Sonic Velocity Data Research Articles

Predicating of Formation Pore Pressures in Tertiary Reservoirs Using Geophysical Wireline Log Data

Changes to certain geophysical characteristics, such as sonic transit time with depth, indicate the pressure system of over-pressured zones. In this study, well-log data is used to investigate the distribution of formation overpressures in the tertiary reservoirs of one Iraqi oil field. Three wells located within the tertiary carbonate reservoir of one Iraqi oil field are of interest in this study. To assess the reservoirs, a petrophysics log analysis was carried out. The reservoir's intervals are the area of interest. The sonic velocity data was used to create a normal compaction trend line, and the overpressure zones were identified by observing the reversals in the normal compaction trend line. The pore pressure, fracture pressure, and gradients were calculated using the Eaton method. The method relies on the correlation between the normal transit time found on the normal compaction trend and the observed transit time from the log reading. The sonic log determines the bulk density and matches the density derived from the density log. Abnormal pressures were identified primarily within the area of interest, especially with unit B in Well 1. In Well 2, the abnormal pressures were recognized in zones above the area of interest. Abnormal pressures in Well 3 were identified primarily within all areas of interest. The importance of this work is to help predict overpressure zones in the research area before drilling or workover.

3D basin modeling of the Hils Syncline, Germany: reconstruction of burial and thermal history and implications for petrophysical properties of potential Mesozoic shale host rocks for nuclear waste storage

AbstractJurassic sedimentary sequences suitable for nuclear waste storage in northern Germany consist of organic-lean claystone and were uplifted to < 100 m depth in the Hils Syncline area (southern Lower Saxony Basin). This Hils Syncline, showcasing a northwestward increase in thermal maturity, facilitates the study of shale petrophysical properties influenced by burial history. This study introduces a 3D-thermally calibrated numerical model of the Hils Syncline area to analyze its geodynamic evolution and maturity variations. It provides new vitrinite reflectance and sonic velocity data for modeling calibration and erosion estimation. The Hils Syncline area has undergone continuous subsidence, interrupted by a Cretaceous uplift documented by an erosional unconformity. During the latest Early Cretaceous, Jurassic rocks underwent maximum burial reaching up to several thousand meters depth and temperatures up to 160 °C in the northwest. The Late Cretaceous inversion caused stronger erosion towards the northwest removing up to 3300 m of sediment compared to about 1300 m in the south, according to vitrinite reflectance-based estimations. Numerical modeling results along the study area indicate decreasing porosity and permeability northwestward with increasing thermal maturity. Porosity and vertical permeability decreased to 5–14% and 2.8 × 10–23 to 1.5 × 10–19 m2 [1 mD = 10−15 m2], respectively, while vertical thermal conductivity increased to 1.30–2.12 (W/m/K). These trends of porosity/permeability and thermal conductivity with burial align with sonic velocity and published experimental porosity data, except for the thermally most mature region (Haddessen). This anomaly is tentatively attributed here to localized overpressure generation in the Posidonia Shale during maximum burial, affecting both the underlying Pliensbachian and overlying Doggerian units. Graphical abstract 3D numerical model of the Hils Syncline and surrounding area revealing that a northwestward increase in maximum burial resulted in higher temperatures and varying maturity levels. While most locations align well with calibration data (i.e. measured vitrinite reflectance and porosity), discrepancies arise in the Haddessen/Bensen area. The mismatch between porosity, vitrinite reflectance, and sonic velocity response indicates local overpressure in the northernmost region mainly during the Cretaceous. It was likely caused by gas generation in the Posidonia Shale affecting nearby Lower and Middle Jurassic units.

