Abstract:
An optical sensor for pH is described using a cross-linked network of bisilanes to immobilize a pH sensitive chromophore to a surface potentially exposed to a high pressure, high temperature environment such as wellbore effluents at a downhole location.

Description:
[0001]    The invention relates to an optical pH sensor for use in a wellbore and corresponding methods for analyzing of fluids produced from subterranean formations. More specifically, it relates to an optical pH sensor attached to drillpipe, wireline and/or production logging tools for in-situ analysis of effluents produced from subterranean formation. 
       BACKGROUND OF THE INVENTION 
       [0002]    Analyzing samples representative of downhole fluids is an important aspect of determining the quality and economic value of a hydrocarbon formation. 
         [0003]    As the technology of oil and gas production advances, and environmental regulations become stricter, new demands are put on the industry to identify more cost-effective methods of reservoir control. A leading example of such control is the prediction, monitoring, preventing and removal of scale formation. A key request that directly related to the first three operations is in situ measurement of pH, together with the concentration of the critical ions, in aqueous borehole fluids. In particular, real time measurement of pH will offer valuable prediction of the initiation of nucleation that eventually leads to macroscopic scale formation. Also, in the ever-significant operation of in situ H 2 S detection, it is often a prerequisite that medium pH is known so that total inorganic sulfur can be deduced on the basis of thermodynamic equilibrium. 
         [0004]    Present day operations obtain an analysis of downhole fluids usually through wireline logging using a formation tester such as the MDT™ tool of Schlumberger Oilfield Services. However, more recently, it was suggested to analyze downhole fluids either through sensors permanently or quasi-permanently installed in a wellbore or through one or more sensors mounted on the drillstring. The latter method, where successfully implemented, has the advantage of obtaining data while drilling, whereas the former installation could provide additional value as part of a control system for wellbores and hydrocarbon production therefrom. 
         [0005]    To obtain an estimate of the composition of downhole fluids, the MDT tools uses an optical probe to estimate the amount of hydrocarbons in the samples collected from the formation. Other sensors use resistivity measurements to discern various components of the formations fluids. 
         [0006]    General downhole measurement tools for oilfield applications are known as such. Examples of such tools are found in the U.S. Pat. Nos. 6,023,340; 5,517,024; and 5,351,532 or in the International Patent Application WO 99/00575. 
         [0007]    Rapid and reliable pH measurement at downhole conditions, i.e., elevated temperature and pressure, and the presence of multiphase fluids, represents a formidable challenge to the existing techniques, such as potentiometric measurement of electromotive force based on glass electrodes, due to their poor stability and difficulty in interface renewal. 
         [0008]    Although, their responses to changes in ionic composition are much faster than with potentiometric methods, conventional calorimetric methods using homogeneous reactions with indicator dyes often suffer from a lack of precision. 
         [0009]    The measurement of pH is an art with an extended track record. The mainstream techniques are colorimetry and potentiometry, while most of those relevant to the present work involving entrapping dye molecules into thick polymeric films. In a review article, Cammann outlined the current scope and directions of future development for optical chemical sensors, in particular, for pH applications in: K. Cammann, “Optrode quo vadis?”,  Sensors and Acutators B,  51, 1(1998), while Spichiger-Keller presented the fundamental principles of optical chemical sensors in: U.E. Spichiger-Keller, “Chemical sensors and biosensors for medical and biological applications”, Chapter 6(259-320) Wiley-VCH, Weinheim, 1998. 
         [0010]    Apart from the entrapment into films, a variety of other methods based on either physical and chemical adsorption are used to immobilize the active (color changing) species. The known methods include sol-gel processes or bifunctional agents to bind the active species to a solid substrate. 
         [0011]    Where optical transducers are applied to measure the response of the sensor to illumination, absorbance or transmission measurements can be used. In many cases, a fluorescence signals is monitored. Alternatively, it is known to use evanescent light or total internal reflection (TIR) measurements to detect a change in the optical properties of the active species. Another possible detection mechanism is based on surface plasmon resonance (SPR). Many of the above methods are used together with fiber optics to couple light into the system and connect light source, sensor and optical detector. 
         [0012]    In modifying siliceous or metal oxide surfaces, one technique that has been used is derivatization with bifunctional silanes, i.e., silanes having a first functional group enabling covalent binding to the surface (often an Si-halogen or Si-alkoxy group, as in —SiCl3 or —Si(OCH3)3, respectively) and a second functional group that can impart the desired chemical and/or physical modifications to the surface. This process is generally referred to as silylation. 
