Abstract

Conventional and Unconventional Gas Resources (CGR, UGR) are a focus area for upstream studies in Saudi Aramco and its Permian clastic reservoirs in particular constitute a world class gas resource. Deposited originally in an Aeolian environment, tight rock partial-seals surround a distribution of permeable "sweet spots". Reservoir development is determined by a complex diagenetic process. The highlighted field is a typical example. During initial appraisal, a continuous gas column was interpreted from pre-production pressure data with a clear free water level. Yet, as development progressed, development wells encountered water up structure and pressure depletion showed a complex pseudo-compartmentalization, in contradiction of the initial model assumptions.

As part of on-going reservoir management, an extensive special core analysis program has been conducted on a comprehensive set of some 300 samples encompassing the range in rock quality present in the formation which ranged from highly permeable quartz rich dunes to diagenetically altered paleosols, siltstones and shales. The bulk of samples showed meso- and macro-pore structures typical of silt to clean sandstone dominated rock sequences. This is interesting because although these rock types should be filled with gas, given the expected column height of approximately 1000 ft, none of the rocks examined showed a gas sealing potential of more than a few hundred feet.

One possible explanation scenario is that, although gas is present throughout the system, it preferably forms accumulations in the high permeable zones until the top sealing potential is breached and then migrates up structure. The poorer rocks then saturate to some extent with gas which reduces the buoyancy pressure differential between the sweet spots and the tight matrix which thus increase the column height potential.

In principle, this model could harmonise the observed pre-production continuous gas column with the subsequently drilled up-dip perched water and complex pseudo-compartmentalisation and if accurate, this hybrid concept could provide upside for conventional reservoirs by incorporating marginal previously uncounted gas resources.

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