Observing reservoir dynamics
1. When a reservoir, any reservoir, is put under production, fluid compositions change, for example oil may be partially replaced by water or gas, gas may be expelled from oil, and so on. However, what is also well-documented and of significance is the fact that fluid pressures drop, changing the ‘internal stresses’ on the reservoir rock. Tempting as it may be, however, it is not possible to understand the (time-varying) geophysics of reservoir rocks by theorising an isotropic pore space responding to fluid changes and changing ‘internal stress’…………
2. Several studies have shown that the hydraulic conductivities of faults and fractures in reservoirs can be influenced by geomechanical perturbations due to production operations and it is reasonable to anticipate that such dynamic permeabilities will be manifest as changes in flow-rates at production and injection wells. Heffer & co-workers (Edinburgh University) have shown that statistical correlations in flow-rate fluctuations between wells from fields in the North Sea appear to bear out this expectation; they are characterised by high correlations over very large separation distances between wells, and appear to be stress-related and fault related. Heffer has proposed that the most likely geomechanical mechanism to explain such orientational characteristics of correlations relative to stress state is dilatation or compaction of aligned compliant fractures in en echelon patterns and at critical densities, also previously proposed by others as active in the nucleation of shear failure. This mechanism is also consistent with an independent empirical feature of production data: the observed frequencies of directionalities in flooding schemes.
3. These reservoir engineering observations lead to the conclusion that time-lapse geophysics - any observations of any reservoirs over time - must be based on the understanding of the physics of fluid-filled, parallel, compliant, fractures/micro-cracks – dilating or compacting as the reservoir is produced. This physics, this New Geophysics, has been documented over many years by Crampin, based on understanding and observing the effects of closely-spaced stress-aligned fluid-saturated microcracks on seismic shear-wave splitting (SWS) in the crust and upper mantle. Critically, seismic observations of P-wave propagation and P-waves are relatively insensitive to fluid-saturated microcracks, whereas SWS is wholly determined by parallel microcracks and can be measured with first-order accuracy. Thus SWS is a second-order quantity (small changes in shear-wave velocities) that can be read with first-order accuracy- thus there is tremendous resolution.
4. Consequently, there are significant implications for geophysical, especially seismic, monitoring of reservoir dynamics:
• First of all, we can say that conventional 4D seismics – towed streamer surveys for example – only discern changes in P-wave reflectivity and thus offer at best an incomplete view of reservoir dynamics, one that is unquantifiable, allowing only empirical comparisons,
• Secondly, a complete, quantifiable, view of reservoir dynamics requires 3C seismic acquisition (and strengthens the case for permanent installations).
• Thirdly, changes in stress can be monitored by changes in SWS so that stress-accumulation before fractures in reservoirs (and earthquakes and volcanic eruptions) can stress-forecast the time, magnitude, and estimate location of impending fractures (and earthquakes and eruptions).