Offshore E&P - Still More To Learn

Abstract

Offshore, shallow or deep, is a realm unto itself in which technology and economics are entwined and constantly evolving. The technical agenda for the 2006 Offshore Technology Conference (OTC) May 1–4 in Houston promises to deliver the next chapter in offshore E&P. Approximately 40 technical sessions and more than 300 technical presentations are planned. Seven papers chosen by OTC 2006 program organizers emphasize that the one sure thing is that the pressure is on the industry to maintain a healthy balance between high risk, steep cost, and reasonable reward. The technical papers discussed in this article are listed below. Recent Findings in Anisotropy Estimation The authors of paper OTC 17866 estimate parameters of depth-variant vertical transverse isotropy (VTI) from the first-arrival times recorded in a 3D vertical seismic profile (VSP) survey over a deepwater Gulf of Mexico (GOM) project. They use both conventional slowness inversion and a recently developed travel-time extrapolation technique. The authors are not aware of any depth-dependent estimates of anisotropy from VSP data done before their study. Their work confirms that 3D VSP acquisition geometry is ideally suited for estimating in-situ seismic anisotropy; however, the locality of anisotropy measurements is achieved only when local quantities are used. This is not the case when the horizontal slowness components are involved in the inversion process because they are influenced by lateral velocity variations in the entire overburden. "Correcting the horizontal slownesses for lateral velocity heterogeneity requires knowing the seismic velocity model with accuracy hardly attainable in practice," the paper states. A VSP survey was shot in a spiral pattern, with waveforms recorded by 28 geophones placed in 100-ft increments at depths ranging from 17,528 to 20,228 ft. The first-break times were picked from the data. The authors apply both the conventional slowness and novel travel-time extrapolation techniques and obtain comparable estimates of anisotropy. The travel-time extrapolation method, however, better illustrates the accuracy of knowing the lateral velocity variations needed to achieve the locality of anisotropy estimates. "One subsurface feature—a salt body—profoundly influences our ability to estimate anisotropy from the VSP data," say the authors. "Its presence causes a decrease of the first-break times clearly seen in the northern portion of the survey. The salt outline is easily identified in the travel-time data (Fig. 1)." This case study reveals the problem with deriving anisotropy from the slowness surfaces; the presence of salt produces slowness distortions that are impossible to fit with any anisotropic model, forcing the authors to rely only on the data recorded outside the salt. Realistic constraints of parameters are discussed. "Our most interesting finding was the negative sign of the an ellipticity coefficient η," the authors say. "It remains to be understood whether the negative sign of η bears lithologic or any other significance."