Homogeneous Sandstone Research Articles

The Ichthys Field: challenges of geological modelling for the field development

Preparation for the Ichthys Field development has been underway since the final investment decision was made in January 2012. The Ichthys Field comprises two reservoirs: the Brewster Member of the Early Cretaceous Upper Vulcan Formation; and the Jurassic Plover Formation. The development will be phased in separate drilling campaigns, commencing with the core area of the Brewster Member reservoir. This reservoir has geological challenges, including: complexity of porosity and permeability distribution in the generally homogeneous sandstone reservoir; and, high gas saturation immediately above and below the free water level. The Ichthys Subsurface team has built geological models of the Brewster Member reservoir. The following methodologies were adopted into the geological models: broad-band seismic inversion data to distribute reservoir porosity away from well control; K Nearest Neighbour methodology to predict permeability from logs utilising core measurements; Cathode-luminescence analysis to understand permeability reduction within the field; imbibition Sw height functions to model high gas saturation immediately above and below the free water level; and, scenario-based geological uncertainty modelling. A series of geological models were utilised to optimise development well locations and to ensure sufficient production from the reservoir.

The Detectability of Free-phase Migrating CO2: A Rock Physics and Seismic Modelling Feasibility Study

Subsurface monitoring is essential for the successful implementation and public acceptance of CO 2 storage. Injected CO 2 will need to be monitored to verify the successful containment within its intended formation, and to ensure no loss of containment within the storage complex. The ability for seismic techniques to monitor structurally trapped CO 2 has been successfully demonstrated due to the changes in the acoustic properties of the reservoir produced by the displacement of brine by less dense and more compressible CO 2 . However, the ability for seismic methods to detect free-phase migrating CO 2 is still moderately understood. In order to assess the feasibility for seismic monitoring of a migrating front, we estimate the time-lapse signal over a theoretical, clean, homogeneous sandstone reservoir through the application of a three-stage model-driven workflow consisting of fluid-flow, rock physics and seismic forward modelling. To capture the range of responses which could be encountered, two end-member fluid distribution models were used: uniform saturation and the modified patchy saturation model. Analysis of the time- lapse survey highlights the importance of determining and understanding the fluid distribution model impacting the range of velocities prior to generating and interpreting the seismic response. This change in velocity is shown to be directly related to the volume of CO 2 occupying the pore-space of a migrating plume front. This highlights the fact that the detectability of a migrating front is a site specific issue which not only depends on the geophysical parameters of the seismic survey but also on the geological variations and spatial distribution in the reservoir.

Changes of petrophysical properties of sandstones due to interaction with supercritical carbon dioxide – a laboratory study

Changes of petrophysical, petrological, mineralogical, mechanical and chemical parameters were studied on sandstones from the Hessian Depression and sandstones from Neidenbach (Eifel) before and after alteration with CO2. The experiments were performed in a wide pressure and temperature range (10 85 wt%, density from 2.62–2.70 g/cm3, porosity from 25 %, and permeability from <10−17 up to 10−12 m2. In both experimental setups, porosity increased by less than 2 vol%. The increase in permeability was less than one order in magnitude for setup (1), and more than 1.5 orders of magnitude for setup (2). The mineralogical composition was unchanged within the detection limit of X-ray powder diffraction (XRD), whereas X-ray fluorescence analysis (XRF) indicated mobilisation of Ca, Mg, Al, and K. In addition, dissolution was confirmed by the chemical analysis of recovered artificial brines using inductively-coupled plasma optical-emission-spectroscopy (ICP-OES), which showed an increase in the concentration of Ca, Mg, Al, and K after the scCO2-experiments. Partial solution of feldspar and clay was detected by optical inspection and SEM analysis. Low frequency electrical conductivity experiments (spectral induced polarization, SIP) exhibited both a significant increase in conductivity, which could be explained by dissolution at narrow pore throats thus causing a higher degree of interconnection of the pore system, and a shift of the phase angle, which indicates changes of the geometry of the pore-surface area. The uniaxial compressive strength was measured before and after scCO2-treatment on a set of homogeneous sandstones from Neidenbach (Eifel, Germany). These data were compared with natural analogues, e.g . bleached and unbleached sandstones from the Hessian depression. The uniaxial compressive strengths of untreated and scCO2-treated samples were found to fit the range reported for sandstones.

