Pressure‐dependent seismic response of fractured rock

Geophysics ◽  
2004 ◽  
Vol 69 (4) ◽  
pp. 885-897 ◽  
Author(s):  
Evgenii A. Kozlov
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Water Saturation ◽  
Fractured Rock ◽  
Gas Production ◽  
Fluid Replacement ◽  
Seismic Velocities ◽  
Amplitude Versus Offset ◽  
Model Predictions ◽  
Avo Attributes

Effects of external stress and pore pressure variations on the seismic signature of fractured rocks remain of interest to geoscientists and practicing geophysicists. Commonly, the effects are modeled theoretically, assuming fracture faces to be rough surfaces contacting each other via the surface asperities. The model proposed here differs from other models of this kind in that (1) fracture roughness is described by a single parameter and (2) a controlled degree of hydraulic connectivity between fractures and equant pores is introduced. This adds to the model's convenience and makes it applicable to a wide variety of reservoirs. The model predictions of seismic velocities in fractured rock at variable stress are consistent with experimental data. For fixed effective stress, the model predictions coincide with those obtained using the model with ellipsoidal fractures of certain average aspect ratio and the same fracture porosity. Apart from known effects, the model introduced predicts an amplification of the stress variation influence on fracturing‐induced anisotropy with an increase of connected equant porosity, a decrease of VP/VS with effective stress, and implicit frequency dependence of the VP/VS relation. It is also shown that amplitude versus offset (AVO) anomalies caused by fluid replacement can be seriously distorted if the fluid replacement is accompanied by significant variations of pore pressure, as, for example, at intense gas production. Neglecting these effects can lead to erroneous conclusions on shear modulus dependence on the pore fluid type. Qualitatively, in rocks with azimuthally aligned fracturing, the increase of effective stress affects AVO gradient in about the same way as the increase of water saturation parameter Vw. In contrast, the AVO intercept is not affected by variations of effective stress, while fluid replacement effect on the intercept is significant. Potentially, this can help distinguish the effects of pore pressure variations and fluid replacement on the AVO attributes.

An integrated pore-pressure model and its application to hydrocarbon exploration: A case study from the Mahanadi Basin, east coast of India

2014 ◽  
Vol 2 (1) ◽  
pp. SB17-SB26 ◽  
Author(s):  
Ajesh John ◽  
Ashutosh Kumar ◽  
Karthikeyan G. ◽  
Pankaj Gupta
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
High Pressures ◽  
Flow Direction ◽  
Hydrocarbon Exploration ◽  
Seismic Velocities ◽  
Gas Reservoirs ◽  
High Quality ◽  
Late Pliocene ◽  
Mahanadi Basin

An integrated pore-pressure modeling approach was adopted to understand the basin architecture from a pressure perspective and its inference toward possible hydrocarbon occurrence. Kriging-based 3D pore-pressure modeling was used with offset well data and seismic velocities to establish the pressure stratigraphy of the northeast coast (NEC) field (southern part) in the Mahanadi Basin. Late Pliocene sediment is moderately pressured ([Formula: see text]), whereas early Pliocene sediment is normally pressured ([Formula: see text]) and compacted, representing a regional seal for this part of the basin. Miocene represents the onset window for major undercompaction and associated high pressures ([Formula: see text]) in conformance with the regional pressure trend. Overpressure distribution and its mechanisms in the late Miocene level across the NEC field shows distinct patterns with highly elevated pressures ([Formula: see text]) in the northern part resulting from a hybrid unloading mechanism, whereas moderate to high pressure ([Formula: see text]) toward the southern part is associated with undercompaction. Regional pressure correlation across the study area suggests a pressure dependent habitat of hydrocarbons in the Miocene and late Pliocene levels. Pressure distribution and an excess pressure pattern within the Miocene stratigraphy shows a regression trend from north to south, possibly indicating a preferred subsurface fluid flow direction, which is supported by high-quality gas reservoirs discovered in the southern part of the study area. A similar but reverse pressure regression trend is observed within the late Pliocene stratigraphy, which is also validated by the presence of gas reservoirs in the northern part of the study area. Major hydrocarbon reservoirs in the Miocene and Pliocene stratigraphy from the southern part of study area exhibit a strong correlation with effective stress distribution. High-quality gas reservoirs are mostly associated with high effective stress ([Formula: see text]), whereas a high probability for reservoirs to be water wet are observed below this threshold value.

scholarly journals Tight gas production model considering TPG as a function of pore pressure, permeability and water saturation

