Abstract:
A method for monitoring oil based mud filtrate contamination is provided including steps of analytically dividing a fluid stream into two parts, determining a gas/oil ratio for a native fluid determining an apparent gas/oil ratio for the contaminated fluid and determining on a volume fraction, an oil based contamination level based upon the gas/oil ratio for the native fluid and the apparent gas/oil ratio for the contaminated fluid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     FIELD OF THE INVENTION 
     Aspects relate to the field of oil well services. More specifically, aspects relate to oil based drilling mud filtrate contamination monitoring. 
     BACKGROUND INFORMATION 
     Oil based drilling mud (OBM) filtrate contamination monitoring (OCM) is one of the biggest challenges in downhole fluid analysis. Conventional systems and algorithms are not capable of providing adequate results for OBM contamination monitoring, particularly with focused sampling interface modules. Accurate and quantitative OBM contamination measurement is a key enabler of quality sampling and quality downhole fluid analysis (DFA). New algorithms are highly demanded for this purpose. 
     Conventional systems do not disclose or suggest any capability that gas/oil ratios may be used in oil based mud filtrate contamination monitoring quantitatively. Previous attempts at developing a relationship have failed as conventional fluid analyzers display a negative gas/oil ratio in Oil Based mud filtrate. This limits its use in quantifying Oil Based mud contamination. Extrapolating contamination free gas/oil ratios determined by asymptotic fitting methods does not work, especially for focused probes and/or new developed probes and packers. 
     SUMMARY 
     In one example embodiment, a method for monitoring OBM contamination, is disclosed, comprising analytically dividing a fluid stream into two parts, determining a gas/oil ratio for a native (or OBM filtrate contamination free) fluid, determining an apparent gas/oil ratio for the native fluid, and determining on a volume fraction, an oil based mud filtrate contamination level based upon the gas/oil ratio for the native fluid and the apparent gas/oil ratio for the native fluid. 
     A novel procedure is provided for oil based mud filtrate contamination monitoring and determination of oil based mud filtrate contamination level. Based on the definition of gas/oil ratio, a simple formula is developed to relate oil based mud filtrate contamination level in volume fraction in stock tank oil (STO) to apparent gas/oil ratio which is measured by downhole fluid analysis. The end point for native (contamination free) oil can be determined in different ways using multiple sensors in downhole fluid analysis. Additionally, density itself can be used for oil based mud filtrate contamination monitoring using a mixing rule. When combining the oil based mud contamination level results from gas/oil ratio, density and pressure gradients with those from optical density calculations, confidence is significantly gained in particular when all the results are close. In addition, the oil based mud filtrate contamination monitoring algorithms can be applied not only for individual guard and sampling flowlines but also for combined guard and sampling flowlines. These formulas and algorithms can be used for oil based mud filtrate contamination monitoring in real time and postjob analysis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a density vs. gas/oil ratio for three (3) fluids. 
         FIG. 2  is a density vs. gas/oil ratio for three (3) fluids. 
         FIG. 3  is a density vs. gas/oil ratio for three (3) fluids. 
         FIG. 4  is a graph of gas/oil ratio vs. live density. 
         FIG. 5  is a gas/oil ratio fitting results using Equation (7). 
         FIG. 6  is a plot of ln(v obmSTO ) vs. ln(V). 
         FIG. 7  is a plot showing a conversion factor as a function of gas/oil ratio, M gas  and ρ STOStd    
         FIG. 8  is a tool string using the methodology disclosed. 
         FIG. 9  is a plot of gas/oil ratio and density variations with pumpout volume. 
         FIG. 10  is plot of gas/oil ratio and density relationship. 
         FIG. 11  is plot wherein oil based mud level can be obtained by both gas/oil ratio and density values. 
     
    
    
