Abstract:
An electromagnetic energy transmitter is provided for borehole to surface electromagnetic (BSEM) surveying. The transmitter is lowered into a well borehole to a desired location in a well borehole to emit electromagnetic energy for the surveys. The transmitter affords more accurate measurements of its position in the well borehole. The transmitter also senses pressure and temperature conditions at its borehole location to provide indications of detected potential problems in the well from the survey or other sources, and to reduce risks and enhance safety and quality of the operations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electromagnetic energy source or transmitter for borehole to surface electromagnetic surveying and mapping of subsurface formations. 
     2. Description of the Related Art 
     Electromagnetic methods to obtain data regarding subsurface earth formations and their constituent fluid contents have been used for several purposes. Among these have been petroleum reservoir characterization and front-tracking in enhanced oil recovery operations. 
     One of these electromagnetic methods has been what is known as the borehole-surface or borehole to surface electromagnetic method (BSEM). Two electrodes have been used in the borehole-surface electromagnetic energy method. The first electrode has been in a well borehole of what is known as the transmitter well, transmitting electromagnetic energy, and the other, which may be a ground electrode, has been at the earth&#39;s surface along with a receiver array. The receiver array has been located at spaced positions on the surface conforming to the reservoir of interest to detect the energy field after passage through the earth from the first or transmitter electrode. 
     In a typical operation, Borehole to Surface Electromagnetic (BSEM) utilized an electromagnetic source in the borehole and an array (typically 600-2000 or more) of receivers on the surface, thus allowing the mapping of the fluid (typically oil and water) distribution in large areas of the reservoir a few (2-4) kilometers away from the well in which the transmitter electrode had been positioned. 
     The transmitter electrode located in the well was activated at depths of interest. The signal emitted on activation could be a single frequency or multiple frequencies. The resultant electromagnetic field which then occurred was sensed in the time and frequency domains by the receiver array. Surveys of this type could then be repeated after passage of a period of time from the transmitter well to track the subsurface fluid migration. 
     An interface in a subsurface formation between solids and liquids produces induced polarization and frequency scattering responses to the emitted signals and the responses were received and recorded. The recorded data was processed and analyzed to map boundaries of subsurface reservoirs of interest and evaluate other nearby formations. The information obtained was important in assessing the sweep efficiency, or the percentage of original oil displaced from a formation by a flooding fluid, and in locating potential bypassed oil zones, thus ultimately increasing oil recovery. 
     So far as is known, no provision has been made to obtain a precisely accurate measurement of the depth position of the transmitter downhole. An indirect measurement was possible only from measurements of the length of cable passing from the cable reel or drum in the wireline truck into the well. However, this length measurement did not take into account elongation of the cable at increasing depths in the well. This gave rise to an inability to accurately determine well depth measurements of formations and correlate actual depth of the transmitter emissions of energy with data representative of subsurface conditions. 
     During BSEM surveying, other well logging operations with other well logging tools present in the well borehole were not, so far as is known, conducted. The purpose of this was so that the transmitter electrode could be easily moved to desired depths in the well. Thus, there was no capability to monitor downhole well conditions during the BSEM survey. Thus, so far as is known, no provision was made to detect incipient abnormal conditions which might provide advance notice of one or more of possible problems, such as overheating of the transmitter electrode, starting of an ignition in the well, a gas kick, overpressure, or the like. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention provides a new and improved electromagnetic energy transmitter mounted with a wireline for electromagnetic surveys of subsurface earth formations from a well borehole which has a casing installed along its extent into the earth to a location near a formation of interest, the casing being formed of lengths of tubular members connected at end portions to adjacent tubular members by casing collars. The electromagnetic energy transmitter includes an electromagnetic energy source emitting electromagnetic energy in the form of electric current when activated, and a control circuit activating the conductive bar to emit electromagnetic energy for a selected time and duration. The electromagnetic energy transmitter also includes a sonde body housing the control circuit. The sonde body is adapted to be lowered by the wireline in the well borehole to the location near the formation of interest. An upper connector subassembly is mounted above the sonde body connecting the control circuit to the wireline and permits the flow of electrical current to the electromagnetic energy source. A lower connector subassembly is mounted below the sonde body and connects the electromagnetic energy source to the control circuit. The electromagnetic energy transmitter also includes a casing collar locator mounted in the sonde body to provide indications of movement of the sonde body past casing collar in the casing during movement of the transmitter through the well borehole. The electromagnetic energy transmitter further includes a fluid pressure sensor mounted in the sonde body for measuring fluid pressure in the well borehole at the location of the sonde body; and a temperature sensor mounted in the sonde body for measuring temperature in the well borehole at the location of the sonde body. 
