Velocity-effective Stress Research Articles

Stress-Dependent Petrophysical Properties of the Bakken Unconventional Petroleum System: Insights from Elastic Wave Velocities and Permeability Measurements

The net-effective stress is a fundamental physical property that undergoes dynamic changes in response to variations in pore pressure during production and injection activities. Petrophysical properties, including porosity, permeability, and wave velocities, play a critical role and exhibit strong dependence on the mechanical stress state of the formation. The Williston basin’s Bakken Formation represents a significant reservoir of hydrocarbons within the United States. To investigate this formation, we extracted core plugs from three distinct Bakken members, namely Upper Bakken, Middle Bakken, and Lower Bakken. Subsequently, we conducted a series of measurements of ultrasonic compressional and shear wave velocities, as well as pulse decay permeabilities using nitrogen, under various confining pressures employing the Autolab-1500 apparatus. Our experimental observations revealed that the ultrasonic wave velocities and permeability display a significant sensitivity to stress changes. We investigated existing empirical relationships on velocity-effective stress, compressional-shear wave velocities, and permeability-effective stress, and proposed the best models and associated fitting parameters applicable to the current datasets. In conjunction with the acquired datasets, these models have considerable potential for use in time-lapse seismic monitoring and the study of production decline behavior. The best fitting models can be used to forecast the petrophysical and geomechanical property changes as the reservoir pore pressure is depleted due to the production, which is critical to the production forecast for unconventional reservoirs.

Velocity-based pore pressure prediction in a basin with late-stage erosion: Delaware Basin, U.S.

Pore pressure is a critical parameter for the exploration and development of hydrocarbons, and it is difficult to predict in unconventional basins that have been unloaded by erosion. We predict pore pressure from mudrock velocities in the Delaware Basin, which is an unconventional basin with significant late-stage erosion. We use an elastoplastic model to describe mudrock decompaction due to unloading, resulting in a velocity-effective stress relationship that differs from a mudrock's normal compaction trend. We establish this velocity-effective stress relationship using well log and measured pore pressure data in a well with a known erosional value. We next use this relationship to estimate the amount of erosion at each new prediction well, and then predict vertical effective stress with depth. We calculate pore pressure as the lithostatic stress less the vertical effective stress. We demonstrate the predictive capability of our approach in four wells spanning the eastern portion of the basin where the amount of erosion differs, and show that our approach better matches the measured pore pressures than the ones predicted by two conventional methods.

Contributions of Loading and Unloading to Overpressure: Analytical Results from The Shelfal Area of The Lower Kutai Basin, Indonesia

DOI:10.17014/ijog.9.1.1-13A modified Bowers Method is applied to analyze the contribution of loading and unloading to total overpressure magnitude in the shelfal area of the Lower Kutai Basin. The method combines Bowers Method with density-sonic cross plots to differentiate contributors to overpressure. This method has a strong physical background, and has proven to give satisfactory overpressure estimation in the studied area. Several processes related to overpressure and compaction in the studied area have also been deduced: 1) top of overpressure is caused by unloading due to gas generation, 2) gas generation on its own can produce overburden-scale overpressure magnitude, 3) loading starts to contribute to overpressure at depth, where the lithology is dominated by mudrocks, 4) effective stress history experienced by mudrocks before unloading, and 5) the presence of two compaction lines and therefore two associated velocity-effective stress relations.

Multi-channel 2D seismic constraints on pore pressure- and vertical stress-related gas hydrate in the deep offshore of the Mahanadi Basin, India

Estimation of pore pressure and in-situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the Mahanadi basin. The basin located at northern side of eastern continental margin of India (ECMI) contains gas hydrate in clay/silt sediments. The pore pressure and vertical stress are mapped on two 2D-multi channel seismic data of lines (MH-38A and MH-38B) with aid of information of three wells (namely NGHP-01-19, NGHP-01-09 and NGHP-01-08). Initially, the coefficients of best fit curve have been computed from velocity-effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress. The vertical stress has been computed from both the seismic and log data. Therefore, the pore pressure is predicted by subtracting the effective stress from the vertical stress. The pore pressure have been mapped in gas hydrate stability zone (GHSZ) and sediments below bottom simulating reflector (BSR). The pore pressure and vertical stress gradient are 10.11 MPa/km and 10.67 MPa/km, respectively. The pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit (R2) varying from 0.82 to 0.95. Normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the BSR indicating presence of free gas. The pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the Mahanadi basin.