         [0013]    Silylation has been used for optical pH sensing purpose and is for example described by F. Baldini et al. in: F. Baldini and S. Bracci, “Optical-fibre sensors by silylation techniques”,  Sensors and Actuators B,  11, 353(1993) using bromophenol blue as chromophore immobilized on controlled-pore glasses (CPG). 
         [0014]    Calibration of pH sensors is a challenging issue. In one known approach a pH-independent wavelength is monitored while probing wavelengths where the entrapped dye indicator showed maximal transmittance. In addition to the wavelength where the variation of analyte&#39;s concentration is probed, another wavelength was probed, which is unaffected by the measurement but is subject to other intrinsic changes in the rest of the system, such as light source fluctuation and/or changes in optical fiber transmission mode. The latter changes are common at both wavelengths and can hence be eliminated. A pH sensor designed as such, using physically adsorbed methylene blue on the tip of an optical fibre, resulted in a rather wide pH range of 3-10 with a resolution of 0.015 units. An extra advantage of this “calibration-free” approach is the dampened temperature dependence of the resultant device, due to similar coefficient of the individual molar adsorptivities. 
         [0015]    Though mature and effective in their own right, none of these aforementioned techniques is capable of direct applications to elevated temperature, high pressure and complex chemical compositions. The device that was able to operate in conditions most closely resemble those encountered in oilfield industry was an in situ pH sensor for hydrothermal fluids, as reported by Ding and co-worker in: K. Ding and W. E. Seyfried, Jr., “Direct pH measurement of NaCl-bearing fluid with an in situ sensor at 400° C. and 40 megapascals”,  Science,  272, 1634(1996). Using a yttria-stablized zirconia membrane as the working electrode, they measured the potentiometric change as a result of pH variation under supercritical conditions. But this approach suffered from the slow response time (&gt;20 minutes) and lack of stability, where the Ag/AgCl reference electrode was in direct contact with the fluids. Also, Gervais et al designed a pH sensor, using a fluorescein indicator, which was able to operate at pressure up to 250 MPa, as described in: M. Hayert, J-M Perrie-Cornet and P. Gervais, “A simple method for measuring the pH of acid solutions under high pressure”,  J. Phys. Chem. A.  103, 1785(1999). The measurement of pH under wellbore conditions is made even more complex because of the pressure induced dissociation of weak acids. For example, neutral water undergoes a shift of about −0.73 pH/100 MPa. 
         [0016]    More recently a method for pH measurement under downhole condition has been described in the published international patent application WO 2004/048969 A1. A colorant is added to a sample taken by a downhole monitoring system suspended into the wellbore from a wireline. The color change of the colorant is monitored by a suitable spectral analyzer and can be made indicative of, for example, the pH of the sample fluid. 
         [0017]    Whilst there are numerous examples of optical pH sensors in other technical fields such as physiological application, the oilfield industry lacks simple and robust sensor to measure the pH under downhole conditions. It is therefore an object of the present invention to provide a sensor for pH measurement at high pressure and/or high temperatures. It is a further object of the present invention to provide downhole sensors and sensing methods for pH. 
       SUMMARY OF THE INVENTION 
       [0018]    The invention describes a pH detection system based on the chemical immobilisation of a monolayer of chromophoric indicators, in a uniform orientation, onto a transparent substrate via a two-step reaction scheme. The coupling chemistry is based on bisilane agents, particularly bi-functional organosilane agents, where one terminal functional group self-assembles onto the substrate and forms a cross-linked network that stabilizes these molecules, while the other silane group is available for the binding of target dye indicators. Changes in the indicators&#39; UV or visible spectra such as UV or visible spectra, as a result of pH variation, are probed, preferably via a spectrometer set up, to monitor an amplified internal reflectance signal of the absorbance. 
         [0019]    In a preferred embodiment, molecular scale membranes are applied to readily manipulate the surface wettability towards water and oil phase, while affording additional protection from fouling to the reactive centers. 
         [0020]    The detection scheme is arranged to have little temperature dependence and dependence on the indicator concentration or optic path length of the internal reflectance element (IRE). 
         [0021]    Such a system may be assembled into an appropriate tool for continuous measurement of pH, and derived therefrom other species or compounds such as P CO2 , of borehole fluids in either wireline and/or production logging processes. 
         [0022]    Preferably assembled as a rugged, no-moving-parts device, such a sensor can be readily integrated into a drill string, production logging tool and open hole formation tester tool such as the MDT™ tool of Schlumberger. 