Direct assessment of the hydraulic structure of the plate boundary at the toe of the Nankai accretionary prism

SUMMARY The Nankai Trough is a locus of slow slip, low-frequency earthquakes, and large magnitude (Mw > 8) earthquakes. It is usually assumed that high pore pressure contributes substantially to earthquake dynamics. Hence, a thorough understanding of the hydraulic regime of the Nankai accretionary prism is needed to understand this diversity of behaviours. We focus on the toe of the accretionary prism by studying data from Hole C0024A, part of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project, that intersected the décollement at 813 m below seafloor (mbsf) about 3 km away from the trench. We contribute to this understanding by innovatively integrating drilling and logging data to derive high-resolution hydraulic profiles along the borehole. A quantitative re-analysis of the variation in the downhole annular pressure monitored during drilling show localized fluid flow from the formation to the borehole in excess of 0.05 m3 s−1, especially in the damage zones at the footwall of the décollement. To validate the fluid flow profile, pore pressure was estimated independently from empirical relationships between pore pressure, porosity and P-wave velocity, obtained from consolidation experiments and Eaton-type methods based on drilling or sonic velocity data. The formation fluids are becoming significantly overpressurized with depth in the few hundred metres above décollement. The hydraulic profile suggests that the core of the décollement acts as a barrier inhibiting upward fluid convection, whereas the damage zone acts as an efficient longitudinal channel able to diffuse high pressure from the deeper part of the subduction.

Understanding the synergistic impact of stress release and cementation on sandstone using sound waves — Implications for exhumation estimation

Exhumation is the process that encompasses uplift and erosion, leading to the removal of overburden and the release of effective stress exerted on rocks. When estimating exhumation magnitude using the compaction trend method, it is commonly assumed that the physical properties of rocks are insensitive to stress reduction. However, recent laboratory evidence has indicated that porosity exhibits weaker sensitivity to stress release compared with velocity that can be significantly affected by stress release. This raises questions regarding the validity of irreversible compaction assumed by compaction trend method. It remains unclear whether the impact of stress release can be observed in real rocks in exhumed areas because there is a lack of methods to directly measure the impact of stress release on field data. In addition, studying real rocks is further complicated by the presence of rock diagenesis and its interaction with stress release. To address these knowledge gaps, this study uses stress-dependent burial and uplift modeling and interprets an extensive well-log data set using the modeling-derived evaluation metrics. Conceptual modeling suggests that methods that neglect the combined effect of cementation and stress release tend to underestimate the exhumation magnitude. Furthermore, we discover that the disparity between porosity sensitivity and velocity sensitivity to stress release can be leveraged to derive a metric we call “porosity inconsistency” that can serve as a qualitative and quantitative measure for identifying and evaluating stress release in sandstone using geophysical field measurements. We gather a significant amount of sonic velocity and porosity data from normally compacted and uplifted clean sandstones in the Norwegian Sea and the Barents Sea. Notably, we observe significant porosity inconsistency in the exhumed well 6510/2-1 in the Norwegian Sea. In the Barents Sea, which has experienced extensive Cenozoic exhumation, the well data reveal a varying pattern of porosity inconsistency increasing toward the north and decreasing toward the west. This distribution of porosity inconsistencies in the Barents Sea wells not only aligns with the spatial variation of exhumation reported in various studies but also exhibits a positive correlation with the magnitude of exhumation. Furthermore, the exhumation magnitude derived from velocity-depth trends is considerably lower than the magnitude obtained from porosity/density-depth trends for wells displaying significant porosity inconsistency. These observations provide support for the predictions made by the conceptual modeling. The results of this study enhance our understanding of the synergistic impact of stress release and cementation on sandstone. Moreover, these findings have implications for pore pressure prediction and core evaluation in exhumed areas. They also provide insights into the feasibility and interpretation of time-lapse data of reservoir injection, for which the effective stress is likely to decrease due to pore pressure buildup.

