Recovery Behaviour of a Partly Illitized Sandstone Gas Reservoir

Abstract Diagenesis is defined as any chemical, physical or biological change undergone by a sediment (rock) after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. The diagenetic changes that occur in the rock result in the alteration of some of the original petrophysical properties of the rock. Porosity and permeability, amongst others, have been established to be altered by diagenesis. It is common knowledge in the industry that the amount of hydrocarbon recovered from a reservoir is dependent, amongst other factors, on the hydrocarbon initially-in-place in the reservoir and the intra reservoir rock pore space connectivity. The hydrocarbon initially-in-place is a function of the reservoir rock porosity and the pore space connectivity is a measure of the permeability of the reservoir rock. Thus the recovery from a reservoir rock affected by diagenesis, based on the aforementioned effects of diagenesis on reservoir rock porosity and permeability, is largely dependent on the extent of diagenesis that took place in the reservoir rock. While reservoir performance behaviour is well established in sandstone reservoirs with diagenetic clays occurring throughout the reservoir, though may be scattered here and there in the reservoir rock, the case of a reservoir rock with a clay-affected layer directly below a clay-free layer, but is one sand pack, is unique. This paper discusses the recurring increases in gas recovery in a partly illitized sandstone reservoir using the North Morecambe field in the East Irish Sea Basin as case study. Illite, a diagenetic clay mineral, has the characteristic of lining the pores of the reservoir rock. Its main effect is reservoir permeability alteration. The North Morecambe field is a sandstone reservoir with two flow zones: an illite-affected zone overlain by an illite-free zone. More than half of the gas-in-place in the field is located in the illite-free zone. Reservoir dynamic simulation and historical production data analysis carried out on the field indicates differential draining of the flow zones: the illite-free zone producing much better than the illite-affected zonecrossflow from the illite-affected zone to the illite-free zonethat though the illite-free zone of the reservoir is being preferentially drained, in comparison to the illite-affected zone, the illite-affected zone is nonetheless being efficiently drained through crossflow into the illite-free zone The study carried out and presented in this paper highlights best practise approach in completion strategy for efficient draining of the illitized part of a partly illitized sandstone reservoir. It further establishes a cubic, not a linear, behaviour as the reservoir performance behaviour to be expected from a partly illitized sandstone reservoir.