Method for detecting formation pore pressure by detecting pumps-off gas downhole

Abstract

Methods and systems are described for drilling a well while distinguishing circulated gas or air from pumps-off gas in a drilling fluid at downhole pressure and temperature. A well is drilled with a drilling fluid, drill string, and drill bit. Drilling fluid is pumped through the drill string, drill bit, and into an annulus between the drill string and a wellbore. The drill string comprises one or more sensors sensing a parameter indicative of circulated gas or air in the drilling fluid flowing through the drill string, one or more sensors being behind and near the drill bit. The sensors measuring gas in the drill string may be at the same level as gas detectors in the annulus. The measurements are communicated to a human-readable interface at the surface, allowing an operator to determine if pressure of the wellbore fluid is greater than formation fluid pressure.

1 . A method of drilling a well while distinguishing circulated gas or air from pumps-off gas in a drilling fluid at downhole pressure and temperature, the method comprising:
 a) drilling a well with a drilling fluid, a drill string, and a drill bit from an earth surface through a formation, the drilling fluid being pumped through the drill string, drill bit, and into an annulus between the drill string and a wellbore, the drill string comprising one or more sensors sensing a parameter indicative of circulated gas or air in the drilling fluid flowing through the drill string, at least one of the sensors located in the drill string behind and near the drill bit;   b) measuring, while drilling, a parameter of the drilling fluid indicative of circulated gas or air inside the drill string behind and near the drill bit using the sensors;   c) supporting at least one annulus gas sensor by the drill pipe behind and near the drill bit to sense an amount of gas in the drilling fluid in the annulus behind and near the drill bit during pumping at a depth of the at least one annulus gas sensor;   d) detecting an amount of gas in the drilling fluid in the annulus behind and near the drill bit using the at least one annulus gas sensor during drilling periods before and after a period when pumping has stopped; and   e) communicating the result of steps (b)-(d) to a human-readable interface at the surface while drilling to allow an operator to compare the amount of gas in the drilling fluid in the annulus behind and near the drill bit during the periods before and after the period when pumping has been stopped, and thus determine if pressure of the wellbore fluid is greater than formation fluid pressure behind and near the drill bit.   
   
   
       2 . The method of  claim 1  wherein the measuring of the parameter of the drilling fluid indicative of circulated gas or air inside the drill string comprises measuring the parameter in the drilling fluid flowing through the drill string at a same level as an annulus gas sensor monitoring gas present in fluid flowing through the annulus. 
   
   
       3 . The method of  claim 1  comprising tracking gas levels volumetrically as a function of drilling fluid volumes pumped to recognize when recirculated gasses or air pass sensors monitoring gas content in the annulus. 
   
   
       4 . The method of  claim 3  further comprising correcting annulus gas volume to remove effects of circulated gas or air. 
   
   
       5 . The method of  claim 1  further comprising sensing parameters indicative of circulating gas or air using sensors placed along the drill string to assess the movement of recirculated gasses or air out of the well, or to detect influxes of gases up the well. 
   
   
       6 . The method of  claim 1  wherein the measuring comprises measuring one or more physical properties selected from density, velocity, temperature, pressure, conductivity, and resistivity of drilling fluid containing recirculating gasses and/or air at downhole pressures and temperatures. 
   
   
       7 . The method of  claim 6  wherein the physical property is measured in real-time, and the real-time measurements compared with measurements obtained using control samples to determine the actual gas content at downhole conditions. 
   
   
       8 . The method of  claim 1  wherein the measuring employs a technique selected from pulse-echo, density, ultrasonic, velocity, sonic impedance, acoustic impedance, and combinations thereof. 
   
   
       9 . The method of  claim 1  further comprising assessing formation pressure relative to wellbore pressure and adjusting a parameter of the drilling fluid. 
   
   
       10 . The method of  claim 9  wherein the parameter of the drilling fluid is selected from weight, density, specific gravity, API gravity, thermal conductivity, pH, viscosity, compressibility, thermal conductivity, salinity, and water activity. 
   
   
       11 . A method for detecting pumps-off gas in drilling fluid in a wellbore during drilling from an earth surface and penetrating a plurality of subterranean formations, the method comprising:
 a) pumping drilling fluid through a drill pipe extending into a wellbore to provide pressure on the drilling fluid in the drill pipe and discharging drilling fluid from a bottom end of the drill pipe into a drill bit and an annulus between an outside of the drill pipe and an inside of the wellbore to drill the wellbore to a greater depth;   b) supporting at least one gas sensor by the drill pipe near the bottom of the drill pipe and positioned and sensing the amount of gas in the drilling fluid in the annulus at a depth of the at least one sensor;   c) periodically stopping the pumping of the drilling fluid for one or more time periods; and   d) comparing amounts of gas detected by the at least one sensor in the drilling fluid in the annulus at the level of the at least one sensor during pumping time periods to determine a change in an amount of gas in the annulus resulting from the periodic stopping of pumping.   
   
   
       12 . The method of  claim 11  wherein the wellbore is drilled to a greater depth at an under-balanced condition. 
   
   
       13 . The method of  claim 11  wherein the wellbore is at an under-balanced condition during one or more of the periods when pumping has been stopped. 
   
   
       14 . The method of  claim 11  wherein the wellbore is at an under-balanced condition at a hydrostatic drilling fluid pressure. 
   
   
       15 . The method of  claim 11  wherein a pore pressure of the subterranean formations penetrated by the drill pipe is between a drilling fluid pumping pressure and a hydrostatic drilling fluid pressure. 
   
   
       16 . The method of  claim 11  wherein the at least one sensor is mounted in a section of drill pipe. 
   
   
       17 . The method of  claim 11  wherein the at least one sensor is mounted on the outside of the drill pipe. 
   
   
       18 . The method of  claim 11  wherein the sensor is selected from the group consisting of pulse-echo, density, ultrasonic, velocity, sonic impedance and acoustic impedance sensors. 
   
   
       19 . The method of  claim 11  wherein the amount of gas in the drilling fluid during a period when pumping has been stopped is compared to a previous amount of gas detected during a previous period when pumping had been stopped. 
   
   
       20 . The method of  claim 11  wherein a plurality of sensors are positioned at a plurality of locations along a length of the drill pipe. 
   
   
       21 . The method of  claim 11  wherein at least one of the pluralities of sensors is positioned at a distance from about 1 to about 1500 feet [460 meters] above the drill bit along a length of drill pipe. 
   
   
       22 . The method of  claim 11  wherein the wellbore is at an under-balanced condition during pumping. 
   
   
       23 . The method of  claim 1  wherein an upper portion of the wellbore is cased. 
   
   
       24 . A system for distinguishing circulated gas or air from pumps-off gas in a drilling mud or fluid at downhole pressure and temperature, comprising:
 a) one or more sensors for measuring a parameter indicative of circulated gas or air in mud flowing through the drill string, at least one of the sensors located in a drill string behind and near a drill bit, and at least one sensor for measuring a parameter indicative of gas or air in an annulus behind and near the drill bit;   b) means for communicating the parameters to a human-readable interface at the surface; and   c) a human-readable interface from which an operator can compare amounts of gas present in the annulus during pumps-on periods which occur before and after a pumps-off period to determine if a pressure of the wellbore fluid is greater than the formation fluid pressure using the amount of circulated gas or air present inside the drill string and the annulus behind and near the drill bit.