     The present invention also provides a new and improved method of electromagnetic surveying subsurface earth formations from a well borehole which has a casing installed along its extent into the earth to a location of interest near a formation of interest, the casing being formed of lengths of tubular members connected at end portions to adjacent tubular members by casing collars. According to the present invention electromagnetic energy source with a sonde body connected therewith is lowered to the location of interest in the borehole. A measure is formed with the casing collar locator of the number of casing collars past which the source and sonde body travel during the step of lowering to determine the depth of the source and sonde body in the borehole based on the measured number of casing collars. The casing collar locator is then deactivated when the source and the sonde body are at the location of interest. Electromagnetic energy is then emitted from the source at the location of interest to travel through the subsurface formations for electromagnetic energy surveying of the subsurface earth formations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram, taken partly in cross-section, of a borehole to surface electromagnetic survey system disposed in a well borehole to obtain borehole to surface electromagnetic survey data according to the present invention. 
         FIG. 2  is an enlarged view of a portion of the well casing of the structure illustrated in  FIG. 1 . 
         FIG. 3A  is a schematic diagram of an upper portion of a borehole to surface electromagnetic transmitter according to the present invention. 
         FIG. 3B  is a schematic diagram of an intermediate portion of a borehole to surface electromagnetic transmitter according to the present invention. 
         FIG. 3C  is a schematic diagram of a lower portion of a borehole to surface electromagnetic transmitter according to the present invention. 
         FIG. 4  is an example display of well log data from conventional well logs regarding subsurface formations as a function of depth in a well borehole. 
         FIG. 5  is a plot of induced polarization data obtained from subsurface formations over a range of depths during borehole to surface electromagnetic surveying and mapping of subsurface formations adjacent the well borehole in which the well log data of  FIG. 4  was obtained. 
         FIG. 6  is a plot of induced polarization data obtained from subsurface formations over a different range of depths than  FIG. 5  during borehole to surface electromagnetic surveying and mapping of subsurface formations adjacent the well borehole in which the well log data of  FIG. 4  was obtained. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, a borehole to surface electromagnetic (BSEM) survey system B is shown schematically in  FIG. 1  in connection with a well borehole  10  which has been drilled into the earth through rock in subsurface earth formations F having hydrocarbon fluids of interest. An electromagnetic energy transmitter T ( FIGS. 3A ,  3 B and  3 C) according to the present invention is mounted with a wireline  12  for electromagnetic surveys of the subsurface earth formations F from the well borehole  10 . As is typical, the well borehole  10  has a casing  14  ( FIGS. 1 and 2 ) installed along its extent into the earth to a location near a reservoir. A typical casing string  14  extends several thousands of feet from wellhead  15  at or above ground level to a lowermost casing section or casing shoe  16  within the wellbore  10 . Below the depth of the casing shoe at  16 , the lower portion of the well where no casing is present is what is known as open hole  17 . 
     The casing  14  is formed of lengths of tubular joint members  18  ( FIG. 2 ) connected at upper and lower end portions  18   a  and  18   b  to adjacent tubular members  18  by casing collars  20 . The ends of each tubular joint or segment  18  of casing string  14  are externally threaded, and the collars  20  are internally threaded to mate with the threaded portion of the adjacent casing members  18 . As is conventional, where two pieces of casing pipe  18  are joined with a collar  20 , there may in some wells be a small gap between the adjacent ends of the two sections of casing. Alternatively, in what is known as “flush joint” casing, no gap is present between the ends of adjacent casing member sections which are held in abutting relationship by collar  20 . 