RhoVe method: A new empirical pore pressure transform

A new empirical pore pressure transform has been developed that includes many of the advanced, state-of-the-art concepts that are useful in today’s pore pressure estimation and theory. The rhob-velocity-effective stress (Rho-V-e) method produces a model-driven, stand-alone set of “virtual” rock property relationships, which at intermediate positions are consistent with Bowers method default values for the Gulf of Mexico. The RhoVe method uses a single transform to convert both compressional sonic and bulk density to common estimates of effective stress and pore pressure where convergence of the two transformed properties offers a robust solution.Velocity-density conversion functions are mathematically linked to a continuous series of velocity-depth normal compaction trend functions. The calculations are limited by bounding end-member curves that provide a basis for intermediate (fractional) solutions of velocity-effective stress and density-effective stress relationships that are applied to a well of interest.Paired “virtual” velocity-depth compaction trends were iteratively solved by using published theoretical smectite and illite porosity trends and velocity-depth normal compaction trends. By using the RhoVe-derived velocity-density functions that match the well of interest in cross-plot, normal effective stress for each end-member and intermediate solution can be calculated. Effective stress is calculated by taking the difference between the integrated density-depth virtual overburden profile, converted from velocity-depth, and the inclusion of hydrostatic pressure. The method produces robust solutions as tested on multiple deep water Gulf of Mexico wells, and extends the predictability of high-velocity, low-effective stress rock types such as those found in the Deepwater Gulf of Mexico Wilcox-equivalent Paleogene and older section. The velocity-effective stress trend curves can also improve pore pressure characterization of the overlying overburden section extending to the mud line. Advantages of the RhoVe method are that it can be made interactive and fast, relative to the application of other acoustic transform methods.This paper attempts to build on previous efforts by other workers to include the role of clay type, clay volume and diagenesis on altering velocity-effective stress relationships and presents a technique in which the effects of clay diagenesis and other factors may be captured and utilized empirically for pore pressure analysis and prediction.

Typical disequilibrium compaction caused overpressure of Paleocene Dongying Formation in northwest Liaodongwan Depression, Bohai Bay Basin, China

Bohai Bay Basin is an important petroliferous basin in China. Petroleum potential in Liaodongwan Depression of offshore Bohai Bay Basin has been proved by previous exploration and production, and overpressures were encountered. Summarized by the DST measurements and mud weight data of over 60 wells in Liaodongwan Depression, the abnormally high pressures preserved in Paleocene Dongying (Ed) Formation distribute over the entire depression and could reach as high as 64.2MPa around the depth of 3420m. Through the well logs analysis, typical corresponding characteristics to disequilibrium compaction can be found: higher in acoustic transit time and lower in density than normal compaction trend. Based on these corresponding features the pore pressure prediction was carried out for all the wells in the area. Sonic velocity-effective stress relationship and sonic velocity-density cross plots were employed to investigate the origin of overpressure in Ed Formation. Sedimentation rates and geothermal features were studied as well. Combining all the analysis, the overpressure of Ed Formation in northwest Liaodongwan Depression was dominantly generated by disequilibrium compaction. With this widely distributed overpressure, thick layers of mudstone of Ed Formation serve as a regional overpressured caprock.

‘Vertically transferred’ overpressures in Brunei: Evidence for a new mechanism for the formation of high-magnitude overpressure

