3-D Seismic Interpretation: Time-lapse seismic
We saw in chapter 5 how seismic data can be used in favourable cases to infer the nature of fluid fill (gas, oil or brine) in a reservoir. An application of this is to follow the way that fluids move through a reservoir during production, by carrying out a baseline seismic survey before production begins and then repeat surveys over the production lifetime. Where 3-D surveys are repeated in this way, they are often referred to as 4-D seismic , with the idea that time is the extra dimension over standard 3-D. Differences in seismic amplitudes or travel-times between the surveys can reveal the movement of fluid contacts (e.g. where produced oil has been replaced by brine) or the extent of pressure changes that affect reservoir properties. As sketched in figs. 8.1 and 8.2, it is not always straightforward to separate out the causes of the changes. At a producing well, the pressure drops, and if it drops far enough then gas will come out of solution. This will result in a decrease in both P velocity and in density, resulting in a drop in acoustic impedance. On the other hand, the drop in pore pressure causes an increase in P velocity and density. At a water injector (fig. 8.2), we replace oil by water, which in itself would increase P velocity and density; however, the injection also increases the pore pressure, which causes the reverse effect on P velocity and density.
PRM system monitoring of injection and production processes for safe operation
The challenges of safe oil recovery and injection projects in areas with a challenged overburden, requires increased fieldand well surveillance in order to avoid leak to surface events, as previously observed at e.g. the Tordis and the Frade field. Monitoring with Permanent Reservoir Monitoring (PRM) systems through installation of seismic ocean bottom nodes offers a strong monitoring solution, which can contribute with useful information contributing to the understanding of the structural integrity of the caprock; injection well integrity; drilling hazards; as well as fluid movement and stress changes build ups, all needed in order to avoid unwanted events as surface leaks. A PRM system further offers solutions for the collection of data from active seismic surveys and the collection of ambient seismic noise data, which can be used obtaining a better understanding of the shallow structures of the overburden. For surveillance of microseismic events, induced through either production or injection processes, a real-time alarm system is required, in order for the appropriate actions to be made when the induced microseismic events propagates towards the surface before reaching the seabed. Such a system is foreseen to utilize down well pressure data, microseismic data and possible other data available.
Aristocratic Century: The sinews of power: political
In his excellent book on the Secretaries of State, Mark Thomson offered ‘the paradox of continuing domination of the aristocracy and the simultaneous insignificance of the House of Lords, per se ’. It is true that, by the beginning of the eighteenth century, the supremacy of the House of Commons in financial matters was a wellestablished convention. The claims of the Commons to an undisputed control over money matters, which had been urged before the Civil War, were resumed immediately after the Restoration and a protracted dispute with the House of Lords began. In 1661 the Lords resolved that there were no precedents in history for this new claim by the House of Commons. In 1671, the Commons retorted that taxation ought not to be questioned by the Lords and in 1678 they reiterated that supply was ‘the undoubted and sole right of the Commons’ and ‘ought not to be changed or altered by the House of Lords’. The Lords' attempt in 1692 to assess their own taxes was also abandoned, though they retired under cover of a rather insubstantial smoke-screen that they maintained their power of amendment to be a ‘fundamental, inherent and undoubted right from which their lordships can never depart’. That established, they departed. Geoffrey Holmes quotes a nice example from 1702 when the bishop of Carlisle found himself the only peer answering to a money bill, ‘no other lord in the House regarded what was doing, this being only (pro forma) to preserve a seeming right to dissent from, or amend, any part of a money bill’.