Transformation of CSEM data for determination of resistivities by seismic data processing methods.

大多数石油和天然气产自地震勘探发现的多孔地下或海底储层。正常情况下，孔隙中充满了盐水。地震信号无法区分含油油藏和含水油藏，因此需要通过钻井来确定油藏中存在的流体。四分之三的探井没有发现碳氢化合物，是“干燥的”。通过减少“干井”数量，可以降低寻找新碳氢化合物储量的成本，根据水深和井深的不同，每口井的成本为100万至1亿英镑。碳氢化合物是电阻性的，而盐水是导电的。因此，电磁方法有可能在钻井前区分水和碳氢化合物，从而减少干井的数量。自2002年以来，石油工业开发了一种进行海上电磁勘探的方法，称为可控源电磁（CSEM）方法。尽管它还不成熟，并且有许多技术问题需要解决，但它已成为一种公认的海底油气勘探工具。电阻率与电磁波传播电场和磁场的传播受介质导电性的影响。导电性是电阻率的倒数：介质越容易导电，它的电阻就越小。随着电磁场的传播，它们会失去能量，因为电流在导电材料中流动：介质的导电性越大，能量的损失就越大。这限制了调查的深度。海水的电导率约为3 S/m（电阻率1/3 ω -m）；北海第三系沉积物的电导率约为1 S/m（电阻率为1 ω -m）。电阻率为1 ω -m的砂岩储层在饱和烃时电阻率可高达1000 ω -m。多年来，这一点已经从测井资料中得到了证实。因此，当存在碳氢化合物时，电阻率对比可以达到两到三个数量级。地震阻抗对比要小得多：对于油/水界面，对比只有几个百分点。天然气的阻抗对比比石油高，但少量天然气可以产生与大量天然气相同的响应。很难从地震反应中量化气体的数量。CSEM方法是对地震方法的补充：它可以提供有关储层流体的额外定量信息。我们所关心的是如何从CSEM数据中确定电阻率。传统上，电磁数据是通过反演来解释的。即在计算机中采用与现场数据相同的采集几何形状合成地球电阻率模型的响应，并将结果与现场数据进行比较。调整电阻率模型，直到合成数据与现场数据在可接受的误差范围内匹配。从反演中得到的只是地球物理学家放进模型中的东西。它不是由测量数据得出的。这与地震方法形成了对比，在地震方法中，地震速度是通过排列地震到达的地震数据来确定的。这一原则是地震资料处理的基础。地震数据符合波动方程：波在传播时不损失能量。电磁波场服从扩散方程。电磁数据的计算已被表示为波的加权和。波是计算中的中间步骤。我们建议将扩散电磁数据转换到这个中间域，在那里它们可以像地震数据一样被操纵，从而可以直接确定电阻率：理论认为电阻率应该与确定的速度的平方成正比。

INTRODUCTION Most oil and gas is produced from porous underground or subsea reservoirs discovered by seismic surveys. Normally the pores are filled with salt water. Seismic signals do not distinguish oil-saturated from water-saturated reservoirs, so wells are drilled to determine the fluids present in the reservoir. Three out of four exploration wells find no hydrocarbons and are "dry." The cost of finding new hydrocarbon reserves can be reduced by reducing the number of 'dry' wells drilled - £1 million to £100 million per well, depending on water depth and depth of well. Hydrocarbons are electrically resistive, whereas salt water is conductive. It follows that electromagnetic methods have the potential to distinguish between water and hydrocarbons before drilling and thus reduce the number of dry wells. Since 2002 a method for conducting offshore electromagnetic surveys has been developed for the oil industry, known as the controlled source electromagnetic (CSEM) method. It has become an accepted tool in the search for sub-sea hydrocarbons, although it is still immature and there are many technical problems to solve. RESISTIVITY AND ELECTROMAGNETIC WAVE PROPAGATION The propagation of the electric and magnetic fields is affected by the electrical conductivity of the medium. Electrical conductivity is the reciprocal of resistivity: the more easily a medium is able to conduct electricity the less resistive it is. As the electromagnetic fields propagate they lose energy because electric current flows in conducting material: the greater the conductivity of the medium, the greater the loss of energy. This limits the depth of investigation. Sea water has a conductivity of about 3 S/m (resistivity 1/3 ohm-m); North Sea Tertiary sediments have a conductivity of about 1 S/m (resistivity 1 ohm-m). A sandstone reservoir with a resistivity of 1 ohm-m can have a resistivity as high as 1000 ohm-m when saturated with hydrocarbons. This has been well known for years from well logs. So the resistivity contrast can be two or three orders of magnitude when hydrocarbons are present. Seismic impedance contrasts are much smaller: for an oil/water interface the contrast is a few per cent. Gas gives a higher impedance contrast than oil, but a small amount of gas can give the same response as a large volume of gas. It is difficult to quantify the amount of gas from the seismic response. The CSEM method is complementary to the seismic method: it can provide additional quantitative information about the reservoir fluids. THE PROBLEM We are concerned with the determination of the resistivities from the CSEM data. Conventionally, EM data are interpreted by inversion. That is, the response of a resistivity model of the earth is synthesized in a computer using the same acquisition geometry as for the field data, and the result is compared with the field data. The resistivity model is adjusted until the synthesized data match the field data within an acceptable error. What comes out of the inversion is only what has been put into the model by the geophysicists. It has not been derived from the measured data. This contrasts with the seismic method in which seismic velocities are determined from the seismic data by lining up seismic arrivals. This principle is fundamental to seismic data processing. THE PROPOSALSeismic data obey the wave equation: the wave travels without losing energy. Electromagnetic wave fields obey the diffusion equation. The calculation of the electromagnetic data has been formulated as a weighted sum of waves. The waves are an intermediate step in the calculation. We propose to transform the diffusive electromagnetic data to this intermediate domain, where they may be manipulated just like seismic data, enabling the resistivities to be determined directly: the theory says the resistivities should be proportional to the square of the determined velocities.