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Are Basin Models Better Suited to Predict Reservoir Overpressures than Analytical Formulas?

X. Guichet1, G. Smagghe2, J-L. Rudkiewicz1, and C. Sejourne2
1IFP, 1 et 4 avenue de Bois Préau, 92500 Rueil-Malmaison
2TOTAL - Exploration & Production - Avenue Larribau - 64018 Pau – France

The extent of compaction is strongly influenced by burial history and the lithology of sediments. Sedimentation rate often exceeds the ability of sediments to drain in rapidly formed basins. As a result, pore fluid is overpressured because it supports overlying material. Wells drilled at the crest of large overpressure structures are at considerable risk of mechanical failure. High pore pressures generated by disequilibrium compaction can also be transmitted by water flow along laterally extensive inclined permeable aquifers. (Yardley and Swarbrick, 2000). A particular point called Centroid can be defined: Centroid is the depth in a dipping sandstone bed where sandstone reservoir and surrounding shales are in pressure equilibrium.
Our work presents a fluid flow modelling sensitivity analysis of lateral transfer for synthetic anticline-like 3D geological structures, using basin modelling software Temis3D® (Becip-Franlab), and compare it to Flemings's analytical undrained Previous HitmodelNext Hit (Flemings et al., 2002). Temis3D® software is a coupled numerical simulation program that evaluates pore pressure, porosity, Previous HitoverburdenNext Hit and other petroleum parameters through time. 3D blocks were built using analytical expression provided by Cowie and Scholz (1992)'s Previous HitmodelNext Hit to compute the geometry of the horizons (anticline and semi-anticline). To achieve our objectives we evaluate the parameters which control the magnitude of the overpressure and the appearance of the Centroid effect. Simulations were performed for varying height of the anticline, height of the shale deposit, sedimentation rates, shape and volume of the sandstone body. The main result of the modelling exercise is that the dominant parameter controlling the magnitude of the overpressure increase, in the sandstone body, is the evolution of surrounding shale porosity and permeability with depth. Earlier work by Flemings et al., 2002 show that once both the geometry of the sandstone body and the load are known, the use of the undrained Previous HitmodelNext Hit is attractive because it doesn’t need many assumptions on the petrophysical properties of the shale, solely the history of Previous HitoverburdenNext Hit must be well defined. However in our work we observe that overpressures obtained by the undrained Previous HitmodelNext Hit are systematically higher than the overpressures obtained by Temis3D® calculations, when overpressure dissipation through the shales occurs after the end of the rapid sedimentation episode. Nevertheless, to integrate the processes of overpressure diffusion, we are confronted to the tricky Previous HitestimationNext Hit of permeability as a function of depth. Therefore the appealing simplicity of the undrained Previous HitmodelNext Hit is lost.
In summary this study shows that an accurate forecast of reservoir overpressure is actually determined at the first order by the complex relationships between sedimentation rate, fluid flow patterns and shale compaction. Unfortunately, simple models based on a robust but simplified physic such as the Flemings et al.'s undrained Previous HitmodelNext Hit, can not provide a correct Previous HitestimationNext Hit of the absolute magnitude of the sandstone overpressure. On the contrary, 3D basin modelling seems to be well adapted for the Previous HitestimationTop of the lateral transfer, as a source of additional overpressure. Moreover the set of the 3D simulations, performed in this study, permits to evaluate the Centroid Concept for a large number of conditions and allows an accurate quantification of the overpressure in the sandstone body if the overpressure in the stand shales is well known.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands