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- Volume 19, Issue 2, 2013
Petroleum Geoscience - Volume 19, Issue 2, 2013
Volume 19, Issue 2, 2013
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The petrophysics of shale gas reservoirs: Technical challenges and pragmatic solutions
Authors Vivian K. Bust, Azlan A. Majid, Joshua U. Oletu and Paul F. WorthingtonThe host rocks of shale gas accumulations act as source, seal and reservoir. They are characterized by complex pore systems with ultra-low to low interparticle permeability and low to moderate porosity. The word ‘shale’ is used in the sense of a geological formation rather than a lithology, so shale gas reservoirs can show marked variations in rock type from claystones, marlstones and mudstones to sandstone and carbonate lithological ‘sweet spots’. The pore space includes both intergranular and intrakerogen porosity. The density of natural fractures varies markedly, and pore throat connectivity is relatively ineffective. Moreover, in-situ gas pore volume has to take account of both free and adsorbed gas, an evaluation exercise that is complicated by pronounced variations in water salinity. All these characteristics present major challenges to the process of petrophysical evaluation. The petrophysical responses to these issues are severalfold. First, a broader calibrating database of core measurements is required at key wells, especially as regards mineralogy, porosity and permeability data, shale/mudstone sample analyses, total organic carbon, gas desorption isotherms, and the analysis of extracted formation waters. Second, at least in the key wells, an extended suite of logs should include an elemental analysis log, magnetic resonance imager, electrical micro-imager, and a dipole sonic log. These databases lead to a rock-typing scheme that takes better account of dynamic properties and fracturability. They also allow reservoir partitioning based on exclusivity of empirical interpretative algorithms, e.g. quartz content vs. producibility. These responses comprise key elements of a functional petrophysical system that encompasses fit-for-purpose interpretation methods, such as a pseudo-Archie approach, i.e. the application of the Archie equations with non-intrinsic exponents. This system is presented as a workflow for application in shale gas reservoirs, for which bulk density retains a major influence on computed gas in place. The benefits of this approach are especially strong in reserves reporting of these unconventional gas reservoirs.
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Quantification of hydrodynamic effects on capillary seal capacity
Authors T. Manzocchi and C. ChildsWhether a reduction in overpressure across a hydrocarbon column and its seal will reduce the column height that the seal is capable of supporting has been much debated. Recent studies have focused on measuring water relative permeability at high capillary pressure, and have concluded that hydrocarbon columns may be sufficiently permeable to water for the seal capacity to be unaffected by hydrodynamic flow in most situations. A new method for estimating the capillary seal capacity is developed which properly takes into account viscous as well as gravity and capillary forces. A reassessment of existing analyses indicates that hydrodynamic flow has a greater influence on the seal capacity than previously thought. Sets of sensitivity models, including a range of reservoir and seal geometrical and petrophysical characteristics, indicate that – as a general rule – hydrodynamic flow should not be neglected when assessing capillary seal capacity. The sealing capacity of thick top seals above reservoirs with a basal aquifer are least likely to be affected by hydrodynamic flow. The sealing capacity of membrane fault seals, irrespective of whether they have a basal or edge aquifer, may be influenced by realistic overpressure differences. Simple end-member equations define the possible influence of hydrodynamic flow in a particular situation, and only if this is significant should a more comprehensive evaluation be considered.
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Recovery efficiency from a turbidite sheet system: numerical simulation of waterflooding using outcrop-based geological models
A series of waterflood simulations were performed to investigate the effect of basinal position and facies permeability within a turbidite sheet system on oil recovery efficiency. Simulations used three-dimensional outcrop models of the Peïra Cava system, comprising gravel, sandstone, thin-bedded heterolithic and mudstone facies.
Recovery efficiency declines with increasing permeability heterogeneity and is influenced by the interaction of vertical bed-permeability trends and flood-front gravity slumping. The occurrence of gravels with permeabilities lower than overlying sandstones produces optimum recoveries. High permeability gravels act as thief zones, enhanced by downward gravity slumping, reducing normalized recovery by up to 34 %. The effect of thief zones on recovery is related to their permeability contrast, abundance, thickness, lateral continuity, vertical position within permeable units and the permeability of underlying facies.
Proximal to distal stratigraphic variations produce relatively small differences in normalized recovery of up to 13 % in models with the highest permeability heterogeneity. Differences in recovery are interpreted to reflect spatial trends in facies architecture, which determine the effectiveness of high permeability gravel thief zones. The poorest recovery is recorded from the medial model where recovery is lower than distal areas because of higher gravel abundance and thicknesses and lower compared to proximal areas because of the higher lateral continuity of gravels and underlying low-permeability mudstones.
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The characteristics of fracture networks in the Shiranish Formation of the Bina Bawi Anticline; comparison with the Taq Taq Field, Zagros, Kurdistan, NE Iraq
Authors A.H. Awdal, A. Braathen, O.P. Wennberg and G.H. SherwaniThe Zagros Fold and Thrust Belt of NE Iraq hosts a prolific hydrocarbon system. Reservoirs are commonly found in fractured Cretaceous carbonates (Shiranish Formation) such as in the Taq Taq Field located in the Kirkuk Embayment of the Zagros foothills. Data providing information on fractures in the Taq Taq Field are core, image logs and flowmeters from wells, and surface observations. For comparison, an outcrop study has been undertaken around the Bina Bawi Anticline (10 km from Taq Taq Field), where the same stratigraphical unit is exposed in a continuous, lenticular-shaped belt. Fracture data have been collected using scanlines on bedding surfaces in the limbs and hinge of the anticlines.