New insights into the Pore Pressure Estimation from Sonic Velocity Data—Application of the equivalent depth method in Chinese Basins

Pore pressure prediction is of fundamental importance in petroleum exploration and production operations. Because there is no direct method to measure pore pressure in shale, the indirect method (the equivalent depth method) for estimating pore pressure from sonic velocity data is widely used. It has been found in the application of the equivalent depth method in several Chinese sedimentary basins that the calculated pressures from sonic logging data are very consistent with the DST or RFT pressures measurements in some basins and are very divergent in others. The consistence of calculated and measured excess pressures in the basin with different excess pressure generation mechanisms such as in the cases in Tarim basin, Bohai Bay basin and Jiuquan basin showed that the pressure calculated by the equivalent depth method accounts for all excess pressure generation mechanisms such as disequilibrium compaction, tectonic compression, aquathermal pressuring, hydrocarbon generation and clay diagenesis. The divergence of calculated and measured excess pressures was found in the organic-rich source rocks in Triassic in Ordos basin and in Cretaceous in Songliao basin. Compared with common shales, organic-rich shales are composed of more solid organic matter, which can influence the sonic velocity values. Our study shows that the sonic velocity decreases dramatically when the TOC is higher than 2%. So the pore pressures calculated by the equivalent depth method from sonic velocity data may much higher than the actual pressures in organic-rich shale. The use of the equivalent depth method may mislead in predicting pore pressure of organic-rich shale without considering the influence of total organic carbon content.

METHODS TO ESTIMATE EROSION IN SEDIMENTARY BASINS

Net erosion, the difference between the present‐day and maximum burial depths of a reference unit, may have a major impact on hydrocarbon prospectivity in a sedimentary basin. Erosion may affect all the components of a petroleum system, from source rock to reservoir to seal. In most cases, vitrinite reflectance (VR), temperature and sonic velocity data, which are often readily available, can be used to determine net erosion in a region based on the thermal and mechanical evolution of sedimentary layers with burial. This paper revisits these methods and discusses the determination of net erosion from these datasets. Furthermore, it is shown that a closer look at the data is warranted if the estimates derived from complementary VR/temperature and velocity datasets significantly diverge. Such differences can be reconciled by critically examining the datasets and the regional geology, resulting in erosion estimates from both datasets which are within 100 m of each other. Lastly, a fully automated, process‐driven method combined with multi‐objective optimization algorithms and that takes all three datasets into account is showcased while determining net erosion for three wells located in the Norwegian Barents Sea. One of the benefits of this method is that it explores a wide range of likely scenarios that would best match the different datasets. Furthermore, the method can also automatically flag datasets that are inconsistent with each other by returning an overall low fit score. These datasets can then be critically examined to determine their reliability and to arrive at a more consistent erosion estimate, reducing the error margin to about 100 m.

Prediction of over pressured zones from well logs in the Niger delta

The changes of some geophysical properties such as sonic compressional velocity with depth is an indication of the pressure system of over-pressured zones. Once these anomalous pressures are not forecasted correctly before drilling, calamitous occurrences, for instance kicks, and blowouts may ensue. This paper investigated the distribution and origin of overpressures in the Niger delta sedimentary basin using well log data. A Normal Compaction Trend line was generated by means of sonic velocity data and the overpressure zones were detected by observing the reversals of the sonic velocity data. The outcome of the formation pressure gradient prediction infers that the commencement of overpressure in the study area occurs at 7500ft with a resultant pressure gradient of 0.51psi/ft. Meanwhile, the findings of the assessment of sonic velocity and density cross-plots revealed disequilibrium compaction as the primary cause of overpressure. The prominent zones of overpressure were detected to appear at depths between 11100ft to 11300ft with an associated pressure gradient of 0.52psi/ft. to 0.53 psi/ft. This work will help in precisely predicting zones of overpressure in the research area prior to drilling.

