scholarly journals Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones

Geophysics ◽  
2020 ◽  
Vol 85 (5) ◽  
pp. MR271-MR283
Author(s):  
Ismael Himar Falcon-Suarez ◽  
Laurence North ◽  
Ben Callow ◽  
Gaye Bayrakci ◽  
Jon Bull ◽  
...  
Keyword(s):  
Stress Field ◽  
Pore Pressure ◽  
P Wave ◽  
Electromagnetic Properties ◽  
Electrical Anisotropy ◽  
High Porosity ◽  
Field Orientation ◽  
Sandstone Sample ◽  
Rock Fabric ◽  
Stress Dependent

Seismic and electromagnetic properties generally are anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, [Formula: see text], and attenuation [Formula: see text]) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20–35 MPa) and pore (2–17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale all affect the elastic and electrical anisotropies. Strong linear correlations ([Formula: see text]) between [Formula: see text], [Formula: see text], and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore-pressure changes (effective stress coefficients [Formula: see text]), whereas P-wave properties provide simultaneous information about the confining (from [Formula: see text], with n slightly less than 1) and pore pressure (from [Formula: see text], with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities.

Joint Focal Mechanism Inversion Using Downhole and Surface Monitoring at the Decatur, Illinois, CO2 Injection Site

2020 ◽  
Vol 110 (5) ◽  
pp. 2168-2187 ◽  
Author(s):  
Nadège Langet ◽  
Bettina Goertz-Allmann ◽  
Volker Oye ◽  
Robert A. Bauer ◽  
Sherilyn Williams-Stroud ◽  
...  
Keyword(s):  
Stress Field ◽  
Pore Pressure ◽  
Monitoring Network ◽  
Microseismic Monitoring ◽  
Fault Reactivation ◽  
Co2 Injection ◽  
Injection Point ◽  
Field Orientation ◽  
Fracture Pressure ◽  
East West

ABSTRACT The three-year CO2 injection period at the Illinois Basin - Decatur Project site (Decatur, Illinois, United States) produced a number of microseismic events distributed in very distinct spatiotemporal clusters with different orientations. Further characterization of the microseismicity encompasses the determination of the event source mechanisms. Initially, the microseismic monitoring network consisted solely of borehole sensors, but has been extended with surface sensors, thereby significantly improving the data coverage over the focal sphere. This article focuses on 23 events from the northernmost microseismic cluster (about 2 km from the injection point) and takes advantage of both, surface and downhole, recordings. The resulting strike-slip east–west-oriented focal planes are all consistent with the east–west orientation of the cluster in map view. The injection-related increase of pore pressure is far below the formation fracture pressure; however, small stress-field changes associated with the pore-pressure increase may reach as far as to the investigated cluster location. Monte Carlo modeling of the slip reactivation potential within this cluster showed that the observed maximum stress-field orientation of N068° is the optimum orientation for fault reactivation of the east–west-oriented cluster. Our results suggest that the east–west orientation of the investigated cluster is the main reason for its activation, even though the cluster is about 2 km away from the low-pressure injection point.

Quantitative CO2 monitoring workflow

2020 ◽  
Author(s):  
Bastien Dupuy ◽  
Anouar Romdhane ◽  
Peder Eliasson
Keyword(s):  
Pore Pressure ◽  
Gravity Data ◽  
Rock Physics ◽  
P Wave ◽  
Added Value ◽  
Monitoring Data ◽  
Storage Site ◽  
High Porosity ◽  
Uncertainty Range ◽  
Fluid Saturation

