Methods and apparatus for the downhole characterization of formation fluids

Abstract

Methods and apparatus for investigating a hydrocarbon bearing geological formation traversed by a borehole are disclosed. A borehole tool is used to acquire a sample of fluid in the formation. Compositional analysis of the fluid sample is conducted to provide a determination of the composition of the sample. The sample composition is then related to a model of the thermodynamic behavior of the fluid; i.e., the mass fractions of the fluid components are used as inputs to an equation of state (EOS) to predict the phase behavior of the fluid.

1 - 29 . (canceled)  
   
   
       30 . A method of investigating a hydrocarbon bearing geological formation traversed by a borehole, comprising: 
 a) acquiring a sample of fluid in the formation with a formation fluid sampling tool located in the borehole;    b) conducting a compositional analysis of the fluid sample located in the sampling tool while said sampling tool is in the borehole, wherein said compositional analysis includes an identification of methane, and an identification of at least one additional hydrocarbon or group of hydrocarbons;    c) relating the compositional analysis to a model of the thermodynamic behavior of the fluid.    
   
   
       31 . A method according to  claim 30 , further comprising: 
 d) based on said relating, predicting a phase behavior of fluid remaining in the formation.    
   
   
       32 . A method according to  claim 30 , further comprising: 
 d) based on said relating, generating a pressure-temperature plot of at least one of said sample and fluid remaining in the formation.    
   
   
       33 . A method according to  claim 32 , wherein: 
 said pressure-temperature plot includes an indication of a critical point.    
   
   
       34 . A method according to  claim 32 , wherein: 
 said pressure-temperature plot includes an indication of at least one of a bubble line and a dew line.    
   
   
       35 . A method according to  claim 32 , wherein: 
 said pressure-temperature plot includes an indication of a critical point, an indication of a bubble line, and indication of a dew line, and an indication of an ambient condition of the formation.    
   
   
       36 . A method according to  claim 31 , further comprising: 
 e) determining whether said fluid in the formation is either condensate or volatile oil.    
   
   
       37 . A method according to  claim 36 , further comprising: 
 if said fluid in the formation is determined to be condensate, finding an indication of a dew pressure for said fluid at an ambient temperature in said formation;    comparing said dew pressure to an ambient reservoir pressure; and    using results of said comparing, indicating a desired maximum drawdown pressure for said acquiring a sample.    
   
   
       38 . A method according to  claim 37 , further comprising: 
 adjusting a drawdown pressure for said acquiring step based on said desired maximum drawdown pressure.    
   
   
       39 . A method according to  claim 36 , further comprising: 
 if said fluid in the formation is determined to be a volatile oil, finding an indication of a saturation pressure for said fluid at an ambient temperature in said formation;    comparing said saturation pressure to an ambient reservoir pressure; and    using results of said comparing, indicating a desired maximum drawdown pressure for said acquiring a sample.    
   
   
       40 . A method according to  claim 36 , further comprising: 
 if said fluid is determined to be a volatile oil, acquiring multiple samples at different drawdown pressures in order to find a bubble point for the fluid in the formation.    
   
   
       41 . A method according to  claim 30 , wherein: 
 said conducting a compositional analysis comprises measuring an optical absorption spectrum of said fluid and conducting a pseudo-compositional analysis by translating said absorption spectrum into concentrations or mass fractions of said methane and at least one said compositional group of hydrocarbons.    
   
   
       42 . A method according to  claim 41 , wherein: 
 said at least one said group of hydrocarbons includes a first group containing ethane, propane, butane, and pentane fractions, and a second group containing hexane and heavier components (C 6 H 14 +).    
   
   
       43 . A method according to  claim 31 , further comprising: 
 repeating steps a) and b) for multiple fluid samples, wherein step d) comprises predicting phase behavior with a level of certainty.    
   
   
       44 . A method according to  claim 31 , further comprising: 
 repeating steps a) through d) at multiple locations in the borehole.    
   
   
       45 . An apparatus for investigating a hydrocarbon bearing geological formation traversed by a borehole, comprising: 
 a) a borehole tool including means for acquiring a sample of fluid in the formation and means for conducting a compositional analysis of the sample of fluid, wherein said compositional analysis includes an identification of methane, and an identification of at least one additional hydrocarbon or group of hydrocarbons; and    b) means for relating the compositional analysis to a model of the thermodynamic behavior of the fluid.    
   
   
       46 . An apparatus according to  claim 45 , wherein: 
 said means for conducting a compositional analysis includes optical means for analyzing the sample.    
   
   
       47 . An apparatus according to  claim 45 , wherein: 
 said means for relating includes means for predicting a phase behavior of fluid remaining in the formation.    
   
   
       48 . An apparatus according to  claim 47 , further comprising: 
 c) means for generating a pressure-temperature plot of at least one of the sample and fluid remaining in the formation.    
   
   
       49 . An apparatus according to  claim 48 , wherein: 
 said pressure-temperature plot includes an indication of a critical point.    
   
   
       50 . An apparatus according to  claim 47 , wherein: 
 said pressure-temperature plot includes an indication of at least one of a bubble line and a dew line.    
   
   
       51 . An apparatus according to  claim 48 , wherein: 
 said pressure-temperature plot includes an indication of a critical point, an indication of a bubble line, and indication of a dew line, and an indication of an ambient condition of the formation.    
   
   
       52 . An apparatus according to  claim 45 , wherein: 
 said means for acquiring a sample of fluid includes means for adjusting a drawdown pressure of said apparatus and means for monitoring acquired samples in order to find a bubble point for the fluid in the formation.    
   
   
       53 . An apparatus according to  claim 45 , wherein: 
 said means for conducting a compositional analysis comprises means for measuring an optical absorption spectrum of said fluid and for conducting a pseudo-compositional analysis by translating said absorption spectrum into concentrations or mass fractions of said methane and at least one said group of hydrocarbons.    
   
   
       54 . An apparatus according to  claim 53 , wherein: 
 said at least one said group of hydrocarbons includes a first group containing ethane, propane, butane, and pentane fractions, and a second group containing hexane and heavier components (C 6 H 14 +).