Abstract:
A method for determining wettability of a solid, such as a reservoir rock material, is described. The method includes disaggregating the material, for example by grinding and placing the disaggregated material on the surface of the fluid. The wettability is analyzed based on whether a portion of the material floats on or sinks into the fluid. The method is well suited for heterogeneous solid materials that have mixed wetting characteristics and/or have varying surface types. The fluid can be evaluated as a potential treatment fluid or a component thereof that can be used for treating the rock formation. For example, the potential treatment fluid can include a surfactant or an oxidizing agent. A simple observation can be made whether substantially all of the material placed on the surface of the fluid sinks into the fluid, or the portions of floating and sinking material can be weighed.

Description:
FIELD 
       [0001]    The patent specification is generally related to hydrocarbon recovery from reservoirs. More particularly, this patent specification relates to methods to characterize unconventional reservoirs and the effect of treatments on reservoir material leading to enhanced hydrocarbon recovery from such reservoirs. 
       BACKGROUND 
       [0002]    Shale reservoirs throughout the world are known to contain enormous quantities of gaseous and liquid hydrocarbons. However, some aspects of the production mechanisms operative in these reservoirs are not well understood. Until fairly recently, the wettability of gas reservoirs has not been of much concern. With the exploitation of gas reserves in coal seams and shales, the so-called unconventional reservoirs, the question of wettability takes on much greater importance. 
         [0003]    In order to develop methods to efficiently recover gas from a shale reservoir, it is very useful to gain a good understanding of the chemical nature of the shale. A productive exploitation of the shale reservoir will likely require the introduction of a fluid into the reservoir. Therefore, how that fluid interacts with the formation material is to a great degree determined by the extent to which the fluid wets the formation. Reliable test methods have been developed to measure the wettability of a material, but the methods are often beyond the capabilities of most field laboratories. Thus, it is desirable in cases hydrocarbon recovery from unconventional reservoirs such as coal seams and shale reservoirs to have improved techniques for understanding the wettability characteristics of the reservoir material that can be carried out quickly, simply and can be made in a field setting. 
       SUMMARY 
       [0004]    According to some embodiments, a method for determining a wettability characteristic of a solid material with a fluid is provided. The method includes disaggregating the solid material, for example by grinding, to form disaggregated solid material; placing the disaggregated solid material on the surface of the fluid; and determining the wettability characteristic based at least in part on whether a portion of the disaggregated solid material floats on or sinks into the fluid. According to some embodiments, the solid material is a sample of a rock formation or subterranean rock formation from which hydrocarbon recovery is desired. The method is well suited to heterogeneous solid materials that have mixed wetting characteristics and/or have varying surface types. 
         [0005]    The fluid can be evaluated as a potential treatment fluid or a component thereof that can be used for treating the rock formation. For example, the potential treatment fluid can include a surfactant or an oxidizing agent. 
         [0006]    According to some embodiments, an observation is made whether substantially all of the disaggregated solid material placed on the surface of the fluid sinks into the fluid. Additionally, the portions of the disaggregated solid material that float on and sink into the fluid can be weighed. 
         [0007]    According to some embodiments, the disaggregating includes grinding and sieving the material through mesh having a size of between about U.S. Standard mesh size 140 and U.S. Standard mesh size 200. 
         [0008]    According to some embodiments, a formation treating fluid for the solid material selected is based in part on the described method. According to some embodiments, a method of enhancing hydrocarbon recovery from a subterranean formation penetrated by a wellbore is provided that includes, providing the selected treatment fluid; and pumping the fluid through the wellbore and into the subterranean rock formation so as to treat the formation. 
         [0009]    According to some embodiments, a method of selecting an appropriate wellbore service fluid for treating a subterranean formation penetrated by a wellbore is provided that includes disaggregating a portion of the subterranean formation to form disaggregated sample material; placing the disaggregated sample material on a surface of each of a plurality of candidate fluids; and selecting a candidate fluid from the plurality based on whether at least a portion of the disaggregated solid material floats on or sinks into the selected candidate fluid. Associated methods and systems for treating a subterranean formation penetrated by a wellbore are provided. According to some embodiments, methods and systems for determining characteristics of a subterranean formation are a provided. 
         [0010]    As used herein the term “shale” refers to mudstones, siltstones, limey mudstones, and/or any fine grain reservoir where the matrix permeability is in the nanodarcy to microdarcy range. 
         [0011]    As used herein the term “gas” means a collection of primarily hydrocarbon molecules without a definite shape or volume that are in more or less random motion, have relatively low density and viscosity, will expand and contract greatly with changes in temperature or pressure, and will diffuse readily, spreading apart in order to homogeneously distribute itself throughout any container. 