<p>CO<sub>2</sub> storage operators are required to monitor storage safety during injection with a long-term perspective (Ringrose and Meckel, 2019), implying that efficient measurement, monitoring and verification (MMV) plans are of critical importance for the viability of such projects. MMV plans usually include containment, conformance and contingency monitoring. Conformance monitoring is carried out to verify that observations from monitoring data are consistent with predictions from prior reservoir modelling within a given uncertainty range. Quantitative estimates of relevant reservoir parameters (e.g. pore pressure and fluid saturations) are usually derived from geophysical monitoring data (e.g. seismic, electromagnetic and/or gravity data) and potential prior knowledge of the storage reservoir.</p><p>In this work, we describe and apply a two-step strategy combining geophysical and rock physics inversions for quantitative CO<sub>2</sub> monitoring. Bayesian formulations are used to propagate and account for uncertainties in both steps (Dupuy et al., 2017). We apply our workflow to data from the Sleipner CO<sub>2</sub> storage project, located offshore Norway. At Sleipner, the CO<sub>2</sub> has been injected at approx. 1000 m deep, in the high porosity, high permeability Utsira aquifer sandstone since 1996 with an approximate rate of 1 million tonnes per year. We combine seismic full waveform inversion and rock physics inversion to show that 2D spatial distribution of CO<sub>2</sub> saturation can be obtained. Appropriate and calibrated rock physics models need to take into account the way fluid phases are mixed together (uniform to patchy mixing) and the trade-off effects between pore pressure and fluid saturation. For the Sleipner case, we show that the pore pressure build-up can be neglected and that the derived CO<sub>2</sub> saturation distributions mainly depend on P-wave velocities and on the rock physics model. The CO<sub>2</sub> saturation is larger at the top of the reservoir and the mixing tends to be more uniform. These mixing properties are, however, one of the main uncertainties in the inversion. We discuss the added value of a joint rock physics inversion approach, where multi-physics (electromagnetic, seismic, gravimetry), and multi-parameter inversion can be used to reduce the under-determination of the inverse problem and to better discriminate pressure, saturation, and fluid mixing effects.</p><p>Acknowledgements:</p><p>This publication has been produced with support from the NCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Aker Solutions, Ansaldo Energia, CoorsTek Membrane Sciences, Emgs, Equinor, Gassco, Krohne, Larvik Shipping, Lundin, Norcem, Norwegian Oil and Gas, Quad Geometrics, Total, Vår Energi, and the Research Council of Norway (257579/E20).</p><p>References:</p><p>Dupuy, B., Romdhane, A., Eliasson, P., Querendez, E., Yan, H., Torres, V. A., and Ghaderi, A. (2017). Quantitative seismic characterization of CO<sub>2</sub> at the Sleipner storage site, North Sea. Interpretation, 5(4):SS23–SS42.</p><p>Ringrose, P. S. and Meckel, T. A. (2019). Maturing global CO<sub>2</sub> storage resources on offshore continental margins to achieve 2DS emissions reductions. Scientific Reports, 9(1):1–10.</p>

scholarly journals Simulating stress-dependent fluid flow in a fractured core sample using real-time X-ray CT data

2016 ◽  
Author(s):  
Tobias Kling ◽  
Da Huo ◽  
Jens-Oliver Schwarz ◽  
Frieder Enzmann ◽  
Sally Benson ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Preferential Flow ◽  
Input Parameter ◽  
Matrix Material ◽  
Core Sample ◽  
High Porosity ◽  
Sandstone Sample ◽  
X Ray ◽  
The Matrix ◽  
Stress Dependent

Abstract. The objective of the current study is to investigate and validate stress-dependent single fluid flow in a fractured core sample using in situ X-ray computed tomography (CT) scans and a finite-volume method solving the Navier-Stokes-Brinkman equations. The permeability of the fractured sandstone sample was measured stepwise during a loading-unloading cycle (0.7 MPa to 22.1 MPa and back) to validate the numerical results. Simultaneously, the pressurized core sample was imaged with a medical X-ray CT scanner with a voxel dimension of 0.5 × 0.5 × 1.0 mm3. Fracture geometries were obtained by CT images based on the Missing Attenuation (MA) approach. Simulation results revealed both, qualitative plausibility and a quantitative approximation of the experimentally derived permeabilities. The qualitative results indicate flow channeling along several preferential flow paths with less pronounced tortuosity. Significant changes in permeability can be assigned to temporal and permanent changes within the fracture due to applied stresses. The applied fluid flow simulations also incorporate potential fracture-matrix interaction and permeability anisotropy within the matrix caused by high-porosity layers. The deviations of the quantitative results appear to be mainly caused by the low resolution affecting the accurate capturing of sub-grid scale features and the reproduction of the actual connectivity. Furthermore, the threshold value CTmat (1862.6 HU) depicting the matrix material represents the most sensitive input parameter of the simulations. Small variations of CTmat (±17.7 HU in this study) can cause enormous changes in simulated permeability by up to a factor of 2.6 ± 0.1 and, thus, has to be defined with caution. Finally, our results are also compared with other studies showing similar results. Based on these observations various recommendations to improve CT image quality, model quality, aperture calibration and validation of qualitative fluid flow are provided.