Research Article| November 01, 2007 ‘Vertically transferred’ overpressures in Brunei: Evidence for a new mechanism for the formation of high-magnitude overpressure Mark R.P. Tingay; Mark R.P. Tingay 1Australian School of Petroleum and School of Earth and Environmental Sciences, University of Adelaide, Adelaide 5005, Australia Search for other works by this author on: GSW Google Scholar Richard R. Hillis; Richard R. Hillis 2Australian School of Petroleum, University of Adelaide, Adelaide 5005, Australia Search for other works by this author on: GSW Google Scholar Richard E. Swarbrick; Richard E. Swarbrick 3GeoPressure Technology Mountjoy Research Centre, Stockton Road, Durham DH1 3UZ, UK Search for other works by this author on: GSW Google Scholar Chris K. Morley; Chris K. Morley 4PIT Exploration and Production Public Company Limited, 555 Vibhavadi-Rangsit Road, Bangkok 10900, Thailand Search for other works by this author on: GSW Google Scholar Abdul Razak Damit Abdul Razak Damit 5Brunei Shell Petroleum, Jalan Utara, Seria KB3534, Brunei Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (11): 1023–1026. https://doi.org/10.1130/G23906A.1 Article history received: 05 Apr 2007 rev-recd: 09 Jul 2007 accepted: 15 Jul 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mark R.P. Tingay, Richard R. Hillis, Richard E. Swarbrick, Chris K. Morley, Abdul Razak Damit; ‘Vertically transferred’ overpressures in Brunei: Evidence for a new mechanism for the formation of high-magnitude overpressure. Geology 2007;; 35 (11): 1023–1026. doi: https://doi.org/10.1130/G23906A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Overpressures in sedimentary basins are commonly assumed to be the result of two distinct and separate mechanisms: disequilibrium compaction and fluid expansion. However, the potential for overpressures to be redistributed or transferred to other pressure compartments over time has been seldom considered and rarely demonstrated. Pore-pressure data and velocity-effective stress plots from 61 fields across the Baram Delta province of Brunei (northwest Borneo) reveal that two different types of overpressure occur in stratigraphically defined sections: the basal pro-delta shales contain overpressures generated by disequilibrium compaction, whereas the overlying sand/shale deltaic sequence contains overpressures that appear to be generated by fluid expansion. However, the geology of the deltaic sequences and high magnitude of the pore pressures precludes the overpressures in the deltaic sequences being generated by any conventional fluid expansion mechanism, such as kerogen-to-gas maturation or clay diagenesis. The fluid expansion overpressures are located in fields that were inverted during the Pliocene, an event that resulted in large-scale fluid migration from the pro-delta shales into the deltaic sequences, including charging of the numerous oil fields in the inner shelf. Hence, we propose that the overpressures in Brunei provide the first evidence for a new overpressuring mechanism whereby overpressured fluids have been “vertically transferred” from the pro-delta shales into the deltaic sequences during basin inversion. Furthermore, vertical transfer may be a mechanism for explaining overpressures observed in basins that have been recently uplifted or inverted. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Velocity-Effective Stress Response of CO2-Saturated Sandstones

Three differing sandstones, two synthetic and one field sample, have been tested ultrasonically under a range of confining pressures and pore pressures representative of in-situ reservoir pressures. These sandstones include: a synthetic sandstone with calcite intergranular cement produced using the CSIRO Calcite In-situ Precipitation Process (CIPS); a synthetic sandstone with silica intergranular cement; and a core sample from the Otway Basin Waarre Formation, Boggy Creek 1 well, from the target lithology for a trial CO2 pilot project. Initial testing was carried on the cores at “room-dried” conditions, with confining pressures up to 65 MPa in steps of 5 MPa. All cores were then flooded with CO2, initially in the gas phase at 6 MPa, 22°C, then with liquid-phase CO2 at a temperature of 22°C and pressures from 7 MPa to 17 MPa in steps of 5 MPa. Confining pressures varied from 10 MPa to 65 MPa. Ultrasonic waveforms for both P- and S-waves were recorded at each effective pressure increment. Velocity versus effective pressure responses were calculated from the experimental data for both P- and S-waves. Attenuations (1/Qp) were calculated from the waveform data using spectral ratio methods. Theoretical calculations of velocity as a function of effective pressure for each sandstone were made using the CO2 pressure-density and CO2 bulk modulus-pressure phase diagrams and Gassmann effective medium theory.Flooding the cores with gaseous phase CO2 produced negligible change in velocity-effective stress relationships compared to the dry state (air saturated). Flooding with liquid-phase CO2 at various pore pressures lowered velocities by approximately 8% on average compared to the air-saturated state. Attenuations increased with liquid-phase CO2 flooding compared to the air-saturated case. Experimental data agreed with the Gassmann calculations at high effective pressures. The “critical” effective pressure, at which agreement with theory occurred, varied with sandstone type. Discrepancies are thought to be due to differing micro-crack populations in the microstructure of each sandstone type. The agreement with theory at high effective pressures is significant and gives some confidence in predicting seismic behaviour under field conditions when CO2 is injected.

Effective stress and the elastic response of reservoir sandstones and shales

SUMMARY Effective stress is a key concept in determining reservoir pore pressures from seismic velocity data. It is usually defined as the difference, ∆P between confining (overburden) pressure, Pc and pore pressure, Pp. However , a more general definition is σ ´ =Pc - nPp where n is an effective stress coefficient. Seismic velocity is observed both in the field and in laboratory experiments to vary non-linearly with effective stress. Furthermore, a form of hysteresis in velocity is observed when reservoir rocks are subjected to differing pore pressure histories. The resulting hysteresis can be quantified with the effective stress coefficient. This coefficient is frequently derived from laboratory velocity-effective stress data at ultrasonic frequencies. This paper describes some recent laboratory based, rock physics experiments involving pore pressure history induced velocity hysteresis and the implications for reservoir 4D monitoring.

