) is an effective method to divert acid to other low permeability zones. The VES acid is able to be self-viscosified with its consumption through the gelling of the surfactant. Since the high permeability zone receives more acid, the gelling process occurs first in the zone so that the acid is diverted to other low permeability zones. However, to the best of our knowledge in the open literature, the diverting mechanism of the VES acid has not been understood in spite of many successful applications in oil and gas fields. In this paper, we thoroughly study the diverting mechanism through the analyses of the former experimental results and our own experimental observations. According to the diverting mechanism, a new model which describes the diverting behavior of the VES acid is developed. The results show that the diverting mechanism is dependent on both the spent acid zone and the viscosity of the spent acid. The spent acid zone and the viscosity of the spent acid keep increasing with the injection of VES acid, which leads to the continuous increase of the pressure difference. However, the spent acid zone is thinner for low permeability zone than for high permeability zone. As a result, the maximum pressure ratio follows a linear increase relationship with the permeability in the log–log coordinate system. In addition, the non-uniformly distributed viscosity of the spent acid leads to the sharp drop of the pressure difference in the vicinity of breakthrough. The new model couples the two-scale continuum model describing the conventional acidizing with three equations describing the concentration of Ca ions, viscosifying behavior and reaction rate. Through a series of simulations, the results are in good agreement with the experimental ones, which means that the new model captures the diverting mechanism of the VES acid.

The Development Technology Policy of Secondary Oil Recovery in the Overpressure Carbonate Reservoir

Kenkiyak oilfield in kazakstan is a low porosity, extremely low permeability and overpressure carbonate reservoir. There are different reservoir and fracture characteristics in different region. The formation pressure decline seriously and water cannot be injected into the low permeability zone. Referring to the domestic and oversea research achievement, integrating regional geologic characteristics, numerical simulation results and reservoir engineering research results, we optimize a series of the development technology policy, including the reasonable gas and water injection modes and injection opportunity, the suitable well patterns and well spacing. Meanwhile, the development mode of energy supplement in the extremely low permeability and overpressure reservoir is explored.

Hydraulic conditions at the source zone and their impact on plume behavior

A laboratory sandbox model (105 × 60 × 15 cm) was filled with different types of sediments to arrange different hydraulic conditions at the tracer’s injection zone. Tracer-input functions were monitored at the injection location and tracer breakthrough curves (BTCs) were measured at a downgradient detection plane. Numerical transport simulations analogous to the sandbox experiments were conducted in a numerical model parameterized with the properties of the sediments used in the sandbox. The results of the study demonstrate that the hydraulic conditions at the source zone have a major influence on plume development. Tracer injection into high-permeability zones causes early arrival, and enhanced transverse and reduced longitudinal spreading. Tracer injection into low-permeability zones causes late arrival, and reduced transverse and enhanced longitudinal spreading. It was further found that using simplified tracer-input functions in the analysis of BTCs can lead to biased transport parameters. The results suggest that transport parameter estimation based on tracer experiments can benefit from monitoring of the tracer-input function and subsequent consideration of the measured tracer-input function in the analysis of BTCs.

Tracing groundwater with low-level detections of halogenated VOCs in a fractured carbonate-rock aquifer, Leetown Science Center, West Virginia, USA

Measurements of low-level concentrations of halogenated volatile organic compounds (VOCs) and estimates of groundwater age interpreted from 3H/3He and SF6 data have led to an improved understanding of groundwater flow, water sources, and transit times in a karstic, fractured, carbonate-rock aquifer at the Leetown Science Center (LSC), West Virginia. The sum of the concentrations of a set of 16 predominant halogenated VOCs (TDVOC) determined by gas chromatography with electron-capture detector (GC–ECD) exceeded that possible for air–water equilibrium in 34 of the 47 samples (median TDVOC of 24,800pgkg−1), indicating that nearly all the water sampled in the vicinity of the LSC has been affected by addition of halogenated VOCs from non-atmospheric source(s). Leakage from a landfill that was closed and sealed nearly 20a prior to sampling was recognized and traced to areas east of the LSC using low-level detection of tetrachloroethene (PCE), methyl chloride (MeCl), methyl chloroform (MC), dichlorodifluoromethane (CFC-12), and cis-1,2-dichloroethene (cis-1,2-DCE). Chloroform (CHLF) was the predominant VOC in water from domestic wells surrounding the LSC, and was elevated in groundwater in and near the Fish Health Laboratory at the LSC, where a leak of chlorinated water occurred prior to 2006. The low-level concentrations of halogenated VOCs did not exceed human or aquatic-life health criteria, and were useful in providing an awareness of the intrinsic susceptibility of the fractured karstic groundwater system at the LSC to non-atmospheric anthropogenic inputs. The 3H/3He groundwater ages of spring discharge from the carbonate rocks showed transient behavior, with ages averaging about 2a in 2004 following a wet climatic period (2003–2004), and ages in the range of 4–7a in periods of more average precipitation (2008–2009). The SF6 and CFC-12 data indicate older water (model ages of 10s of years or more) in the low-permeability shale of the Martinsburg Formation located to the west of the LSC. A two-a record of specific conductance, water temperature, and discharge recorded at 30-min intervals demonstrated an approximately 3-month lag in discharge at Gray Spring. The low groundwater ages of waters from the carbonate rocks support rapid advective transport of contaminants from the LSC vicinity, yet the nearly ubiquitous occurrence of low-level concentrations of halogenated VOCs at the LSC suggests the presence of long-term persistent sources, such as seepage from the closed and sealed landfill, infiltration of VOCs that may persist locally in the epikarst, exchange with low-permeability zones in fractured rock, and upward leakage of older water that may contain elevated concentrations of halogenated VOCs from earlier land use activities.