2020 ◽  
Vol 17 (5) ◽  
pp. 1356-1369
Author(s):  
Atif Zafar ◽  
Yu-Liang Su ◽  
Lei Li ◽  
Jin-Gang Fu ◽  
Asif Mehmood ◽  
...  
Keyword(s):  
Pressure Gradient ◽  
Pore Pressure ◽  
Water Saturation ◽  
Great Influence ◽  
Gas Production ◽  
Production Model ◽  
Threshold Pressure ◽  
Tight Gas ◽  
Gas Field ◽  
Threshold Pressure Gradient

Abstract Threshold pressure gradient has great importance in efficient tight gas field development as well as for research and laboratory experiments. This experimental study is carried out to investigate the threshold pressure gradient in detail. Experiments are carried out with and without back pressure so that the effect of pore pressure on threshold pressure gradient may be observed. The trend of increasing or decreasing the threshold pressure gradient is totally opposite in the cases of considering and not considering the pore pressure. The results demonstrate that the pore pressure of tight gas reservoirs has great influence on threshold pressure gradient. The effects of other parameters like permeability and water saturation, in the presence of pore pressure, on threshold pressure gradient are also examined which show that the threshold pressure gradient increases with either a decrease in permeability or an increase in water saturation. Two new correlations of threshold pressure gradient on the basis of pore pressure and permeability, and pore pressure and water saturation, are also introduced. Based on these equations, new models for tight gas production are proposed. The gas slip correction factor is also considered during derivation of this proposed tight gas production models. Inflow performance relationship curves based on these proposed models show that production rates and absolute open flow potential are always be overestimated while ignoring the threshold pressure gradients.

Experimental efforts to access 4D feasibility and interpretation issues of Brazilian presalt carbonate reservoirs

2019 ◽  
Vol 7 (4) ◽  
pp. SH1-SH18
Author(s):  
Guilherme Fernandes Vasquez ◽  
Marcio Jose Morschbacher ◽  
Julio Cesar Ramos Justen
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Carbonate Rocks ◽  
Time Lapse ◽  
Carbonate Reservoirs ◽  
Fluid Replacement ◽  
S Wave ◽  
Confining Stress ◽  
Reservoir Properties ◽  
Stress Coefficient

Brazilian presalt reservoirs comprise carbonate rocks saturated with light oil with different amounts of [Formula: see text] and excellent productivity. The occurrence of giant-size accumulations with such productivity generates the interest in production monitoring tools, such as time-lapse seismic. However, time-lapse seismic may present several challenges, such as imaging difficulties, repeatability, and detectability of small variations of reservoir properties. In addition, when assessing time-lapse seismic feasibility, the validity of Gassmann’s modeling for complex, heterogeneous carbonate rocks is arguable. Other questions include the pressure variation effects on the seismic properties of competent rocks. The effective stress is a linear combination of confining stress and pore pressure that governs the behavior of physical properties of rocks. Many applications assume that the effective stress for elastic-wave velocity is given by the difference between confining stress and pore pressure, whereas another common approach uses the Biot-Willis coefficient as a weight applied to the pore pressure to estimate the effective stress. Through a series of experiments involving ultrasonic pulse transmission on saturated core plugs in the laboratory, we verified the applicability of Gassmann’s fluid substitution and estimated the empirical effective stress coefficients related to the P- and S-wave velocities for rock samples from two offshore carbonate reservoirs from the presalt section, Santos Basin. We observed that Gassmann’s equation predicts quite well the effects of fluid replacement, and we found that the effective stress coefficient is less than one and not equal to the Biot-Willis coefficient. Moreover, there is a good agreement between the static and dynamic Biot-Willis coefficient, which is a suggestion that the presalt rocks behave as a poroelastic media. These observations suggest that more accurate time-lapse studies require the estimation of the effective stress coefficient for the particular reservoir of interest.

Sorption-Induced Permeability Change of Coal During Gas-Injection Processes

2008 ◽  
Vol 11 (04) ◽  
pp. 792-802 ◽  
Author(s):  
Wenjuan Lin ◽  
Guo-Qing Tang ◽  
Anthony R. Kovscek
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Gas Adsorption ◽  
Gas Production ◽  
Co2 Injection ◽  
Permeability Reduction ◽  
Permeability Change ◽  
Gas Composition ◽  
Adsorption Equilibria ◽  
Study Results

