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Abstract Drilling in deepwater presents risks associated to fault reactivation, what includes severe drilling fluid losses, connection of different reservoirs and, in the worst case, connection of the reservoir to the sea floor, causing oil and gas seeps. Recent accidents along the Brazilian coast showed that such phenomena are real and associated risks should be evaluated before any well planning. This work presents a method to identify and quantify risks of fault reactivation while drilling. An upper limit on the safe drilling window is proposed to account for these risks. The method also includes tridimensional stress analysis, where maps, slices and cubes with reactivations pressures over the faults are determined, what can be used to reduce risks by changing well positioning, casing points and mud weight plan. The proposed method is based on two main pillars. The full 3D geomechanical characterization of the area, which is done through the 3D-Mechanical Earth Model, a consistent way to determine the stresses around faults; and a proper criterion to determine the potential risk of fault reactivation during drilling. For this, a criterion of slip tendency analysis based on frictional constrains is adopted. The method can be applied under two methodologies. The first one represents the general case, where enhanced fault and in situ stresses characterizations are required. Through this approach, each fault plane is analyzed and more precise values of fault reactivation pressure are obtained. The second one is a simplification of the first one, and considers that all media is potentially faulted, where each potential fault is critically orientated with respect to the far field stress. This approach is recommended for exploratory or semi-exploratory scenarios, where faults are not very well mapped and main stresses directions are unknown. A case study for a planning phase of a drilling campaign on a deepwater field located in Santos Basin, Brazil is presented. Fault reactivation risks were identified and quantified through the proposed method. Safe mud weight windows were determined accounting for fault reactivation pressures, which was valuable during well planning. Recommendations for casing points and mud weight plans could be done including fault reactivation risks. Examples of risk mitigation by avoiding problematic zones and by re-positioning planned wells were presented. Results showed that the proposed method is an important tool to identify, quantify and reduce fault reactivation risks while planning any drilling campaign.
- Geology > Structural Geology (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- South America > Colombia > Casanare Department > Llanos Basin > Cusiana Field > Mirador Formation (0.99)
- South America > Brazil > Brazil > South Atlantic Ocean > Santos Basin (0.99)
ABSTRACT: This work illustrates how the 3D Finite Element (FE) method can be used to gain information on the reactivation potential and recent behaviour of faults. The Upper Rhine Graben (URG), Germany is used as a case study to demonstrate the proceedure. The URG is an intra-plate graben that developed during the Tertiary in an area with a complex pre-Tertiary (Variscan) crustal architecture. Several fault trends characteristic of Variscan and Tertiary tectonic phases can be distinguished within the highly faulted graben. The URG is presently characterized by relative low deformation rates and seismicity. Since the area is highly populated and industrialized, knowledge on the reactivation potential of the numerous faults is of high societal importance. The northern part of the URG is represented as a series of fault blocks bounded by frictional contact surfaces. Using the commercial FE solver Abaqus, the 3D state of stress for this model is obtained. The modelled stress state serves as a basis for the calculation of slip tendency and dilation tendency parameters, which describe the likelihood of the faults to slip or dilate respectively. The slip tendency results enable distinction between fault segments that are relatively prone to shear reactivation in the present-day stress field. The dilation tendency provides information on tensile failure of these segments. Additionally, the relative displacements of the fault blocks within the model show fault segments with large displacements as well as locked segments. 1.INTRODUCTION In this paper, the potential for reactivation of upper crustal faults in an abandoned continental rift within the present-day stress field is investigated. The contemporary state of stress is approximated using the method of 3D finite element modelling. The geometrical complexity and rheological heterogeneities modelled induce a highly variable (i.e. non-Andersonian; Figure 1) stress field within the model domain as a function of the boundary conditions applied. Figure 1. The 3 Andersonian stress regimes and the associated faulting types. Close to the surface, which can carry no shear stresses, the principal stress vectors are parallel to the vertical and horizontal stress components. Figure modified after [1].(available in full paper) The state of stress obtained from the modelling is then used to calculate the Slip Tendency [2] and Dilation Tendency [3] for the upper crustal fault surfaces included in the models. These static risking parameters are then compared to predicted slip magnitudes along the faults and geological reference data to evaluate their reactivation potential. 2.MODELLING APPROACH The in-situ state of stress affecting the URG area was approximated using a series of simplified mechanical earth models (MEMs). The upper crust is defined as two rheological domains describing the graben interior (i.e. Caenozoic sediments) and it's shoulder regions (i.e. Palaeozoic and Mesozoic rocks). Within the models, individual fault-blocks are separated by frictional contact surfaces which enable slip to occur during the analysis [4]. A uniform coefficient of friction of µ= 0.6 was assigned to all fault surfaces considered [5]. Linear elastic properties (i.e. density, Young's modulus and Poisson's ratio) for each element in the two rheological domains are defined as a function of lateral density variations of the crustal domains and their depth.