Mapping the distribution of weathered Pleistocene wadi deposits in Southern Jordan using ASTER, SPOT-5 data and laboratory spectroscopic analysis

In the arid regions of the Levant, ancient wadi fills act as a terrestrial sedimentary archive with a high potential for preserving archaeological findings. This current study combines remote sensing with laboratory VIS-spectroscopy to investigate the spatial distribution of alluvial wadi fills in a small catchment in Southern Jordan. Due to its homogeneous sandstone geology, the composition of the alluvial sediments is highly influenced by the local bedrock whilst fluvial relocation and surface weathering processes initiated a secondary alteration of dominant iron oxides (Fe3 +). The differences in mineralogical composition of the sediments enable the detection and mapping of wadi deposits by remote sensing using different spectral combinations of ASTER and SPOT-5 satellite images. Additionally, laboratory measurements of reflectance spectra were applied on selected surface samples and soil sections from the study area in order to verify the information derived from remote sensing and to quantify the degree of surface weathering and pedogenic processes. The results show that an initial transformation from hematite to goethite is the dominant process related to the recent arid conditions in the study area. Furthermore, it is possible to predict potential new archaeological finding areas using remote sensing techniques.

Reservoir Geology, Hydrocarbon Reserves and Production in the Croatian part of the Pannonian Basin

3 of gas (52 fi elds), were recovered in the Croatian part of the Pannonian Basin System during 64 years of exploitation (1941- 2005). The production peak was attained between 1980-1989, when exploitation began in 12 new fi elds. Based on their cumulative production, the Croatian oil and gas fi elds can be divided into four groups, and the condensate fi elds into three groups. Such a division has been supported by analysis of recovery, number of reservoirs, porosity and permeability, age and lithology of reservoir rocks. The longest production period is assumed for the fi rst group of fi elds; for oil it is approximately 55 years, for condensate 46 and gas 36 years. In the favourable fi rst group the aver- age number of reservoirs is 16 for oil and 11 for gas. Lithological composition is highly favourable, because reser- voirs are represented mostly by sandstones of Pannonian and Pontian age with high porosities and permeabilities. A relatively homogeneous sandstone lithology, including good regional seals like marls, enables an increase in recov- ery through the use of secondary and tertiary recovery methods. Also, water-fl ooding will remain the dominant sec- ondary-recovery method for increased production in the future.

Technology Focus: Reservoir Simulation (July 2011)

Technology Focus Simulation is about building a mathematical model of the reservoir and then using it to predict, for the purpose of making decisions. That may be restating the obvious, but there are a few points that bear a little re-emphasis. First, it is only a model. Second, if the model does not predict adequately, it does not serve its purpose. There is a growing tendency in some quarters to use very simple models—mainly because they are faster. As my colleague Ed May recently put it, “I know everyone pushes for faster, faster, but doing it right has definite advantages.” Following my recent move into well testing, I have been rereading the late Laurie Dake’s The Practice of Reservoir Engineering. Dake often is regarded as hostile to simulation, but what he really was about was understanding your assumptions, and using the most appropriate model for the decision to be made. At the 2011 SPE Reservoir Simulation Symposium, many papers focused on re-examining original basic assumptions. Correcting or removing those assumptions (improving the model) often led not only to better predictions, but to faster simulation. For example: Is the most recent solution of the well model the best starting point for the next solution? As the industry develops more-complex reservoirs (e.g., shales, naturally fractured, and heavy oil), we are called on to apply our simulation tools to new environments, and we must check constantly that the assumptions that are implicit in those tools are still valid. For example: Do assumptions for light-oil flow in homogeneous sandstone also apply to gas flow in shale? The need to check assumptions applies as much to the data as to the tools. In simulation support, we often see data sets in which hundreds of warning messages about bad data are ignored or even suppressed, and the complaint is about the final failure to converge. Is that discontinuity in the pressure/volume/temperature data really a property of the fluid, or just a measurement artifact? A good way to consistently challenge your assumptions is to read the best of the best technical articles: JPT is here for you! Reservoir Simulation additional reading available at OnePetro: www.onepetro.org SPE 144401 • “Design and Examination of Requirements for a Rigorous Shale-Gas-Reservoir Simulator Compared to Current Shale-Gas Simulators” by Juan Andrade, SPE, University of Oklahoma, et al. SPE 140882 • “Multilateral Complex-Well Optimization” by P.I. Crumpton, Schlumberger, et al. SPE 137507 • “Reservoir Characterization and Flow Simulation of a Low-Permeability Gas Reservoir: An Integrated Approach for Modeling Tommy Lakes Gas Field” by H. Deng, University of Calgary, et al. (See Journal of Canadian Petroleum Technology, May 2011, page 32.) SPE 141935 • “An Adaptive Newton’s Method for Reservoir Simulation” by Pengbo Lu, ExxonMobil, et al. SPE 142093 • “A New Method To Model Relative Permeability in Compositional Simulators To Avoid Discontinuous Changes Caused by Phase-Identification Problems” by Chengwu Yuan, SPE, University of Texas at Austin, et al.

