--> TFEM & BSEM Techniques Applied in Oil E&D Stages

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TFEM & BSEM Techniques Applied in Oil E&D Stages


We understand that hydrocarbon reservoir distribution and variation are controlled by many factors, including complex surface and underground conditions (Niu, et al., 2005). Drilling risks exist not only due to the above factors, but also because of the fluid movement and variation of oil, gas and water during production period. Therefore, any single geophysical method cannot completely eliminate drilling risks (Xiong, 1997). Electromagnetic (EM) method is given more expectation on the prediction of subsurface oil, gas and water distribution (He and Wang, 2002; He, et al., 2010). Years’ experience indicates that resistivity is related to oil, gas and water saturation in the reservoir formation rock pore space. Higher oil or gas saturation usually leads to higher resistivity (Xiao and Xu, 2006). Therefore, resistivity anomaly can be used to predict and interpret hydrocarbon-bearing possibility in the subsurface formation. Recent years, another parameter, induced polarization (IP), has gained more and more attentions. Induced polarization (IP) log has been applied for oil or gas interpretation within logging industry. Many laboratory core tests and oilfield tests show that high oil or gas saturation strongly presents high induced polarization. Oil-saturated rocks are tested to study the low frequency resistivity dispersion effect, mainly caused by the IP effect. These tests indicate that IP effect becomes stronger when oil and gas saturation increased (Xiao and Xu, 2006). Traditional EM methods only study the secondary resistivity and IP anomalies in shallow layers. He (2007) suggested a three-layered anomaly mode for hydrocarbon reservoir, which can directly detect and evaluate hydrocarbon reservoir target. The new methods are named high-power time and frequency domain electromagnetic method (TFEM) and bore hole to surface electromagnetic (BSEM). A high-power surface or downhole large current dipole source is used to directly excite deeply-buried hydrocarbon reservoir targets, thus the resistivity and IP responses from target reservoir can be observed and measured on the surface. In 2005, a hydrocarbon detection technique with IPR (IP*Resistivity) anomaly has been developed (He, 2007). It is believed that a hydrocarbon-saturated formation has higher resistivity than a water-saturated formation, therefore high resistivity anomaly will appear in EM (resistivity) data. An IP anomaly can indicate the existence of potential hydrocarbon fluids. Only in the case of both high resistivity and high potential of hydrocarbon fluids, it can be interpreted that the target may contain hydrocarbon. Resistivity-related anomalies might be represented by amplitude or dual-frequency amplitude anomaly, or resistivity anomaly, etc. Polarization-related anomalies also might be represented by some other parameters, such as dual-frequency phase, polarization, time constant, etc. Here, induced polarization is the major anomaly for hydrocarbon potential assessment. With these two kinds of anomalies, we can distinguish either the igneous rocks, which show high resistivity anomaly and low polarization without hydrocarbon, or the disseminated pyrite, which usually shows low resistivity and high polarization, from real hydrocarbon reservoir. Since 2000, more than one hundred TFEM and BSEM projects have been carried out by BGP both, inside or outside China. These completed projects contribute to reduce drilling risk and assessment of enhance oil recovery (EOR). The statistics shows that the success rate can be up to 70-80%.