- Europe > Germany (0.86)
- North America > United States > California (0.29)
- Geology > Structural Geology > Tectonics > Extensional Tectonics (1.00)
- Geology > Structural Geology > Fault > Dip-Slip Fault > Normal Fault (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
ABSTRACT: The reactivation of geological faults in oil and gas reservoirs during the production process may lead to severe consequences. During fluid injection, the stress state of the surrounding formation is modified, which may induce slippage, seismicity and breach of the reservoir seal. The numerical modeling of a reservoir crossed by faults requires a special treatment to model the existing discontinuities. Often the geometric description of these features is complex and the explicit modeling in a finite element mesh is very cumbersome. This work presents a 3D equivalent continuum formulation for the description of discontinuities such as faults in an anisotropic continuum finite element approach. This formulation is based on the principle of equivalent deformation and failure is governed by the Mohr-Coulomb criterion for both rock and fault. In this approach, the intact rock deformation and the relative deformation of the fault are calculated according to Hooke's law and a traction-separation law, respectively. A synthetic model of a reservoir crossed by a fault was analyzed and its results are compared with models that explicitly consider faults via interface elements. The numerical results demonstrate that the proposed methodology can be an attractive alternative to model fault reactivation in geomechanical problems. 1 INTRODUCTION Geological discontinuities, such as faults, are preexisting in most oil and gas field. In active reservoirs the fluid pressure has been modified by injection/depletion and the stress states of the rock formation, including the inherent geological faults, are altered. If the stress components acting on a fault plane are sufficiently altered, the fault may slip and reactivate (Byerlee 1978, Sibson 1977). Indeed, the slippage of a geological fault may induce seismicity, land subsidence, and cause well collapse, as reported in many studies of oil field integrity. Key factors in triggering fault reactivation are shear stress accumulation and fluid pressures, which can generate rupture by reducing fault strength (Segall et al. 1994, Morton et al. 2006, Chan & Zoback 2007). Recently, the hydromechanical behavior of fault zones has been studied in research works related to the assessment of the geological risk induced by injection of CO2 (Rutqvist & Tsang 2005, Rutqvist et al. 2007, Cappa & Rutqvist 2011, Pereira et aí. 2014).
- North America > United States (0.29)
- South America > Colombia > Risaralda Department > Pereira (0.25)
- Europe > Norway > Norwegian Sea (0.25)
- South America > Brazil > Rio de Janeiro (0.16)
- Research Report > New Finding (0.49)
- Research Report > Experimental Study (0.35)
- Geology > Structural Geology > Fault (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Geological Subdiscipline > Economic Geology > Petroleum Geology (0.40)
- Europe > France > Nouvelle-Aquitaine > Lacq Basin > Lacq Field (0.98)
- Europe > United Kingdom > England > London Basin (0.91)
Coupled deformation and fluid flow modelling of simple fault systems and implications on hydrocarbon preservation potential
Zhang, Y. (CSIRO Exploration and Mining ) | Gartrell, A. (CSIRO Petroleum Resources ) | Underschultz, J.R. (CSIRO Petroleum Resources ) | Dewhurst, D.N. (CSIRO Petroleum Resources )
ABSTRACT: The relationship between fault geometries and fault deformation behaviors, as well as its controls on fluid flow in reservoirs during extensional reactivation, is explored using a series of coupled deformation and fluid flow numerical models. The results show that the initial length of faults in a fault population is the primary control in determining strain distribution between faults and in transporting fluids through the top seal from the reservoir horizon. During extensional reactivation, longer faults tend to accommodate greater shear strain and accumulate greater throws than smaller faults. Longer faults are also more important fluid upflow conduits, because their greater shear strain results in greater dilation and better connectivity. 