         [0012]    As used herein the term “supercritical fluid” means any primarily hydrocarbon substance at a temperature and pressure above its thermodynamic critical point, that can diffuse through solids like a gas and dissolve materials like a liquid, and has no surface tension, as there is no liquid/gas phase boundary. 
         [0013]    As used herein the term “oil” means any naturally occurring, flammable or combustable liquid found in rock formations, typically consisting of mixture of hydrocarbons of various molecular weights plus other organic compounds such as is defined as any hydrocarbon, including for example petroleum, gas, kerogen, paraffins, asphaltenes, and condensate. 
         [0014]    As used herein the term “condensate” means a low-density mixture of primarily hydrocarbon liquids that are present as gaseous components in raw natural gas and condense out of the raw gas when the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    The present disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
           [0016]      FIG. 1  illustrates a system for enhancing recovery of hydrocarbons from a low-permeability hydrocarbon reservoir, according to some embodiments; 
           [0017]      FIG. 2  shows a typical example of a conventional Zisman plot; 
           [0018]      FIG. 3  is flow chart showing steps involved performing a floatation test on disaggregated material to determine wettability characteristics, according to some embodiments; 
           [0019]      FIG. 4  is a plot showing results of a floatation test conducted using a disaggregated sample, according to some embodiments; and 
           [0020]      FIG. 5  is a flow chart showing steps for combining the technique of  FIG. 3  with other experimental configurations, according to some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following description provides exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration of the disclosure. Rather, the following description of the exemplary embodiments will provide those skilled in the art with an enabling description for implementing one or more exemplary embodiments. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope of the invention as set forth in the appended claims. 
         [0022]    Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, systems, processes, and other elements in the invention may be shown as components in block diagram form in order not to obscure the embodiments in unnecessary detail. In other instances, well-known processes, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. Further, like reference numbers and designations in the various drawings indicate like elements. 
         [0023]    Also, it is noted that individual embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process may be terminated when its operations are completed, but could have additional steps not discussed or included in a figure. Furthermore, not all operations in any particularly described process may occur in all embodiments. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function. 
         [0024]    Furthermore, embodiments of the invention may be implemented, at least in part, either manually or automatically. Manual or automatic implementations may be executed, or at least assisted, through the use of machines, hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium. A processor(s) may perform the necessary tasks. 
         [0025]    According to some embodiments, a simple method to measure the extent to which a particular fluid will wet reservoir material is provided. The techniques described allow for good estimates of the wettability of reservoir material to be made. The techniques are straight-forward, inexpensive, and require only small samples from the reservoir, rather than whole cores. Further, the techniques specifically and quantitatively address mixed-wet systems. Finally, the techniques may be used to evaluate the effect of various treatments on formation surfaces of various types, and selective treatment may result. 
         [0026]      FIG. 1  illustrates a system for enhancing recovery of hydrocarbons (in this example gas  100 ) from a low permeability hydrocarbon reservoir  102 , according to some embodiments. The system utilizes a borehole  103  which is formed by drilling through various layers of rock (collectively, overburden  104 ), if any, to the low permeability reservoir  102 . The reservoir  102  is described as a shale reservoir. However, according to some embodiments other types of reservoirs can benefit. For example, according to some embodiments the reservoir  102  is another type of reservoir having low permeability. It is believed that many of the techniques described herein can practically be applied to any reservoirs having low matrix permeability (i.e. between 100 nanodarcies (nD) and 500 mD, where 1 D=9.87×10 −13  m 2 ). According to some embodiments, the reservoir  102  is heterogeneous and/or has mixed wet characteristics. 
         [0027]    The recovery enhancing system of  FIG. 1  includes a fluid storage tank  106 , a pump  108 , a well head  110 , and a gas recovery flowline  112 . The fluid tank  106  contains a treating fluid formulated to promote imbibition in the low permeability reservoir  102 . For example, the treating fluid may be an aqueous solution including surfactants that result in a surface tension adjusted to optimize imbibition based at least in part on determination or indication of the wettability of the shale, permeability of the shale, or both. The treating fluid  114  is transferred from the tank to the borehole using the pump  108 , where the treating fluid comes into contact with the reservoir. The physical characteristics of the treating fluid facilitate migration of the treating fluid into the shale reservoir. In particular, the treating fluid enters the pore space when exposed to the reservoir, e.g., for hours, days, weeks, or longer. Entrance of the treating fluid into the pore space tends to displace gas from the pore space. The displaced gas migrates from a portion of the reservoir  116  to the borehole  103  through the pore space, via the network of natural and/or induced fractures. Within the borehole, the gas moves toward the surface as a result of differential pressure (lower at the surface and higher at the reservoir) and by having a lower density than the treating fluid. The gas is then recovered via the pipe (flowline) at the wellhead. The recovered gas is then transferred directly off site, e.g., via flowline  112 . 
