Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
The “low-frequency” range of seismic recordings has been of interest to global seismologists for many decades, but only recently has there been a growing interest in utilizing low-frequency passive seismic information for hydrocarbon exploration purposes. Examples of applications include passive seismic monitoring, interferometry methods using the ambient background field, and generally utilizing lower frequency energy for imaging and seismic inversion. Sources of low-frequency ambient background noise are often not well-understood, but a good understanding is essential to avoid pitfalls during data analysis and interpretation. In this study, I analyze a passive seismic data set from the North Sea with regard to ambient noise sources in the frequency range 1–10 Hz. As we shall see, we can find a multitude of explainable ambient noise sources, some of which are external, and some of which are related to the sensor and recording packages themselves. External noise sources include boats, seismic operations, and other fixed seabed installations. Environmental noise includes wind-generated ocean waves, water currents, and wildlife. The design of the sensor and recording unit in combination with the soft seabed sediments gives rise to resonances particularly detrimental to the frequency range of interest. These resonances are exacerbated by presumably strong water currents in the survey area. Boats can be observed at distances of more than 160 km.
- Europe > United Kingdom > North Sea (0.24)
- Europe > Norway > North Sea (0.24)
- Europe > North Sea (0.24)
- (2 more...)
Surface Tiltmeter Mapping Shows Hydraulic Fracture Reorientation in the Codell Formation, Wattenberg Field, Colorado
Wolhart, Stephen Lee (Pinnacle Technologies) | McIntosh, Gregory Edward (Kerr-McGee Rocky Mountain Corp.) | Zoll, Michael Bruce (Noble Energy Inc.) | Weijers, Leen (Pinnacle Technologies)
Abstract This paper reports on a study conducted to measure the orientation of fractures created during initial fracture and refracture treatments in the Codell formation of the Wattenberg Field, Colorado. The Codell is a thin, low permeability sandstone that is laterally continuous across the field area. Due to the low permeability, stimulation is required for economic development of the Codell. Refracturing of Codell wells in Wattenberg Field has been successfully performed for the last decade and several reasons have been suggested for the success of Codell refracs including reorientation of fractures. Surface tiltmeter mapping was performed during this study on several initial fracture and refracture treatments in the field to determine fracture orientation. The average azimuth of initial fracture treatments was N66°E. Significant reorientation occurred on some refrac wells and the average azimuth of refracture treatments was N29°E although highly variable. It has been observed that fracture orientation can vary due to changes in pore pressure as the result of asymmetrical depletion and/or injection or due to structural features. In this case we believe that asymmetrical depletion is affecting the in-situ stress orientation and thus the fracture orientation. There were also indications of fracture complexity with some treatments showing a change in fracture orientation during the treatment and several showing higher than normal horizontal components. The results from this study help explain the success of refracture treatments in the Codell. Refracture reorientation allows existing wells to contact new reservoir increasing gas recovery per well. There may be the potential to fracture wells for a third time (re-refracs) for additional fracture Energy; Greg McIntosh, Anadarko Petroleum; and reorientation and improved gas recovery. Mapping and production results are discussed in this paper. Background The objective of surface tiltmeter mapping on this project was to determine the fracture orientation of initial fracture treatments and refracture treatments of the Codell formation in the Wattenberg Field. The primary focus of the project was to determine if refracture reorientation was playing a role in the restimulation success in the Codell. Refracture reorientation allows an existing well to contact new reservoir and improve ultimate recovery. The secondary objective was to determine fracture orientation in the J-Sand, a target zone below the Codell, in order to optimize well locations. A mix of fracture treatments was monitored during the project, initial treatments in both the Codell and J-Sand and refrac treatments in the Codell, in order to determine original fracture azimuth and refracture azimuth. Codell Formation The Wattenberg Field is located in the Denver-Julesburg (DJ) Basin as shown in Figure 1. The producing area is a multiple pay accumulation in a basin center which is associated with a geothermal anomaly. Productive zones within the field include the J (Muddy), Codell, Terry (Sussex) and Hygiene (Shannon) sandstones and the Niobrara chalks and shales as shown in Figure 1.