         [0023]    These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the following detailed description, appended drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0024]      FIGS. 1A-1D  illustrate the preparation of a transparent carrier with a cover layer of chromophores in accordance with an example of the invention; 
           [0025]      FIG. 2  shows elements of a sensor in accordance with an example of the invention; 
           [0026]      FIG. 3  shows absorbance spectra at different levels of pH; 
           [0027]      FIG. 4  is a plot of a calibration curve using the ratio of two absorbance peaks in the spectra (A575/A440) of  FIG. 3 ; 
           [0028]      FIG. 5  is a perspective view, partially cut-away, of a sensor in accordance with an example of the present invention in a downhole tool; 
           [0029]      FIG. 6  illustrates an example of a sensor in accordance with the invention as part of a wireline formation testing apparatus in a wellbore; 
           [0030]      FIG. 7  shows a wellbore and the lower part of a drill string including the bottom-hole-assembly, with a sensor in accordance with the invention; and 
           [0031]      FIG. 8  shows a sensor located downstream of a venturi-type flowmeter in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    The concentration of protons in an aqueous medium arises from the chemical equilibrium of water molecules presented in equation 1 and the definition of pH is given in equation 2: 
         [0000]      H 2 O  -&lt;   &gt;- H + +OH −   [1] 
         [0000]      pH=−log [H + ]  [2] 
         [0033]    The alteration of the colors of dyes by the interactions with acids and/or alkalines is one of the oldest observations in chemistry. The color change results from a rearrangement of the indicator molecules when hydrogen ions are (partially) released or taken up. Typical indicator molecules or chromophores have two tautomeric forms, each having a different absorption spectrum. As the pH of the solution varies, the relative size of each tautomer&#39;s optical absorption peak changes in proportion to the changing relative concentration of the two individual forms. 
         [0034]    The present invention employs a unique approach for interface modification, that is, using a bi-functional organosilane reagent. In the example an alkyl chain is used terminated at each end with a group according to the formula —SiX3 where X can be am alkoxy-group (e.g. —OMe or OEt) or a halogen (e.g. —Cl or Br). 
         [0035]    As shown in  FIG. 1A , one terminal functional group binds to the surface of a transparent substrate or carrier  10 , e.g. glass, treated with a mixture of sulfuric acid and hydrogen peroxide to enhance the number of reactive sites (—OH). 
         [0036]    The surface  10  is then exposed to alkoxy-terminated ((MeO) 3 ) bifunctional silane  11 . In  FIG. 1B , the silane is shown linked to the surface. The chemistry of the binding mechanism is thought to be analogous to the reaction 
         [0000]      SiX3+H2O -&lt;   &gt;- Si(OH)3+3HX  [3] 
         [0000]    where X denotes one of the terminal groups. 
         [0037]    The other functional group (-&lt;) subsequently binds to the chromophore  12  via a condensation reaction with its hydrolysable group. Such silane reagents involved in the formation of covalent bonds offer a bridge between inorganic surfaces and organic species, and are stable in both thermal and temporal terms. Ordinary silane bonds are known to survive up to 500° C. in water vapor over a period of 1000 hours. The effective coverage of the silane reagents at the interface provides a marked improvement in composite properties and, hence, substantially extends the lifetime of the modification layer leading ultimately to a much-improved long-term stability under harsh conditions. These properties are described for example in: G. B. Harper, “Reusable glass-bound pH indicators”,  Anal. Chem.,  47, 348(1975). 
         [0038]    In the example of  FIG. 1C , the active species or chromophore  12  of  FIG. 1B  is a phenolsulphonphthalin also referred to as Phenol Red. The Phenol Red molecules are chemically immobilized in a uniform orientation as a single layer at the interface. It should be understood that many similar chromophores are known in the art and it is considered being a mere matter of adaptation to use chromophores other than Phenol Red for the purpose of this invention. 
         [0039]    The adaptation of the surface  10  to a specific purpose can be further enhanced. In the example of  FIG. 1D , molecular (scale) membranes are added to provide an effective protection for the chromophores  12 . The interfacial wettability is manipulated towards a specific phase. For example, long chain molecules  13  terminated with polyethylene glycol, co-adsorbed alongside the indicators and usually in the (stretched) length range of 9-16 nm, result in a robust hydrophilic surface. Likewise, hydrophobicity may be obtained using pure hydrocarbon chains terminated with methyl groups. In addition to enhancing the sampling procedure in the target phase, these membranes  13  also offer effective protection for the reactive centers from fouling. As experiments show, the interfacial chromophores remain functional after experiencing extended reflux up to temperatures of ˜160° C. in the mixture of crude oil, high salinity water, surfactant and solid particles. 