Petrophysical characterization of the South Georgia Rift Basin for supercritical CO2 storage: a preliminary assessment

The Triassic–Jurassic South Georgia Rift (SGR) Basin, buried beneath Coastal Plain sediments of southern South Carolina, southeastern Georgia, western Florida, and southern Alabama, consists of an assemblage of continental rift deposits (popularly called red beds), and mafic igneous rocks (basalt flows and diabase sills). The red beds are capped by basalts and/or diabase sills, and constitute the target for supercritical CO2 storage as part of a Department of Energy funded project to study the feasibility for safe and permanent sequestration. The purpose of this research is to evaluate subsurface suitability for underground CO2 storage in terms of the local and regional distribution of porous and permeable reservoirs. In addition, unlike shale-capped CO2 reservoirs, very little is known about the ability of basalts and diabase sills to act as viable seals for CO2 storage. New results demonstrate the presence of confined porous rocks that may be capable of storing significant quantities of supercritical CO2. Reservoir thicknesses as high as 420 m and an average porosity as high as 14 % were obtained. The SGR Basin manifests distinct porosity–permeability regimes that are influenced by the depositional environments. These are: a high-porosity, medium/low-permeability zone associated with lacustrine deposits, a medium/low-porosity, low-permeability zone dominated by fluvial fine- to very fine-grained sandstone, and an extremely low porosity and permeability zone characterized by fluvial and alluvial-fan deposits. Analyses further show that the basalt flows and diabase sills are characterized by low porosity as well as high seismic velocities and densities that are favorable to caprock integrity.

Modeling contaminant transport and remediation at an acrylonitrile spill site in Turkey

The August 1999 earthquake in Turkey damaged three acrylonitrile (AN) storage tanks at a plant producing synthetic fiber by polymerization. A numerical modeling study was carried out to analyze the groundwater flow and contaminant (AN) transport at the spill site. This study presents the application of a numerical groundwater model to determine the hydrogeological parameters of the site, where such data were not available during the field surveys prior to the simulation studies. The two- and three-dimensional transient flow and transport models were first calibrated using the first 266days of observed head and concentration data and then verified using the remaining 540-day observed data set. Off-site migration of the contaminant plume was kept under control within the site boundaries owing to the favorable geology of the site, the characteristics of the local groundwater flow regime and the pumping operations. As expected, the applied pump-and-treat system was effective at high-permeability zones, but not fully effective at low-permeability zones. The results of long-term simulations for unconfined aquifer showed that the size of the plume in the high permeability zone shrank significantly due to the dilution by natural recharge. However, in the low permeability zone, it was not significantly affected. The study showed that accurate and sufficient data regarding the source characteristics, concentration and groundwater level measurements, groundwater pumping rates and their durations at each of the extraction points involved in the pump-and-treat system along with the hydrogeological site characterization are the key parameters for successful flow and transport model calibrations.