Downscaled Capillary Pressure and Relative Permeability of Small Cores for Network Model Validation

This paper presents the results of drainage capillary pressure and relative permeability measurements made on cores having different bulk volumes ranging from 0.5 to 12 cm3. The aim of the experiments was to provide reliable experimental data which can be used to validate the predictive value of micro-CT based network models for capillary pressure and relative permeability. The micro-CT based network models use realistic networks constructed from the X-ray images of the rock samples having a typical bulk volume of 0.3 cm3. Experimental data for drainage capillary pressure were obtained using the centrifuge technique. The results of the largest cores were verified by the data obtained on the same sample using the porous plate technique. Relative permeability data were obtained by history matching the unsteady state displacement data. Homogeneous model sandstones (Berea and Bentheim) and carbonate (Mt. Gambier) were used in the experiments. Air-brine and oil-brine fluid-systems were used for drainage capillary pressure and relative permeability measurements, respectively. The relative permeability data were compared with those predicted from empirical and geometry based models using capillary pressure data. Good agreement was obtained for the drainage capillary pressure measured on all samples used. The residual saturations obtained from the cores used in the displacement experiments were also in good agreement. The models were found to predict relative permeability of oil and water with varying degrees of success. For water relative permeability, the Pirson model predicts the experimental data successfully while the Corey, Corey-Brooks/Burdine and van Genuchten/Burdine models predict the data of oil relative permeability better than the others. The results demonstrate for the first time that reliable drainage capillary pressure and relative permeability measurements can be made on small model sandstone and carbonate cores of representative scales used in micro-CT-imaging.

The Analysis and Theory Research on the Factor of Multiple Fractures During Hydraulic Fracturing of CBM Wells

The multiple fracture theory is the hydraulic fracturing frontier theory, especially for the coal the fractures, physical and mechanical properties of which is different from homogeneous sandstone. The study of multiple fracture theory is difficult and faces many problems. According to coal rock mechanics and structural characteristics and based on the analysis of stress field of coal near by the hydraulic fracture, The results of the theory study shown that sub-fracture of tension easily formed in the area near the well and sub-fracture of shear formed in the area far from the well more easily in process of coal fracturing. And the paper revealed the development rule of hydraulic fracture.

High Resolution Imaging of Unstable, Forced Imbibition in Berea Sandstone

An experimental investigation is presented of immiscible, high-mobility ratio forced imbibition in a representative linear homogeneous sandstone. Water floods with mobility ratios (from 1 to 155) at various water injection rates were conducted. Fine-scale (order mm3) in situ water saturation history was collected via X-ray computed tomography (CT). Three-dimensional images were constructed of stable displacement and the initiation and growth of unstable water fingers. Interestingly, viscous fingers do not lead the displacement front by significant distances, counter to experience in miscible systems. In this homogeneous porous medium, both water (displacing phase) injection rate and oil (displaced phase) viscosity have an obvious effect on the stability of the water front. As the oil viscosity and displacement rate increase, the water front becomes less stable. In addition, the so-called shock mobility ratio, as computed from steady-state relative permeability, is found to be predictive regarding displacement front stability. When the shock mobility ratio is greater than 1, the displacement is always unstable. Steady-state relative permeability, however, is found to be a function of viscosity ratio for unstable displacements.

