Recent studies have revealed a strong correlation between the directionality of reservoir flow in waterfloods and the local orientation of horizontal earth stresses. Field applications are described of a novel technique of determining interaction between wells as an indicator of flow directionality. This technique calculates the Spearman rank correlation coefficient between flow rates at pairs of wells. These applications have demonstrated that the reservoir dynamics associated with correlated rate fluctuations have at least some component coupled to geomechanics. Coupled geomechanical-fluid flow numerical modelling is able to explain some of the observations and so offers an improved predictive tool for planning and managing waterfloods and determining optimal locations for infill wells. Introduction One of the most important parameters in designing the pattern of wells for waterflooding a reservoir is the directionality in horizontal flow; i.e. any preferred lateral direction for fluid flow across the reservoir. Recent studies have revealed a strong correlation between the directionality of reservoir fluid flow and the local orientation of modern-day maximum horizontal principal earth stress (Shmax). Even more surprisingly, this correlation holds equally well for the set of reservoirs which would not normally be described as "naturally fractured" as it does for those that obviously do contain open, conductive natural fractures. It has been conjectured that this correlation is explained by coupled processes in which the conductivities of natural fractures and faults (of which, generally speaking, there is a large population even in "unfractured" reservoirs) are altered by the geomechanical changes induced by the flooding process. According to the concepts of the metastability and self-organized criticality of the lithosphere, perturbation by even minor stress changes is likely. This conjecture was given credence by coupled numerical modelling of the geomechanical, fluid flow and heat flow processes involved in waterflooding; generic modelling of the progress of a flood front around a single injector well gave rise to similar patterns of directionality as observed in the aggregated field data. Coupled modelling therefore provides a potential new tool for improved design of waterfloods and infill drilling projects this will be further demonstrated later in this paper. Directionality The field data which allowed identification of directionality in the correlations with stress derived mainly from tracers, interference or pulse testing, or oil production response to start of nearby water injection. Communication through the reservoir has long been assessed by the strength of the response of oil production to start-up of water injection. However, in a complex schedule of well start-ups, it is often difficult to make unambiguous association between producers and injectors by this means. The technique to be described provides a more rigorous extension of this concept. Rank correlation of rates The basic technique of seeking correlation between well rates to indicate communication in the reservoir has been applied successfully in oilfields of the Former Soviet Union countries for some time.

Length frequency distributions of fractures are shown to have a power-law trend across many orders of magnitude. This allows an estimate to be made of the full number of fractures and faults existing in a reservoir over that which can be measured from core and seismic data. This relationship has been applied to reservoir studies involving the continuity of juxtaposed sands, permeability impairment, and estimating the number of fractures that a horizontal well may intersect. Many more fractures and faults exist in reservoirs than those that can he observed from seismic and core data, which have significant implications for reservoir characterisation, geomechanical properties and waterflood behaviour. Introduction There is a tendency for natural fractures in reservoirs to gain the attention of development teams only when the fractures provide permeability significantly greater than that of the matrix: usually in reservoirs which would not be commercially productive without the presence of those fractures. These reservoirs are deemed as 'naturally fractured' and are, of course, an important resource in the hydrocarbon industry. However, the somewhat loose application of this term tends to lead to the neglect of natural fractures as a component in the description of a much larger set of reservoirs. By the term 'fractures' we encompass both joints (extension fractures) and faults (all fractures whose walls have undergone shear displacements). The wider importance of natural fractures includes the following:Juxtaposition of different lithologies across faults can cause either an interruption in reservoir continuity, or enhancement due to the juxtaposition (across non-sealing faults) of depositionally isolated units.Fractures provide surfaces with an intrinsic conductivity that is almost certainly different from that of the surrounding matrix, occurring either through genesis (e.g. the cataclastic grain comminution in a granulation seam), diagenesis (i.e. mineralisation) or aperture governed by present-day effective stress. These altered surfaces can form a first-order perturbation to the permeability variations arising from depositional processes. In addition, the intrinsic conductivity of a fracture can change as stresses change in the reservoir rock during the development life of a field. There is growing evidence of the dominant influence of fractures in the lateral anisotropy of waterfloods, a process in which the reduction in stresses due to injection probably plays a significant role. The geomechanical behaviour of natural fractures during depletion may manifest itself as an influence upon compaction drive, subsidence and even casing failures. The presence of natural fractures carries particular implications when development of a reservoir with horizontal or high-angle wells is considered. The most obvious change is that high-angle wells have a greater probability than conventional wells of intersecting sub-vertical or vertical fractures. This fact has led to spectacular successes in the exploitation of reservoirs containing conductive natural fractures, for example in the Austin chalk of Texas. Additionally, the influence of natural fractures on the viability of horizontal or high-angle wells may be effective in at least two other ways:The productivity benefit of a high-angle well is very dependent upon the effective ratio of vertical to horizontal permeability. Fractures can increase or decrease this ratio.Conductive natural fractures and faults may provide pathways forextraneous water production from an underlying aquifer or gas production from a gas cap. In all these applications a knowledge of the true frequency distribution of fractures is a necessary component in a complete synthesis of the process. However, direct measurement of the distribution is hindered because reservoir fractures are observable only in the relatively limited ranges in scale of seismic and core data. We examine in this paper the concept that scaling relationships in fracture distributions enable the quantitative interpolation between the ranges of observable fracture sizes. We are concerned here primarily with spatial frequency distributions of fractures. A knowledge of the true form of these is a first step towards being able to predict the effect of fractures on fluid flow, but the subsequent steps of connectivity and conductivity are beyond the scope of this paper. REVIEW OF FRACTURE DISTRIBUTIONS The literature dealing with fractures in rock covers many disciplines and industries. Many assessments of frequency distributions of fracture lengths and spacings have been reported. The distributions have usually been given as one of log normal, negative exponential, gamma, Weibull, or occasionally power law or Pareto. P. 367

