Study On Theory And Method Of Electromagnetic Detection Of Residual Oil In

In recent years, as the total number of the world’s oil exploration regions has decreased considerably, the focus of oil exploration and development has gradually been shifted from new oil-fields to old ones. That is to say, secondary recovery of residual oil in old oilfields has begun to receive more attention. In view of the actual working conditions and the characteristics of oil production wells, this thesis attempts to explore the rationale, methods, and key technologies for electromagnetic detection of inter-well remaining oil. With the support of a project of the National Natural Science Fund, work has been done as follows in terms of modeling, simulation and field experiment:(1) Using a casing as a source to send alternating current at a certain frequency and using another casing as a receiver to observe the induced electromotive force on the receiving casing, systematic research is done on the influence of such factors as current strength, frequency, formation conductivity and well spacing on the change of signals of induction electromotive force on the receiving casing. A graphical description is given. The results show that the received signal decreases with the increase of the frequency of injection current, the received signal decreases with the increase of distance, the received signal decreases with the increase of formation conductivity, and the received signal monotonically increases with the increase of emission current.(2) Placing a probe (an iron core wound with a certain number of turns of wire) into each of two casings, and exerting alternating current on one probe at a certain frequency while using the other probe to receive and observe the model of inductive current density, a study is done on the influence of such factors as current density, frequency, formation conductivity and well spacing on the change of signals of induction electromotive force on the receiving casing. A graphical description is given. The results shows that when the frequency of emitted current increases, the induced current density on the receiving coil shows a trend of gradual decrease; when the amplitude of emitted current increases, the induced current density on the receiving coil increases obviously; when the medium conductivity is 10 S/m, the receiving coil receives maximum induced current; when the distance between wells increases, the induced current on the receiving coil decreases.(3) Placing an iron core wound with two groups of coil in an open hole, with one group of coil used as a transmitter and the other group used as a receiver, and then exerting alternating current at a certain frequency on the transmitting coil so as to allow the receiving coil to receive the transmitted current which is coupled to the current of formation, an analysis is made to obtain some basic information of the formation according to the induced current density on the receiving coil. The results show that, when the distance between wells increases, the density model of the induced current on the receiving coil changes slightly with a trend of decline. When the formation conductivity changes, there is no significant effect on the density model of induced current. The reason may be that the coil distance is too close. The density model of the induced current on the receiving coil tends to decline with the current exerted on the transmitting coil. There is no significant change in the density model of the induced current on the receiving coil.(4) Based on Ampere’s law and the theory of electromagnetic wave propagation, research is done on the response characteristics of electromagnetic signals on casing itself. An analysis is made on the influence of such factors as current source, the distance between current source and casing, the distance between two current sources, and the radius of casing, on the current density on the casing. The results show that, the closer the distance between casing and magnetic probes, the greater the current density on the casing. and the location of the minimum point of current density on the casing is closer to the point where current source is. The current density of the minimum point is determined by the point where current source is located The smaller the radius of casing is, the greater the amplitude change of current density is, and the greater the amplitude of the current density is.(5) The current density distribution varies with the existence and non-existence of remaining oil between wells. When there is no residual oil between casings, the current density of casing axial is mostly concentrated in the upper part of the casings, namely, the current density line on the upper part of the casing is closer. When there is remaining oil, the current density line is more closely distributed in the upper of casing and on either side of the casing. When there exists remaining oil, the current density of selected points is lower than when there is no residual oil. When evenly selecting points which are parallel to casing axial for computing current density value, if there exists oil between casings, then its current density module will be significantly smaller than other points.(6) Repeated field experiments show that, when the wire is exposed to the air, casings should be given shielding wire in order to reduce interference; a filter is needed to filter out 50 Hz frequency interference; signals can be clearly observed by using Oscilloscope; the electromagnetic field caused by the wires in the casing can be effectively shielded by using two shielding casings instead of one.