Unconsolidated sands and weakly cemented sandstones from five different hydrocarbon reservoirs have been studied experimentally in order to better understand the coupling between mechanical deformation and fluid flow response. Steady-state permeability has been measured continuously on core plugs deforming under complex nonhydrostatic stress paths. The degree of consolidation was observed. to have first order control on the rheological behaviour and permeability response of such granular materials. All normally consolidated test reservoir horizons exhibited a strong single function relationship between permeability decline and differential stress, irrespective of stress path ratio or initial stress conditions. The exact nature of this functional relationship is material dependent and related to specific microstructural characteristics. For over consolidated material, permeability reduction with increasing differential stress was observed to increase systematically as the magnitude of the stress path ratio increased, perhaps reflecting the decrease in mechanical strength with increasing stress path ratio associated with the elliptical shape of the yield cap.

Des sables non-consotides et des gres d'une faible cimentation de 5 reservoirs hydrocarbonique differents ont ete etudies experimentalernent pour mieux comprendre Ie couplage qui existe entre une deformation mecanique et la circulation des fluides. Une permeabilite stationnaire a ete mesure continuellement sur des echantillons cylindriques deformes le long des chemins de contrainte complexe non-hydrostatique. Le degree de la consolidation a ete le parametre contrôlant du comportement rheologique et de la permeabilite du materiel granulaire. Tout les horizons de materiel dans le reservoir teste qui etait consolides normalement ont montre une forte relation de fonction simple entre la decroissance de la permeabilite et la contrainte differentielle c'est à dire entre le coefficient du chemin de la contrainte et la contrainte initiale. La nature exact de cette relation depend du materiel et de ses microstructures. Le materiel sur-consolide a montre systematiquernent une reduction de la permeabilite avec la contrainte differentielle croissante en fonction de la croissance de la magnitude du chemin de la contrainte. Ceci pourrait refleter la decroissance de la resistance mecanique avec le coefficient du chemin de la contrainte croissante en function du limite inferieur elliptique de la plasticite du materiel.

Unverfestigter Sand und leicht zementierter Sandstein von 5 verschiedenen Kohlenwasserstoff-Reservoirs wurden in dieser Studie untersucht, urn den Zusammenhang, der zwischer einer mechanischen Verformung und der Bewegung fluider Phasen besteht, besser zu verstehen. Eine "steady-state" Permeabilitaet wurde kontinuierlich an den zylindrischen Proben entlang des komplexen nicht-hydrostatischen Stress-Pfades gemessen. Der Verfestigungsgrad war hierbei der kontrollierende Parameter des rheologischen Verhaltens und der Permeabilitaet des granulaeren Materials. Aile Material-Horizonte, die im Reservoir normal verfestigt waren, zeigten eine deutliche Beziehung zischen der Verringerung der Permeabilitaet und der Auflast, d. h. zwischen dem Koeffizient des Stress-Pfades und der Initial-Auflast. Die genaue Natur dieser Beziehung haengt von den spezifischen Materialeigenschaften und den Mikrostrukturen abo "Ueber't"-verfestigtes Material zeigte systematisch eine Verringerung der Per meabilitaet mit zunehmender Auflast als Funktion des ansteigenden Stress-Pfades. Diese Beobachtung koennte die Abnahme des mechanischen Widerstandes mit ansteigendem Stress-Pfad als Funktion der unteren Plastizitaetsgrenze beschreiben.


Depletion associated with primary production from hydrocarbon reservoirs leads to an overall decrease in pore fluid pressure which in turn causes an increase in effective rock stresses. Increases in effective stresses can lead to induced strains, with deformation being accommodated via compaction. Compaction can enhance productivity by reducing pore volume and thus maintaining reservoir fluid pressure, however associated changes in petrophysical properties (permeability reduction) can significantly restrict flow to the wellbore. Thus for reliable simulation of temporal changes in productivity, it is vital that experimentally determined permeability decline curves as a function of realistic variations in reservoir effective stresses resulting from depletion, are incorporated.


The experimental programme has specifically focused on unconsolidated sands and weak sandstones (collectively referred to here as "granular media"). For these materials, conventional "hard" rock mechanics techniques have proved inapplicable, necessitating the adoption of various concepts and experimental techniques originally developed by the soil mechanics community. A brief summary of the underlying theory is given below.

Stress path ratio

The term "stress path ratio" is used here to quantify the in situ stress changes that occur in the reservoir as a result of pore pressure drawdown.

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