Resolution limits of fluid overpressures from mineralogy, porosity, and sonic velocity variations in North Sea mudrocks

Pore pressure identification in mudrocks largely relies on indirect methods, including analyses of sonic velocity data. The magnitude of pore pressure changes that can be resolved from such data depends on the impact of fluid pressures on well log responses and on the velocity responses to lithological variations that could be mistaken for indications of pore pressure changes. Analysis of relationships between sonic velocity, mineralogy, and porosity from 11 North Sea wells demonstrated porosity variations of up to 5% from the mean value in mudrocks at any given depth between 1 and 3.5 km (3300–11,500 ft). Comparisons of these wells with seven wells from the Norwegian Sea demonstrated that the sonic velocity and density variations are influenced by the content of silica with a biogenic origin, but that they can otherwise not, in general, be related to mineralogy and pore pressure variations. The sonic velocity deviations from the mean value are of the same magnitude as those that would be caused by porosity changes of ±8% in mudrocks of uniform lithology. Furthermore, the deviations are not significantly reduced when the velocity data are compensated for changes in density (porosity). Detection limits for mechanically compacting mudrocks that are at their maximum effective stress are equivalent to static mud density changes of 0.1–0.15 g/cm3. These numbers apply to mudrocks with similar lithological variability to those analyzed in our data set but should be much higher when significant amounts of silica with a biogenic origin are present. The impact of effective stress changes on the sonic velocities of chemically compacting as well as of mechanically unloading rocks is much less because small or no porosity changes follow from the effective stress changes in such cases. The velocity responses for mechanically unloading and stress-insensitive chemical compaction can only be identified from four-dimensional (time-lapse) seismic data unless the rocks are close to the fracturing limit. These observations imply that reasonably accurate velocity-based pore pressure identification in North Sea mudrocks requires a knowledge of local mudrock heterogeneity and compaction that cannot easily be obtained. As a result, sonic velocity data should be treated with considerable caution in pore pressure evaluation.

Quantification of Cretaceous-Cenozoic exhumation in the Otway Basin using sonic velocities and implications for hydrocarbon exploration

Increasing demand for hydrocarbons is prompting increasing exploration in poorly understood and geologically complex basins. These include basins affected by exhumation, where there is often uncertainty regarding the maximum burial depths of source, reservoir and seal horizons. This uncertainty can be reduced by quantifying exhumation magnitudes using techniques such as sonic velocity analysis. The Cretaceous- Cenozoic Otway Basin of the southern Australian margin is an important oil and gas province whose subsidence history was interrupted by exhumation events during the mid-Cretaceous, mid-Eocene and late Miocene. We have used sonic velocity data from 135 wells to quantify the magnitude and extent of exhumation during these events. Here we present estimates of net exhumation based on analyses of Lower Cretaceous Eumeralla Formation shales, which are important source rocks within the basin. Our approach involves measuring the velocities of overcompacted intra-formational shale units identified from gamma ray logs, and determining their displacement on the depth axis from an empirically derived normal compaction trend based on data from wells where the Eumeralla Formation is at maximum burial at presentday. Our results indicate significant values of mid- Cretaceous net exhumation in the Otway Ranges and Colac Trough, with estimates for Olangolah-1 (~2700 m), Stonyford-1 (~1100 m), Tirrengowa-1 (~860 m) and Ingleby-1 (~1000 m) in good agreement with independent estimates of mid-Cretaceous exhumation for these wells based on palaeothermal data. There is also moderate net exhumation around the Merino High (~700 m) but minor (0-200m) net exhumation around the basin?s northern margins and Penola Trough area and moderate (200-600 m) around the Port Campbell Embayment gas fields. Mid-Cretaceous exhumation is interpreted to have been caused by NW-SE directed compression and inversion of the basin. Our results indicate that potential Eumeralla Formation source rocks in wells where large values of mid-Cretaceous exhumation are recorded are most likely over-mature for hydrocarbon generation, in agreement with previous studies of the thermal history of the basin.

