Apparatus and method for determining the dip of an underground formation in a cased or uncased borehole

Abstract

A well logging system that has, as its goal, the collection of information normally acquired by a dip log tool based on conductivity or resistivity measurements, but with the advantages of radiation based well logging systems. Unlike conductivity based dip log systems, the radiation- based system can be implemented in cased or uncased boreholes. The combination system incorporates a number of radially arrayed devices that, though structurally distinct, function much like the arms associated with conductivity dip log devices. The system incorporates radiation detectors positioned in a radial array rather than electrodes. The combination system measures radiation in each of the multiple radial orientations and responds to generated radiation in the respective directions. The sensitivity of the radiation measuring devices is set so as to localize the region of the borehole being measured. Whereas standard radiation well logging systems are less concerned with localizing the measurement, the present system accurately identifies the orientation of the signal and thus the nature and angle of the formation the signal derives from.

1 . A method for determining the dip angle of a stratified underground formation, said method comprising the steps of: 
 generating pulsed neutrons with a non-chemical based neutron source selectable to generate neutrons or remain non-radioactive, said generating means positioned within a housing capable of being lowered or raised within said borehole;    detecting a quantity of gamma radiation received at a plurality of detectors from a specific direction within said underground formation in response to said pulsed neutrons, said detectors being positioned within said housing;    determining a depth to which said housing is lowered into said borehole;    determining an azimuthal orientation of said housing within said borehole with respect to a radial orientation reference;    correlating a measured gamma reading with a specific depth within said borehole and a specific azimuth of said housing;    identifying a characteristic of said underground formation at a specific depth within said borehole and at a specific azimuthal orientation for said housing; and    comparing a plurality of said gamma measurements made in sequence and determining changes in said underground formation and an angle of said formation as a result of said measurements; and    removing oil from the borehole.    
   
   
       2 . The method of  claim 1  further comprising the step of determining a diameter measurement of said cased or uncased borehole and compensating said gamma radiation measurements for effects caused by passage of said pulsed neutrons and said gamma radiation through said borehole volume.  
   
   
       3 . The method of  claim 1  wherein said step of determining said azimuthal orientation of said housing within said borehole comprises providing an orthogonal gyroscopic compass array referenced to a know azimuthal orientation at a surface location and tracking a plurality of motions of said housing as said housing is lowered and raised within said borehole.  
   
   
       4 . The method of  claim 1  wherein said of detecting said gamma radiation comprises providing four detectors oriented in a radial spacing of approximately 90 degrees and detecting said radiation from each of said orientations and said step of generating pulsed neutrons comprises providing an omni directional pulsed neutron source.  
   
   
       5 . The method of  claim 1  wherein said step of generating pulsed neutrons comprises orienting said pulsed neutron source in an equilateral radial arrangement, and said step of detecting said gamma radiation comprises providing a plurality of detectors in an array referenced to said determined azimuthal orientation within said borehole and further comprising the step of identifying an azimuthal orientation of said gamma radiation detected thereby.  
   
   
       6 . A method for determining the dip angle of a stratified underground formation, said method comprising the steps of: 
 generating pulsed neutrons with a non-chemical based neutron source selectable to generate neutrons or remain non-radioactive, said generating means positioned within a housing capable of being lowered or raised within said borehole arid comprising providing an omni-directional pulsed neutron source;    detecting a quantity of gamma radiation received at a plurality of detectors from a specific direction within said underground formation in response to said pulsed neutrons, said detectors being positioned within said housing and comprising four detectors oriented in a radial spacing of approximately 90 degrees, and detecting said radiation from each of said orientations;    determining a depth to which said housing is lowered into said borehole;    determining an azimuthal orientation of said housing within said borehole with respect to a radial orientation reference comprising providing an orthogonal gyroscopic compass array referenced to a know azimuthal orientation at a surface location and tracking a plurality of motions of said housing as said housing is lowered and raised within said borehole;    determining a diameter measurement of said cased or uncased borehole and compensating said gamma radiation measurements for effects caused by passage of said pulsed neutrons and said gamma radiation through said borehole volume;    correlating a measured gamma reading with a specific depth within said borehole and a specific azimuth of said housing;    identifying a characteristic of said underground formation at a specific depth within said borehole and at a specific azimuthal orientation for said housing; and    comparing a plurality of said gamma measurements made in sequence and determining changes in said underground formation and an angle of said formation as a result of said measurements; and    removing oil from the borehole.