TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractNaturally Fractured Reservoir (NFR) characterization represents an increased focus for oil and gas companies as it becomes more and more admitted that they represent a substantial part of their portfolio. However the complexity of the understanding of fractured reservoirs, in terms of fracturing mechanism, fracture density, orientation, and the complexity of their management issues (i.e. infill drilling, water production, steam injection, to list few of these issues) pushed several service and integrated companies to tackle the fractured reservoir characterization challenge. Moreover the use of integrated approaches with the help of 3D seismic and new technologies are started to show successful results. This paper will present two technologies where 3D seismic attributes along with geologic and engineering data are being used to characterize fractured reservoirs. The first technology will show how the use of post-stack seismic in an integrated approach, involving high resolution seismic inversion, spectral imaging and static geological modeling, provides an accurate fracture reservoir model that can be applied in the reservoir simulation and development stage. The second technology will highlight the use of pre-stack seismic to actually image the fracture distribution. Application of these technologies is presented on two different fields.
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This paper presents an innovative integrated workflow applied to the characterization of a fractured chalk reservoir in the Danish North Sea. The methodology uses simultaneous integration of geophysical, geological and engineering data to produce an improved reservoir description. Integrating dynamic flow data with the geophysical and geologic information in 3D, reservoir properties - porosity and effective permeability - are generated using artificial intelligence tools. The strength of this technique lies in the fact that property modeling is not constrained to match upscaled well data and consequently these data serve to validate the outcome. This workflow builds upon a methodology that has been used successfully for the characterization of fracture distribution. The technique has been extended to include the generation of seismically derived models of porosity and matrix permeability. The objective of the approach is to improve the ability to capture the heterogeneity of key reservoir properties, and thus use the resulting reservoir model to both provide improved predictive ability and identify previously undiscovered development opportunities. The application and outcome of this integrated workflow to the Syd Arne field is presented in this paper. The Syd Arne Field The Syd Arne field, operated by Hess Corporation, is located in the Danish part of North Sea. (Fig. 1). The field, presently estimated by the Danish Energy Agency at 185 MMstb of oil and 434 Bscf of gas of initial reserves, was originally discovered in 1969 by the I-1X well. However, it was not until 1995 and the drilling of the RIGS-1 well that the extension and true value of this accumulation was appreciated. The field came on production in 1999 and year-to-date a total of 17 development wells have been drilled - 11 producers and 6 water injectors. The Syd Arne field lies approximately 250 km offshore west of Denmark. It is an elongated anticline, 12 km by 3 km, and is one of the most northerly chalk fields in the Danish North Sea1. The reservoir consists of the Maastrichtian (Upper Cretaceous) to Danian (Paleocene) chalk of the Tor and Ekofisk Formation (Fm). In this interval, the late Maastrichtian is the best reservoir layer and there is currently no dedicated development of the Ekofisk Fm. The field lies at a depth of between 2700–2940 m subsea. The depth map at the Tor level is shown in Fig. 2. Over the crest of the field the oil column is restricted to the thickness of the reservoir. Some general reservoir parameters are listed in Table 1. Following the "rediscovery" of the field in 1995 a 3D survey was acquired. This survey is the basis of all geophysical work on the field for the past decade, despite the somewhat poor data quality. In third quarter of 2005 a combined 3D and 4D survey was acquired, but these data were not available for the present study. The 1995 survey has been reprocessed several times with the objective to improve structural imaging and resolution, the most recent versions being a Pre-Stack Time Migration (PSTM) volume from 2001 and a Pre-Stack Depth Migration (PSDM) volume from 2003 (Fig. 3). As with most chalk fields in the Danish and Norwegian part of the North Sea, Syd Arne is plagued by a gas cloud in the overburden at the center of the accumulation, causing strong attenuation and poor quality of the seismic data in this portion of the field.
Imaging and characterizing fractured shale reservoirs constitute significant challenges. We introduce a workflow and illustrate the application of instantaneous spectral analysis with the aid of a case study on fractured shale reservoirs. With the well log modeling, frequency dependent energy and phase are improved by removing the wavelet overprint. Extracted frequency dependent energy and phase cubes provide an efficient tool to delineate distributions of reservoirs and fault systems. Our results show that the reservoirs distribute along the structure high and are consistent with the paleo-current directions. The attenuation analysis allows observing spectral anomalous variations, representing an alternative tool for hydrocarbon exploration and indirect fracture indicator. Our results also show attenuation anomalies integrated with the production data can guide our predictions of reservoir producibility and indirectly characterize fracture intensity.
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