Gas-saturated Zone Research Articles

Geological factors of gassing of the east inclined longwall # 3 of m3 bed at O.F. Zasiadko mine

The paper deals with the fact that gassing of the East inclined longwall (EIL) # 3 of the m3 bed at the «O.F. Zasiadko mine» are caused by a combination of mining-geological conditions. The main geological factor is the presence of a synclinal fold, characterized by increased fracturing. The map construction of local structures, the use of gravity data and seismological observations allow to effectively isolate gas-saturated zones and, therefore, correct degassing projects and parameters of ways to prepare workings.

Integrated analysis of geoelectric and surface geochemical data for exploration of subsurface hydrocarbon accumulations (Carpathian Foredeep, SE Poland)

The search for hydrocarbon deposits in the Carpathian Foredeep is dominate by seismic methods. However, chances for additional discoveries of oil and gas will be increased by the integration of geochemical methods with non-seismic methods. The authors document mutual relationships between data obtained from surface geochemical survey and surface geoelectric survey. The geoelectric and geochemical surveys were run along 8 linear profiles positioned over the Rudka gas deposit discovered in Miocene sediments. This deposit is located over the margin of the Ryszkowa Wola Horst (Carpathian Foredeep). The Rudka deposit accumulates microbial gas generated under strictly defined conditions (microbial reduction of carbon dioxide). Soil gas samples were collected from a depth of 1.2 m at 177 sites and analyzed for methane, alkanes, alkenes, and non-hydrocarbon components. Maximum concentrations of CH4, total alkanes C2-C5 and total alkenes C2-C4 are 1200.0 ppm, 16.49 ppm and 0.289 ppm, respectively. The high concentrations of methane correlated with high values of C1/∑(C2-C4) ratio indicate the presence of recent (microbial) methane whereas higher alkanes detected in soil gases originated from deep hydrocarbon accumulations. Two methodological variants of magnetotelluric survey were applied: natural field magnetotelluric soundings and controlled-source audiomagnetotelluric profiling. The interpreted resistivity values of Miocene sediments change from 0.6 to 20 Ωm but higher resistivities, locally exceeding 100 Ωm also occur in the near-surface zone. The lateral decreases of resistivity observed in the cross-sections may be linked to tectonic discontinuities. Changes of resistivity values identified in the magnetotelluric profiles and interpreted as the signs of “gas chimneys” correlate well with anomalous concentrations of total alkanes C2-C5.The multistage analysis of the results of surface geochemical and geoelectric surveys demonstrated that gas-saturated zones may extend beyond the known contour of the Rudka gas deposit. These results demonstrate that geoelectric and geochemical data collected in the near-surface zone are valuable complements of conventional hydrocarbon exploration methods.

Numerical Modeling of CO2 and Brine Leakage through Open Fracture in a Fault Zone: Open Channel Flow or Darcy Flow

Understanding fluids migration and leakage risk along the fault zone is necessary to guarantee the safety of CO2 geological storage. The validity of Darcy’s law gets challenged in dealing with the flow in open fractures since the occurring of turbulence flow. In this study, we develop a 2D model with usage of T2Well, an integrated wellbore-reservoir simulator, to investigate the leakage problem along open fractures which are embedded in a fault zone from the deep injection reservoir to shallow aquifers. The results record a positive feedback of gas expansion and pressure response in fracture, which causes a quick downward propagation of highly gas saturated zone from the top of fracture and an easy gas breakthrough in the shallower aquifers. The decreasing of aperture size of fracture significantly enhances the leakage rates in fracture, but with less influences as aperture increases. In comparison, the Equivalent Porous Media models show a good approximation with the momentum model of large apertures but poor for the small one. Nevertheless, the differences are small in terms of final CO2 distribution among various aquifers, suggesting that Darcy’s law may be still “effective” in solving flow problem along fractures in a constant injection system at a large time scale.

Review of the hydrocarbon potential of the Steele Shale and Niobrara Formation in Wyoming, USA: A major unconventional resource play?

