Stefan M. Luthi

Fracture apertures from electrical borehole scans

Three-dimensional finite-element modeling was performed to investigate the response to fractures of the Formation MicroScanner (Mark of Schlumberger), which records high-resolution electrical scans of the borehole wall. It is found that the equation W=cARmbRx0l describes, over two orders of magnitude of resistivity contrasts between borehole mud and the formation, the relationship between fracture width W (in mm), formation sensitivity Rx0, mud resistivity Rm, and the additional current flow A caused by the presence of the fracture. A is the additional current which can be injected into the formation divided by the voltage, integrated along a line perpendicular across the fracture trace. Coefficient c and exponent b are obtained numerically from forward modeling. Tool standoffs of up to 2.5 mm and fracture dips in the range from 0° to 40° were found to have an insignificant effect on the above relation. A three‐step approach to detect, trace,and quantify fractures is used. Potential fractures in Formation...

Contribution of research borehole data to modelling fine-grained turbidite reservoir analogues, Permian Tanqua-Karoo basin floor fans (South Africa).

Outcrop analogue studies can be augmented and constrained by drilling research wells through the same stratigraphic interval. Close-to-outcrop wells help to validate outcrop observations with well log and core data and thus improve the use of such data in actual field developments. Research wells located further away from the outcrops increase the spatial data coverage and can give important insight into regional facies distributions and net:gross changes. In the Tanqua–Karoo Basin (South Africa), seven wells were drilled into fine-grained sand-rich basin-floor fans and interfan mudstones to supplement outcrop data. Three close-to-outcrop wells proved useful in establishing characteristic log responses of the main architectural elements identified from the nearby outcrops. Lithofacies were correctly identified in more than 80% of cases using an artificial neural network. Borehole images provided detailed information on sedimentary structures, including a wealth of palaeocurrent data from climbing ripples that significantly enhanced the interpretations based only on outcrops. Wells sited away from the outcrops supplied information on lateral thickness and facies trends and intrafan stacking patterns, which helped to define the stratigraphic evolution of the fans. The combined data indicate that deposition was controlled in part by subtle basin-floor topography, and that intrafan lobe switching took place, leading to internal subdivisions that potentially caused effective compartmentalization of the basin-floor fan.

Integrated high-resolution stratigraphy of a Middle to Late Miocene sedimentary sequence in the central part of the Vienna Basin

Integrated high-resolution stratigraphy of a Middle to Late Miocene sedimentary sequence in the central part of the Vienna Basin In order to determine the relative contributions of tectonics and eustasy to the sedimentary infill of the Vienna Basin a high-resolution stratigraphic record of a Middle to Late Miocene sedimentary sequence was established for a well (Spannberg-21) in the central part of the Vienna Basin. The well is located on an intrabasinal high, the Spannberg Ridge, a location that is relatively protected from local depocentre shifts. Downhole magnetostratigraphic measurements and biostratigraphical analysis form the basis for the chronostratigraphic framework. Temporal gaps in the sedimentary sequence were quantified from seismic data, well correlations and high-resolution electrical borehole images. Stratigraphic control with this integrated approach was good in the Sarmatian and Pannonian, but difficult in the Badenian. The resulting sedimentation rates show an increase towards the Upper Sarmatian from 0.43 m/kyr to > 1.2 m/kyr, followed by a decrease to relatively constant values around 0.3 m/kyr in the Pannonian. The sequence reflects the creation of accommodation space during the pull-apart phase of the basin and the subsequent slowing of the tectonic activity. The retreat of the Paratethys from the North Alpine Foreland Basin during the Early Sarmatian temporarily increased the influx of coarsergrained sediment, but eventually the basin acted mostly as a by-pass zone of sediment towards the Pannonian Basin. At a finer scale, the sequence exhibits correlations with global eustasy indicators, notably during the Sarmatian, the time of greatest basin subsidence and full connectivity with the Paratethyan system. In the Pannonian the eustatic signals become weaker due to an increased isolation of the Vienna Basin from Lake Pannon.

Toward a quantitative definition of mechanical units: New techniques and results from an outcropping deep-water turbidite succession (Tanqua-Karoo Basin, South Africa)

The physical properties of reservoirs are strongly influenced by distributed fracture fields. Outcrop studies are commonly used to determine them but have provided unsatisfactory results because the definition of mechanical units, i.e., (groups of) layers displaying homogeneous fracture patterns, is difficult and typically conducted in a qualitative manner. We have developed a systematic methodology to acquire and process fracture patterns in outcrops and to define their relation with stratigraphy. We dedicate particular attention to the vertical distribution of fractures in a sedimentary succession, commonly composed of layers of different thicknesses and compositions. The method makes full use of geographic information system technologies and allows for direct digital acquisition in the field leading to time-efficient acquisition. Data are processed with a newly developed routine that permits an objective description of the changes of fracture characteristics along the stratigraphy of the outcrop. The operator is then able to define the most suitable fracture stratigraphy. The integration of results from different outcrops is thought to provide a tool for predicting fracture distributions in subsurface target areas.