- North America > United States > Colorado > Weld County (1.00)
- North America > United States > Colorado > Larimer County (1.00)
- North America > United States > Colorado > Denver County (1.00)
- (3 more...)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.55)
Abstract Presented in this paper is a novel technique of hydraulic fracture azimuth determination. Full size oriented cores, retrieved after microfracturing, were scanned using X-ray computed tomography (CT) to evaluate the fractures. The SEM/EDAX analysis was applied to confirm the presence of barite particles from drilling mud inside hydraulically induced fractures. The study, performed on core samples from the same sandstone formation of two adjacent fields, showed the good agreement of fracture azimuth data, obtained from CT analysis. Postfrac well production history indicates a significant hydrocarbon productivity increase, without interference to surrounding wells. Introduction Orientation of hydraulically induced fractures has a significant impact on final results of hydraulic fracturing operations. Knowledge of hydraulic fracture orientation can be useful in many reservoir applications. Hydraulic fracture azimuth prediction becomes important in terms of improving recovery efficiency in case of producing wells, or optimizing the areal sweep efficiency in case of water flooding or EOR applications using injection wells. When designing the injection pattern and selecting optimum well locations, fracture azimuth should not be ignored. For waterflooding producing wells located perpendicular to the fracture direction will experience better areal sweep efficiency, than wells, situated parallel to the fracture direction. On the other hand, if fracture orientation can not be predicted, and spacing of wells is less than the designed propped fracture length - the wing of fracture can aim to the neighboring well, causing the failure of both wells. Also, if geological condition cause favorable fracture direction, the wing of fracture can reach the hydraulically isolated part of the reservoir, making it recoverable. A number of techniques and methods for mapping or predicting fracture orientation can be found in literature. These can be summarized as:active fracturing techniques (tiltmeter arrays, triaxial borehole seismic), openhole logging techniques (borehole elongation orientation, television camera, sonic televiewers, impression packers), and predictive oriented core techniques (strain relaxation, compressional-wave velocity, thermal expansion, differential strain curve, fracture point load test or residual stress measurement). Although each of them under proper conditions can give more or less accurate and reliable results, each has limitations. In this paper, a new approach to indirect fracture azimuth measurement, based on oriented core analysis is described. The method involves microfracturing technique (Fig. 1) and X-ray CT scanning of oriented cores. During drilling, just after entering a zone of interest for future stimulation by hydraulic fracturing, the drilling process is temporarily interrupted and microfrac job, using relatively small volume of water base - barite weighted mud is performed. This is followed by coring operation and 3 to 10 m of full diameter oriented core is taken from the bottom of the well. The drilling procedure is then continued. Conventional core analysis is performed by visual inspection or by the use of goniometer to characterize fractures, if found on the core surface. X-ray computed tomography is used for visualization and investigation of fractures inside the rock body. Consecutive CT scanning of oriented core (Fig. 2), is made by taking axial cross sections subsequently reconstructed as tomogram images. Since the core orientation during scanning is known and fixed, fracture azimuth is easily determined (see Fig. 3). Analysis of tomogram series furnishes data on fracture growth and position in the core. Hydraulically made fractures can be filled with mud used for microfrac operation. The presence of solid particles, particularly high density barite, can be easily detected in CT tomograms, to distinguish hydraulically initiated fractures from naturally generated ones. Also, the traces of microfrac fluid can be analyzed after location and detection in the fracture by CT scanning, using the other analytical methods, such as SEM/EDAX or chemical analysis of selected rock specimens. Using the data from tomograms, 3-D reconstruction of hydraulically initiated fracture was made. P. 69
- North America > United States (0.30)
- Europe > Croatia (0.29)
- Geology > Mineral > Sulfate > Barite (0.68)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.55)
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying (0.88)
- North America > United States > Colorado > DJ (Denver-Julesburg) Basin > Wattenberg Field (0.99)
- Europe > Austria > Pannonian Basin (0.99)
In the oil & gas industry, surface seismic imaging is critical in defining the sub-surface structure & stratigraphy but could suffer from poor seismic resolution due to complex geology. This paper describes applications of Walkaway and Walkaround VSPs recorded over an oilfield with low surface seismic resolution located in the offshore Arabian Gulf. Two further challenges needed to be addressed: i) estimation of azimuthal anisotropy, and ii) calibration of PS seismic data. Walkaway VSP was proposed as a viable solution to obtain the higher seismic resolution by placing the receiver below the strong shallow reflectors. A multi-azimuthal (4 orthogonal lines) walkaway VSP survey was acquired based on pre-survey modeling to achieve the objectives of higher resolution and calibration of PS data obtained during OBC acquisition. Walkaround VSP was also acquired in two settings: a deep setting to estimate the azimuthal anisotropy to provide information on presence of natural fractures in the main producing reservoirs, and a shallow setting to investigate the causes of a major lost circulation zone. 3-Component downhole receivers and a powerful seismic source (6-Gun Cluster) were used to acquire good quality signal below the near surface attenuating layers. Passing through an elaborate 3-component processing chain, the walkaway survey resulted in hi-resolution subsurface PP and PS images (both in time & depth). These images provide detailed sub-surface features which were not observed in the existing surface seismic, resulting in improved reservoir characterization, as well as calibration of the surface PS seismic data. Walkaround VSP data were interpreted in terms of fracture azimuth (Fast Shear direction) and fracture density variation with depth (ratio of transverse to radial amplitudes). The Walkaround VSP results show good correlation with core data across two discrete reservoir intervals. Walkaround VSP data recorded across the shallow lost circulation zone showed strong anisotropy that coincides precisely with the lost circulation zone indicating high fracture density, with the fast velocity azimuth being consistent with the regional stress field. It was the first time that high resolution VSP images (vertically and spatially) were generated for this field. High resolution walkaway VSP images were utilized to enhance the existing horizon and fault interpretation and reservoir characterization. The Walkaround VSP also helped to understand the azimuthal anisotropy and presence of natural fractures near the well in the main producing reservoir.
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.34)
- Geology > Structural Geology > Tectonics (0.94)
- Geology > Geological Subdiscipline > Stratigraphy (0.89)
- Geology > Geological Subdiscipline > Geomechanics (0.68)
A method of computation for calculating steeply dipping strata is presented. It takes full advantage of the velocity information afforded by well shooting and is simple in application. Charts are derived from the velocity data which will give depth and position of the reflecting point together with angle of dip and azimuth of dip immediately from the time and differential time of a reflection taken from a seismic record.