Sub-surface geology and velocity structure of the Krafla high temperature geothermal field, Iceland: Integrated ditch cuttings, wireline and zero offset vertical seismic profile analysis

The Krafla geothermal area in northern Iceland comprises one of the best studied examples of a high temperature geothermal field associated with an active volcanic rift zone. Of key importance to improved resource exploration and development in volcanic areas such as Krafla, is the interpretation and understanding of the subsurface geology. Within this study we present results from an integrated analysis of the downhole volcanic stratigraphy from the K-18 borehole within the Krafla caldera based on combined wireline, ditch cuttings, and zero-offset VSP (vertical seismic profile) analyses. This study presents the first published sonic log velocity data from a high temperature geothermal borehole in Iceland and clearly demonstrates the importance of borehole velocity data for improving volcanic facies interpretations. The shallow subsurface geology of the K-18 site from c. 0–380m comprises an inter-layered sequence of sub-aerial basaltic lavas, hyaloclastites and tuffaceous lithologies of both felsic and basic composition, which are progressively replaced by basaltic sheet intrusions with increasing depth. An interval of variably basic to more evolved mixed tuffaceous facies is identified based on cuttings analysis, gamma and sonic velocities between c. 790–1120m depth. Discrete high sonic Vp units cut the lower c. 100m of this interval and are interpreted as either sheet intrusions or lava interiors. At c. 1800m, a sharp increase in P-wave velocity from c. 4.5 to c. 5.5km/s, represents the transition from a mixed lava and sheet intrusion dominated sequence, into a dense basic intrusion forming the local basement that persists to the bottom of the borehole at c. 2215m. Reduced travel time analysis of processed zero-offset VSP results reveal good correspondence with the major stratigraphic boundaries in the borehole, supporting the potential for VSP studies to robustly characterize complex volcanic stratigraphy in high temperature geothermal fields. Finally, the application of percentage-based ditch cuttings analyses methods for volcanic facies analysis in geothermal boreholes is tested and assessed to have future potential.

Multiple episodes of regional exhumation and inversion identified in the UK Southern North Sea based on integration of palaeothermal and palaeoburial indicators

Abstract We present a consistent synthesis of palaeothermal (apatite fission track analysis (AFTA) and vitrinite reflectance) data from UK Southern North Sea wells with the regional pattern of exhumation defined from sonic velocity data. Cenozoic exhumation across most of the region began in the Paleocene between 63 and 59 Ma. Amounts of removed section are around 1 km across the offshore platform, increasing to 2 km or more on the Sole Pit axis. Neogene exhumation within this area began between 22 and 15 Ma, and led to removal of up to 1 km of section. Along the eastern flank of the Sole Pit axis, sonic data define a pre-Chalk event, and AFTA data from these wells show that exhumation began between 120 and 93 Ma. This timing correlates with events defined from AFTA data in the Sorgenfrei–Tornquist Zone, further east, presumably reflecting a response to regional tectonic stresses. East of the Sole Pit axis, AFTA and sonic velocities suggest that Neogene exhumation dominates, while further east towards the central parts of the North Sea Mesozoic sediments appear to be at maximum burial today except for local effects related to salt movement. The multiple episodes of exhumation and burial defined here have important implications for exploration.

A new method for rock brittleness evaluation in tight oil formation from conventional logs and petrophysical data

Brittleness is a critical indicator for hydraulic fracturing candidate screening in unconventional reservoirs. Current rock brittleness estimation models are often inferred from mechanical parameters and mineralogical data, which primarily use empirical equations. However, the absence of shear sonic velocity data and insufficient mineral data sometimes restricts its wide application. In this article, our objective is to illustrate the application of a data-driven approach for rock brittleness estimation that employs computational intelligence technologies (multilayer perception and radial basis function models) that use conventional well logs as inputs. To reflect the local rock type variation with depth, we first updated the typical mineralogy based brittleness calculation formulas. A database of the well logs, mechanical parameters, X-ray diffraction (XRD) and QEMSCAN mineralogy results collected from a single well in the Santanghu tight oil formation in the Xinjiang basin, China was then constructed. Rock brittleness tests were performed using a multilayer perception model and radial basis function model with different inputs. The comparison of the rock brittleness results produced by the log-based soft computing technologies, mechanical-based method and mineralogy-based method demonstrated that the data-driven approach is flexible and has sufficient accuracy. According to the performance indicators, the predictive performance of the radial basis function model was found to be better than that of the multilayer perception model. This study shows that soft computing technologies can be used to infer missing data when the mineralogical data are inadequate and are less dependent on acoustic full-wave logging, and they are therefore more applicable and practical than traditional empirical formulas.