1. INTRODUCTION Fault reactivation and associated rock deformation represent a major risk to trap integrity and hydrocarbon preservation. Data from the Timor Sea indicate that most of the faults in the region have been reactivated during the Late Tertiary extensional event [1, 2]. The distribution of fault movement and rock deformation on the fault populations of the region are inhomogeneous, and this critically controls the breach or preservation of oil traps. To advance our understanding of the relationship between fault attributes (length and orientation) and fault deformation and their impact on fluid flow in reservoirs and faults, we have constructed a group of coupled mechanical deformation and fluid flow numerical models containing simple fault geometries. This paper presents the results of these models. Extensive previous studies have been conducted on rock deformation and fluid flow associated with faults in the fields of petroleum geology and economic geology. For example, Connolly and Cosgrove [3, 4] investigated stress patterns around dilatant fault jogs under strike-slip conditions and inferred fluid flow patterns based on mean stress patterns from their photoelastic analogue modelling experiments. Using a numerical modeling approach, Zhang et al. [5] simulated rock dilation and fluid flow localization around fault dilation jogs under compressional and strike slip conditions. McLellan et al. [6] numerically simulated deformation and fluid flow in an extensional basin model containing one shallow dipping fault, a structural scenario for the Hamersley Basin. Gartrell et al. [7] numerically modeled extensional reactivation of a triple fault intersection, one of structure types in the Timor Sea, and further explored fluid flow patterns for such fault architecture. Walsh et al. [8] showed that fault displacement rates correlate with fault size, based on the results of their deformationonly, particle-material models on normal faults. All these efforts illustrated strain localization or fluid flow focusing into fault zones (in particular, fault intersections, jog structures or longer faults), under contractional, strike slip or extensional conditions. However, the mechanism by which the fault length factor can affect strain partitioning and fault movement (e.g. down throw under extensional settings) in a fault population still needs further work. This is particularly important for the evaluation of integrity and hydrocarbon preservation, and is thus the focus of the present study. It also needs to be mentioned that this work is only concerned with the extensional reactivation of pre-existing faults, rather than fault initiation and growth [9].
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.34)
- Geology > Geological Subdiscipline (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.51)
- Geology > Structural Geology > Fault > Dip-Slip Fault > Normal Fault (0.35)
Evaluation of Fault Sealing Ability and Prediction of Fault Critical Reactivation Pressure in Water Flooding Reservoir
Hou, YaWei (Tianjin Branch of CNOOC Ltd.) | Zhao, ChunMing (Tianjin Branch of CNOOC Ltd.) | Su, YanChun (Tianjin Branch of CNOOC Ltd.) | Wu, TingTing (Tianjin Branch of CNOOC Ltd.) | Luo, XianBo (Tianjin Branch of CNOOC Ltd.) | Yang, QingHong (Tianjin Branch of CNOOC Ltd.) | Huang, Kai (Tianjin Branch of CNOOC Ltd.)
Abstract Fault reactivation induced by excessive reservoir water pressure in oil fields is suspected as one of the possible perpetrators that caused crude oil eruption to the surface. This can lead to significant financial losses related to environment cleanup and curtailed oil production. During fluid production and injection, changes in stress have a significant influence on reservoir behavior. Fault sealing ability is one of the key factors that control hydrocarbon preservation. If these faults are reactivated, their permeabilities will likely increase, facilitating fluid migration and potentially compromising the hydraulic integrity of the caprocks that seal the reservoir. Therefore, during a waterflooding process, it is necessary to limit the injection pressure in order to avoid fault reactivation. A traditional approach to fault reactivation prediction provides a deterministic critical reservoir pressure without proper regard to the uncertainties in the model input parameters and the predicted results. A probability distribution of the fault reactivation critical reservoir pressure provides a better means to calculate a risk weighted Expected Net Present Value for management to make better decisions on water flooding and to anticipate potential consequences. In this study, a geomechanical model was established for the BZ Field, Bohai Bay, China. Using the geomechanical model, first, a fault seal analysis was performed and indicated that all faults were sealed in sands under initial stress and pore pressure conditions. Second, fault reactivation reservoir pressures for major faults in the field were predicted assuming a frictional sliding frictional coefficient of 0.5, which was derived using frictional faulting theory by the geological events provider for the nearby PL Field. Monte Carlo simulations were run to consider the uncertainties of frictional sliding coefficient, and especially the the minimum horizontal stress among the input parameters.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.30)