4.1 An Introduction to 4D Seismic


This introduction is partially based on Chapter 19 in Petroleum Geoscience (edited by K. Bjørlykke) and on unpublished material we have gathered from work at Statoil and at NTNU over two decades.


4D, or time-lapse, seismic is a phrase used to describe the process of using two or more seismic surveys acquired over the same area or field to find changes that have occurred over calendar time. For hydrocarbon reservoirs, these changes might represent production related changes such as pore pressure, temperature or fluid saturation. 4D seismic might also be used to monitor seasonal changes (near surface effects), earthquakes (before and after) or, for instance, underground storage of CO2 As a common reservoir monitoring tool today, 4D seismic


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has gone through a tremendous development over the past three decades. In the beginning monitoring was done by repeating 2D seismic lines, and then by repeating large 3D surveys. Today, several fields can be monitored by trenched receiver cables at the seafloor, enabling frequent and close to continuous reservoir monitoring. Te benefit of this development is two-fold: increased hydrocarbon production by infill drilling, and early detection of unwanted and unforeseen reservoir developments, such as gas breakthrough and sudden pressure increases. Te first 4D commercial seismic surveys were acquired in


America in the early 1980s for heavy oil fields. Heavy oil is viscous, and steam injection was used as a way to heat the oil and thereby reduce the viscosity. In this way the oil migration towards the producing wells was improved, and oil production increased. During the heating process, the P-wave velocity of the oil decreased, and this change was clearly observed on repeated seismic data acquired before and after the heating process. Te major breakthrough for commercial 4D seismic acquisition in the North Sea was the Gullfaks 4D study launched by Statoil in 1995. By comparing seismic data before and after the start of production, Statoil identified several (about 20) targets that were undrained. Statoil estimated the added value of this study to be of the order of US$ 1 billion.


4.1.1 An Early North Sea 4D – Still Alive!


One of the first 4D studies performed in the North Sea, however, had a more dramatic background: an underground blowout that Saga Petroleum encountered when drilling exploration well 2/4-14, which occurred in January 1989, and lasted for almost a year. During this period, Saga Petroleum acquired several 2D seismic lines close to the well. Figure 4.2 shows a vertical cross-section from the 3D seismic data that was acquired in 1991, two years after the blowout. We can clearly see vertical chimneys along the two vertical well bores. Te chimney in well 14 is more pronounced and stretches further into the overburden, compared to well 15. When we use time-lapse seismic data as shown in


Figure 4.3, we clearly see huge and dramatic changes in the overburden. At 520 ms (two-way traveltime) we observe a new event that is hardly visible on the 1988 data. Tis is interpreted as gas leaking vertically


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Figure 4.1: Artistic view of the rotated Jurassic fault blocks constituting the Gullfaks reservoir (left). Solid black lines represent wells; light brown-yellow layers represent sandstones, and blue-grey layers represent shales. The right- hand figure illustrates the connection between the top reservoir sandstone layer (dashed red line) and the corresponding seismic response before and after ten years of production. The original oil-water contact is shown as a dashed green line on the time-lapse seismic data, clearly demonstrating the production effect on the data. Also notice the decreased traveltime for the seismic event below the oil-water contact. This reduction in velocity is caused by water replacing oil in the lower part of the reservoir rock.


outside well 14 from a leakage point through the casing at approximately 900m depth, and into a thin and almost horizontal sand layer at approximately 490m depth. Figure 4.3 shows a clear example of the most common interpretation


Figure 4.2: Vertical cross-section of a seismic line acquired in 1991 (after the blowout) intersecting the blowout well (2/4-14) and the relief well (2/4-15). Notice the gas accumulation along the vertical well paths of both wells, and the horizontal accumulation of gas (leaking from the well and marked by the red arrow).


Statoil


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