The study area, located in the northern rim of the Greater Green River Basin, Wyoming, is a large anticlinal feature with a complex structural history and extensive fracturing. The targets are the Late Cretaceous Steele Shale and the Niobrara Formation, having a combined thickness of 3700–4900ft. The productive intervals have a depth ranging from 8500 to 13,000ft. Total Organic Carbon content in one of the wells drilled, the Bare Ring Butte 4–36, ranges from 0.5wt.% to 4.0wt.% and averages 2.0wt.%. Hydrogen Index ranges from 50 to 220mg HC/g TOC. Gas content averages 30scf/t (as-received) and porosity for the inferred pay zone is estimated at 3–6%. Matrix permeability is low, estimated to be 130nD (nano-Darcy). Mineralogy in the above well consists of 40wt.% quartz, with illite, fracture-filling calcite and chlorite also present. The shales are overpressured, have a gradient of 0.7psi/ft., and they lie within the oil and condensate liquids windows. At Bare Ring Butte 4–36, vitrinite reflectance (%Ro, maximum) ranges from 0.85% at 8000ft. to 1.20% at 10,200ft. A distinct change in the maturation and vitrinite reflectance gradients with depth is evident in this well, which coincides with the top of the overpressure zone. The overpressure zone has a thin wet (oil) zone (34–46° API) at the top, followed by a thick gas-saturated zone underneath. A similar trend was also seen in at least four more wells in the Antelope Arch area. Using an imaging tool, three highly-fractured zones with a combined thickness of 600ft. have been identified in the Steele/Niobrara interval in the Bare Ring Butte 4–36 well. The recoverable gas resource is estimated to be 5Bcf/160acres (1/4 section) using a recovery factor of only 5%. In the early 1980s, vertical wells tested 1.4 to 3.7Mmcf/d following natural completion in the Steele Shale and Niobrara Formation. The closest analogue to the Steele Shale and Niobrara Formation is the stratigraphically-equivalent Baxter Shale in Wyoming. Questar E&P has drilled a number of vertical and horizontal wells in the Baxter Shale. Vertical wells that targeted fractured zones had IP (initial production) rates ranging from 1.8 to 9Mmcf/d. Questar estimated a EUR (estimated ultimate recovery) of 8–16Bcf for vertical wells drilled on 160-acres with recovery factors of 7–15%. A horizontal well (3000ft. long) that encountered a natural fracture swarm had an IP rate of 9.1Mmcf/d in the first 4–5days before it stabilized at 2.7Mmcf/d. With today's improved drilling and stimulation techniques, horizontal wells targeting the highly-fractured zones could improve productivity considerably. Because of the great thickness, highly-fractured character, and overpressured nature, the Steele Shale and Niobrara Formation could have greater resource potential than the normally-pressured Barnett Shale in the core area of the Fort Worth Basin.

Seismic Attenuation and Velocity Dispersion to Discriminate Gas Hydrates and Free Gas Zone, Makran Offshore, Pakistan

Gas hydrates gained a remarkable attention as an unconventional energy resource recently. In order to interpret gas hydrates (part of fluid) and free gas saturated zone accurately, it is essential to implement new technique related to seismic attenuation and velocity dispersion. P wave attenuation and velocity dispersion in porous media made promising imprints for exploration of gas hydrates. The most prominent phenomenon for attenuation and velocity dispersion in porous media is wave induced fluid flow in which wave inhomogeneities are larger than pore size but smaller than wavelength. Numerical simulation technique is applied to analyze frequency dependent velocity dispersion and attenuation in gas hydrates and free gas layer in Makran offshore of Pakistan. Homogeneous and patchy distribution patterns of gas hydrates and free gas within pore spaces of host sediments at lower and higher frequency regime are considered. It is noted that the attenuation and velocity dispersion increase with the increase in gas hydrates saturation. The maximum attenuation is observed at 66% saturation of gas hydrates in the area under investigation. However, in case of water and gas mixture the maximum attenuation and velocity dispersion occur at low gas saturation (~15%). Therefore, based on our numerical simulation, velocity dispersion and attenuation can be used as seismic attributes to differentiate various gas saturations and gas hydrates saturation for Makran offshore area of Pakistan.