Ocean bottom seismometer data modeling to infer gas hydrate saturation in Krishna-Godavari (KG) basin

The velocities derived from seismic refraction data/well log/multi-channel seismic (MCS) data serve as the starting point for the saturation estimation of gas hydrates/free gas. Combined with well log data, the estimation of gas hydrate saturation by applying rock physics models to sonic P-wave velocity (Vp) can be greatly improved. The presence of gas hydrates usually changes the shear modulus of the sediment, and to make a reasonable quantification, both P-wave (Vp) and S-wave (Vs) velocities are to be considered, wherever available. The sonic S-wave velocity (Vs) data, if available, provides additional constraints in estimating saturation of gas hydrates. In 2010, CSIR-National Geophysical Research Institute acquired MCS and ocean bottom seismometer (OBS) station data in the Krishna-Godavari (KG) basin for the delineation of gas-hydrate reservoirs and quantifying the resource potential. This data is utilized in the present work, and the gas hydrate saturation is estimated using the seismically derived Vp and Vs in the effective medium theory (EMT). The Vp is determined using the MCS data and the hydrophone component (OBS-H) data of the OBS station that records the PP reflections, while Vs is determined from PS converted waves, recorded on the radial component (OBS-R) of the OBS station data. The results have delineated a five layer subsurface model with an average P-wave velocity of 1.8 km/s in the gas hydrate layer and 1.45 km/s in the free gas layer. The average S-wave velocity is of the order of 0.64 km/s in the gas hydrate layer and 0.5 km/s in the free gas layer. Gas hydrate saturation, computed by employing effective medium theory (EMT), shows that the average gas hydrate saturation varies from 16% to 18% of pore space along the profile.

Constraining Late Cretaceous exhumation in the Eromanga Basin using sonic velocity data

Exhumation in sedimentary basins can have significant consequences for their petroleum systems. For example, source rocks may be more mature than their present-day burial depths suggest, increased compaction can result in reduced reservoir quality, and seal integrity problems are commonly encountered. The Eromanga Basin in central Australia experienced an important phase of exhumation during the Late Cretaceous, though the magnitude and spatial distribution of exhumation is poorly constrained. In this study exhumation magnitudes have been determined for 100 petroleum wells based on sonic transit time analyses of fine grained shales, siltstones and mudstones within selected Cretaceous stratigraphic units. Observed sonic transit times are compared to normal compaction trends (NCTs) determined for suitable stratigraphic units. The Winton Formation and the Bulldog Shale/Wallumbilla Formation were chosen for analysis in this study for their homogenous, fine-grained and laterally extensive properties. Exhumation magnitudes for these stratigraphic units are statistically similar. Results show net exhumation in the southern Cooper-Eromanga Basin (<500 m [~1,640 ft]) and higher net exhumation magnitudes (up to 1,400 m [~3,937 ft]) being recorded in the northeastern margins of the basin. Gross exhumation magnitudes show significant variation across short distances suggesting different tectonic processes acting upon the basin. Independent vitrinite reflectance and apatite fission track analysis data, available for a subset of wells, give statistically similar exhumation magnitudes to those that have been calculated through the compaction methodology, giving confidence in these results. The effect on source rock generation is illustrated through 1D basin modelling where exhumation is shown to impact the timing and type of the hydrocarbons generated. The improved quantification of this exhumation permits a better understanding of the Late Cretaceous tectonics and palaeogeography of central Australia.