     DETAILED DESCRIPTION 
     Through aspects described herein, it is now possible to use the value of gas/oil ratio for oil based mud filtrate contamination monitoring. The oil based mud filtrate contamination monitoring formula is derived from the definition of gas/oil ratio and oil based mud filtrate contamination level in volume fraction on the basis of dead oil (stock tank oil, STO). Confidence is significantly gained using gas/oil ratio as oil based mud filtrate contamination monitoring due to this theoretical base. 
     Additionally, the new generation of downhole fluid analysis, like in situ fluid analyzer, avoids negative gas/oil ratio (normalizing GOR to zero for dead oil) in the algorithm and the assumption of zero gas/oil ratio for pure oil based mud filtrate is valid. 
     Contamination free GOR 0  for native oil can be determined from different methods, which can gain confidence for the analysis. For example, (1) density derived from pressure gradients and GOR 0  from a linear relationship between density and gas/oil ratio measured by downhole fluid analysis; (2) GOR 0  from the asymptotic fitting method is also used for reference. The linear relationship between density and gas/oil ratio is confirmed by laboratory and field data. 
     In an asymptotic fitting method, a new and robust optimization method is provided to reduce arbitrariness in determining the exponential constant of the power function asymptote. 
     Oil/gas ratio can be measured by downhole fluid analysis based on downhole optical spectra using optical densities at multiple hydrocarbon channels, referred to as apparent gas/oil ratio. In-field practice, apparent gas/oil ratio was used to guide downhole reservoir fluid sampling along with other sensor measurements downhole during cleanup, especially for focused probes and new developed probes and packers. Once apparent gas/oil ratio reaches a stable value with time or/and pumpout volume, one is able to start sampling. Gas/oil ratio can be used as well to determine oil based mud filtrate contamination levels and then for oil based mud filtrate contamination monitoring during cleanup. 
     It is reasonably assumed that pure oil based mud filtrate has no gas/oil ratio (no gas dissolved in pure oil based mud filtrate) and cannot be vaporized into the gas phase at a single stage flash at standard conditions (the flash process reaches equilibrium). Based on the definition of gas/oil ratio, a simple formula is derived for the first time to relate oil based mud filtrate contamination level in volume fraction in stock tank oil (STO) to gas/oil ratio. Therefore, one endpoint gas/oil ratio for pure oil based mud filtrate is zero, and the other endpoint gas/oil ratio for native oil can be determined in different ways using multiple sensors in downhole fluid analysis. 
     Using gas/oil ratio and multiple sensors in downhole fluid analysis as oil based mud filtrate contamination monitoring has the following advantages:
         1. Using all optical density information in hydrocarbon channels.   2. Large gas/oil ratio contrast (e.g. 0 scf/bbl for oil based mud filtrate and 20-50000 scf/bbl from heavy oil to gas condensate) between pure oil based mud filtrate and native oil.   3. Linear (near linear) relationship between gas/oil ratio and density confirmed from laboratory and field data, which allows to extrapolate gas/oil ratio to native oil based on density from pressure gradients and other methods, which also allows to extrapolate density of pure oil based mud filtrate by setting gas/oil ratio to zero.   4. Engineers may estimate endpoint gas/oil ratio for native oil in a sense from information of nearby wells, nearby downhole fluid analysis stations and the like.   5. Gas/oil ratio and other fluid properties for native oil can be obtained as by-product without oil based mud correction.   6. If the result becomes close by integrating multiple sensor oil based mud filtrate contamination algorithms such as gas/oil ratio, density and optical density, confidence about the answer is significantly gained.   7. Gas/oil ratio cannot be used for oil based mud filtrate contamination monitoring in previous generations of downhole fluid analysis such as optical fluid analyzer, live fluid analyzer and advanced fluid analyzer but a new generation of downhole fluid analysis such as in situ fluid analyzer because the gas/oil ratio algorithm in optical fluid analyzer, live fluid analyzer and advanced fluid analyzer does not normalize gas/oil ratio to zero (negative gas/oil ratio occurs for low gas/oil ratio fluids) at the low end.       

     For a native live fluid, the single stage flash gas/oil ratio is defined as the ratio of the volume of the flashed gas that comes out of the live fluid solution, to the volume of the flashed oil (also referred to as stock tank oil, STO) at standard conditions (typically 60° F. and 14.7 psia) 
                     GOR   0     =       V   gas       V     oil   ⁢           ⁢   0                 (   1   )               
where GOR 0 , V gas  and V oil0  are the gas/oil ratio of the native fluid, the flashed gas volume and the volume of flashed native (oil based mud filtrate contamination free) STO at standard conditions respectively.
 