The influence of addition of coal tar pitch (CTP) and expired pharmaceuticals (EP) on properties and composition of pyrolysis products for lower and higher rank coals

Changes in electrical resistivity and tangent of dielectric losses of pyrolyzed coals were determined along with the yield of organic material soluble in dichloromethane (OMSDCM) obtained from coals plasticized with addition of CTP and EP. No proportionality between the height of the maximum of electrical resistivity parameter of pyrolysates in the plasticity state and the yield of OMSDCM was noticed. The pyrolysates (obtained with and without additives) corresponding to the plasticity state and OMSDCM were analyzed with the FT-IR method. It was stated that the additives cause greater changes of structural-chemical parameters in OMSDCM than in the pyrolysates. It was observed that a steady change in composition of the extracted material takes place under the influence of additives in the plasticity state. The amount of determined polycyclic aromatic hydrocarbons (PAHs) decreases almost by 5 times in composition of OMSDCM from the zone of maximum plasticity of LRC under the influence of CTP, and from the weakened zone of HRC – almost by 1.43 times. The addition of EP causes an increase in amount of the determined PAHs in OMSDCM from the weakened zone of HRC by 1.84 times; in the composition of OMSDCM from the gas-saturated zone of LRC the content of PAHs determined during the experiment decreases.

Numerical identification of a viscoelastic substitute model for heterogeneous poroelastic media by a reduced order homogenization approach

The paper deals with the computational homogenization of pressure diffusion processes in a poroelastic medium. The underlying physical phenomena are of interest for the interpretation of seismic data with applications in hydrocarbon production and geothermal energy. Pressure diffusion is assumed to take place on a length scale much smaller than the observer scale. Thus, the macroscopic observer is not able to measure the properties of the poroelastic medium directly but notices an intrinsic viscous attenuation. Under these circumstances, the macro-scale can be interpreted as a single-phase solid with (apparent) viscoelastic properties. In this paper, we establish a numerical upscaling procedure based on a volume averaging concept. This enables us to identify the material properties of the viscoelastic substitute model in a numerically efficient manner. For this purpose, the poroelastic medium on the small scale is modelled in terms of the momentum balance of the biphasic mixture and a coupled diffusion equation. We approximate the poroelastic pressure field on the small scale by a linear combination of pressure modes forming a reduced orthogonal basis and being identified by a Proper Orthogonal Decomposition (POD) technique. From the superposition principle, the evaluation of the poroelastic continuity equation results in a proper identification of the evolution equations defining the apparent viscoelastic model. In comparison to the nested FE2 solution schemes, the reduced order approach only requires a small amount of off-line precomputations and, therefore, causes very low numerical costs. The proposed method is validated for the simple setup of a layered porous rock with alternating water-and gas-saturated zones.

Application of multivariate statistical methods for characterizing heterolithic reservoirs based on wireline logs – example from the Carpathian Foreland Basin (Middle Miocene, SE Poland)

Principal Components Analysis (PCA) and Cluster Analysis (CA) were applied for well log data derived from heterolithic intervals drilled in two boreholes (Mrowla-1 and Cierpisz-2) in the Miocene fill of the Carpathian Foredeep. Both boreholes penetrated similar basement elevations conductive for structural trapping of hydrocarbons in an overlying thin-bedded heterolithic reservoir, which produces gas in commercial quantities in one borehole. The PCA was used to reduce data space preserving sufficient amounts of parameters for a differentiation between thin layers of sandstones and mudstones and between gas- and water-saturated horizons. In both boreholes, the number of logs was reduced to four significant principal components (PCs). Differences between gas-saturated and water-saturated layers were found. CA was used for the classification and grouping of data according to natural petrophysical features of the analysed rocks. The group corresponding to gas-saturated zones was found in the Cierpisz-2 borehole. It is concluded that PCA and CA can provide useful information for a more reliable identification of gas-saturated horizons

Synthesis of deep multichannel seismic and high resolution sparker data: Implications for the geological environment of the Krishna–Godavari offshore, Eastern Continental Margin of India