Both the Bina Bawi Anticline and Taq Taq Field show a systematic relationship between fracture sets and fracture lineaments, with a dominance of NE–SW-oriented structures. This orientation is perpendicular to the major folds and parallel to the maximum horizontal in situ stress. There are three fracture populations in the Bina Bawi Anticline, classified according to their relationship with the fold axis and bedding: (i) NW–SE-striking fractures normal to bedding: (ii) NE–SW-striking fractures normal to bedding; and (iii) conjugate oblique fracture sets subnormal to bedding. Both fracture intensity and fracture terminations are controlled by the location within the anticline; the hinge zone displays the highest intensity and the most fracture-abutting terminations. Cross-cutting relationships suggest that a prefolding stage of NE–SW tensional fractures predates folding-related tensional and shear fractures. Few uplift fractures can be indicated. We propose that the former fracture set (joints) formed in a foreland setting and was controlled by the far-field stresses, whereas later fracturing occurred due to outer arc extension during flexing of the Bina Bawi and Taq Taq anticlines. Our comparative analysis of outcrop and well data underline the importance of representative analogue data for reservoir modelling and production strategies.
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Using 3D seismic data to map shallow-marine geohazards: a case study from the Santos Basin, Brazil
Authors Andy Sharp and Gianluca BadaliniThis paper documents a study that used high quality, conventionally processed, speculative 3D seismic data for two specific purposes: first, to identify and avoid any health, safety, security and environmental (HSSE) issues regarding the water bottom and, secondly, to avoid shallow (<1 s below the mud line) geohazards when drilling the wells. The study integrated existing legacy well data, new 3D data from the BM-S-52 concession and a targeted programme of water-bottom grab samples from the BM-S-52 concession, Santos Basin, offshore Brazil. Two aspects of the analysis are described: the first addresses the problem and solution adopted for characterizing the water-bottom environment in this area; the second addresses the interpretation of the data for shallow geohazards.
The results of this case study have been accepted by the regulatory authorities as representing sufficient evidence of risk evaluation that no further dedicated site-survey data acquisition is required. This has led to savings in time and in costs which would otherwise have been expended on deep-water site-survey campaigns. Furthermore, the use of 3D seismic data for well-location planning brings benefits in improved imaging of the near-mud-line geology, full coverage over the whole concession, operational flexibility for changing well location and timely availability of data. The analysis workflow may also be applicable in other, similar deep-water areas.
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Seismic data conditioning and neural network-based attribute selection for enhanced fault detection
Authors A. Chehrazi, H. Rahimpour-Bonab and M. R. RezaeeIn this study of the Dorood oil field, offshore Iran, 3D seismic data were utilized to identify a complex fault pattern in the highly faulted and fractured Fahliyan Formation. To enhance data quality and improve attribute accuracy and detection power, a steering cube was first computed based on a sliding 3D Fourier analysis technique, using the concept of directivity. The steering cube, which contains dip and azimuth information for each trace, was utilized for calculation of dip-steered filters and attributes. We applied the dip-steered median filter to remove random noise and to enhance laterally continuous seismic events by filtering along the structural dip. Several fault identification attributes, such as dip, curvature, coherency and similarity, and a meta-attribute of a ridge enhancement filter, were extracted from dip-steered, noise-attenuated data. A supervised, fully connected multi-layer perceptron neural network was constructed to select and combine the most sensitive fault attributes. The neural network, trained at identified fault and non-fault locations, was applied to the whole seismic volume to generate a cube of fault probability.
Interpretations of major faults and fractures were integrated with geological, reservoir engineering and production data to highlight the role of these heterogeneities on dynamic reservoir properties. Faults and fractures in the Fahliyan reservoir were identified in general to have the effect of decreasing reservoir permeability. While most of the faults recognized have locally a sealing capacity effect, when the fault throws are large enough to bring into contact the Manifa and Yamama reservoirs, they act as a conduit for fluid communication and, wherever a well crosses a major fault, early water breakthrough is observable.
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Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil
Authors Andréa B. Bahlis and Luiz F. De RosUpper Cretaceous sandstones are the main clastic reservoirs of the Santos Basin, eastern Brazil, showing high porosity at depths greater than 4000 m. Porosity preservation in these deep sandstones is ascribed to the inhibition of quartz cementation and pressure dissolution by authigenic pore-lining chlorite. Santos sandstones are mostly fine-grained, lithic arkoses, rich in volcanic rock fragments (VRF). Chlorite, the most abundant diagenetic constituent, occurs as coatings, rims, rosettes and replacing grains. Chlorite precipitation was favoured by the presence of eodiagenetic smectite coatings and by the abundance of VRF. Detrital heavy minerals, biotite and mud intraclasts were also sources and/or substrates for chlorite authigenesis. The chloritization of VRF, biotite and heavy minerals resulted in precipitation of abundant TiO2 minerals. Fluid flow patterns, burial and thermal history also played a role in chlorite authigenesis and reservoir quality evolution. Discontinuous and thin coatings and rims were not effective in inhibiting quartz cementation, while those too thick severely reduced the permeability. The rosette aggregates exerted no inhibition on quartz cementation, while reducing intergranular porosity. Systematic studies are necessary for modelling the diagenetic patterns of Santos Basin sandstones, in order to contribute to the reduction of risks involved in the exploration of these reservoirs.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)
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