Study on the interaction between a dislocation and impurities in KCl:Sr2+single crystals by the Blaha effect Part III Influence of heat treatment on various characteristics

Strain-rate cycling tests associated with ultrasonic oscillation were conducted at 80–239 K for two kinds of KCl:Sr2+ (0.05 mol% in the melt) single crystals: quenched and annealed specimens. Examining the relationship of temperature and dislocation velocity-effective stress exponent, m*, estimated from the data obtained in this series, we could find the suitable force-distance relation between a dislocation and the impurity. The force-distance relation for the quenched specimen approached to the Fleischer's model taking account of the Friedel relation rather than the Fleischer's model. As for the annealed specimen, the SQ was the most appropriate of the three models: the SQ, the PA, and the TR indicate a square, a parabolic, and a triangular force-distance profile respectively. The three force-distance relations are taken account of the Friedel relation. By annealing the quenched specimen, m* became low at a given temperature. This may have been caused by the following two phenomena. First, the concentration of weak obstacles to dislocation motion decreased after the heat treatment. Secondly, the resistance to movement of a dislocation in the quenched specimen was weakened by annealing it, e.g., F0 was reduced to about one-third and ϕ0 increased from 154 to 172 degrees. F0 and ϕ0 are the force acted on the dislocation and the bending angle of dislocation by the weak obstacle such as the impurity. Therefore, it may be deduced that the dislocation velocity in the quenched specimen is more sensitive to the effective stress due to the impurities than that in the annealed specimen at the temperature.

Velocity-effective stress laws and hysteresis in Carnarvon Basin sandstones

P-wave and S-wave velocities, attenuation and the Vp/Vs ratio show different responses in saturated and dry Carnarvon basin sandstones under increasing differential pressures. At low differential pressures, Vp/Vs ratios are high under oil saturation but low in the same rocks under dry (gas-saturated) conditions. Velocities and attenuation are also shown to be sensitive to pore pressure, not just differential pressure. Under identical low differential pressure conditions, velocity is higher in sandstones when pore pressure is high during simulation of inflationary overpressure conditions. Thus the velocity- differential pressure relationship is not unique. However, the use of the measured Biot effective stress coefficient results in a unique relationship. It is postulated that higher velocity and different Vp/Vs ratios are caused by loss of compliance in microfractures due to stiffening by increasing fluid pressure.

Study on the applicability of the Friedel relation with dislocation velocity-stress exponent

It was investigated at 77–178 K whether the Friedel relation is appropriate for the interaction between a dislocation and the impurity in KCl:Mg2+ (0.035 mol% in the melt) single crystal. This was carried out by fitting of me=0 to the temperature versus dislocation velocity-effective stress exponent, m*, curves with respect to the two models: one is the Fleischer's model taking account of the Friedel relation and the other the Fleischer's model. The former model is termed the F-F. me=0 is the m value at zero strain. m is a dislocation velocity-applied stress exponent. The m* for the F-F was examined from

CALIBRATING CORE, LOG AND SEISMIC DATA TO ASSESS EFFECTIVE STRESS AND HYDROCARBON SATURATION, COOPER BASIN, SOUTH AUSTRALIA

