a
b
c
Figure 4.59: Seismic 4D maps from Snorre. (a): Water injection in a well between 1997 and 2001 is associated with amplitude dimming (blue colours). (b): Example of complex 4D amplitude response from mixed gas and water injection during 2001–2006. In 2005, significant amounts of gas were injected. Increased gas saturation leads to amplitude brightening (red colours). (c): The WAG cycle during 2006–2009 has largely cancelled the seismic 4D effect. The lesson is that with a large time span between seismic surveys, it becomes difficult to interpret the 4D effect related to the complex injection and production history.
the surveys. Due to the fast-changing production and reservoir conditions, it turned out to be difficult to interpret the 4D data and relate the 4D signal to production effects. Te reservoir changes in some cases could add up to a strong 4D signal, and in other cases the reservoir changes could more or less cancel the seismic 4D effect (Figure 4.59). In addition, pressure variations and related seismic time-shift effects added to the complexity. To be able to understand the total 4D response and use the result both qualitatively and quantitatively, the mixed 4D effects needed to be decomposed into individual components. To this end, it was recognised that improved 4D data quality and more frequent data acquisition would simplify Snorre’s complex 4D story.
4.7.2 Monitoring a Complex WAG Cycle
The ability to monitor a complex WAG cycle (Figure 4.60) was demonstrated by the observations of injector well A. The first survey was acquired in the spring of 2014, the second survey in the autumn of 2014, and the third survey in the spring of 2015. Well A injected water during the period from spring to
autumn in 2014, encompassing the first two seismic PRM surveys. During this period the water injection is clearly mapped seismically as an amplitude increase between the two surveys, as displayed in Figure 4.61. Later, prior to the third PRM survey, the well converted to gas injection, which is clearly indicated seismically by an amplitude decrease between the second and third surveys. When comparing the seismic difference between the first and third surveys, the amplitude increase due to water injection interacts destructively with the amplitude decrease due to gas injection during this 12-month period of monitoring. Tis leads to no observable amplitude
a b c
Figure 4.60: Injection profile for well A, alternating between water and gas injection, with the corresponding PRM surveys during spring 2014, autumn 2014, and spring 2015. The different colours correlate to different sleeves which regulate flow into the different reservoir units.
changes seismically. Te increased amplitude between spring 2014 and autumn
2014 was interpreted to be the result of the increased pressure in the area due to the water injection of an existing well. Te 4D signal progresses southwards through a fault, which had previously been considered restrictive to reservoir communication, with the 4D signal now terminating against another fault further south. With the new frequent 4D data available, the fault, further south, is now interpreted to be more sealing than previously assumed. Between autumn 2014 and spring 2015 the decrease in amplitude observed on the seismic, as well as its areal extent, was interpreted as a decrease in injection pressure, which also confirmed southward progression, and the southerly sealing fault. Tese observations were used as input into the geomodel and to validate reserve calculations for future wells to be drilled in the area.
Figure 4.61: Amplitude increase (red) between spring 2014 and autumn 2014 (a), and amplitude decrease (blue) between autumn 2014 and spring 2015 (b). The amplitude effect is largely cancelled between spring 2014 and spring 2015 (c).
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