     In connection with borehole to surface electromagnetic (BSEM) surveys, the transmitter T and wireline cable  12  are suitably supported at the wellhead  15  such as by a sheave wheel  22 , which is used to raise and lower the transmitter T in the wellbore  10 . During the borehole-surface or borehole to surface electromagnetic surveys, two electrodes are used. A first electrode  30  ( FIG. 3B ) of the transmitter T according to the present invention is in the well borehole  10 , which serves as the transmitter well to transmit electromagnetic energy of desired frequency and amplitude into earth formations around the well borehole for travel through the subsurface earth formations F. The other electrode  32  ( FIG. 1 ), which may be a ground electrode, is at the earth&#39;s surface  34  along with a receiver array A indicated schematically in  FIG. 1 . The receiver array A is composed of electromagnetic energy receivers  36  located at spaced positions on the earth over a surface area conforming to dimensions of a reservoir of interest. Receivers in the receiver array A detect the transmitted energy field after passage through the earth from the transmitter electrode T. A borehole to surface electromagnetic survey allows mapping of the fluid (typically oil and water) distribution in large areas of the reservoir a few (typically 2-4) kilometers away from the well in which the transmitter electrode had been positioned. Parameters of interest in such a survey are resistivity and induced polarization or IP, as will be set forth. 
     The transmitter electrode T located in the well  10  is activated at depths of interest. The resultant electromagnetic field is sensed in the time and frequency domains by the receiver array A. Surveys of this type are repeatable at required intervals over a period of time to track migration of subsurface fluids. 
     The electromagnetic energy transmitter T includes a conductive metal bar or rod  40  of copper or other similar conductive material. The conductive electrode energy source  40  is operatively connected to a control circuit  42  which responds to control signals sent from the surface from a transmitter vehicle V at the surface over the wireline  12  and activates the conductive electrode  40  to emit electromagnetic energy of the desired frequency and amplitude for a selected time and duration during BSEM surveying. 
     According to the present invention, the borehole depths at which the BSEM survey electromagnetic energy is emitted by the transmitter T during surveys are obtained in a manner to be set forth. The borehole depth readings are recorded along with the sensed electromagnetic fields corresponding to emissions at that depth in a suitable data memory in a computer or data processor in a logging vehicle or truck L ( FIG. 1 ). Once recorded, the BSEM data and depth measurements are transferred as needed into the data processing system or computer for on site processing and analysis and are available for further processing and analysis elsewhere. Records of the time and content of the electromagnetic energy specified by control signals are also furnished from the transmitter vehicle V to data recoding computer or processor equipment in the logging vehicle or truck L. 
     The electromagnetic energy transmitter T also includes a sonde body  44  ( FIGS. 3A and 3B ) connected to the wireline  12  by an upper connector subassembly  46 . The transmitter T is adapted to be lowered by the wireline  12  in the well borehole  10  to the various depths indicated as adjacent or near the formations of interest for BSEM surveying. The upper connector subassembly  46  is mounted above the sonde body  44  operatively coupling the control circuit  42  to the wireline  12  to provide electrical energy as well as mechanical connection for the transmitter T. The upper connector subassembly  46  permits the flow of electrical current to provide power for signals emitted by the electromagnetic energy source  40  during surveys and passage of control signals to the control circuit  42 . 
     With the present invention, the electromagnetic energy transmitter T is provided with a casing collar locator  50  mounted within the sonde body  44  and electrically connected through connector subassembly  46  and wireline  12  with surface electronics in the logging vehicle L to provide indications of movement of the transmitter T and sonde body  44  past casing collars  20  in the casing string  14  during movement of the transmitter T through the well borehole  10 . The casing collar locator  50  may be one of several available types, such as those available from Sondex (General Electric Co.) of Hampshire, UK. In the casing collar locator  50 , magnetic sensors detect the presence of casing collars  20  by sensing larger metallic mass at the location of the casing collar at the ends of the sections  18  of casing than along the length of the casing sections  18 . 
     Electronic circuitry within the casing collar locator  50  forms electrical signals, usually in the form of pulses, as the locator passes successive casing collars  20  during movement of the transmitter T through the well borehole  15 . The casing collars  20  are located at defined known lengths from each other according to the known distance or length of a casing section  18  between its ends  18 . Thus a count of the number of casing collars  20  passed during movement of the transmitter to a target depth such as shown at  52  or  54 , for example, either in the open hole region  17  or within the casing string  14  indicates accurately for the purposes of the present invention the depth of the transmitter T. The casing collar locator  50  thus measures the position of the transmitter T relative to the last casing point or casing shoe at depth  16 . The casing collar locator  50  is provided with on-off switching capability so that measurements are not being made with the locator during the transmission of electromagnetic signals from the transmitter T. Thus, the casing collar locator  50  is sensing and transmitting signals indicating the presence of casing collars only at those times when the locator is passing through the casing shoe  16  before entering in the target zone. 
