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Mechanics
A mathematical model is proposed that describes electrical conductivity variation in the near-well zone during drilling formations containing three immiscible phases: oil, gas, and a small amount of native salt water. It is assumed that borehole drilling is performed using a clay-water solution, the mass-exchange process between the invaded mud filtrate and immobile native water is infinitely fast, and flow of the gas phase occurs by piston-like displacement. The redistribution of the immiscible phases is described by the conventional Buckley-Leverett equations. The actual electric resistivities field is transformed into the apparent resistivity field by probabilistic convolutions with a kernel as logarithmic normal distribution of the probe sensitivities. The centers of sensitivities locating on ring square are corresponding to half length of every probes. The electromagnetic response dispersion locking on probes can be taken equal. This assumption are confirmed by calculations of the real examples. The model is used to the data interpretation of electromagnetic sounding in wells.
It is shown that as a result of penetration of drilling mud filtrate when boring in productive strata containing three immiscible phases occurs, the electric resistance in the near-well zones can be written as a piecewise constant function of the radial coordinate. This function generally consists of four steps, the lower of which is detected by probes as a resistivity annulus, i.e., a zone with decreased induction resistivity. Calculations show that even a small amount of gas in the formation has a significant effect on the electromagnetic log shape. In particular, for small gas saturation of formation, the induction resistivity curves have a local maximum in the neighborhood of the centers of sensitivities of the first two probes in HFIIL devices, along with the minimum characteristic of oil formations.
The results are demonstrated as the data interpretation of sounding for real Siberian oil fields.
Note. Abstracts are published in author's edition
Last update: 06-Jul-2012 (11:52:45)