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Concept
Unconventional (oil & gas) reservoir
Summary
Unconventional (oil & gas) reservoirs, or unconventional resources (resource plays) are accumulations where oil & gas phases are tightly bound to the rock fabric by strong capillary forces, requiring specialised measures for evaluation and extraction.
Petroleum reservoir
Oil and gas are generated naturally at depths of around 4 or 5 kms below Earth’s surface. Being lighter than the water, which saturates rocks below the water table, the oil and gas percolate up through aquifer pathways towards Earth's surface (through time) by buoyancy. Some of the oil and gas percolate all the way to the surface as natural seepages, either on land or on the sea floor. The rest remain trapped underground where the oil and gas are prevented from reaching the surface by geological barriers, in a range of trap geometries. In this way, underground pockets of oil & gas accumulate by displacing water in porous rock, which, if permeable, are referred to as conventional reservoirs. A well drilled into these reservoirs normally flow oil and gas through natural buoyancy, driven to the well bore where pressure differences are relatively high. Where the pressures are low, flow can be assisted with pumps (e.g. nodding donkeys).
In the early days of the oil industry, there was no need for stimulation to improve recovery efficiency, because supply vastly outstripped demand and leaving "difficult" oil in the ground was economically expedient. Two world wars, followed by huge economic growth resulted in surging demand for cheap portable energy, while the availability of new conventional oil and gas resources declined. The industry initially sought to enhance recovery of trapped oil and gas, using techniques like restricted, or low volume hydraulic fracturing to stimulate the reservoir further, thereby reducing the volume of oil and gas left in the ground to an economic minimum. By the turn of the millennium, a new kind of energy resource was required, particularly by the USA, who were driven to achieve energy independence.
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