         [0040]    The mono-molecular layered interfacial structures constructed in accordance with the invention are interrogated using an optical system such as the spectrophotometer outlined in  FIG. 2 . The device probes the change in the optical absorption in an internal reflectance mode. Thus, light  24  from a suitable light source (e.g. a light emitting diode, not shown) generates internally reflected optical waves  241  propagating through the carrier  20  and its modified interface. The amplitude of the evanescent wave of light decays exponentially with distance from the interface and hence the sensitivity is confined within a thin surface layer. The depth of penetration, defined as the distance where the intensity of the evanescent oscillation falls to e −1  of its value at the interface, is given as below: 
         [0000]        d   p =λ 1 /2π(sin 2  φ− n   2 ) 1/2   [3] 
         [0000]    where λ 1  (=λ 0 /n 1 ) is the wavelength of the incidence light entering the optical window (medium 1, optically denser) at an angle φ and λ 0  its wavelength in a vacuum, while n represents the ratio of refractive index n 2 /n 1 . The number of contact reflections within the internal reflectance element is given by: 
         [0000]        N=/ ½ L  cot φ/ t   [4] 
         [0000]    where L is the length of the optical window or carrier  20  and t its thickness. The surface modification chemistry and optical alignment are arranged in such a fashion that the chemical changes in the course of the measurement are well covered by the evanescent wave propagation and captured by an optical detector  25 . 
         [0041]    These devices respond rapidly to the chemical changes at the interface, due to the absence of the hindrance of conventional protective membrane. In the example of  FIG. 3 , the absorbance ratio of the two bands that represent the protonated and deprotonated form of the dye, respectively, is measured. The spectra of  FIG. 3  are taken at pH values of 3.3, 4.8, 6.6, 8.5, and 9.7, respectively. With increasing pH, the absorbance at 440 nm shrinks while at 575 nm it increases. Using the dimensionless A(575)/A(440) ratio, the pH measurement is made insensitive to the absolute values of chromophore concentration or optical path length. This approach leads to measurements independent of total dye concentration and optical path length of the device; the measurements also show reduced temperature dependence. 
         [0042]    As shown in the graph of  FIG. 4 , the measurement of the ratio of two different bands appears to result in a near linear dynamic range of &gt;pKa±3.5 units for a phenolphthalein derivative. 
         [0043]    The pH measurement can be readily used to measure the concentration of other species, such as CO 2 , another important species in borehole fluids, according to the following formula: 
         [0000]      pH= pK   1 +Log([CO 2 ]−0.0449 αP   CO2 )/0.0449 αP   CO2    [5] 
         [0000]    where K 1  is the apparent acid constant, [CO 2 ] is the total concentration of all carbonate species and free carbon dioxide, the factor 0.049 (mol/L) the reciprocal molar volume under normal conditions, α the Bunsen absorption coefficient and P CO2  the partial pressure in bar. 
         [0044]    An alternative temperature-dependent formula is: 
         [0000]      pH=7.3232+(Log [HCO 3   − ]−3)−Log  P   CO2 −((160− T )0.0041)   [6] 
         [0000]    where [HCO 3   − ] is measured in mg/L, P CO2  in psi and T in ° F. 
         [0045]    A sensor assembly  50  using the electrode configuration as shown in  FIG. 2 , can be coupled to a flowline  53  in a manner described in  FIG. 5 . The body  51  of the sensor is fixed into the end section of an opening  52 . The body carries the optical interface  511  with the chromophores and contacts  512  that provide connection points for power supply and signal feed-through from and to the sensor through a small channel  521  at the bottom of the opening  52 . A sealing ring  513  protects the contact points and electronics from the wellbore fluid that passes under operation conditions through the sample channel  53 . The active surface may be exposed directly to the fluid flow, or alternative and as shown, a permeable membrane  514  may protect the optical interface  511  from direct contact with the fluid passing through the flowline  53 . 
         [0046]    The sensors of the present invention such as described in the example of  FIG. 5  or alternatives thereof can be used in a variety of measurements, some of which are described below in greater detail. 
         [0047]    In the following various possible downhole applications of the novel sensor are described making reference to  FIGS. 6-8 . 
