Advances in 4D Seismic and Geophysical Monitoring of Deepwater Fields

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 27828, “Advances in 4D Seismic and Geophysical Monitoring of Deepwater Fields,” by Kanglin Wang, Paul Hatchell, David Chalenski, and Jorge Lopez, Shell, prepared for the 2017 Offshore Technology Conference, Houston, 1–4 May. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission. Integrated surveillance is critical for understanding reservoir dynamics and improving field management. A key component of the surveillance is areal monitoring of subsurface changes by use of time-lapse geophysical surveys such as 4D seismic. The complete paper reviews the advances in these technologies with recent examples from the Gulf of Mexico (GOM) and deepwater Brazil. Data-Quality Improvement 4D seismic has played a pivotal role in monitoring offshore fields for some time. However, until recently, its application in the GOM had been limited, largely because the effects of the Loop Current and infrastructures make it difficult to repeat the feathering of streamers used in conventional 4D-seismic acquisition. The key for 4D success is to repeat everything in baseline and monitor surveys to make sure the time-lapse difference observed in the data reflects real subsurface changes rather than differences in data-acquisition conditions or processing work flows. To solve the challenge of streamer-positioning repeatability, an operator used ocean-bottom nodes (OBNs) to achieve highly repeated surveys at the Mars Field, and excellent 4D results were obtained. Since then, 4D seismic has been deployed successfully at the portfolio scale in the GOM and that operator’s 4D surveillance strategy has been changed from streamer seismic to ocean-bottom seismic (OBS), which includes OBNs and ocean-bottom cables. In addition to source and receiver positions, other conditions in data acquisition can play important roles in 4D repeatability—for example, the variation of water properties during a seismic survey and between different surveys including water depth (tides) and seismic velocity in water. Such variations have been a well-known issue that limits the repeatability of 4D seismic, in particular for OBS data where multiple migration is often used. Seismic travel-time analysis is traditionally used to estimate errors and make corrections, but the process is often complicated by coupled factors including tides, water velocities, positioning errors, and the clock drift of OBNs, which collectively make it difficult to achieve precise water static corrections. To address this challenge, a seafloor device called the Pressure Inverted Echo Sounder (PIES) was developed for direct measurement and continuous monitoring of water- column properties during marine seismic surveys. A PIES is deployed at the seafloor and monitors two-way water time by transmitting an acoustic signal and measuring the time it takes to be reflected back from the surface. Such information has been used to make travel-time corrections of seismic data for enhanced data repeatability in the operator’s 4D processing projects.