Proven hydrocarbon occurrence in the Krishna Godavari (KG) basin, a part of the Eastern Continental Margin of India initiated intense exploration activities to discern the potential hydrocarbon bearing structures/payoff zones. Recent discovery of gas hydrates in its offshore triggered detailed exploration measures adopting advanced seismic data acquisition techniques supported by deep drilling to understand its genesis. Interpretation of Multichannel Seismic (MCS) sections highlighted several distinct horizons from Lower Cretaceous to Recent in the sediment overburden, and the nature and configuration of the underlying crust. The sections also depicted the presence of growth faults, shale bulging, toe thrust faulting, delta progradation, active sliding/slumping, buried channels, pinchout structures and reflection free zones, indicating the different types of sedimentary processes, varied geological environments and neotectonic activity in the KG offshore. High resolution sparker (HRS) data revealed subsidence, gas/fluid expulsion pathways, occasional diapiric intrusions and growth faulting in the upper slope, and toe thrust faulting, upthrust and diapiric intrusions in the lower slope. The mid slope is characteried by a well stratified E–W trending basin bounded by upthrust and diapirism. The entire region is associated with gas escape features (blanking zones, columnar type gas vents, fault controlled mud/shale diapirs, large dimension gas saturated zones and pockmarks). The growth faults, shale bulging and toe thrusting seen in the MCS data since Oligocene time could be linked to shale diapirism, upthrust, gas/fluid expulsion, subsidence and mini-basin formation observed on the shallow HRS records. Synthesis of these datasets suggests that the distribution of gas hydrate in the shallow sediments may be controlled by the deeper structural and tectonic configuration. Therefore, knowledge about shallow sedimentary column and its connectivity with deeper strata through faults/fractures and other structural features, and the geological architecture are significantly important to evaluate the genesis of gas hydrates.

The application of well logging and seismic modeling to assess the degree of gas saturation in Miocene strata (Carpathian Foredeep, Poland)

The presence of natural gas in the pore space of reservoir rocks results in a significant decrease in P-wave velocity. Even low gas saturation can generate seismic anomalies (DHI) and false image of gas accumulation of economic importance. This article presents an attempt to evaluate gas saturation from 2D seismic section in the Miocene sandstone strata in the south-eastern part of the Carpathian Foredeep. The ESTYMACJA program and the Biot-Gassmann model were used to study the dependence between elastic parameters and saturating fluids (water and gas) recorded in wells. Series of calculations was carried out using a method of fluid substitution for various gas saturation. The applicability of seismic data for evaluating gas saturation of reservoir beds was assessed with the use of 1D modelling (synthetic seismograms) and 2D modelling (theoretical seismic section) calculated for different gas saturation. The proposed methodology can be used to identify low and high gas-saturated zones and contour the reservoir.

METHOD OF DETERMINING THE RESIDUAL GAS COMPONENT OF COAL-ROCK MASS DONBASS

Method of determining the residual gas component to determine the rock mass zones most suitable for gas and mining areas unsafe coal seams. Definitions of gas-saturated zones and the potential unintended releases in rock mass is carried out for changes in quality and quantity of gas. 

Gas hydrate and free gas petroleum system in 3D seismic data, offshore Angola

Evidence of gas hydrate and free gas occurrences in a 3D seismic volume from the West-Central Coastal Province of the Congo Fan, offshore Angola, illustrates all the components of a complete petroleum system. Analysis and interpretation are based on the information in attributes calculated from three 3D time-migrated common-angle seismic volumes; the attributes include seismic amplitude, spectral components, dip magnitude, amplitude variation with angle, and instantaneous frequency. The source is organic-rich muds associated with late Cretaceous to early Tertiary channels, the migration paths are along growth faults, and the traps are partly defined by the gas hydrate stability zone (for the gas hydrate), partly by the bottom-simulating reflector (for the subhydrate free gas), and partly by faults (for both). The spatial distribution of free gas is further supported by the associated seismic bright spots, and also by the attenuation of high frequencies of P-waves that traverse the gas-saturated zone. Locally higher temperatures, associated with upward fluid circulation along fault zones, facilitate gas transmission through the gas hydrate and forms gas chimneys that extend to the sea floor.

Mathematical modeling of seismic gravity effect of oil and gas reservoir in crystalline basement

A new seismic and gravitational method for the decompacted zones detection in crystalline basement, based on a multi-crack-cavernous pore (gas or oil saturated) model of crystalline reservoir is proposed. To estimate the capabilities of the proposed method mathematical modeling of effective elastic and acoustic properties of granite reservoir rocks with different pore space structure is realized. The algorithm and software for solving the problem of joint inversion of seismic and gravimetric data based on Born's approximation for the space-time Green's function with root mean square background velocity for the horizontally-layered medium has been developed and tested. It has been determined that the most reliable estimation of density and seismic velocity can be obtained by joint inversion. Convincingly is illustrated on an example of the Prykerch shelf model that the presence of decompacted oil or gas saturated zone in granite basement can be confidently identified based on results of seismic and precise gravity observations.