     The transmitter T of the present invention thus compensates for any potential bias or distortion in the accuracy of depth locations at which the transmitter T is activated which are induced by the elongation of the wireline cable from surface to the target depth. This has been found to be satisfactorily accurate even when the transmitter is located at a depth in open hole  17 . Normally there are only a few feet of open hole section at the end of a cased well. The possible elongation of the cable in the last few feet of open-hole has been found to be negligible compared to the thousands of feet in the cased section  14 . 
     The electromagnetic energy transmitter T in accordance with the present invention is also provided with a pressure and temperature sensing capability which includes a fluid pressure sensor  55  and a temperature sensor  60  mounted in the sonde body  44 . The fluid pressure sensor  55  measures fluid pressure in the well borehole at the location of the sonde body  44  within the wellbore  10 . The fluid pressure sensor  55  is electrically connected with surface electronics in the logging vehicle L to provide indications of fluid pressure at the location of transmitter T. The pressure sensor  55  may be one of several available types, such as those available from Omega Data Services Limited of Aberdeen, Scotland. 
     The temperature sensor  60  measures fluid pressure in the well borehole at the location of the sonde body  44  within the wellbore  10 . The temperature sensor  60  is electrically connected with surface electronics in the logging vehicle L to provide indications of temperature conditions at the location of transmitter T. The temperature sensor  60  may be one of several available types, such as those available from Omega Data Services Limited of Aberdeen, Scotland. 
     According to the present invention, it is now possible to monitor the downhole conditions of pressure as well as temperature during BSEM surveys. In this manner, well crews are able to identify and take steps to prevent a potential problem from occurring. Examples of such potential problems are overheating of the transmitter electrode T; an ignition starting in the well borehole, a gas kick in the well, an overpressure condition, and the like. Accordingly, survey crews and well crews are able to sense and detect conditions which might give rise to the risk of blowout or ignition, or might affect the quality of data. 
     It has also been found that due to the very low electromagnetic frequency typically used in BSEM surveys, energy emitted during the surveys does not affect the pressure and temperature measurements sensed by the sensors  55  and  60 , respectively. The electromagnetic current could, however, affect pressure and temperature conditions downhole. The present invention by including pressure sensors and temperature sensors integrated in the BSEM transmitter T is able to detect possible anomalous increase of temperature or pressure, or both, due to a number of reasons. Examples are overheating of the BSEM transmitter electrode or antenna  30 , with the risk of melting the transmitter T or wireline cable  12 ; an anomalous hydrocarbon overpressure bubble entering the well; and possible ignition of gases started downhole, whether or not triggered by the electromagnetic current emitted. The pressure and temperature readings sensed with the present invention are important for timely preventive measures to be taken at the surface, such as stopping transmission of the BSEM signals, activating the well control procedures, emergency measures as required. 
     A lower connector subassembly  70  ( FIG. 3B ) is mounted below the sonde body  44  and connects the conductive metal bar  40  of electrode  30  source to the control circuit  42  so that electrical power is provided to the metal bar  40  to emit electromagnetic energy of the desired frequency and amplitude for a selected time and duration during BSEM surveying. 
     The conductor bar  40  is a solid bar of requisite thickness for mechanical strength formed of copper and is, for example about 0.8 m in length. A weight bar connector  72  is mounted at a lower end of conductor bar  40  to connect a swivel connector subassembly  74  with upper portions of the transmitter T. The swivel connector subassembly  74  provides as indicated schematically at  76  for pivotal movement and connection of a weight bar member  78  of a suitably heavy material to the upper portions of the transmitter T. The weight bar member  78  assists, as is conventional, in proper orientation and movement of the transmitter T in the well borehole  10 . Typically, as indicated at  80  a nose plug is mounted below the weight bar  78  for facilitating movement of the transmitter T through the well borehole  10 . 