Résumé -Présentation générale des effets géomécaniques induits par l'injection d'eau dans les réservoirs -Au cours de l'injection d'eau, l'augmentation des pressions interstitielles et la diminution de température entraînent une diminution des contraintes effectives horizontales, notamment à proximité des puits d'injection. Ce déchargement mécanique peut activer des failles et des fractures naturelles au sein du réservoir, du recouvrement, des couches inférieures ou des fractures hydrauliques à proximité des puits d'injection. Ces effets géomécaniques induisent des modifications anisotropes de la perméabilité, généralement en augmentation, qui devraient être intégrées dans la simulation du réservoir, particulièrement du point de vue de la configuration des puits, afin d'en obtenir les meilleurs avantages commerciaux. , over-or under-burden, or Abstract -Geomechanical Influences in Water Injection Projects: An Overview -During water injection, increasing pore pressures and decreasing temperatures imply reductions in effective horizontal stresses, particularly in proximity to injection wells. This unloading can lead to activation of the natural faults and fractures by brittle failure in the body of the reservoir

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An areal model of a fractured/faulted reservoir with 49 wells is developed that incorporates fully-coupled geo-mechanics and fluid flow. It is a generic example of a pattern waterflood although it is inspired by a parallel study of the Gullfaks reservoir in the North Sea, in which stress-related, fault-related and long-range correlations in rate fluctuations are observed. Based on this model, three scenarios are examined in terms of different initial stress states prior to production, each of which involves 36 months of production and injection in the presence of fracture sets and faults. The results support the concept that the long-range, stress-related and fault-related characteristics of correlations in rate fluctuations, observed not only in the Gullfaks data, but also in several other fields worldwide, are symptomatic of a system near a geomechanical critical point. These characteristics are not observed in models that are sub-critical. Short-range rate correlations are likely to exist where there are highly permeable zones between producers and injectors. Long-range rate correlations occur only within critically-stressed regions where there is active shearing or fault reactivation. The modelling results are consistent with field evidence suggesting that incipient shearing is an important mechanism coupled with reservoir flow behaviour.

[1] Earthquake triggering data exhibit long-range spatiotemporal correlations of the power-law form C(l ) $ l À and anomalously-slow temporal diffusion of the mean triggering distance of the form: hli $ t H , with H < 0.5. We examine spatio-temporal correlations in subsurface effective stress state caused by fluid injection and extraction at well sites in a hydrocarbon reservoir using a multivariate statistical regression model, and observe long-range correlations in flow rate that cannot be caused by Darcy flow alone. Significantly-correlated well pairs align with the directions of incipient horizontal-displacement tensile and shear failure in the present-day stress field, while the contours of the first principal component of the regression matrix closely follow the macroscopic fault pattern in the main producing horizon. The correlation function for well pairs has a power-law form with % 0.5, and the mean correlation distance increases with H % 0.33, implying a similar critical-point response to perturbations in effective stress as the earthquake data.

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