Comparison of Positron Emission Tomography and X-ray radiography for studies of physical processes in sandstone

The migration of moisture in cores of porous homogeneous sandstone of Ledian age (Belgian Eocene, Tertiary) is monitored with PET ( Positron Emission Tomography) and micro-focus X-ray radiography. In the case of PET, a nuclear medical imaging technique, 55Co-EDTA ( Ethylene diamine Tetra acetic acid) and R- 18F were used as water-soluble tracers. The X-ray projection method has evolved from the better-known medical technique and allows a fast and accurate determination of the two-dimensional transient moisture content profiles. Results indicate that both techniques can deliver important information concerning physical processes in situ.

The Effects of Macropores in Heterogeneous Porosity Systems on Electrical Resistivity Behavior

To better understand the petrophysical properties, such as electrical resistivity behavior, capillary pressures and displacement behavior, of hydrocarbon bearing carbonate rocks with a macropore system, a porous plate capillary desaturation cell was built. Lucia's carbonate classification was selected to be used in this research. Electrical end effects were minimized by utilizing a four-electrode system. An oil/ water system and a gas/ water system have been studied. The effects of vugularity on the petrophysical properties were evident from the experiments. For a constant porosity, as vugularity increases, cementation exponent (m) and saturation exponent (n) increase systematically. The irreducible water saturation, the capillary pressure curves and the displacement behaviors are also substantially different from homogeneous sandstone experiments.

A Numerical Study of Acoustic Wave Propagation in Partially Saturated Poroelastic Rock

AbstractBased on the Biot theory we use a high‐order staggered finite‐difference algorithm to carry out numerical experiments on wave propagation in a rock sample partially saturated by multi‐phase fluids with random fluid distributions. The ultrasonic source frequencies are 25, 50, 75kHz and 100kHz, respectively. The first modeling sample is homogeneous isotropic sandstone partially filled with two phase pore fluids (water and gas), which are defined by their different elastic characteristic parameters of saturation, viscosity, compressibility, and density, respectively. We assume that the two different pore‐fills occupy different macroscopic regions and the wavelength of acoustic wave is much larger than the scale of these regions. The result shown that the P‐wave velocity and attenuation change with fluid saturations (water and gas) and central frequencies of the acoustic source. These numerical results are qualitatively in agreement with the White and Gassmann's models. The second modeling sample is partially filled with 3 phase pore fluids (water, oil, and gas). We keep the saturation of water with a constant and change the saturations of oil and gas. Under these conditions, the velocity of P‐wave shows an increasing trend with the increasing saturation of oil. The snapshot of the acoustic wave field shows that the energy of P‐wave is converted into several kinds of slow P‐waves and S‐waves which occur at the interfaces between water, oil and gas. These modeling results have great significance in seismic exploration and acoustic well logging interpretations.

The Cyborg Astrobiologist: scouting red beds for uncommon features with geological significance

The `Cyborg Astrobiologist' (CA) has undergone a second geological field trial, at a red sandstone site in northern Guadalajara, Spain, near Riba de Santiuste. The Cyborg Astrobiologist is a wearable computer and video camera system that has demonstrated a capability to find uncommon interest points in geological imagery in real-time in the field. The first (of three) geological structures that we studied was an outcrop of nearly homogeneous sandstone, which exhibits oxidized-iron impurities in red and and an absence of these iron impurities in white. The white areas in these ``red beds'' have turned white because the iron has been removed by chemical reduction, perhaps by a biological agent. The computer vision system found in one instance several (iron-free) white spots to be uncommon and therefore interesting, as well as several small and dark nodules. The second geological structure contained white, textured mineral deposits on the surface of the sandstone, which were found by the CA to be interesting. The third geological structure was a 50 cm thick paleosol layer, with fossilized root structures of some plants, which were found by the CA to be interesting. A quasi-blind comparison of the Cyborg Astrobiologist's interest points for these images with the interest points determined afterwards by a human geologist shows that the Cyborg Astrobiologist concurred with the human geologist 68% of the time (true positive rate), with a 32% false positive rate and a 32% false negative rate. (abstract has been abridged).