Experimental investigation of the effect of compliant pores on reservoir rocks under hydrostatic and triaxial compression stress states

2019 ◽  
Vol 56 (7) ◽  
pp. 983-991
Author(s):  
Hua Yu ◽  
Kam Ng ◽  
Dario Grana ◽  
John Kaszuba ◽  
Vladimir Alvarado ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Triaxial Compression ◽  
Matrix Material ◽  
Volumetric Strain ◽  
Confining Pressure ◽  
Differential Pressure ◽  
Sandstone Sample ◽  
Stress States ◽  
Rock Springs ◽  
Stage 1

The presence of compliant pores in rocks is important for understanding the stress–strain behaviors under different stress conditions. This paper describes findings on the effect of compliant pores on the mechanical behavior of a reservoir sandstone under hydrostatic and triaxial compression. Laboratory experiments were conducted at reservoir temperature on Weber Sandstone samples from the Rock Springs Uplift, Wyoming. Each experiment was conducted at three sequential stages: (stage 1) increase in the confining pressure while maintaining the pore pressure, (stage 2) increase in the pore pressure while maintaining the confining pressure, and (stage 3) application of the deviatoric load to failure. The nonlinear pore pressure – volumetric strain relationship governed by compliant pores under low confining pressure changes to a linear behavior governed by stiff pores under higher confining pressure. The estimated compressibilities of the matrix material in sandstone samples are close to the typical compressibility of quartz. Because of the change in pore structures during stage 1 and stage 2 loadings, the estimated bulk compressibilities of the sandstone sample under the lowest confining pressure decrease with increasing differential pressure. The increase in crack initiation stress is limited with increasing differential pressure because of similar total crack length governed by initial compliant porosity in sandstone samples.

scholarly journals Comparison of Mechanical Behavior and Acoustic Emission Characteristics of Three Thermally-Damaged Rocks

Energies ◽  
2018 ◽  
Vol 11 (9) ◽  
pp. 2350 ◽  
Author(s):  
Jun Peng ◽  
Sheng-Qi Yang
Keyword(s):  
Mechanical Properties ◽  
Acoustic Emission ◽  
Mechanical Behavior ◽  
Thermal Damage ◽  
Strain Curve ◽  
Treatment Temperature ◽  
P Wave ◽  
High Temperature Treatment ◽  
High Porosity ◽  
Compression Tests

High temperature treatment has a significant influence on the mechanical behavior and the associated microcracking characteristic of rocks. A good understanding of the thermal damage effects on rock behavior is helpful for design and stability evaluation of engineering structures in the geothermal field. This paper studies the mechanical behavior and the acoustic emission (AE) characteristic of three typical rocks (i.e., sedimentary, metamorphic, and igneous), with an emphasis on how the difference in rock type (i.e., porosity and mineralogical composition) affects the rock behavior in response to thermal damage. Compression tests are carried out on rock specimens which are thermally damaged and AE monitoring is conducted during the compression tests. The mechanical properties including P-wave velocity, compressive strength, and Young’s modulus for the three rocks are found to generally show a decreasing trend as the temperature applied to the rock increases. However, these mechanical properties for quartz sandstone first increase to a certain extent and then decrease as the treatment temperature increases, which is mainly attributed to the high porosity of quartz sandstone. The results obtained from stress–strain curve, failure mode, and AE characteristic also show that the failure of quartz-rich rock (i.e., quartz sandstone and granite) is more brittle when compared with that of calcite-rich rock (i.e., marble). However, the ductility is enhanced to some extent as the treatment temperature increases for all the three examined rocks. Due to high brittleness of quartz sandstone and granite, more AE activities can be detected during loading and the recorded AE activities mostly accumulate when the stress approaches the peak strength, which is quite different from the results of marble.