         [0028]    Generally in hydrocarbon recovery from subterranean formations, sample material from a reservoir formation is scarce. Therefore, analysis techniques that make use of only small samples is advantageous. According to some embodiments, sample sizes on the order of 5 g have been found to be sufficient. According to some embodiments, a measurement is made using disaggregated material, and it is understood that grinding of the sample exposes sufficient fresh surface area so as to ensuring that the test fluid is exposed to a surface very representative of that found in the undisturbed reservoir. 
         [0029]    The use of disaggregated material is not new and the method is known to be used to evaluate the properties of extremely low permeability materials. For example, see: Schettler, P. D., Parmely, C. R., Lee, W. J., “Gas Storage and Transport in Devonian Shales” SPE Formation Evaluation, September 1989; Schettler, P. D., Parmely, C. R., “Contributions to Total Storage Capacity in Devonian Shales”, SPE 23422 (1991); and 
         [0030]    Luffel, D. L., Hopkins, C. W., Schettler, P. D., “Matrix Permeability of Gas Productive Shales”, SPE 26633 (1993). 
         [0031]    Properties that can be measured using disaggregated material include permeability, porosity, and adsorption characteristics. As an example, disaggregation provides a way to determine the matrix permeability of highly fractured samples. Shales often exhibit natural fractures—even on the scale of laboratory samples. It has been found that the use of disaggregated materials provides a logical means to isolate the matrix permeability. 
         [0032]    It is believed that the grinding of the core has only minor impact on the surface properties of the material. While the process of grinding alters the reservoir material physically, the fresh surfaces that result from grinding are believed to be quite representative of the chemical nature of the formation in its natural state. Furthermore, the surfaces of samples shaped by drilling or sawing using either oil or water lubricants do not accurately reflect in-situ properties. 
         [0033]    The wettability of a porous medium is of great interest. While reliable test methods have been developed to measure the wettability of a material, the methods are often beyond the capabilities of most field laboratories. According to some embodiments a technique is described that is based on the Zisman plot. A conventional Zisman plot is often prepared by plotting the contact angle formed by a test fluid on the surface of a medium versus the surface tension of the test fluid.  FIG. 2  shows a typical example of a conventional Zisman plot. For further information on Zisman plots, see: Fuerstenau, D. W.; Williams, K. S.; Narayanan, K. S., and Diao, J. L.; “Assessing Oxidation and the Wettability of Coal by a Film Flotation Technique”, ACS Division of Fuel Chemistry Preprints, 32(1) pp 417-424 (1987) (hereinafter “Fuerstenau et al.”), which is incorporated herein by reference. 
         [0034]    The Zisman plot  210  of  FIG. 2  shows that the medium is not strongly water-wetting, but can be wetted by water with the addition of a surfactant. For instance, the contact angle is 80° when the test fluid (water) contains no surfactant, but the contact angle is 0° when the surface tension of the test fluid is lowered to 20 dynes/cm. 
         [0035]    According to conventional practice, the contact angle is measured using a prepared surface and a goniometer. However, this conventional approach is unsuitable for studying typical reservoir specimens given the strong likelihood of contamination and/or alteration during preparation of the sample surface. Furthermore, in the case where a reservoir sample is quite heterogeneous, which is often the case, the results obtained will vary depending on where the drop of test fluid is placed. Accordingly, the use of a conventional goniometer method is best suited for homogeneous, non-porous substrates. 
         [0036]      FIG. 3  is a flow chart showing steps involved in performing a floatation test on disaggregated material to determine wettability characteristics, according to some embodiments. According to some embodiments, in step  310  a formation sample is ground, for example using a SPEXSamplePrep 8000D mixer/mill. In step  312 , the resulting material is dry sieved with 4 inch diameter Stainless Steel Retsch sieves on a Retsch AS200 sieve shaker and the 140/200 mesh size material fraction is retained. It has been found that this mesh size gives a fine powder that is well defined, has a narrow range of particle sizes. According to some embodiments, it has been found that a mesh size of between about U.S. Standard mesh size 140 and U.S. Standard mesh size 200 is suitable for many applications. According to some other embodiments, the suitable mesh size can be determined though a separate test. A floatation test, such shown in  FIG. 3  is carried out for several different decreasing particle size ranges. When the wettability characteristics tend to stabilize, this indicates that the heterogeneity size has been approximately reached, and this is the size range that is suitable. 
