Jia, Tianxia (BP America) | Regone, Carl (Formerly with BP) | Yu, Jianhua (Formerly with BP) | Gangopadhyay, Abhijit (BP America) | Pool, Robert (BP America) | Melvin, Colin (BP America) | Michell, Scott (BP America)
Microseismic events have been widely used for the monitoring of hydraulic fracturing (Duncan & Eisner, 2010). Common methods of microseismic monitoring use downhole or buried arrays, both of which are expensive and can be operationally difficult to deploy. Surface arrays, therefore, have the potential to mitigate such issues. In this paper, we used finite difference modeling based on the SEAM II model to test a surface patch array's effectiveness in locating and imaging microseismic events. We detected the modeled microseismic events on both noise-free and noisy data using different layout geometries of the patches. The results show that surface patch arrays could generate reasonably good locations of the modeled microseismic events. Based on the modeling results, BP acquired their first microseimic dataset for fracture monitoring using a surface patch array. The array reasonably recorded the microseismic events. Beyond the traditional event locations, we also turned the ambient noise recorded by the surface patch array into signal using the Spatial Auto-Coherency (SPAC) technique (Jia, 2011). We noted that the Rayleigh wave phase velocity calculated from ambient noise correlates very well with the shallow subsurface geology.
Presentation Date: Tuesday, October 18, 2016
Start Time: 11:10:00 AM
Presentation Type: ORAL
In an effort to better understand imaging challenges in the deep water Gulf of Mexico, in 2011 we constructed a large 3D model loosely based on the complex salt geology of the Garden Banks protraction area of the deep water GoM. We then simulated a regional WAZ (wide azimuth towed streamer) seismic survey over this model, producing data that on casual inspection could be mistaken for real. We then performed velocity-model building and imaging on this dataset as if it were real. In particular, the team performing the analysis never saw the correct model.
For much of the model, especially where the salt was relatively simple, we found that the resulting velocity model was quite accurate, even if lacking in fine detail. Reverse-time migration of the seismic data through these parts of the velocity model produced an imperfect but usable image. In other places the salt structures were misinterpreted, causing large-scale errors in the migration velocity model, which resulted in an unusable, shattered image below the salt.
We conclude that traditional velocity-model-building techniques can miss features that occur at too large a scale. Reliably imaging under complex salt in the Gulf of Mexico may require new velocity-model-building methodologies to be developed that are specifically designed to deal with the problem of large velocity heterogeneities.
A previous study (Etgen et al., 2014) showed that fine-scale features associated with abrupt velocity heterogeneities (in particular, top of salt) can damage the seismic image. In this study our goal was to examine the other end of the velocity heterogeneity size spectrum, by simulating acquisition and processing of a large 3D WAZ dataset in an area of complex salt containing large structural features.
We found that existing models available to us were unsuitable for this purpose. We concluded that:
1) The model needed to be 3D, because complex salt geology is intrinsically 3D. A 2D model cannot adequately represent the challenge.
2) The model needed to be very large, big enough to contain modern wide-azimuth long-offset acquisition geometries with plenty of room to spare, to avoid the results being contaminated by edge effects and to allow us to test multiple novel acquisition strategies using the same synthetic dataset. It also needed to be deep enough to contain diving waves out to wide offsets. The final model design was 18 km deep by 85.5 km by 106 km wide.
We use finite-difference (FD) modeling to study the effect of spatial sampling on coherent noise suppression in 3D land seismic surveys. We compute realistic signal and noise wavefields separately and then combine them in various ways to form 3D surveys with variable S/N. We use two types of noise wavefields: source generated and ambient. We measure the S/N level for source-generated noise (SGN) inside the direct- arrival noise cone so that it represents the relative level of signal to scattered noise, and we use S/N levels of 0 dB and -35 dB so that we have both easy and difficult noise problems. We set the level of ambient noise so that the amplitude of the near-offset, direct-arrival, ground-roll is roughly equal to that for the 3D seismic survey sources. We use grid geometries, and we compare a source grid with 25 meter spacing paired with receiver grids having 25, 50, 100, and 200 meter spacings to the reciprocal cases for their relative performance in coherent noise suppression. In the process, we achieve fold levels of 400, 1600, 6400, and 25,600. When the S/N for SGN is high (i.e. the coherent noise is mostly direct-arrival and the scattered noise is no stronger than the signal), either dense sources and sparse receivers or sparse sources and dense receivers yield high quality images. However, as the S/N due to SGN decreases, dense receiver grids become favored. When ambient noise is included, the balance is further shifted in favor of dense receivers. Furthermore, adequate coherent noise suppression for very low S/N requires an increase in the density of the sparser of the two grids.
In land areas, such as desert terrains, where access for both sources and receivers is relatively unrestricted, dense source grids using vibrators and simultaneous shooting methods coupled with a much sparser receiver layout have often been used. The questions we wish to address in this study are the following: Suppose that a sufficient number of receivers are available so that either a dense source grid or a dense receiver grid may be deployed (where by dense we mean that the spacing is sufficiently small so that the noise wavefield is not spatially aliased). Is one favored over the other? Does this conclusion depend on whether all the noise is source generated? How does the presence of strong levels of ambient noise such as caused by vehicles, motors, compressors, or generators affect the balance? What happens when the noise is spatially aliased in both the source and receiver domains? What is the required sampling interval for the sparser of the two domains as a function of S/N?
Michell, Summers (BP Houston USA) | Summers, Tim (BP Houston USA) | Shoshitaishvili, Elena (BP Houston USA) | Etgen, John (BP Houston USA) | Regone, Carl (BP Houston USA) | Barley, Brian (BP Cairo Egypt) | Keggin, Jim (BP Cairo Egypt) | Benson, Mark (BP Cairo Egypt) | Rietveld, Walter (BP Cairo Egypt) | Manning, Ted (BP London UK)
Copyright 2007, Offshore Technology Conference This paper was prepared for presentation at the 2007 Offshore Technology Conference held in Houston, Texas, U.S.A., 30 April-3 May 2007. This paper was selected for presentation by an OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference.
3D finite-difference (FD) modeling studies conducted over subsalt structures in the deep-water Gulf of Mexico confirm the deficiencies of narrow azimuth towed streamer (NATS) surveys and predict significant improvement in image quality with wide azimuth methods. FD modeling has provided important design parameters for two separate approaches for wide azimuth surveys: OBS nodes distributed on the ocean floor in a regular grid coupled with a dense grid of sources on the surface, and a Wide Azimuth Towed Streamer (WATS) method using multiple standard marine seismic vessels in a novel configuration. These two methods complement each other. OBS nodes may be used effectively where field development results in many obstacles that might interfere with towed streamer methods, where the required size of the 3D survey is not too extensive, or where very long offsets are required for all azimuths. The WATS method becomes more efficient as the survey size increases, and key parameters scale to provide a wide range of cost vs. data quality options that make the method suitable for field development or exploration.
No preview is available for this paper.