AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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A Mathematical Model for Evaluation of Caprock Sealing Capacity from Mercury Intrusion Data and Some Modelled Results

Abstract

Caprock seal absence or failure is one of the main oil exploration risks. Sealing capacity of a caprock depends on the pore size of its 3D pore network. Mercury intrusion measures pore size of a rock’s pore network. This article presents a methodology for constructing a 3D pore network model from mercury intrusion measurements and a theory to derive the sealing capacity of a caprock from the constructed 3D pore network model. A 3D pore network model has two elements: the pore network, and the shape and alignment of a pore. Based on the fact that mercury intrusion measures connected pore networks, and that mercury invades pores from largest to smallest, an “inversion” method, involving modeling the mercury intrusion, has been developed to construct the 3D pore network model given a pore size distribution. In the 3D pore network model, the pore shape is defined by two frusta of cones connected at their base. Pores align at an angle, which is compaction dependent. A sealing capacity theory is derived based on the understanding that a caprock starts to leak when its largest pore string (a connection of the largest pores, is penetrated by oil which is subjected to a pressure above a threshold. The threshold pressure is the sealing capacity of the caprock. The threshold pressure, or the sealing capacity is the capillary pressure of the smallest pore of the penetrated pore string. Sealing capacity of a caprock can be easily derived from the constructed 3D pore network model. Sealing capacity of 29 mudstone samples have been modelled. The modelled results show that sealing capacity of caprock is a function of porosity and clay content. As a rule of thumb, a mudstone caprock with >40% clay content and <0.2 porosity can hold more than a 100 moil column.