Experimental Investigation of Depletion- and Injection-Induced Changes in Poromechanical, Transport, and Strength Properties of High-Porosity Sandstone

SPE Journal ◽  
2021 ◽  
pp. 1-21
Author(s):  
Saeed Rafieepour ◽  
Stefan Z. Miska ◽  
Evren M. Ozbayoglu ◽  
Nicholas E. Takach ◽  
Mengjiao Yu ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Microstructural Analysis ◽  
Optical Microscope ◽  
Strength Properties ◽  
Rock Properties ◽  
Absolute Permeability ◽  
High Porosity ◽  
Confining Stress ◽  
Stress Changes

Summary In this paper, an extensive series of experiments was performed to investigate the evolution of poromechanical (dry, drained, undrained, and unjacketed moduli), transport (permeability), and strength properties during reservoir depletion and injection in a high-porosity sandstone (Castlegate). An overdetermined set of eight poroelastic moduli was measured as a function of confining pressure (Pc) and pore pressure (Pp). The results showed larger effect on pore pressure at low Terzaghi’s effective stress (nonlinear trend) during depletion and injection. Moreover, the rock sample is stiffer during injection than depletion. At the same Pc and Pp, Biot’s coefficient and Skempton’s coefficient are larger in depletion than injection. Under deviatoric loading, absolute permeability decreased by 35% with increasing effective confining stress up to 20.68 MPa. Given these variations in rock properties, modeling of in-situ-stress changes using constant properties could attain erroneous predictions. Moreover, constant deviatoric stress-depletion/injection failure tests showed no changes or infinitesimal variations of strength properties with depletion and injection. It was found that failure of Castlegate sandstone is controlled by simple effective stress, as postulated by Terzaghi. Effective-stress coefficients at failure (effective-stress coefficient for strength) were found to be close to unity (actual numbers, however, were 1.03 for Samples CS-5 and CS-9 and 1.04 for Sample CS-10). Microstructural analysis of Castlegate sandstone using both scanning electron microscope (SEM) and optical microscope revealed that the changes in poroelastic and transport properties as well as the significant hysteresis between depletion and injection are attributed to the existence and distribution of compliant components such as pores, microcracks, and clay minerals.

The Mechanics of Rock Failure Due to Water Jet Impingement

1974 ◽  
Vol 14 (01) ◽  
pp. 10-18 ◽  
Author(s):  
S.E. Forman ◽  
G.A. Secor
Keyword(s):  
Rock Mass ◽  
Stress Field ◽  
Pressure Distribution ◽  
Pore Pressure ◽  
Static Pressure ◽  
Jet Impingement ◽  
Water Jet ◽  
Rock Properties ◽  
Rock Surface ◽  
Pressure Loading

Abstract The initiation of fracture in a rock mass subjected to the impingement of a continuous water jet has been studied. The jet is assumed to place a quasistatic pressure loading on the surface of the rock, which is treated as a saturated, porous-elastic, isotropic, and homogeneous half-space. While this pressure loading is held constant, the impinging water flows through the rock according to Darcy's law and pressurizes the fluid in the pores. The pore pressure distribution couples with the stress field due to the surface loading to produce an effective stress field, which can start tensile fracturing directly under the load. At various time intervals after initial impingement, the effective-stress field is computed using finite element methods and the results, together with the Griffith criterion for tensile failure, produce the loci of the zones of fracture initiation. The behavior of these zones is displayed as a function of the two jet parameters - pressure and nozzle diameter - and the five rock properties: Young's modulus, Poisson's ratio, tensile strength, porosity and permeability, and time. To experimentally verify that pore pressure plays an important role in the mechanism of rock fracture due to jet impingement, thin sheets of copper (0.001 to 0.005 in.) were placed between a continuous jet (up to 20,000 psi) and the surface of a block of Indiana limestone. The purpose of the copper sheet was to allow the pressure of the jet to be transmitted to the rock, but to prevent water from entering the pore structure. Using pressure substantially greater than the threshold pressure of pressure substantially greater than the threshold pressure of limestone (3,500 psi) where penetration always occurred in the absence of the copper sheet, placement of the sheet was sufficient to prevent any visible damage from occurring to the rock surface, provided the jet did not penetrate the copper first. provided the jet did not penetrate the copper first Introduction The method by which a water jet penetrates and fractures a rock mass is highly complicated and poorly understood. This is mainly because the rock is subjected during the impact to several separate processes, each of which can cause failure. Failure can result from the effects of dynamic stress waves, static pressure loading and erosion. The degree of failure caused by each mechanism is, of course, dependent on the rock properties and jet parameters. parameters. In the first few microseconds of impingement, a subsonic jet pressure on the rock surface reaches the so-called "water hammer" pressure on the rock surface reaches the so-called "water hammer" pressure of pvv(c) and then drops to the nozzle stagnation pressure pressure of pvv(c) and then drops to the nozzle stagnation pressure of approximately 1/2 pv2. (p = fluid density, v = jet velocity, and v(c) = velocity of compression waves in the liquid.) During this initial period of impact, large-amplitude compressive waves are caused to emanate from the point of impingement. Upon reflection off a free surface, these waves become tensile and can cause spalling failures. This mode of failure is usually important with pulsed jet impingement. For continuous jets the spalling effects are small and will be neglected for this study. During the impingement process, the water of the jet flows into the accessible pore space of the rock mass. Since in a continuous jetting process the jet applies a quasi-static pressure loading to the rock surface, the water in the pores is pressurized while the surrounding rock mass is simultaneously stressed. The intent of this paper is to describe the role played by this static pressure loading coupled with the pore-pressure distribution, or pressure loading coupled with the pore-pressure distribution, or the "effective stress," in the first moments of penetration. In studying the process, we will take into account the influence of jet parameters and rock properties. In the course of the impingement process, the jet pressure loading is constantly being redistributed over the crater as it is formed. During this progressive removal of material, erosion is also contributing. The process of erosion is in itself highly complex, so no attempt will be made to characterize it here. EFFECTS OF STATIC PRESSURE DISTRIBUTION-ZERO PORE PRESSURE It has been shown by Leach and Walker that a water jet emanating from the nozzle depicted in Fig. 1 applies a quasi-scatic pressure loading to the surface upon which it is impinging. SPEJ P. 10