     The contaminated fluid is divided into two components: the pure oil based mud filtrate and the native fluid. If the reservoir fluid is contaminated by oil based mud filtrate and it is assumed that the oil based mud filtrate exists only in the flashed liquid (oil) phase (i.e., the oil based mud filtrate has no gas/oil ratio), then gas/oil ratio of the contaminated fluid can be expressed as in equation two (2): 
                         GOR   =       ⁢       V   gas       V   STO                   =       ⁢       V   gas         V     oil   ⁢           ⁢   0       +     V   obm                       (   2   )               
where the total volume of STO (V STO ) is the summation of the oil based mud filtrate volume (V obm ) and native STO volume (V oil0 ) at standard conditions. Divided both numerator and denominator by V oil0  on the right-hand side, Equation (2) can be rearranged as:
 
                         GOR   =       ⁢         V   gas     /     V     oil   ⁢           ⁢   0             (       V     oil   ⁢           ⁢   0       +     V   obm       )     /     V     oil   ⁢           ⁢   0                       =       ⁢       GOR   0         (       V     oil   ⁢           ⁢   0       +     V   obm       )     /     V     oil   ⁢           ⁢   0                         (   3   )               
where the definition of gas/oil ratio, i.e. Equation (1), is used for the native fluid. Furthermore, Equation (3) can be rewritten as:
 
                           GOR     GOR   0       =       ⁢       V     oil   ⁢           ⁢   0         (       V     oil   ⁢           ⁢   0       +     V   obm       )                   =       ⁢     1   -       V   obm       (       V     oil   ⁢           ⁢   0       +     V   obm       )                     =       ⁢     1   -     v   obmSTO                     (   4   )               
where v obmSTO  is the oil based mud filtrate contamination level in volume fraction in stock tank oil (STO) at standard conditions.
 
Therefore, the oil based mud filtrate contamination level in volume fraction based on STO can be related to gas/oil ratio by:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           v 
                           obmSTO 
                         
                         = 
                           
                         ⁢ 
                         
                           1 
                           - 
                           
                             GOR 
                             
                               GOR 
                               0 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               GOR 
                               0 
                             
                             - 
                             GOR 
                           
                           
                             GOR 
                             0 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Equation (5) can be used for downhole oil based mud filtrate contamination monitoring in real time. Apparent gas/oil ratio can be measured by downhole fluid analysis at a series of time during cleanup. The endpoint, GOR 0  (gas/oil ratio for the native fluid), can be determined by the following different ways. Then the most suitable gas/oil ratio is selected for GOR 0 . 
     GOR 0  from Density and Pressure Gradients 
     Gas/oil ratio is typically in a linear relation with live fluid density. To test the relationship, gas condensate, black oil and heavy oil have been mixed with three types of oil based mud filtrates (esters, mineral oil and olefins) at 10 wt %, 25 wt % and 40 wt % oil based mud filtrate based on STO, respectively, and then the gas/ratio ratio and density are measured for all the mixtures. The results are shown in  FIGS. 1, 2 and 3 . The results clearly show that the relation between density and gas/oil ratio is linear. 
     The real time in situ fluid analyzer data also show the linear relationship between gas/oil ratio and live density as illustrated in  FIG. 4 . It can be seen that the higher density data in the low gas/oil ratio range are off the trend because the fluids may contain some solids and/or be compressed due to a pressure increase at the beginning of the cleanup process. 
     Because downhole fluid analysis measures apparent gas/oil ratio and density during cleanup, a linear relation can be determined from the cleanup data by selecting a suitable time interval. Pretest pressure (pressure gradient) data can be used to determine density of the contamination free fluid—density endpoint for the native fluid. Thus, the linear relation between gas/oil ratio and density can be extrapolated in terms of the density obtained from the pressure gradient. As a result, the endpoint GOR 0  can be determined. Once GOR 0  is obtained, oil based mud filtrate contamination level can be estimated by Equation (5) at a series of time (pumpout volume) based on apparent gas/oil ratio measured by downhole fluid analysis. On the other hand, this linear relation can be used to obtain the density of pure oil based mud filtrate by setting gas/oil ratio to zero. 
     GOR 0  from Density Regression and the Linear Relation Between GOR and Density 
     During cleanup, live fluid density can also be fitted by the following empirical expression:
 
ρ=ρ 0   −βV   −γ   (6)
 
where ρ and V are the density and pumpout volume (can be replaced by time t) measured by downhole fluid analysis; ρ 0 , β and γ are three adjustable parameters. Once good density data regression is obtained, density (ρ 0 ) for the native fluid can be extrapolated by assuming that the pumpout volume (time) approaches infinity. Then GOR 0  for the native fluid can be determined from the linear relationship between gas/oil ratio and density mentioned previously. For the focused flow, V can be replaced by the volume in the sample line instead of total volume (summation of sample and guard line volumes).
 
