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AAPG Methods in Exploration
Author(s)
George B. Asquith
American Association of Petroleum Geologists
Volume
5
Copyright:
American Association of Petroleum Geologists
© American Association of Petroleum Geologists 1985, ISBN 978-1-62981-151-2$24.00
ISBN electronic:
9781629811512
ISBN print:
0891816550
Publication date:
January 01, 1985
Will a reservoir produce hydrocarbons? This is a particularly troublesome question in carbonates because, frequently, the answer is anything but straightforward.
Despite the best geology put together from carefully crafted depositional and seismic models, only after a well is drilled into a carbonate reservoir, can a geologist decide whether or not the well will give up commercial quantities of hydrocarbons or, indeed, any hydrocarbons at all. Besides information from surrounding wells, data from drill stem tests, cores, cuttings, and open-hole logs ensure the best basis for making a decision about a well's productivity; unfortunately, drill stem tests or core data are not always available so the geologist is forced to fall back on open-hole logs for most of his or her information.
Because of unique pore characteristics in carbonate rocks and their affect on resistivity logs, geologists can easily make some incorrect judgements. They sometimes decide a well is productive when it's not, or they sometimes overlook a good well.
Problems occur because carbonate reservoirs can have several types of porosity which include intergranular, intercrystalline, vuggy, moldic, and fracture. In addition to these different types of porosity, the pore size may be large (megaporosity) or very small (microporosity). The different pore types and sizes result from both depositional and diagenetic processes.
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Citation
1985. "Use of Bulk Volume Water", Handbook of Log Evaluation Techniques for Carbonate Reservoirs, George B. Asquith
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Abstract
The bulk volume of water (BVW) in a reservoir is simply the product of the water saturation (Sw) times the porosity (ɸ), or stated mathematically: BVW = Sw × ɸ. It is often one of the most important parameters that we can use to evaluate a carbonate reservoir's potential. This is true because bulk volume water indicates whether or not a reservoir is at irreducible water saturation (Swirr). Remember that at irreducible water saturation, a reservoir produces water-free hydrocarbons because all the formation's water is held through surface tension or capillary pressure by the grains.
A reservoir at irreducible water saturation (Swirr) exhibits bulk volume water values that are constant throughout (Dewan, 1983). This means that when BVW is calculated at different points through an interval, the values should be the same or very close to the same for an essentially water-free completion.
Besides mathematically determining BVW, it can also be determined by crossplotting. Selection of the technique used is a matter of personal preference. When you are constructing a crossplot, porosity (ɸ) and water saturation (Sw, calculated by standard or modified Archie equation) are crossplotted on a bulk volume water chart. The bulk volume water chart has hyperbolic lines representing lines of constant BVW values. (BVW charts with hyperbolic lines already printed on them are available from different logging companies.) Data should crossplot on or parallel to one of the hyperbolic lines when a zone is at or near irreducible water saturation, and completion should be mostly water-free. If the data points exhibit scatter from the lines, then the reservoir is not at irreducible water saturation and will produce water, possibly with some hydrocarbons.
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