         [0048]    In  FIG. 6 , there is shown a formation testing apparatus  610  held on a wireline  612  within a wellbore  614 . The apparatus  610  is a well-known modular dynamic tester (MDT, Mark of Schlumberger) as described in the co-owned U.S. Pat. No. 3,859,851 to Urbanosky U.S. Pat. No. 3,780,575 to Urbanosky and U.S. Pat. No. 4,994,671 to Safinya et al., with this known tester being modified by introduction of a sensor  616  as described in detail above ( FIG. 5 ). The modular dynamics tester comprises body  620  approximately 30 m long and containing a main flowline bus or conduit  622 . The analysing tool  616  communicates with the flowline  622  via opening  617 . In addition to the novel sensor system  616 , the testing apparatus comprises an optical fluid analyser  630  within the lower part of the flowline  622 . The flow through the flowline  622  is driven by means of a pump  632  located towards the upper end of the flowline  622 . Hydraulic arms  634  and counterarms  635  are attached external to the body  620  and carry a sample probe tip  636  for sampling fluid. The base of the probing tip  636  is isolated from the wellbore  614  by an o-ring  640 , or other sealing devices, e.g. packers. 
         [0049]    Before completion of a well, the modular dynamics tester is lowered into the well on the wireline  612 . After reaching a target depth, i.e., the layer  642  of the formation which is to be sampled, the hydraulic arms  634  are extended to engage the sample probe tip  636  with the formation. The o-ring  640  at the base of the sample probe  636  forms a seal between the side of the wellbore  644  and the formation  642  into which the probe  636  is inserted and prevents the sample probe  636  from acquiring fluid directly from the borehole  614 . 
         [0050]    Once the sample probe  636  is inserted into the formation  642 , an electrical signal is passed down the wireline  612  from the surface so as to start the pump  632  and the sensor systems  616  and  630  to begin sampling of a sample of fluid from the formation  642 . The sensor  616  is adapted to measure the concentration of protons of the formation effluent. 
         [0051]    A bottle (not shown) within the MDT tool may be filled initially with a calibration solution to ensure in-situ (downhole) calibration of sensors. The MDT module may also contain a tank with a greater volume of calibration solution and/or of cleaning solution which may periodically be pumped through the sensor volume for cleaning and re-calibration purposes. 
         [0052]    A further possible application of the novel sensor and separation system is in the field of measurement-while-drilling (MWD). The principle of MWD measurements is known and disclosed in a vast amount of literature, including for example U.S. Pat. No. 5,445,228, entitled “Method and apparatus for formation sampling during the drilling of a hydrocarbon well”. 
         [0053]    In  FIG. 7 , there is shown a wellbore  711  and the lower part of a drill string  712  including the bottom-hole-assembly (BHA)  710 . The BHA carries at its apex the drill bit  713 . It includes further drill collars that are used to mount additional equipment such as a telemetry sub  714  and a sensor sub  715 . The telemetry sub provides a telemetry link to the surface, for example via mud-pulse telemetry. The sensor sub includes a novel pH sensor  716  as described above. The sensor units  716  collects fluids from the wellbore and hence from oil-bearing layers such as layer  742  via a small recess  717  protected from debris and other particles by a metal mesh. 
         [0054]    During drilling operation wellbore fluid enters the recess  717  and is subsequently analyzed using sensor unit  716 . The results are transmitted from the data acquisition unit to the telemetry unit  714 , converted into telemetry signals and transmitted to the surface. 
         [0055]    A third application is illustrated in  FIG. 8 . It shows a Venturi-type flowmeter  810 , as well known in the industry and described for example in the U.S. Pat. No. 5,736,650. Mounted on production tubing or casing  812 , the flowmeter is installed at a location within the well  811  with a wired connection  813  to the surface following known procedures as disclosed for example in the U.S. Pat. No. 5,829,520. 
         [0056]    The flowmeter consists essentially of a constriction or throat  814  and two pressure taps  818 ,  819  located conventionally at the entrance and the position of maximum constriction, respectively. Usually the Venturi flowmeter is combined with a densiometer  815  located further up- or downstream. 
         [0057]    The novel pH sensor  816  is preferably located downstream from the Venturi to take advantage of the mixing effect the Venturi has on the flow. A recess  817  protected by a metal mesh provides an inlet to the unit. 
         [0058]    During production wellbore fluid enters the recess  817  and is subsequently analyzed using sensor unit  816 . The results are transmitted from the data acquisition unit to the surface via wires  813 . 
         [0059]    A sensor in accordance with the present invention will also be applicable as a formation evaluation probe. This can have direct impact on the evaluation of the distribution, size and properties of the different pay zones with a given reservoir. It can also be used in production logging process as a diagnosis of, for example, hydrogen sulphide. 
         [0060]    Various embodiments and applications of the invention have been described. The descriptions are intended to be illustrative of the present invention. It will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.