Changing fracture configurations in a field undergoing depletion inferred from flowrate fluctuation correlation analysis.

Transcription

1 P2-1-4 Changing fracture configurations in a field undergoing depletion inferred from flowrate fluctuation correlation analysis. Kes Heffer Reservoir Dynamics Ltd, Leatherhead, Surrey, UK Introduction Anisotropic heterogeneities in permeabilities are vital factors in determining reservoir recoveries. It has long been known that even when the anisotropy of horizontal permeabilities is homogeneous the recovery factor of a waterflood can change by 10s of percentage points depending upon how the well configuration is oriented relative to the axes of anisotropy. Towards delineating the communication paths in a reservoir between wells many authors have analysed correlations in the fluctuations that occur in production and injection flowrates at wells in oilfields. The flowrate fluctuations, which are often of high amplitude and occur on all timescales, can be caused, not only by reservoir communications, but also through interventions by operators in changing chokes or in well operations such as workovers or stimulation; or by in- or near-wellbore due to such effects as sanding, scaling or wax or asphaltene deposition. In analysing flowrate fluctuation correlations most authors have concentrated on interpreting solely hydraulic properties (timeindependent permeabilities and porosities) over short spatial ranges (Jansen & Kelkar, 1996, 1997a&b; Refunjol & Lake,1997; Soeriawinata & Kelkar, 1999; Albertoni & Lake, 2002; Lee et al., 2008, 2009). In contrast, Heffer et al. (1995) found that flowrate correlations between injectorproducer wellpairs are generally significantly higher when the wellpairs subsurface locations are separated in a direction sub-parallel to the direction of the maximum horizontal principal stress axis (Shmax). In a study of flowrate fluctuations in 6 North Sea fields Heffer (2012a & b) found that the diffusivity tensor of flowrate fluctuations, derived from the time behaviour of correlations over a more local region around both injector and producer wells, showed peaks in the shearing directions at ~30 degrees to Shmax. Additionally Heffer et al. (1995), Main et al. (2007) and Heffer (2012a,b) found long-range correlations between temporal fluctuations in flowrates at pairs of wells in oilfields, whether injectors or producers: the average correlations slowly fell with lag distance between the pair of wells, more slowly than might be reasonably explained by normal Darcy diffusion. These characteristics are consistent with the theory and evidence indicating that the earth s crust is in a near-critical state (e.g. Harper & Szymanski, 1991; Main, 1996; Bak, 1997; Crampin, 1999; Zoback et al., 2002; Al-Kindy &Main, 2003; Rundle et al., 2003). This implies that there are percolating paths of faults, joints, incipient fractures and other discontinuities that are near mechanical failure in the prevailing stress states or that provide connectivity for conductivity (Madden, 1983). The characteristics are also consistent with the common observations of shear-wave splitting (or birefringence) of seismic waves, where the polarisations of the faster waves are sub-parallel to the direction of Shmax (e.g. Crampin, 1987; Crampin, 1994). This has been taken to indicate that most in-situ rocks are pervaded by stress-aligned fluid-saturated microcracks. Shear-wave splitting has also been observed to be extremely sensitive to changes in stress conditions in the crust and to also show long-range influence (Crampin, 2003; Crampin et al., 2003; Wuestefeld et al., 2011). Heffer (2012a, 2015) indicated how the stress interactions between stress-aligned (micro-) cracks in rock, at or above a critical density (which is in the middle of the range commonly observed in shear-wave splitting), can explain a) the long-range sensitivities of geomechanical influences; b) the peak in sensitivity at about 30 degrees to the maximum horizontal principal stress axis (Shmax; also the crack strike); and c) the independent observations of preferential directionalities of secondary recovery flooding projects sub-parallel to Shmax.

2 Application to a fractured reservoir undergoing depletion Flowrate fluctuation correlation analysis can offer not just evidence that geomechanical changes are an intimate component of the physics of reservoir behaviour, but also a means of inferring orientations and locations of dynamic fractures in reservoirs, which are likely to be the most conductive features hydraulically. Following on from several cases of analyzing flowrate fluctuation correlations in waterfloods, this paper describes a case study of a fractured carbonate reservoir undergoing depletion (i.e. no secondary recovery scheme in place). The analysis was performed over two separate time windows. Conventionally one would expect the influence of fractures to diminish as fluid pressures decreased with depletion and effective stresses acting on fractures increased. This case study indicates that over both time windows the main correlation paths were associated with the main faulting trend in the field. However, whereas in the earlier reservoir history the correlation paths appeared to follow particular faults over extensive distances across the reservoir; in the latter history, the correlations were more localized towards the periphery of the field (figure 1). It is surmised that the poroelastic shrinkage of the rock with depletion in the centre of the field has progressively led to greater dilation and normal shears at the locations of higher gradients in fluid pressure (and therefore effective stress) at the periphery. Conversely, later correlation paths are less extensive across the interior of the field area, consistent with the compression of fractures under higher effective stresses. Peripheral localization of normal shears has been observed elsewhere in microseismicity projects (e.g. Segall, 1989) and satellite radar surveys (e,g, Tamburini, 2012, example of Tengiz field). These concepts would ideally be tested for this field with geomechanical modelling, for which the flowrate correlation analysis would provide useful calibration. If supported, the implied configuration of higher fracture-related permeabilities around the reservoir periphery surrounding a central zone of lowered permeabilities has strong implications for how best to continue development of the field, especially the pattern of potential secondary recovery schemes.

3 Fig 1. First principal component (fluctuations giving the greatest contribution to the overall variance) of the interwell flowrate correlations interpolated across a fractured carbonate reservoir undergoing depletion in (above) earlier years; (below) later years. These indicate a change from early localized trends coincident with extensive faults across the reservoir area to more concentrated trends near the periphery of the field in later years. References Albertoni, A. & Lake, L.W Inferring interwell connectivity from well-rate fluctuations in waterfloods. SPE 75225, presented at the 2002 SPE/DOE Thirteenth Symposium on Improved Oil Recovery, Tulsa, Oklahoma. Al-Kindy, F. H. & I. Main, 2003, Testing self-organized criticality in the crust using entropy: A regionalized study of the CMT global earthquake catalogue: Journal of Geophysical Research, v.108, no. B11, p Bak, P, 1997, How Nature Works the Science of Self-Organized Criticality: Oxford, Oxford University Press, 476 p. Crampin, S., 1999, Implications of rock criticality for reservoir characterization, Journal of Petroleum Science & Engineering, v. 24, p Crampin, S., 1987, Geological and industrial implications of extensive-dilatancy anisotropy, Nature, 328, Crampin, S., 1994, The fracture criticality of crustal rock, Geophys. J. Int., 107, Crampin, S., 2003, The new geophysics: Shear-wave splitting provides a window into the crackcritical rock mass, The Leading Edge, June 2003.

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