Principle of CO₂ sequestration

Block diagram illustrating the principle of CO₂ sequestration. Unwanted CO₂ produced with the gas from the Sleipner field Ty formation is injected into the Utsira formation repository. The 1999 and 2001 time-lapse seismic sections (lower right) show that the injected CO₂ is in place and that the volume has increased substantially. This is further corroborated by the thickness maps of the most extensive layer (upper right). Note that the 1996 survey was carried out prior to the CO₂ injection; the 1999 and 2001 surveys were carried during the CO₂ injection (courtesy Statoil).

The repository is the Utsira formation---a thick water-bearing sandstone some 1,000 meters below the seabed---into which about 1 million tons of CO₂ has been reinjected annually since 1996. Time-lapse seismic has been used to monitor the formations' behavior. The method is particularly suitable because the velocity of sound waves can easily be used to differentiate between water-bearing (higher-velocity) and gas-bearing (lower-velocity) sandstones.

So far, the results show that the CO₂ indeed resides in the formation and that no leakage can be detected---a highly encouraging situation with respect to the trap's behavior in years to come. These conclusions are based on the geological interpretation, simulation, and seismic modeling of results obtained from three surveys: a baseline survey carried out in 1994, prior to CO₂ injection; and two monitoring surveys carried out in 1999 and 2001, during CO₂ injection.

The long-term effect of subsurface CO₂ sequestration is being documented by the Saline Aquifer CO₂ Storage project (SACS).

Note that subsurface gas storage has previously been used in other ways. An early example is that of a French company, Gaz de France. Gas de France has used onshore reservoirs to store gas during off-peak seasons ready for winter demands. Stock levels are monitored by seismics.

Other Images in Chapter 7

An example of the sailing path of a marine vessel in a towed-streamer survey

Images - Chapter 8 An example of the sailing path of a marine vessel in a towed-streamer survey. Note that the time for turning from one sailing line to another is about nine hours for vessels carrying streamers that are 10 km long. The dotted line indicates the turning legs of the sailing path.

This figure illustrates a typical sailing path of a 3D survey; the vessel travels back and forth, shooting and collecting data along many parallel lines, resulting in seismic data generated along lines 25 to 50 meters apart. Note that it takes about nine hours to turn from one sailing line to another for a vessel carrying 10-km-long streamers. Today, data are recorded even when the vessel is turning.

A shot diagram

Images - Chapter 7 A display of source and receiver distribution of a 2D seismic line in the so-called shot diagram. The rows corresponding to common-shot gathers and columns to common-receiver gathers. The diagonal is the zero-offset section, and all the other lines parallel to the diagonal are common-offset gathers (also known as common-offset sections). The lines perpendicular to the diagonal are the CMP gathers.

Another illustration of towed streamer acquisition

Images - Chapter 7 Another illustration of towed streamer acquisition

Interference noise

Images - Chapter 7 An illustration of interference noise in seismic data before and after. This figure shows the stack of seismic interference noise contaminated shots from another line in the Gulf of Mexico. Interference noise is clearly visible. Attenuation of seismic interference noise can be achieved by the use f-x prediction filters. Courtesy of Western Geco.

Other examples of structural traps

Images - Chapter 1 Structural traps. (A) Tilted fault blocks in an extensional regime. The seals are overlying mudstones and cross-fault juxtaposition against mudstones. (B) Rollover anticline on thrust. Petroleum accumulations may occur on both the hanging wall and the footwall. The hanging wall accumulation is dependent on a subthrust fault seal, whereas at least part of the hanging wall trap is likely to be a simple, four-way, dip-closed structure. (C) Lateral seal of a trap against a salt diapir and compactional drape trap over the diapir crest. (D) Diapiric mudstone associated trap with lateral seal against mud wall. Diapiric mud associated traps share many common features with that of salt. In this diagram, the diapiric mud wall developed at the core of a compressional fold. (E) Compactional drape over a basement block commonly creates enormous low-relief traps. (F) Gravity-generated trapping commonly occurs in deltaic sequences. Sediment loading causes gravity-driven failure and produces convex-down (listric) faults. The hanging wall of the fault rotates, creating space for sediment accumulation adjacent to the fault planes. The marker beds (grey) illustrate the form of the structure that has many favourable sites for petroleum accumulation. Adapted from Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience. Oxford: Blackwell Science.