Laboratory measurements of acoustic properties of representative rock samples, simulating in-situ effective stress and fluid saturation, provide useful guides for calibrating and interpreting seismic and sonic log data. This paper addresses some of the major implications arising from a petro-acoustic study for the evaluation of reservoir depletion of Cooper Basin gas reservoirs using logs and seismic. Measurement of P- and S-wave velocities on cores under varying pressure conditions reveals that the stress dependency of Cooper Basin rocks is very large, while core porosity remains effectively unchanged.The saturation heterogeneity at pore-scale, which is shown in capillary pressure data, controls the velocity- saturation in partially water-saturated samples. The steady decrease of P-wave velocity as saturation decreases from the high saturation range to near irreducible conditions suggests a simultaneous drainage of water from pores with a variety of high to moderate aspect ratios, while microcracks (low aspect ratio pores) retain water. Closure and degree of saturation of the low aspect ratio pores control the velocity-effective stress and velocity-saturation relationships at low saturation and stress conditions.The velocity dispersion due to frequency difference between ultrasonic laboratory measurements on cores and theoretical low (seismic) frequency is about 1%, and thus laboratory-measured velocities are comparable with sonic log and seismic data in the Cooper Basin. The potential of the velocity ratio (Vp/Vs) for detection of fluid type and the saturation status at in-situ reservoir effective stress, and prediction of Vs from Vp, are demonstrated for the Cooper Basin rocks. Acoustic measurements on cores, wireline data and seismic modelling are used to predict the expected change in seismic response as the reservoir depletes. Synthetic seismic profiles indicate that the zero-offset reflectivity of a shale to reservoir interface decreases by 28% for a 30 MPa pressure depletion in a typical gas expansion drive reservoir. Such changes should be easily measurable between repeated surveys, suggesting that time-lapse seismic for the monitoring of in-situ effective stress and saturation may have application in Cooper Basin reservoirs. Although these findings refer specifically to the Cooper Basin, the methods used and results of this study may be applicable elsewhere.

Effects of Superimposed Ultrasonic Oscillatory Stress on the Deformation of Fe and Fe-3%Si Alloy

When ultrasonic oscillations are applied during static testing, the static flow stress decreases. The purpose of the present work is to elucidate to what extent ‘the stress superimposition effect’ can explain this phenomenon. On the basis of the stress superimposition, the decrements in static flow stress, Δτ, were calculated neglecting mean internal stress. From the measurements of specimen temperatures and the comparison between the measured and calculated decrements, the following conclusive remarks were drawn.(1) The effect of the temperature rise on the decrements was negligible in the superimposed stress range investigated.(2) The theory and experimental results both showed that the larger the dislocation velocity-effective stress exponent, the larger became the decrease in static flow stress.(3) The measured decrements were larger than the calculated by a factor of about 1.5. They slightly decreased with increase in strain rates and slightly increased with increasing τA at which the superimpositions of stress were conducted.(4) The results obtained above were qualitatively explained by the change in the fraction of mean internal stress to the flow stress.

The effect of grain size and cross-head velocity on the lower yield strength during inhomogeneous discontinuous yielding in iron and steel

The validity of the relation, $$\underline {Ag} $$ , has been checked with experimental results on the lower yield strength, σ aF of various irons and steels determined at room temperature over wide ranges in cross-head velocity,V c, and grain boundary intercept or grain diameter,l. Good agreement was found with the proposed relation form *=4, wherem * is the dislocation velocity-effective stress exponent, and α=1/3. σ i , the internal stress, ranged between 5000 and 13,000 psi andN αθK’ between 4.3×10−15 and 19.3×10−15. In the original relation, the value of α was one andK’ was given asK, which was assumed to be a material constant. The modification which became necessary indicates that possibly the density of mobile dislocations and/or the resistance of grain boundaries to propagating Luder’s bands depend on Luder’s band velocity and strain. The validity of the relation at very high and very low cross-head velocities or strain rates is discussed. The relation should apply in an intermediate range of cross-head or dislocation velocities wherem * should remain constant and should show an apparent decrease at both higher and lower velocities. The basis for this prediction is discussed and results are presented at low strain rates.

Interpretation of high temperature plastic deformation in terms of measured effective stresses

The microdynamics of elevated temperature deformation in single and polycrystalline Fe-3 % Si is examined in terms of measured minimum effective stresses. Effective stresses are obtained through the use of a new experimental technique based upon the response of dislocations to the internal and applied stresses. Primary and steady state creep are examined with respect to the measured minimum effective stresses. Modified stress relaxation and strain rate sensitivity techniques are used to study dislocation velocities at elevated temperatures. Dislocation density measurements are made in both steady state creep and stress relaxation experiments and are related to measured effective stresses. Results indicate that the effective stress dependence of the steady state strain rate can be described by e ̇ g3 s ∝ σ min ∗2 where σ min ∗ is the minimum effective stress. Measurements of the tota within the subgrains yielded, p m ∝ σ min ∗ under the assumption that a constant fraction of the total dislocation density within the subgrains is mobile. The indirect measurements of dislocation velocity indicate that at elevated temperatures both the internal stress and mobile dislocation density change reversibly as a unique function of the effective stresses during stress relaxation and strain rate sensitivity tests. Comparison of steady state strain rates with measured dislocation densities supports the assumption of a linear dislocation velocity-effective stress relation.