Use of Variography in Permeability Characterization of Visually Homogeneous Sandstone Reservoirs with Examples from Outcrop Studies

Sandstones of different ages provide economically significant oil, gas, and water reservoirs. In sandstones where heterogeneities are not visually obvious, it is particularly difficult to predict the location of permeability barriers and the scale at which high and low permeability zones occur, yet this is critical in providing information on hydrocarbon reservoir performance. This study uses variogram analysis to investigate spatial variation in permeability in visually homogeneous reservoir sandstone successions. Air permeability measurements were taken using unsteady state probe permeametry following regular grid schemes with centimeter spacing. Spatial variation in permeability was characterized using omnidirectional and directional variograms. This study combines variography with geological interpretation to assess the degree of heterogeneity of permeability in visually homogeneous sandstone successions. Variography indicates spatial dependence and short-range variation at 1 cm grid spacings that is not apparent at a larger 5 cm grid spacing in the visually homogeneous sandstones studied. The range of the models fitted to the variograms provide a potentially important index of spatial variability in permeability for different depositional settings including aeolian, fluvial, shallow marine, and marine/mass- flow turbidite.

Fossilized worm burrows influence the resource quality of porous media

Burrow-associated, selective dolomitization in the Yeoman Formation limestone (Ordovician, Williston basin) is characterized by distinct textural heterogeneity. Physical parameters such as permeability, porosity, tortuosity, and dispersivity are therefore difficult to assess. This study compares the relative dispersivities of three geologic media: homogeneous sandstone, fractured limestone, and burrowed dolomitic limestone. Results show that the flow paths present in burrow-associated dolomite are tortuous, and that the interaction between the flow paths and the matrix is extensive. Such rocks act as dual-permeability systems in the subsurface. Hydrocarbon production from such deposits will be strongly influenced by burrow-related heterogeneity, and its influence should be carefully considered before secondary recovery schemes are implemented.

White's model for wave propagation in partially saturated rocks: Comparison with poroelastic numerical experiments

We use a poroelastic modeling algorithm to compute numerical experiments of wave propagation in White's partial saturation model. The results are then compared to the theoretical predictions. The model consists of a homogeneous sandstone saturated with brine and spherical gas pockets. White's theory predicts a relaxation mechanism, due to pressure equilibration, causing attenuation and velocity dispersion of the wavefield. We vary gas saturation either by increasing the radius of the gas pocket or by increasing the density of gas bubbles. Despite that the modeling is two dimensional and interaction between the gas pockets is neglected in White's model, the numerical results show the trends predicted by the theory. In particular, we observe a similar increase in velocity at high frequencies (and low permeabilities). Furthermore, the behavior of the attenuation peaks versus water saturation and frequency is similar to that of White's model. The modeling results show more dissipation and higher velocities than White's model due to multiple scattering and local fluid‐flow effects. The conversion of fast P‐wave energy into dissipating slow waves at the patches is the main mechanism of attenuation. Differential motion between the rock skeleton and the fluids is highly enhanced by the presence of fluid/fluid interfaces and pressure gradients generated through them.

A criterion for crack initiation and propagation of intact rocks under creep condition