GOR 0  and Density from Nearby Wells or/and Nearby Downhole Fluid Analysis Stations
 
     Both GOR 0  and density (ρ 0 ) for the native fluid can be obtained from data of nearby wells or/and nearby DFA stations in the same well. 
     GOR 0  from the Plot of Apparent GOR Vs. Pumpout Volume (Time) Data 
     When gas/oil ratio becomes unchanged (derivative of gas/oil ratio with respect to pumpout volume (time) is zero) even changing flowrate in guard or sampling flowline, that gas/oil ratio is taken as GOR 0 . This method may be used in field practice for focused sampling and new developed probes and packers. 
     GOR 0  from Fitting to Apparent GOR vs. Pumpout Volume (Time) Data 
     During cleanup, apparent gas/oil ratio can also be fitted by:
 
GOR=GOR 0   −βV   −γ   (7)
 
GOR 0 , β and γ are the three regression parameters and they are determined by fitting the GOR and pumpout volume (time) data during cleanup. Setting V to infinity, GOR 0  is assumed to be the GOR for the native fluid.
 
Equation (7) can be rearranged as
 
ΔGOR=GOR 0 −GOR=β V   −γ   (8)
 
Combining Equation (5) and Equation (8) the following is obtained:
 
                           v   obmSTO     =       ⁢         GOR   0     -   GOR       GOR   0                   =       ⁢       β   ⁢           ⁢     V     -   γ           GOR   0                     (   9   )               
If it is assumed GOR 0  from the apparent gas/oil ratio vs. V (or t) plot, the result is:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ln 
                           ⁡ 
                           
                             ( 
                             
                               v 
                               obmSTO 
                             
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           ln 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   GOR 
                                   0 
                                 
                                 - 
                                 GOR 
                               
                               
                                 GOR 
                                 0 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ln 
                             ⁡ 
                             
                               ( 
                               
                                 β 
                                 
                                   GOR 
                                   0 
                                 
                               
                               ) 
                             
                           
                           - 
                           
                             γln 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             V 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     A linear regression method may be used to obtain β and γ. The constraints can be applied to the regression: 0≦η≦1; GOR≦GOR 0 ; ⅓≦γ≦2 (γ constraint can be changed according to different packers and probes). In plots ln(v obmSTO ) vs. ln(V) or ln(t). a straight line can be observed. The slope is γ and the interception is ln(β/GOR 0 ). Because GOR 0  is assumed, β can be determined. GOR 0  is then updated; updating β and γ is followed. The process may be repeated and the most suitable GOR 0  may be found for the best fit for the graph as well as other objectives. An example is shown in  FIGS. 5 and 6  and the data come from in situ fluid analyzer measurements.  FIG. 5  illustrates gas/oil ratio fitting results using Equation (7).  FIG. 6  gives the ln(v obmSTO ) vs. ln(V) plot. It can be seen that a nice linear relationship is observed. 
     All these methods can be used to obtain GOR 0  for the native fluid. Finally, a most suitable GOR 0  is selected for oil based mud filtrate contamination level estimation. 
     Once GOR 0  is obtained and the pumpout flowrate is known, the time required for sampling to reach a certain oil based mud level can be calculated by: 
                     Δ   ⁢           ⁢   t     =       Δ   ⁢           ⁢   V       Q   pump               (   11   )               
where Δt, ΔV, and Q pump  are the time required to reach a specified OBM level, the pumpout volume required to reach the specified OBM level, and the pumpout volume flowrate (assuming to be a constant).
 
Using Density as Oil Based Contamination Monitoring
 
     Again, the contaminated fluid is divided into two components: the pure oil based mud and the native fluid. It is assumed that the mixing of the oil based mud filtrate and native fluid is ideal, i.e., producing no excess volume:
 
 V   mol   =x   obm   V   obm   mol +(1− x   obm ) V   0   mol   (12)
 
where V mol  and x are the molar volume and mole fraction. Subscripts obm and 0 represent the pure oil based mud filtrate and native fluid. The molar volume and mole fraction can be changed into density (ρ) and oil based mud filtrate volume fraction (v obm ) at downhole conditions by:
 
ρ=ν obm ρ obm +(1−ν obm )ρ 0   (13)
 
     Rearranging Equation (13), the oil based mud filtrate volume fraction is expressed as 
                     v   obm     =         ρ   0     -   ρ         ρ   0     -     ρ   obm                 (   14   )               
v obm  can be related to the weight fraction of oil based mud contamination at downhole conditions by:
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           w 
                           obm 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               v 
                               obm 
                             
                             ⁢ 
                             
                               ρ 
                               obm 
                             
                           
                           ρ 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ρ 
                               obm 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   ρ 
                                   0 
                                 
                                 - 
                                 ρ 
                               
                               ) 
                             
                           
                           