Application of Borehole Images to Three-Dimensional Geometric Modeling of Eolian Sandstone Reservoirs, Permian Rotliegende, North Sea

High-resolution electrical borehole images of the Permian Rotliegende Sandstone (North Sea) clearly reveal a vertical succession of eolian and fluvial bedding facies. Porosities, which are found to be primarily a function of these facies, show a hierarchy of reservoir heterogeneities related to alternating depositional mechanisms. Dips and azimuths were measured in interdune layers, bounding surfaces, and cross-strata using, in addition to electrical images, full-circumference acoustic borehole images and dipmeter results. The vertical sequence and geometry of the bedding elements indicate composite cross-bedding formed by downwind migration of superimposed bed forms alternating with interdune sections containing only few single bed forms. Foreset direction scatter decrea es significantly with increasing set thickness. Statistical modeling of the thickness and foreset direction of cross-bedded sets is shown to constrain the three-dimensional geometry of the sand bodies. At least two size populations are identified, one with maximum cross-bed thicknesses from 5 to 20 ft (1.5 to 6 m) and another with a thickness of 30 ft (9 m). Estimated widths and lengths of cross-bedded sets are 100 and 200 times the thicknesses. Crescentic (semi-elliptic) bed forms with oblique migration account best for the observed azimuth data; the dune crescents probably were laterally linked into a ridge. From bed-form orientation and migration angle, a local paleowind direction from the northeast is suggested, fitting well the Permian paleo-tradewind regime. A few cross-beds in the central part of the sequence indicate wind-p rallel migration and generally thicker bottomset portions; these factors are interpreted as evidence for stronger, less variable winds in the central erg. The results statistically predict distances at which wells are not connected by highly porous layers. An average preferred drainage direction for the reservoir as well as local drainage direction for each cross-bed is obtained. Extent and thickness of the cross-beds may also serve as input for average horizontal and vertical permeability as well as radial inflow performance estimates.

Nuclear Magnetic Resonance Logging

Nuclear magnetic resonance (NMR), or “nuclear induction” as it was initially called, was developed in the late 1940s (Bloch et al., 1946). Its principle is based on the response of nuclei to magnetic fields (for terminology, see table 2.6.1). Some nuclei, particularly protons — which form the nuclei of hydrogen atoms — have a magnetic moment which can be visualized as a spinning bar magnet. These magnetic moments can be detected with suitable measurement set-ups and an estimation of the location and amount of hydrogens in a sample can be obtained. As hydrogen is abundant in both oil and water, NMR logging was soon proposed for oilfield applications. Chevron developed and patented a NMR logging tool in the 1950s, and in 1960 ran the first NMR log (Brown & Gamson, 1960). One of their primary goals was to quantify the amount of tar in some of their Californian reservoirs. When compared to lighter oils and water, hydrogen atoms in heavy oil take longer to respond to a magnetic field and, conversely, lose an acquired precession faster when the magnetic field is turned off (they “relax” faster). To this day, this represents one of the unique and more interesting applications of NMR logging.

Acoustic nonlinear full-waveform inversion on an outcrop-based detailed geological and petrophysical model (Book Cliffs, Utah)

Analog outcrops are commonly used to develop predictive reservoir models and provide quantitative parameters that describe the architecture and facies distribution of sedimentary deposits at a subseismic scale, all of which aids exploration and production strategies. The focus of this study is to create a detailed geological model that contains realistic reservoir parameters and to apply nonlinear acoustic full-waveform prestack seismic inversion to this model to investigate whether this information can be recovered and to examine which geological features can be resolved by this process.Outcrop data from the fluviodeltaic sequence of the Book Cliffs (Utah) are used for the geological and petrophysical two-dimensional model. Eight depositional environments are populated with average petrophysical reservoir properties adopted from a North Sea field. These units are termed lithotypes here. Synthetic acoustic prestack seismic data are then generated with the help of an algorithm that includes all internal multiples and transmission effects. A nonlinear acoustic full-waveform inversion is then applied to the synthetic data, and two media parameters, compressibility (inversely related to the square of the compressional wave velocity vP) and bulk density, ρ, are recovered at a resolution higher than the shortest wavelength in the data. This is possible because the inversion exploits the nonlinear nature of the relationship between the recorded data and the medium contrast properties. In conventional linear inversion, these details remain masked by the noise caused by the nonlinear effects in the data. Random noise added to the data is rejected by the nonlinear inversion, contributing to improved spatial resolution. The results show that the eight lithotypes can be successfully recovered at a subseismic scale and with a low degree of processing artifacts. This technique can provide a useful basis for more accurate reservoir modeling and field development planning, allowing targeting of smaller reservoir units such as distributary channels and lower shoreface sands.