Matrix Diffusion Modeling Applied to Long‐Term Pump‐and‐Treat Data: 1. Method Development

AbstractDespite the installation in the 1980s and 1990s of hydraulic containment systems around known source zones (four slurry walls and ten pump‐and‐treat systems), trichloroethene (TCE) plumes persist in the three uppermost groundwater‐bearing units at the Middlefield‐Ellis‐Whisman (MEW) Superfund Study Area in Mountain View, California. In analyzing TCE data from 15 recovery wells, the observed TCE mass discharge decreased less than an order of magnitude over a 10‐year period despite the removal of an average of 11 pore volumes of affected groundwater. Two groundwater models were applied to long‐term groundwater pump‐and‐treat data from 15 recovery wells to determine if matrix diffusion could explain the long‐term persistence of a TCE plume. The first model assumed that TCE concentrations in the plume are controlled only by advection, dispersion, and retardation (ADR model). The second model used a one‐dimensional diffusion equation in contact with two low‐permeability zones (i.e., upper and lower aquitard) to estimate the potential effects of matrix diffusion of TCE into and out of low‐permeability media in the plume. In all 15 wells, the matrix diffusion model fit the data much better than the ADR model (normalized root mean square error of 0.17 vs. 0.29; r2 of 0.99 vs. 0.19), indicating that matrix diffusion is a likely contributing factor to the persistence of the TCE plume in the non‐source‐capture zones of the MEW Study Area's groundwater‐extraction wells. © 2013 Wiley Periodicals, Inc.

Natural Gas Exploration Using Carbon Isotopic Fractionation Effect: A Case Study of Shanxi Formation, Upper Palaeozoic Group in the Center of Ordos Basin, China

Abstrac t—Carbon isotopic fractionation would occur during the process of natural gas generation and migration. As for 'self-generating and self-preserving' tight sandstone gas reservoir, carbon isotope better keeps the current maturity and migration results. In this paper, three local hydrocarbon generation centers and two relatively lower permeability zones were determined from the study area and it is suggested that natural gas migrated roughly from south to north direction through the analysis of carbon isotope isoline. Thus, we predict three favorable exploration areas using natural gas storage index combined with above isotopic results. It is a common geochemistry phenomenon that carbon isotopic fractionation would occur during the process of natural gas generation and migration. Carbon isotope becomes heavier as organic matter of the gas source rock becomes more mature; meanwhile, carbon isotope appears lighter along the direction of natural gas migration. Therefore natural gas carbon isotope records parent material source, and information of geology and geochemistry action that parent material and its products experience. Researchers in China and other countries have found and confirmed fractionation effect of natural gas carbon isotope, and built quantitative relations of δ13C1 and Ro that suggested specific mathematical relationship for the different type of source rocks. It is verified by indoor research that carbon isotopic fractionation range is wide under low temperature, and hydrocarbons that organic matter discharges contain lighter carbon isotope; as the temperature rises, fractionation effect is reduced, and carbon isotope of generated compounds become heavier(1),(2). It is also concluded by indoor experiments that natural gas always displays carbon isotopic fractionation effect to a different extent when it is injected into and diffuses through several types of lower permeability rocks respectively, in which montmorillonitic clay is the most obvious, kaolinitic clay takes the second and limestone fractionation effect is the least(3)-(6). Moreover, gas isotopic fractionation is influenced by migration dimension and channel. When natural gas flows in bulk through reservoir rocks, carbon isotopic fractionation is not obvious as the gas-rock medium contract area is small and the dwelling time is short; whereas the gas is seeping through the rocks with a smaller scale and slower flow rate, the fractionation is strong because of the long dwelling time and the adequate gas-rock contract, that leads carbon isotope along front of migration direction becomes light. As a result, an application of carbon isotope information is to infer the direction of natural gas migration and the possible enrichment zone. According to the geochemistry analysis and some other geologic evidence of gas reservoir in the eastern-central part of the Ordos basin, this paper just discusses how to predict natural gas migration and favourable exploration areas using carbon isotopic fractionation information.

Steering Parameters for Rock Grouting

In Swedish tunnel grouting practice normally a fan of boreholes is drilled ahead of the tunnel front where cement grout is injected in order to create a low permeability zone around the tunnel. Demands on tunnel tightness have increased substantially in Sweden, and this has led to a drastic increase of grouting costs. Based on the flow equations for a Bingham fluid, the penetration of grout as a function of grouting time is calculated. This shows that the time scale of grouting in a borehole is only determined by grouting overpressure and the rheological properties of the grout, thus parameters that the grouter can choose. Pressure, grout properties, and the fracture aperture determine the maximum penetration of the grout. The smallest fracture aperture that requires to be sealed thus also governs the effective borehole distance. Based on the identified parameters that define the grouting time-scale and grout penetration, an effective design of grouting operations can be set up. The solution for time as a function of penetration depth is obtained in a closed form for parallel and pipe flow. The new, more intricate, solution for the radial case is presented.