Abnormal High Pore Pressure Prediction and Cause Analysis of Salt and Anhydrite Formation of Fauqi Oilfield in Iraq

Abnormal high pressure exists in Lower Fars formation of Fauqi oil field in Iraq and lead to many complicated problems during drilling. There are salt, anhydrite and shale in Lower Fars formation through lithology analysis. Through the crossplot of sonic velocity vs. density data, it can be identified that the high pressure of Lower Fars is cause by the undercompaction of shale. Through composite analysis of different prediction methods of pore pressure, Eaton model is used to calculate the pore pressure of Lower Fars. The pore pressure begins to increase at the MB4 layer and declines to normal value at MB2 layer, and the peak is about 2.20 . The calculated results are in conformity with field situation, which can be applid to guide drilling practice in Fauqi oil field.

Porosity and sonic velocity depth trends of Eocene chalk in Atlantic Ocean: Influence of effective stress and temperature

We aimed to relate changes in porosity and sonic velocity data, measured on water-saturated Eocene chalks from 36 Ocean Drilling Program drill sites in the Atlantic Ocean, to vertical effective stress and thermal maturity. We considered only chalk of Eocene age to avoid possible influence of geological age on chalk compaction trends. For each depth, vertical effective stresses as defined by Terzaghi and by Biot were calculated. We used bottom-hole temperature data to calculate the time–temperature index of thermal maturity (TTI) as defined by Lopatin. Porosity and compressional wave velocity data were correlated to vertical effective stresses and to TTI. Our porosity data showed a broader porosity trend in the mechanical compaction zone, and the onset of the formation of limestone at a shallower burial depth than the porosity data of the Ontong Java Plateau chalk show. Our porosity data do not show or at least it is difficult to define a clear pore-stiffening contact cementation trend as the Ontong Java Plateau chalk. Mechanical compaction is the principal cause of porosity reduction (at shallow depths) in the studied Eocene chalk, at least down to about 5 MPa Terzaghi׳s effective stress corresponding to a porosity of about 35%. This indicates that mechanical compaction is the principal agent of porosity reduction. Conversely, at deeper levels, porosity reduction is accompanied by a large increase in sonic velocity indicating pore-filling cementation. These deep changes are correlated with TTI. This indicates pore-filling cementation via an activation energy mechanism. We proposed a predictive equation for porosity reduction with burial stress. This equation is relevant for basin analysis and hydrocarbon exploration to predict porosity if sonic velocity data for subsurface chalk is available.

Caprock compressibility and permeability and the consequences for pressure development in CO2 storage sites

Large scale CO2 storage has previously been considered for the Vedsted structure located in the Northern part of Jylland in Denmark. Pressure buildup in the Gassum reservoir and transmission to the shallower Chalk Group where the brine–fresh water interface resides need to be investigated as part of site qualification, as overpressure can push brine into the fresh water zone and thereby affecting aquifer performance. Pressure transmission from the reservoir into the surrounding formations, when fractures and faults are ignored, will depend on the properties and thickness of the sealing rock. The most important property to be considered is caprock compressibility and permeability. Laboratory experiments on centimeter-scale plugs and dynamic sonic velocity data from relevant shale formations in Denmark indicate that shale compressibility is lower than often assumed for reservoir simulation studies. The measured compressibility for the Fjerritslev Formation is 0.5×10−5bar−1, which is an order of magnitude lower than the standard compressibility (4.5×10−5bar−1) normally used for reservoir simulation studies. The consequences of this lower compressibility are investigated in a simulation case study and the results indicate that higher overpressure is created in the reservoir and the caprock. Overestimating caprock compressibility can therefore underestimate overpressure within the storage and sealing formations and this can have significant implication in the presence of highly permeable fractures and faults. The caprock permeability is measured on core samples using a geotechnical method of constant rate of strain (CRS) experiments which seem to match the modeled permeability data for the Fjerritslev Formation. We found an average vertical permeability of 0.1μD for the Fjerritslev Formation from the samples measured. The sensitivity of pressure development for the caprock permeability has been studied by varying from one to three orders of magnitude higher and one to two orders of magnitude lower than the measured permeability of 0.1μD. Injecting 60 million tons (Mt) of CO2 at a rate of 1.5Mt/year into the Gassum Formation for 40 years indicates that, with permeability above 1.0μD, overpressure can be transmitted through the 530m thick Fjerritslev Formation caprock and further up into the overburden layers.

