Shale Gas Play Definition using Controlled Source Electro-Magnetic Geophysics

点击翻译按钮获取中文摘要

INTRODUCTION. This project aims to show that the presence of subsurface hydrocarbons, especially shale gas, may bedetected before drilling by performing a controlled-source electromagnetic (CSEM) survey on the earth's surface above thetarget. The application of CSEM for shale gas detection has never been done before. We plan to do this over an existingseismic line in Scotland. Hydrocarbons are electrically resistive. So the objective is to map subsurface resistivity variationsusing the surface CSEM survey data. There are two goals: (1) to acquire good quality transient CSEM data; and (2) torecover the resistivities from the data.CSEM DATA ACQUISITION USING THE MTEM METHOD. Using a field layout similar to that used in seismic reflection,the method has a current source that injects current (~40A) into the Earth via a pair of source electrodes, and a line ofelectric field receivers, each of which measures the voltage response of the earth between a pair of receiver electrodes.The recorded data are processed to recover the impulse response of the earth for each source-receiver pair.EQUIPMENT. The equipment we will use was developed initially in the University of Edinburgh and later in MTEM Limitedand Petroleum Geo-Services (PGS). In 2009 PGS ceased to perform land surveys. In April 2014 PGS donated the MTEMland system to the University of Edinburgh for research purposes. This system, valued at £500k, forms a key part of thisproject. There are more components than are required for the survey, providing redundancy, and minimising the risk ofdowntime due to equipment failure.MTEM FIELD SURVEY. The MTEM survey will be conducted over a shale gas prospect at the Deerdykes site in theMidland Valley of Scotland near Cumbernauld, where shale gas/oil plays are suspected to be present at depths ranging to3000 m below the ground surface. Previous land MTEM surveys have acquired data for targets down to 2000 m. ThePetroleum Exploration and Development License (PEDL) for the site is held by ReachCSG (project Partner), who alreadyhave significant geological and geophysical information relating to the site, including historic 2-D seismic reflection data thatwill be made available to the project. The survey line will pass through a location where Reach have extant planningpermission to drill a shale gas exploration borehole.THE DATA. The data will be obtained with the most up-to-date commercial grade equipment and are expected to be ofexcellent quality. This will be the first CSEM data set ever recorded over a shale gas prospect.RECOVERING THE RESISTIVITIES FROM THE DATA. We will recover resistivities from the MTEM data using forwardmodelling routines to compute synthetic impulse response data that would be obtained over the model for all the sourcereceiverconfigurations used in the survey. The resistivity model will be adjusted to minimise the error between thesynthetic and real data, this is a process known as inversion. The data will be inverted in two ways: first, without any a prioriinformation or constraints; second, using sub-surface rock layer interfaces obtained from the existing seismic reflectiondata, which will be re-processed by project partner ReachCSG.