                             ρ 
                             ⁡ 
                             
                               ( 
                               
                                 
                                   ρ 
                                   0 
                                 
                                 - 
                                 
                                   ρ 
                                   obm 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In Equation (14) and Equation 15, two endpoints—densities of pure oil based mud (ρ obm ) and native fluid (ρ 0 ) should be known. It should be noted that density contrast between the pure oil based mud filtrate and native fluid should be large enough in order to use Equation (14) and (5) for oil based mud filtrate contamination monitoring. 
     Determination of Density of Pure OBM (ρ obm ) 
     The density of pure oil based mud filtrate can be determined
         a) Measure the density of pure oil based mud filtrate if pure oil based mud filtrate (or base oil) is available before logging at different temperatures and pressures covering entire reservoir conditions.   b) According to the linear relationship between gas/oil ratio and density mentioned previously, setting gas/oil ratio to zero, the density obtained is the density of the pure oil based mud filtrate.   c) At the beginning of clearup, 100% oil based mud filtrate may be pumped in flowline. The downhole fluid analysis measured density at the beginning of clearup may be considered to be the density of pure oil based mud filtrate.
 
Determination of Density of Native Fluid (ρ 0 )
       

     The density of native fluid can be determined as follows:
         d) Pretest pressure (pressure gradient) data can be used to determine density of the contamination free fluid—density endpoint for the native fluid.   e) During cleanup, live fluid density can also be fitted by Equation (6). Once good density data regression is obtained, density (ρ 0 ) for the native fluid can be extrapolated when the pumpout volume (time) approaches infinity.       

     Once the two endpoint densities are obtained, Equation (14) and (15) are used to estimate oil based mud filtrate contamination level. 
     Oil based mud contamination level in weight fraction can be converted between standard and downhole conditions by Equation 16 below: 
                             w   obmSTO       w   obm       =       ⁢     (     1   +     GOR   ⁢         M   gas     ⁢     P   Std           ρ   STOStd     ⁢     RT   Std             )                 =       ⁢     conversion   ⁢           ⁢   factor                   (   16   )               
where w obmSTO , ρ STOStd , M gas , R, P Std  and T Std  are the oil based mud filtrate contamination level in weight fraction based on STO at standard conditions, the STO density at standard conditions, the molecular weight of flashed gas at standard conditions, the universal gas constant, the standard pressure (typically 14.7 psia) and standard temperature (typically 60° F.), respectively. ρ STOStd  and M gas  can be estimated by the method proposed in U.S. Pat. No. 7,920,970.  FIG. 7  shows the conversion factor as a function of gas/oil ratio, M gas  and ρ STOStd . For high gas/oil ratio fluids, oil based mud weight fraction based on STO is quite different from that at downhole conditions.
 
     Oil based mud contamination level in volume fraction is then converted between standard and downhole conditions by Equation 17 below: 
                             v   obmSTO       v   obm       =       ⁢       (       ρ   obm       ρ   obmStd       )     ⁢     (       ρ   STOStd     ρ     )     ⁢     (     1   +     GOR   ⁢         M   gas     ⁢     P   Std           ρ   STOStd     ⁢     RT   Std             )                   =       ⁢       (       ρ   obm       ρ   obmStd       )     ⁢     B   o                     (   17   )               
where B o  is the formation volume factor of the contaminated fluid.
 
     If it is assumed that the density ratio of oil based mud filtrate to fluid at reservoir and standard conditions (i.e., the isothermal compressibility of both oil based mud filtrate and fluid) are approximately identical, the same conversion factor can be used for both oil based mud weight and volume fractions. 
     The existing oil based mud filtrate contamination monitoring methods such as the methane and color channel oil based mud filtrate contamination algorithms, multi-channel oil based mud filtrate contamination algorithms can be used as well. 
     An example is given below: 
     The tool string is shown in  FIG. 8 . The PQ, DV-Rod and in situ fluid analysis were used. Only comingling up flow was performed. Gas/oil ratio and density variations with pumpout volume are shown in  FIG. 9 . The gas/oil ratio and density relationship is given in  FIG. 10 . GOR fitting ( FIG. 9 ) shows GOR 0 =595 scf/bbl for the native oil. The native oil and pure OBM densities are obtained by the linear relation between GOR and density, ρ 0 =0.762 g/cm3 and ρ obm =0.875 g/cm3. Therefore, the oil based mud filtrate contamination level can be obtained by both gas/oil ratio and density as shown in  FIG. 11 . They are very close. 
     While the aspects has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure herein.