Method for determining the acoustic characteristics of a mud filter cake

Abstract

At least one acoustic sensor registers a pressure response to a low-frequency non-oscillating pressure signal generated in a borehole by at least one source. At least one characteristic of a transient process of pressure change is determined from the registered signal. A thickness of a mudcake is determined. Then, based on the values received, a mudcake piezoconductivity or a fluid mobility or both are determined.

1 . A method for determining mudcake acoustic characteristics in a borehole, the method comprising:
 registering by at least one acoustic sensor a response to a low-frequency non-oscillating pressure signal generated in a borehole by at least one source;   determining from the registered signal at least one characteristic of a pressure change transient process;   determining a thickness of a mudcake; and   determining based on values obtained a piezoconductivity of the mudcake, a fluid mobility, or both.   
     
     
         2 . The method of  claim 1 , wherein the at least one characteristic of the pressure change transient process is an exponent of a transient component of a solution, a moment of time when the transient component of the solution reaches its maximum, and a value of maximal pressure attained during the transient process. 
     
     
         3 . The method of  claim 1 , wherein a natural source is used as the source of the low-frequency non-oscillating pressure signal. 
     
     
         4 . The method of  claim 2 , wherein the at least one acoustic sensor is installed on a packer. 
     
     
         5 . The method of  claim 2 , wherein the at least one acoustic sensor is installed on a sampling probe. 
     
     
         6 . The method of  claim 2 , wherein the at least one acoustic sensor is installed on a backup shoe. 
     
     
         7 . The method of  claim 1 , wherein at least one artificial source is used as the source of the low-frequency non-oscillating pressure signal. 
     
     
         8 . The method of  claim 7 , wherein the source of the low-frequency non-oscillating signal is at the same time the acoustic sensor. 
     
     
         9 . The method of  claim 7 , wherein the low-frequency non-oscillating pressure signal is excited by a low-frequency modulation of the well pressure. 
     
     
         10 . The method of  claim 1 , wherein vibrometers are used as the acoustic sensors to register the pressure response. 
     
     
         11 . The method of  claim 1 , wherein accelerometers are used as the acoustic sensors to register the pressure response. 
     
     
         12 . The method of  claim 1 , wherein transducers are used as the acoustic sensors to register the pressure response. 
     
     
         13 . The method of  claim 1 , wherein pressure transmitters are used as the acoustic sensors to register the pressure response. 
     
     
         14 . The method of  claim 7 , wherein the source of the low-frequency non-oscillating signal or the acoustic sensor or both are installed on a packer. 
     
     
         15 . The method of  claim 7 , wherein the source of the low-frequency non-oscillating signal or the acoustic sensor or both are installed on a sampling probe. 
     
     
         16 . The method of  claim 7 , wherein the source of the low-frequency non-oscillating signal or the acoustic sensor or both are installed on a backup shoe. 
     
     
         17 . The method of  claim 7 , wherein several sources of the low-frequency non-oscillating signal are installed at different places. 
     
     
         18 . The method of  claim 1 , wherein a thickness of a mudcake is determined based on echo-impulse measurements comprising a supply of short high-frequency signals to a formation and registration of the reflected echo-signals arrival time. 
     
     
         19 . The method of  18 , wherein the short high-frequency signals are supplied at least from two positions, located at different distances from the mudcake.