Estimation of free gas saturation from seismic reflection surveys by the genetic algorithm inversion of a P-wave attenuation model

Many previously proposed methods of estimating free gas saturation from seismic survey data rely on calibration to invasively collected, in situ measurements. Typically, such in situ measurements are used to parameterize or calibrate rock-physics models, which can then be applied to seismic data to achieve saturation estimates. We tested a technique for achieving estimates of the spatial distribution of gas saturation solely from shipboard seismic surveys. We estimated the quality factor from seismic reflection surveys using the spectral ratio method, and then inverted a mesoscopic-scale P-wave attenuation model to find the parameters that matched the modeled attenuation to our estimates of observed attenuation within the range of seismic frequencies. By using a genetic algorithm for this inversion, we not only searched efficiently for a global solution to the nonlinear set of equations that compose the model, but also constrain the search to a relatively broad set of realistic parameter values. Thus, our estimates do not rely on in situ measurements of these parameters, but on distributions of their possible values, many of which may be referenced from literature. We first tested this method at Blake Ridge, offshore North and South Carolina, where an approximately 400-m-deep gas-saturated zone underlies a field of methane hydrates. The extensive field work and subsequent studies at this site make it ideal for validating our method. We also demonstrated the applicability of our method to shallower deposits by presenting results from Finneidfjord, Norway, where the inversion of the P-wave attenuation model recognizes very small gas saturations.

Mapping the productive sands of Lower Goru Formation by using seismic stratigraphy and rock physical studies in Sawan area, southern Pakistan: a case study

This study has been conducted in the Sawan gas field located in southern Pakistan. The aim of the study is to map the productive sands of the Lower Goru Formation of the study area. Rock physics parameters (bulk modulus, Poisson’s ratio) are analysed after a detailed sequence stratigraphic study. Sequence stratigraphy helps to comprehend the depositional model of sand and shale. Conformity has been established between seismic stratigraphy and the pattern achieved from rock physics investigations, which further helped in the identification of gas saturation zones for the reservoir. Rheological studies have been done to map the shear strain occurring in the area. This involves the contouring of shear strain values throughout the area under consideration. Contour maps give a picture of shear strain over the Lower Goru Formation. The identified and the productive zones are described by sands, high reflection strengths, rock physical anomalous areas and low shear strain.

High-resolution seismic and resistivity profiling of a buried Quaternary subglacial valley: Northern Alberta, Canada

High-resolution seismic and resistivity profiles were acquired to assist in the delineation of a buried Quaternary subglacial valley, the existence of which was first discovered by drilling. One aspect of this valley is the glacial sediments that host substantial methane at shallow depths (~50 m or less). Geophysical well logs indicate that the glacial sediments in this area generally have lower compressional wave speeds V P 10 Ω·m than adjacent undisturbed Cretaceous-age rocks, which have V P >2200 m/s and r 300 m) valley. The reflection profile reveals a complex series of events consistent with heterogeneous deposition. One major reflection dips eastward across the valley. The reflection character changes abruptly on either side of this event, with dipping clinoform reflections beneath and nearly horizontal reflections above. These may be related to changes in deposition processes beneath a retreating ice sheet, as has been suggested by earlier workers. The buried valley's basal reflections are complex and possibly reflect glaciotectonic deformation or block plucking. The near-surface methane gas is within, but at the edge of, the valley. The methane gas–saturated zone displays anomalously high resistivities both in the well logs and in resistivity tomography.

Identification of seismic anomalies caused by gas saturation on the basis of theoretical P and PS wavefield in the Carpathian Foredeep, SE Poland

The thin-layer build of the Carpathian Foredeep Miocene formations and large petrophysical parameter variation cause seismic images of gas-saturated zones to be ambiguous, and the location of prospection wells on the basis of anomalous seismic record is risky. A method that assists reservoir interpretation of standard recorded seismic profiles (P waves) can be a converted wave recording (PS waves).