The mechanism and criterion of crack initiation and propagation of rocks were investigated by many researchers. And the creep behaviour of rocks was also theoretically and experimentally studied by some scientists and engineers.The characteristics of crack initiation and propagation of rocks under creep condition, however, are very important for rock engineering and still not paid enough attention by researchers. In this paper, the criterion and mechanism ofcrack initiation and propagation under creep condition were investigated using specimens collected from sandstone rock formations outcropping in the Emei Mountain, the Sichuan Province of China. Cuboid specimens under three point bendingwere used in this investigation. All specimens were classified into four sorts and used for Mode-I fracture or creep fracture tests. The experimental result shows that due to creep deformation,rock crack will inevitably initiate and propagate under a load of K I, which is less than fracture toughness K ICbut not less than a constant (marked as K IC2). K IC2indicates the ability of rock to resist crack initiation and propagation under creep conditions and is less than fracture toughness K IC, defined as creep fracture toughness in this paper.K IC2 should be considered as an important parameter on design and computation of rock engineering. The microstructural mechanism for crack initiation and propagation of rock materials undercreep condition was introduced based on competitive model between softening effect and hardening effect, and the validity of test result was explained. The test result was also verified in rheological theory. When K Iis more than K IC2 but less than K IC, rock crack will initiate and propagate after a time interval of sustained loading under creep condition.In order to find the relation between duration of sustained loading, which can lead to crack initiation and propagation, and the initial stress intensity factor K I, an unequal-interval time sequence forecasting andpredicting model was introduced, and the relation was obtained for homogeneous and isotropic fine-grained red sandstone. Finally a modified fracture toughness formula was given, in which the influence of fracture process zone(FPZ) was fully considered.

Investigation of the fracture process with different loading path triaxial tests on saturated sandstone

This paper discusses a detailed investigation to determine the microcrack closure and opening of a homogeneous isotropic sandstone during four different triaxial tests: undrained deviator triaxial test; undrained triaxial test with constantly increasing stress; drained triaxial test connected with decreasing confining pressure and drained triaxial test with constantly increasing stress. An analysis of the results is presented.

Sensitivity of reflection seismic data to oil‐column height in high‐porosity sandstones

Tuning is the effect of interference between the reflections from the top and bottom of a thin layer on the amplitude of the composite reflection. For a homogeneous sandstone reservoir containing an oil column overlying brine, interference between the reflection from the top reservoir and the oil/water contact is a function of the height of the oil column. If the properties of the sandstone do not vary across the oil/water contact, the SS, PS and SP reflection coefficients from the oil/water contact are small in comparison to the PP reflection coefficient. This allows analytic expressions for the effective PP and PS reflection coefficients from the reservoir to be derived that include all P‐wave multiples within the oil column. For a given source/receiver offset, the component of the wavevector inside the oil column normal to the interface is larger for the PPPP reflection than for the PPPS reflection, due to the asymmetry in the raypath for the PPPS reflection. The PPPS reflection is therefore useful for determining oil‐column heights larger than that discriminated by the PPPP reflection, especially when used at wider offsets.A convenient classification of the AVO response of hydrocarbon‐bearing sandstone reservoirs overlain by shale is the scheme of Rutherford and Williams. Class 1 sands have higher acoustic impedance for normal incidence than the overlying shale, Class 2 sands have nearly the same acoustic impedance as the shale and Class 3 sands have lower acoustic impedance. Synthetic shot gathers calculated for these three classes as a function of oil‐column height show that a combination of the PPPP and the PPPS amplitudes can be plotted as a tuning trajectory, which can be used to determine the oil‐column height. This method is most sensitive for reservoirs that belong to AVO classes 1 and 2, and therefore may be useful in AVO analysis of Class 1 and 2 reservoirs where the traditional AVO indicators (developed for Class 3 reservoirs) do not work very well. These results demonstrate the usefulness of shear waves recorded in the marine environment at wide offsets.

The use of air decks in production blasting in an open pit coal mine

The influence of air deck blasting on blast performance and blast economics and its feasibility has been studied in the production blasting of soft and medium strength sandstone overburden rocks in an open pit coal mine in India. The air deck blasting technique was very effective in soft and medium strength rocks. Its main effects resulted in reducing fines, in producing more uniform fragmentation and in improving blast economics. The fines were reduced by 60–70% in homogeneous sandstones. Oversize boulders were reduced by 80% and shovel loading efficiency was improved by 20–40% in blocky sandstones. The explosive cost was reduced by 10–35% dependent on the type of rock mass. Throw, backbreak and ground vibration were reduced by 10–35%, 50–80% and 30–94% respectively. For a particular rock mass and blast design environment, air deck length (ADL) significantly influenced the fragmentation. ADL as represented by air deck factor (ADF) in the range of 0.10–0.35 times the original charge length (OCL) produced optimum results. ADF beyond 0.35 resulted in poor fragmentation and in inadequate burden movement.