Salt-induced stress anomalies: an explanation for variations in seismic velocity and reservoir quality

ABSTRACT Accurate rock property prediction is often a critical success factor for wells targeting hydrocarbons. This applies not only to reservoir porosity and permeability affecting productivity directly, but also to acoustic velocity, seismic time-to-depth conversion and depth prognosis. A detailed analysis of variation in the overburden rock velocity in the Southern North Sea has shown that Triassic velocity variations of up to 18% occur within short distances (e.g. <1 km). A correlation was found between increased acoustic velocities and the presence of an underlying salt weld. Salt Induced Stress Anomaly (SISA), a geomechanical model, is presented that can explain these observations and is based on the principles of buoyancy and point-loading. In the initial state, prior to salt movement, the vertical effective rock stresses resulting from the overburden weight are transmitted uniformly and cause laterally even compaction in the sediments. However, once the salt layer is able to flow and redistribute itself under the influence of buoyancy forces, the overburden stress will concentrate itself near the salt welds. This locally increased stress gives rise to higher velocities in the overburden. The same stress concentration model can also explain deterioration in the porosities of the Rotliegend reservoir as observed underneath salt welds.

High-resolution reservoir characterization by an acoustic impedance inversion of a Tertiary deltaic clinoform system in the North Sea

Fluvio-deltaic sedimentary systems are of great interest for explorationists because they can form prolific hydrocarbon plays. However, they are also among the most complex and heterogeneous ones encountered in the subsurface, and potential reservoir units are often close to or below seismic resolution. For seismic inversion, it is therefore important to integrate the seismic data with higher resolution constraints obtained from well logs, whereby not only the acoustic properties are used but also the detailed layering characteristics. We have applied two inversion approaches for poststack, time-migrated seismic data to a clinoform sequence in the North Sea. Both methods are recursive trace-based techniques that use well data as a priori constraints but differ in the way they incorporate structural information. One method uses a discrete layer model from the well that is propagated laterally along the clinoform layers, which are modeled as sigmoids. The second method uses a constant sampling rate from the well data and uses horizontal and vertical regularization parameters for lateral propagation. The first method has a low level of parameterization embedded in a geologic framework and is computationally fast. The second method has a much higher degree of parameterization but is flexible enough to detect deviations in the geologic settings of the reservoir; however, there is no explicit geologic significance and the method is computationally much less efficient. Forward seismic modeling of the two inversion results indicates a good match of both methods with the actual seismic data.

Non-Vegetated Playa Morphodynamics Using Multi-Temporal Landsat Imagery in a Semi-Arid Endorheic Basin: Salar de Uyuni, Bolivia

Playas in endorheic basins are of environmental value and highly scientific because of their natural habitats of a wide variety of species and indicators for climatic changes and tectonic activities within continents. Remote sensing, due to its capability of acquiring repetitive data with synoptic coverage, provides a unique tool to monitor and collect spatial information about playas. Most studies have concentrated on evaporite mineral distribution using remote sensing techniques but research about grain size distribution and geomorphologic changes in playas has been rarely reported. We analysed playa morphodynamics using Landsat time series data in a semi-arid endorheic basin, Salar de Uyuni in Bolivia. The spectral libraries explaining the relationship between surface reflectance and surficial materials are extracted from the Landsat image on 11 November 2012, the collected samples in the area and the precipitation data. Such spectral libraries are then applied to the classification of the other Landsat images from 1985–2011 using maximum likelihood classifier. Four types of surficial materials on the playa are identified: salty surface, silt-rich surface, clay-rich surface and pure salt. The silt-rich surface is related to crevasse splays and river banks while the clay-rich surface is associated with floodplain and channel depressions. The classification results show that the silt-rich surface tends to have a positive relationship with annual precipitation, whereas the salty surface negatively correlates with annual precipitation and there is no correlation between clay-rich surface and annual precipitation. Salty surfaces seem to consist primarily of clay due to their similar characteristics in response to precipitation changes. The classification results also show the development of a crevasse splay and avulsions. The results demonstrate the potential of Landsat imagery to determine the grain size and sedimentary facies distribution on playas in endorheic basins.