Upper Paleozoic petroleum system, Ordos Basin, China

The Ordos Basin is a typical lapped basin, including three sequences of strata: early Paleozoic, late Paleozoic and Mesozoic, with a total thickness of 4000–6000 m. Impermeable sealing beds are well developed at the top and base of the upper Paleozoic sequence, separating it from the Mesozoic and the lower Paleozoic strata to form an independent petroleum system. In this petroleum system, the source rocks are widely distributed coals and dark mudstones occurring in the Carboniferous–lower Permian coal measures, with a thickness of 10–15 and 40–60 m, respectively. The reservoirs are mainly early Permian tight sandstones, mostly with a porosity of 4–8% and a permeability of 0.1–1.0×10 −3 μm 2. The regional cap rock is a 100–150 m thick mudstone in the upper Permian strata. The structural framework of the basin is a huge asymmetric syncline, dipping gently toward the east and north, and steeply toward the south and west. Well data show that gas-saturated, gas–water transition and water-saturated zones are developed from the depositional center to the basin edges. The gas-saturated zone mainly lies in the gently dipping slope area of the Shanbei Slope. Toward eastern and northern up-dip directions the water–gas transitional zone occurs, and finally the water-saturated zone, presenting a reverse relation of water on top of gas. An abnormal negative strata pressure is developed in the gas-bearing area, with a pressure coefficient ( C p) ranging from 0.83 to 0.95. Fluid inclusion data indicate that the upper Paleozoic gas pool began to develop around the Wuqi area at about 150 Ma, and it extended toward the north and was largely formed at about 120 Ma, showing there was a regional migration of the gas-water interface from south to north during the gas pool formation. These characteristics appear to show that the northern and eastern margins of the petroleum system are defined by a regional hydrodynamic regime. The critical moment of the petroleum system occurred during the early Cretaceous. The basin experienced a rapid subsidence and a significant geothermal event, resulting in the source rocks being matured into the main stage of gas generation to form a huge amount of gas. The gas migrated into the tight sandstone reservoirs and displaced the rock pore water to form the gas pools and developing the regional hydrodynamic trap.

The influence of mineral matter on transport of coal plastic mass and on volume of coal charge during carbonisation

The influence of a sand additive on the transport of the plastic mass during carbonization of two orthocoking coals was studied using X-rays. For this purpose, special markers were introduced into the coal charge. During heating the coal charge with the markers in it, X-ray pictures were taken. Based on the X-ray pictures, the data on the displacement of the charge and change in its volume were obtained at various carbonisation stages. It was stated that the addition of sand has a varying effect on mass transport processes in the coal plastic layer and on the volume of the coal charge. When coal Z is heated, an increase in the sand amount leads to a rise in the charge volume at the stage of coal grains swelling but it does not affect the processes of transport in the gas-saturated zone of the plastic layer. When heating coal P, a 1.8 times increase in sand reduces the distance over which the plastic mass displaces in the gas-saturated zone. The increase in sand in the two coals reduces the shrinkage of the semi-coke.

Some aspects of structural transformations taking place in organic mass of Ukrainian coals during heating. Part 1. Study of structural transformations when heating coals of different caking capacity

The structure of the plastic layer in coals of different caking capacity was studied using the X-ray method and the influence of the plastic layer structure on the mechanism of coke formation was determined. It is shown that the transportation of the plastic mass in the gas-saturated zone of the plastic layer has a decisive influence on the decrease in volume of the coal charge and on obtaining a denser residue during carbonization. When heating poorly caking and non-caking coals an increase in the charge volume takes place. Using the quantitative X-ray phase analysis an ordered phase amount ( C cryst) was found in solid residues of carbonization and the parameters d 002, L c and L a were determined. It is shown that the gas-saturated zone is generated in the plastic layer only when structural transformations in coals take place at the pre-plastic stage without changing the ratio between the number of carbon atoms in the ordered and non-ordered phases. The transition of well caking coal into the plastic state is accompanied by the activation of segmental and conformal movement of side chains and separate macromolecular fragments of the coal organic mass. In the course of grain swelling of non-caking coal its crystallites degrade. The conclusions drawn on the basis of X-ray studies were confirmed by the EPR-spectroscopy of residues corresponding to plastic zones in coals of different caking capacity.