Summary Our study has two features. First, laboratory experiments measured the change of the permeability of coal samples as a function of pore pressure and injected-gas composition at constant effective stress. Second, adsorption-solution theory described adsorption equilibria and aided interpretation. The gases tested include pure methane (CH4), nitrogen (N2), and carbon dioxide (CO2), as well as binary mixtures of N2 and CO2 of different compositions. The coal pack was initially dry and free of gas, then saturated by each test gas at a series of increasing pore pressures at a constant effective stress until steady state was reached. Thus, the amount of adsorption varied, while the effective stress was held constant. Results show that, (i) permeability decreases with an increase of pore pressure at fixed injection-gas composition, and, (ii) permeability change is a function of the injected-gas composition. As the concentration of CO2 in the injection gas increases, the permeability of the coal decreases. Pure CO2 leads to the greatest permeability reduction among all the test gases. However, 10 to 20% by mole of N2 helps to preserve permeability significantly. According to the mixed-gas adsorption isotherms, adsorption and the selectivity of a particular gas species on coal surfaces is a function of pressure and the gas composition. Therefore, we conclude that loading coal surfaces with adsorbed gas at constant effective stress causes permeability reduction. Finally, gas adsorption and permeability of coal are correlated, simply to extend the usefulness of study results. Introduction Coalbed methane (CBM) has grown to supply approximately 10% of US natural-gas production and is becoming important worldwide as an energy source (EIA 2006). Conventional CBM-recovery procedures stimulate wells and produce CH4 by depressurizing the coalbed. A full understanding of the mechanisms underlying CBM production has yet to be established. Injection of CO2, N2, or mixtures of the two gases enhances CBM recovery significantly (Stevens et al. 1998; Stevens 2001). Coalbeds also present a potential sink for greenhouse gases (GHGs), such as CO2. One issue of particular interest for CO2 injection, and the subject of our study, is the sensitivity of coal permeability to the partial pressure of CO2 in the injection gas.

A comparative study on methods for determining the hydraulic properties of a clay shale

2020 ◽  
Vol 224 (3) ◽  
pp. 1523-1539
Author(s):  
Lisa Winhausen ◽  
Alexandra Amann-Hildenbrand ◽  
Reinhard Fink ◽  
Mohammadreza Jalali ◽  
Kavan Khaledi ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Ion Beam ◽  
Applied Pressure ◽  
Pressure Oscillation ◽  
Stress Conditions ◽  
Data Set ◽  
Clay Shale ◽  
Permeability Coefficients ◽  
Effective Stresses

SUMMARY A comprehensive characterization of clay shale behavior requires quantifying both geomechanical and hydromechanical characteristics. This paper presents a comparative laboratory study of different methods to determine the water permeability of saturated Opalinus Clay: (i) pore pressure oscillation, (ii) pressure pulse decay and (iii) pore pressure equilibration. Based on a comprehensive data set obtained on one sample under well-defined temperature and isostatic effective stress conditions, we discuss the sensitivity of permeability and storativity on the experimental boundary conditions (oscillation frequency, pore pressure amplitudes and effective stress). The results show that permeability coefficients obtained by all three methods differ less than 15 per cent at a constant effective stress of 24 MPa (kmean = 6.6E-21 to 7.5E-21 m2). The pore pressure transmission technique tends towards lower permeability coefficients, whereas the pulse decay and pressure oscillation techniques result in slightly higher values. The discrepancies are considered minor and experimental times of the techniques are similar in the range of 1–2 d for this sample. We found that permeability coefficients determined by the pore pressure oscillation technique increase with higher frequencies, that is oscillation periods shorter than 2 hr. No dependence is found for the applied pressure amplitudes (5, 10 and 25 per cent of the mean pore pressure). By means of experimental handling and data density, the pore pressure oscillation technique appears to be the most efficient. Data can be recorded continuously over a user-defined period of time and yield information on both, permeability and storativity. Furthermore, effective stress conditions can be held constant during the test and pressure equilibration prior to testing is not necessary. Electron microscopic imaging of ion-beam polished surfaces before and after testing suggests that testing at effective stresses higher than in situ did not lead to pore significant collapse or other irreversible damage in the samples. The study also shows that unloading during the experiment did not result in a permeability increase, which is associated to the persistent closure of microcracks at effective stresses between 24 and 6 MPa.

scholarly journals Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

2021 ◽  
Vol 13 (14) ◽  
pp. 2684
Author(s):  
Eldert Fokker ◽  
Elmer Ruigrok ◽  
Rhys Hawkins ◽  
Jeannot Trampert
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Pressure Head ◽  
Groundwater Table ◽  
Surface Velocity ◽  
Seismic Velocities ◽  
Stress Monitoring ◽  
Near Surface ◽  
Velocity Variations ◽  
Velocity Changes

Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

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