     In the operation of the present invention, borehole to surface electromagnetic surveying of subsurface earth formations is performed in the well borehole  10  when the transmitter T with sonde body  44  are lowered to locations of interest in the borehole in the free hole zone  17  borehole below the casing  18 . A measure is formed, during such movement, with the casing collar locator  50  of the number of casing collars  20  past which the transmitter and sonde body travel during lowering and the measurements forwarded to the surface over the wireline  12  and recorded in the logging truck L. In this manner, the depth of the transmitter T in the borehole  10  is measured and recorded based on the measured number of casing collars. The casing collar locator  50  is then deactivated when transmitter T are at a location of interest. Electromagnetic energy is then emitted from the conductive bar  40  at the location of interest to travel through the subsurface formations for electromagnetic energy surveying of the subsurface earth formations. 
       FIG. 4  is a simplified example display of well log data from conventional well logs as a function of borehole depth regarding subsurface formations as a function of depth in the well borehole  10 . The well log or plot in  FIG. 4  illustrate as a function of depth over a range of porosity values from below 5% to about 25% the relative presence of oil as indicated at  100  and water as indicated at  102 . The measurements from which the data displayed in  FIG. 4  were attained from an example well in an existing reservoir. 
     Another measurement of interest in addition to the well logs of  FIG. 4  obtainable from the same subsurface formations is data obtainable from BSEM surveys. One of the parameters obtainable from data from BSEM surveys of subsurface earth formations from a well borehole is Induced Polarization or IP. Plots or maps of induced polarization for an investigative layer in the subsurface formations are utilized in discriminating oil from water zones in the formations near or even within a few kilometers from the well borehole. If the induced polarization maps obtained from BSEM survey data indicate a high induced polarization measure, this indicates that there is a high oil saturation in the investigated layer. Conversely, if the induced polarization maps obtained from BSEM survey data indicate a low induced polarization measure, this indicates that there is a water saturation in the investigated layer. 
       FIG. 5  is a plot or map of induced polarization as a function of surface area or extent based on BSEM survey data for a layer indicated as extending from depth A 1  through depth A 4  in the well which is the subject of the well logs plotted in  FIG. 4 . For ease of reference and analysis, the well log plot is also included in  FIG. 5 . The induced polarization measurements as determined from BSEM surveys are plotted in the color key  104  for the map of  FIG. 5 . The locations or depths so indicated in the well of depths A 1  and A 4  are depicted in the well log plots of  FIGS. 4 and 5 . 
       FIG. 6  is a plot or map of induced polarization based on BSEM survey data for a layer determined according to the present invention as extending from the depth A 1  through depth A 2  in the same well which is the subject of the well logs plotted in  FIG. 4 . For ease of reference and analysis, the well log plot is also included in  FIG. 6 . The depth A 2  is also indicated in the well log plotted in  FIG. 4  along with depths A 1  and A 4 . The map co-ordinates are plotted in the margins of  FIG. 6 . As is evident, the areas which are the subject of  FIGS. 5 and 6  substantially overlap. The induced polarization measurements as determined from BSEM surveys are plotted in the color key  104  for the map of  FIG. 5  which is the same as that of  FIG. 6 . 
     In the induced polarization maps of  FIGS. 5 and 6  the differences of induced polarization response are apparent. In the map of  FIG. 5 , the induced polarization data map provides indications of oil as indicated by the areas in the upper right quadrant of the map when the transmitter is located at the layer between depths A 1  and A 4 . Conversely, the induced polarization data map of  FIG. 6  which refers to the layer A 1  and A 2  indicates the presence of substantially more water in the same general area of the reservoir of interest, with water indicated in the same reservoir area. 
     Accordingly, with the present invention a more precise measure and knowledge of the depth where electromagnetic signal energy is being transmitted is provided. A more accurate reading of the depth location of the transmitting antenna or electrode  30  is available. It can be seen that if the location of the electromagnetic transmitter antenna  30  is properly indicated at A 2  instead of A 4  a markedly different map of fluid distribution is obtained and measured for the selected reservoir layer It can also be seen that thus it is now possible to have an accurate measurement of the depth position of the BSEM transmitter T downhole. 
     The invention has been sufficiently described so that a person with average knowledge in the matter may reproduce and obtain the results mentioned in the invention herein Nonetheless, any skilled person in the field of technique, subject of the invention herein, may carry out modifications not described in the request herein, to apply these modifications to a determined structure, or in the manufacturing process of the same, requires the claimed matter in the following claims; such structures shall be covered within the scope of the invention. 
     It should be noted and understood that there can be improvements and modifications made of the present invention described in detail above without departing from the spirit or scope of the invention as set forth in the accompanying claims.