Preliminary Evaluations Applicable to Predicting the Probability of Occurrence for Compaction and Subsidence Resulting from Fluid Withdrawal in Unconsolidated Shallow Sands
Published:January 01, 1987
F. E.. Ashford, Y. Ghoniem, 1987. "Preliminary Evaluations Applicable to Predicting the Probability of Occurrence for Compaction and Subsidence Resulting from Fluid Withdrawal in Unconsolidated Shallow Sands ", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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Shallow, loosely packed, unconsolidated reservoirs subjected to intense fluid-withdrawal operations generallyexhibit porosity reduction as a function of pressure drop. This phenomenon, reflected as subsurface compaction, causes a reduction in the net reservoir thickness and initiates certain stress propagations that cause disturbances and movements in the overburden strata. Topographical surface subsidence may occur in response to these displacements.
Both compaction and subsidence mechanisms are directly related to specific reservoir parameters during exploitation. These parameters, identified as compaction or subsidence factors, may be classified in terms of reservoir geology, sedimentology, petrophysics, and production.
The results may be applied to areas under study for possible hydrocarbon and/or water resource exploitation, where decisions must be made as to the development of surface facilities and infrastructure, and where the effect on surrounding areas must be qualitatively evaluated.
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Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.