Electrical resistivity of eight igneous rocks and two crystalline limestones was measured at pressures to 10 kb. The rocks were saturated with tap water or salt solution, and the pore pressure was maintained near zero. The dependence of resistivity on temperature, porosity, and pore fluid salinity suggested that conduction was primarily electrolytic throughout the entire pressure range, even though the porosity of some rocks was less than 0.001. Resistivity increased with increasing pressure. The average increase over the 10‐kb range amounted to a factor of 250. The changes of resistivity with pressure parallel changes of compressibility with pressure, being rapid over the first 2 kb and tapering off more gradually at higher pressures. The data suggest that the electrical conduction of these rocks consists of (1) conduction along cracks, below a few kilobars pressures, and (2) volume and surface conduction along a network of pores which persist throughout the entire pressure range. Surface conduction of the rocks saturated with tap water was 10 to 20 times greater than the volume conduction of the pores. The dependence of conductivity on porosity for all the samples saturated with saline solution followed the same empirical law that is observed for porous sedimentary rocks, σ(rock) = σ(solution) × η2.
Changes in electrical resistivity were observed as a function of compressive stress in a variety of crystalline rocks that were subjected to confining pressure of up to 5 kb and to pore pressure of water of 500 bars. In the majority of the rocks, resistivity increased slightly up to about half the fracture stress; just the reverse effect has been noted elsewhere for rocks that were apparently partially saturated. Beyond half and particularly within about 20 per cent of the fracture stress, resistivity dropped typically by an order of magnitude. This sharp decrease corresponded closely to an increase in porosity, or dilatancy, which took place under compressive stress. Detailed study of one rock, Westerly granite, showed that changes in resistivity and, hence, porosity with stress were insensitive to effective pressure, when stress was normalized with respect to fracture stress. This suggests that fracture occurred at a critical crack porosity that was pressure independent. The changes in resistivity with stress that accompany frictional sliding on a fault are insignificant when the measurement volume contains the fault, even though faulted rock under pressure can support high stress.
Monitoring changes in hydrocarbon reservoir geometry and pore-fluid properties that occur during production is a critical part of estimating extraction efficiency and quantifying remaining reserves. We examine the applicability of the marine controlled-source electromagnetic ͑CSEM͒ method to the reservoir-monitoring problem by analyzing representative 2D models. These studies show that CSEM responses exhibit small but measureable changes that are characteristic of reservoir-depletion geometry, with lateral flooding producing a concave-up depletion-anomaly curve and bottom flooding producing a concave-down depletion-anomaly curve. Lateral flooding is also revealed by the spatial-temporal variation of the CSEM anomaly, where the edge of the response anomaly closely tracks the retreating edge of the flooding reservoir. Measureable changes in CSEM responses are observed when 10% of the resistive reservoir is replaced by conductive pore fluids. However, to avoid corrupting the relatively small signal changes associated with depletion, the acquisition geometry must be maintained to a fraction of a percent accuracy. Additional factors, such as unknown nearby seafloor inhomogeneities and variable seawater conductivity, can mask depletion anomalies if not accounted for during repeat monitoring measurements. Although addressing these factors may be challenging using current exploration CSEM practices, straightforward solutions such as permanent monuments for seafloor receivers and transmitters are available and suggest the method could be utilized with present-day technology.
Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea‐floor magnetic records can be replaced by land recordings, producing an effective sea‐surface MT response. Field trials of such equipment in 1-km-deep water produced good‐quality MT responses at periods of 3 to 1000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea‐floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.
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