Trend‐kriged seismic velocities to predict pore pressure and to model effective stress for reservoir characterization in a deepwater basin

2006 ◽  
Author(s):  
S. Noeth ◽  
C. M. Sayers ◽  
P. J. Hooyman ◽  
L. den Boer ◽  
N. Banik ◽  
...  
Keyword(s):  
Pore Pressure ◽  
Effective Stress ◽  
Reservoir Characterization ◽  
Seismic Velocities ◽  
Deepwater Basin

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.

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.

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.

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