Kes Heffer

Hydromechanical modeling of critically stressed and faulted reservoirs

A critical stress state around a faulted reservoir prior to production and injection is an important factor in the hydromechanical responses during production. The purpose of this article is to show how the long-range correlations of production rates observed in several oil fields can be reproduced with hydromechanical modeling of a faulted reservoir subjected to a critical stress state prior to production operations. The modeling implies that the permeability distribution in a reservoir that is in a critical stress state is time dependent. A finite-element model with fully coupled geomechanics and flow was used. The modeling has been applied to an approximation of the complex structure of the Gullfaks reservoir in the North Sea, including the far-field stress regimes and fault systems, although the model is considerably simplified in the search for generic, instead of field-specific, principles. Under a critical stress state, a small change of the effective stress caused by fluid-pressure changes in the reservoir is likely to trigger reservoirwide hydromechanical reactions, irrespective of whether the change was at a local scale or a reservoir scale. Such responses include fault reactivations, volumetric and shear strain changes, induced deformation evolution, and permeability changes. With a permeability enhancement model, permeability increase is expected if fault reactivation and shear strain change occur. In contrast, if the in-situ stress is not at a critical state, the reservoir reacts locally. In this case, the deformation is mainly elastic, and no permeability enhancements occur. Therefore, the impact of inelastic geomechanical interactions (particularly shear deformation) at a critical point is likely to be very influential on reservoir fluid flow. This critical-point behavior gives explanation to the widespread field observations of long-range correlations in well rates, which are inferred to be manifestations of reservoir-scale mechanical responses involving faults, instead of the local hydraulic links that Darcy flow between wells implies. Permeability changes occur during inelastic deformation despite injection pressures being much lower than the confining stress (the minimum total principal stress). The increase in permeability in the reservoir rocks is caused by the dilation normal to the surface of the faults and/or fractures, which is caused by the shearing along the faults and/or fractures, instead of hydrofracturing. This confirms that dilational shearing can develop despite the effective stress regime being compressive. Dilational shearing has a major impact on the deformation of reservoir rock during production and is an important mechanism for generating conductivity on fractures under a fluid pressure that is lower than the confining stress, possibly even in reservoirs under depletion.

Coupled geomechanics-flow modelling at and below a critical stress state used to investigate common statistical properties of field production data

Abstract 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.

The Statistical Reservoir Model: calibrating faults and fractures, and predicting reservoir response to water flood

Abstract This paper describes the new concept of a ‘Statistical Reservoir Model’ to determine significant well-pair correlations. We solve this conceptual problem using a predictive error filter, combined with Bayesian methods that identify those well pairs that are related to each other with statistical significance, for the Gullfaks reservoir in the North Sea. Significant, long-range, correlations in the whole field are found at an optimal time lag of one month. The correlation function for significantly-correlated well pairs, after normalization for the distribution of available wells, shows a long-range power-law decay that is consistent with a critical-point response at the reservoir scale. A principal component analysis shows a strong correlation with the location and orientation of faults that intersect the main producing horizon. A predictive experiment shows that the model performs very well both in history matching and predictive mode on a time scale of about one month.

Identification Of Activated (Therefore Potentially Conductive) Faults And Fractures Through Statistical Correlations In Production And Injection Rates And Coupled Flow-Geomechanical Modelling.

Long-range, stress-related and fault-related characteristics of correlations in fluctuations in flow-rates are explained conceptually in the context of the lithosphere’s near-critical mechanical state and a strong feedback between deformation and local permeability. A more sophisticated statistical model, devised to extract a parsimonious set of flow-rate correlations, has shown similar characteristics. Coupled geomechanicalflow modeling was able to reproduce those characteristics for a generic pattern waterflood perturbed with random noise, but only when loaded to a near-critical state, hence providing strong support for the conceptual model. Coupled modeling of a cross-section representative of the Gullfaks field also demonstrated long-range influences. The matrix of empirical correlations between all well-pairs for a field can be decomposed in various ways. The principal components of the matrix, when interpolated with appropriate spatial correlation functions, have indicated the importance of particular faults in the rate fluctuation history; it is inferred that those faults are mechanically active during the development, and thus are potentially conductive features.