Deformation of Chalk Under Confining Pressure and Pore Pressure

1981 ◽  
Vol 21 (01) ◽  
pp. 43-50 ◽  
Author(s):  
Thomas Lindsay Blanton
Keyword(s):  
Mechanical Behavior ◽  
Pore Pressure ◽  
Hydrostatic Stress ◽  
Confining Pressure ◽  
Low Permeability ◽  
High Porosity ◽  
Pore Collapse ◽  
Soft Matrix ◽  
Pore Pressures ◽  
Ductile Behavior

Abstract Compression tests with and without pore pressure have been run on Danian and Austin chalks. The rocks yielded under increasing hydrostatic stress by pore collapse. The same effect was produced by holding a constant hydrostatic stress and reducing the pore pressure. This pore collapse reduced the permeability. The ultimate strength of the chalks increased with increasing confining pressure. The yield strength increased initially, but at higher confining pressures it decreased until it yielded under hydrostatic stress. Relatively high pore-pressure gradients developed when the chalks. were compressed. In these situations, the mechanical behavior tended to be a function of the average effective stresses. Introduction Hydrocarbons have been found in chalks in the North Sea, the Middle East, the Gulf Coast and midcontinent regions of the U.S., and the Scotian Shelf of Canada1; however, problems have been encountered in developing these reservoirs efficiently because of the unusual mechanical behavior of chalk. Chalks have three characteristics that interact to differentiate their behavior from most reservoir rocks. High Porosity. Porosities may be as high as 80070.1,2 Effects of burial and pore-water chemistry can reduce this porosity to less than 1%, but notable exceptions occur in areas of early oil placement and overpressuring where porosities in excess of 40% have been reported.2,3 Low Permeability Regardless of porosity, chalks have low permeabilities, usually around 1 to 10 md. Soft Matrix. Chalks are predominantly calcite, which has a hardness of 3 on Mohr's scale. These properties create problems in the following areas of reservoir development. Drilling. High porosity combined with a soft matrix material makes for a relatively weak and ductile rock. Efficient drilling involves chipping the rock and ductile behavior inhibits this process. Stimulation. The combination of high porosity and low permeability makes chalks prime candidates for stimulation by hydraulic fracturing or acid fracturing. The best production often is associated with natural fractures.2,3 Man-made fractures could open up new areas to production, but again ductile behavior inhibits the fracturing process. Production. In many cases permeabilities are low enough to trap pore fluids and cause abnormally high pore pressures.2 These high pore pressures help maintain the high porosities at depth by supporting some of the weight of the overburden. As the field is produced and the pore pressure lowered, some of the weight will shift to the soft matrix. The result may be pore collapse and reduction of an already low permeability. These problems indicate a need for basic information on the mechanical behavior of chalks. Determining methods of enhancing brittle behavior could lead to improved drilling and stimulation techniques. The ability to predict and prevent pore collapse could increase ultimate recovery. The approach taken in this study was experimental. Specimens of chalk were subjected to different combinations of stress and pore pressure in the laboratory, and the resulting deformations were measured.

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