Static and dynamic effective stress coefficient of chalk

Deformation of a hydrocarbon reservoir can ideally be used to estimate the effective stress acting on it. The effective stress in the subsurface is the difference between the stress due to the weight of the sediment and a fraction (effective stress coefficient) of the pore pressure. The effective stress coefficient is thus relevant for studying reservoir deformation and for evaluating 4D seismic for the correct pore pressure prediction. The static effective stress coefficient [Formula: see text] is estimated from mechanical tests and is highly relevant for effective stress prediction because it is directly related to mechanical strain in the elastic stress regime. The corresponding dynamic effective stress coefficient [Formula: see text] is easy to estimate from density and velocity of acoustic (elastic) waves. We studied [Formula: see text] and [Formula: see text] of chalk from the reservoir zone of the Valhall field, North Sea, and found that [Formula: see text] and [Formula: see text] vary with differential stress (overburden stress-pore pressure). For Valhall reservoir chalk with 40% porosity, [Formula: see text] ranges between 0.98 and 0.85 and decreases by 10% if the differential stress is increased by 25 MPa. In contrast, for chalk with 15% porosity from the same reservoir, [Formula: see text] ranges between 0.85 and 0.70 and decreases by 5% due to a similar increase in differential stress. Our data indicate that [Formula: see text] measured from sonic velocity data falls in the same range as for [Formula: see text], and that [Formula: see text] is always below unity. Stress-dependent behavior of [Formula: see text] is similar (decrease with increasing differential stress) to that of [Formula: see text] during elastic deformation caused by pore pressure buildup, for example, during waterflooding. By contrast, during the increase in differential stress, as in the case of pore pressure depletion due to production, [Formula: see text] increases with stress while [Formula: see text] decreases.

Estimating Sand–Shale Formation Pore Pressure

This article presents a comprehensive evaluation method of estimating sand–shale formation pore pressure by using sonic velocity and other logging data. The method takes the influence of porosity, density, shale content, effective stress, and some other physical properties of sand–shale formation on sonic velocity into account. The influence and related logging data are combined to estimate the effective stress, and then pore pressure can be obtained by the effective stress principle. Using this method, software was developed and it applied in the well site. Its applications indicate that this method can accurately estimate the abnormal pressure of a sand–shale formation.

Overpressure prediction of the Efomeh field using synthetic data, onshore Niger Delta, Nigeria

For effective and accurate prediction of overpressure in the Efomeh field, located in the Niger delta basin of Nigeria, integrated seismic and borehole analyses were undertaken. Normal and abnormal pore pressure zones were delineated based on the principle of normal and deviation from normal velocity trends. The transition between the two trends signifies the top of overpressure. The overpressure tops were picked at regular intervals from seismic data using interval velocities obtained by applying Dix’s approximation. The accuracy of the predicted overpressure zone was confirmed from the sonic velocity data of the Efomeh 01 well. The variation to the depth of overpressure between the predicted and observed values was less than 10 m at the Efomeh 01 well location, with confidence of over 99 per cent. The depth map generated shows that the depth distribution to the top of the overpressure zone of the Efomeh field falls within the sub-sea depth range of 2655 ± 2 m (2550 ms) to 3720 ± 2 m (2900 ms). This depth conforms to thick marine shales using the Efomeh 01 composite log. The lower part of the Agbada Formation within the Efomeh field is overpressured and the depth of the top of the overpressure does not follow any time-stratigraphic boundary across the field. Prediction of the top of the overpressure zone within the Efomeh field for potential wells that will total depth beyond 2440 m sub-sea is very important for safer drilling practice as well as the prevention of lost circulation.