A study of the structural controls on oil recovery from shallow-marine reservoirs

Abstract

The differences in oil production are examined for a simulated waterflood of faulted and unfaulted versions of synthetic shallow-marine reservoir models with a range of structural and sedimentological characteristics. Fault juxtaposition can reduce the economic value of the reservoirs by up to 30%, with the greatest losses observed in models with lower sedimentological aggradation angles and faults striking parallel to waterflood direction. Fault rock has a greater effect than fault juxtaposition on lowering the economic value of the reservoir models in the compartmentalized cases only – and only when the fault rock permeability model is based on the least permeable published laboratory data. Moderately sealing faults can increase the economic value of reservoirs except when the main flow direction is parallel to the faults. These results arise from the dependence of economic value on both sweep efficiency and production rate. Simple predictors of fault juxtaposition and fault-rock heterogeneity have been established and combined with two-dimensional considerations from streamline theory in an attempt to capture quantitatively the change in economic reservoir value arising from faults. Despite limitations associated with the three-dimensional role of juxtaposition, the results are encouraging and represent a step towards establishing a rapid transportable predictor of the effects of faults on production.

Faults in conventional flow simulation models: a consideration of representational assumptions and geological uncertainties

“…9), this is broadly representative of laboratory measurements of phyllosilicate framework fault rocks from North Sea Brent Province reservoirs (Jolley et al 2007). The transition from juxtaposition-dominated production behaviour to fault rock-dominated behaviour in the suite of models from which this one derives occurs over a permeability range of approximately two orders of magnitude, and the datum permeability model used in this study is situated close to the centre of this range (Manzocchi et al 2008b). Hence, for more permissive permeability models, the behaviour of the two structures (A and B) is fairly similar.…”

“…Production is simulated for up to 30 years, subject to minimum well-specific and field-wide oil production rate cutoffs. Further details of sedimentological, structural and reservoir engineering aspects of the models are given by Howell et al (2008), Manzocchi et al (2008b), Matthews et al (2008) and Stephen et al (2008).…”

“…Hence, for more permissive permeability models, the behaviour of the two structures (A and B) is fairly similar. In less permissive ones the compartmentalization of Structure B is of paramount importance, and the performance of Structure B reservoirs is much worse than Structure A reservoirs, within which tortuous flow paths from injectors to producer exist around the faults (Manzocchi et al 2008b). Therefore, one can expect that oil production in the reservoirs will show more sensitivity to details of the fault systems given the datum fault permeability model used in this study than they would, for example, if a more permissive reference property model were used (the datum case used in this study was selected precisely for this reason).…”

“…Recovery factors for the Structure A versions of these reference models are 41.6% when the more permissive case is used, 41.9% using the datum case and 40.4% using the less permissive case (the higher recovery factor for a lower permeability case is because of increased sweep efficiency, and is discussed in detail by Manzocchi et al 2008b). The same three cases have recovery factors of 42%, 37.5% and 9.8% when the reservoir contains the Structure B fault system.…”

“…The second fault throw effect considered is the inclusion of sub-seismic faults. Although the models are of synthetic reservoirs, both structures derive from seismic interpretation of natural fault systems (see Manzocchi et al 2008b for details) and, like all models of natural reservoirs, they are therefore subject to resolution effects. Three realizations each of two sub-seismic populations are considered for each structure.…”