         [0037]    In Step  314 , the sample is then dried to constant weight at a temperature not to exceed the static reservoir temperature in order to avoid alteration due to thermal effects. 
         [0038]    In step  316 , the test fluids are prepared by addition of methanol, another miscible low-surface tension solvent, or surfactants at varying concentrations to de-ionized water, brine or other suitable base fluid; as such, the surface tensions of the test fluids will vary. In step  318 , small, pre-weighed quantities of the disaggregated reservoir material are placed onto the surface of the test fluid. In step  320 , the degree of wetting is easily determined by simple observation of the extent to which particles either sink or remain floating on the surface of the test fluid. In step  322 , a more sophisticated approach is carried out, according to some embodiments, wherein the material added to the test solution is separated into wetted (particles that sink) and non-wetted (particles that float) fractions and then weighed so as to allow in step  324  for a quantitative assessment of wettability distribution. For further information on useful test procedures, see: Fuerstenau et al.; and Dang-Vu, T.; Jha, R.; Wu, S.; Tannant, D. D.; Masliyah, J.; and Xu, Z,; “Wettability Determination of Solids Isolated from Oil Sands”, Colloids and Suraces A: Physicochem. Eng. Aspects 337 (2009) 80-90, which is incorporated herein by reference. Note that it may be necessary to observe the tests for some time, as wetting may occur gradually. The following discusses a typical result for a mixed-wet sample. 
         [0039]    As shown in plot  410  of  FIG. 4 , various fractions of the disaggregated material are wetted as the surface tension of the test fluid is varied. Therefore the described method, according to some embodiments, is particularly well suited to the study of heterogeneous, also known as mixed-wet, materials. The test of  FIG. 3 , according to some embodiments, is repeated for multiple fluids having different surface tensions such that a direct measure of the distribution of materials according to surface energy such as shown in curve  420 , as opposed to a conventional goniometer test show in the plot of  FIG. 2 , which provides a global average. According to some embodiments, repeating the test of  FIG. 3 , and building multiple profile curves such as shown in  FIG. 4 , is used in selecting suitable treatment fluids for treating subterranean formations. 
         [0040]    The circles (curve  424 ), the asterisks (curve  426 ) and the triangles (curve  428 ) represent increasing hydrophilicity as evidenced by the lower percentage of floaters in each successive curve. Increased hydrophobicity is indicated by the squares (curve  422 ). 
         [0041]    According to some embodiments, different additives that alter wetting characteristics are tested in order to evaluate the impact of a treatment on the formation. In this example, the plot of curve  420  (diamonds) represents an as-received, unaltered formation sample and the other curves  422 ,  424 ,  426  and  428 , represent differential wetting alteration through the use of certain additives. In this way the impact of a treatment can be discerned. 
         [0042]    According to some embodiments the curve in plot  410  are used to evaluate oil wetting induced by different surfactants. In this example, curve  428  (triangles) represents a virgin water-wet rock. The other curves  422 ,  420 ,  424  and  426  represent the extent of oil-wetting induced by different surfactants. According to an alternate embodiment, curve  422  (squares) represents data that were obtained from an as-received, properly selected sample, this result would be representative of a significantly oil-wet formation. 
         [0043]    According to some embodiments, an additional step  326 , in  FIG. 3 , is provided where the particle size distributions of the various fractions are determined. The particle size distribution information allows for an estimate of total surface area. The surface area and surface energy data may be used to estimate contact angles representative of the various fractions. 
         [0044]    As previously mentioned, the surface chemistry of the naturally occurring shale samples is variable and complex, both in its chemical composition, and in its texture. A given sample may contain microscopic regions that have a distribution of different surface energies, and as such widely different wetting characteristics. It has been found in U.S. patent application Ser. No. 12/914,463 (hereinafter “the &#39;463 patent application”) that the initiation of imbibition is often not a linear function as predicted by the model described in that application. It is possible that the observed induction period of the experiment is the result of a distribution of mixed wetting regions on the surfaces of the particles in the pack, and/or the result of pore size distribution in the material. According to some embodiments, an floatation test analysis method as shown in  FIG. 3  can be used to determine how variable the distributions of surface energies are for a given rock, and how prone they are to alteration. 
         [0045]    According to some other embodiments, the floatation test analysis of  FIG. 3  is amenable to other experimental configurations. By using a series of wetting phase fluids with a range of surface tensions (such as a controlled series of water methanol solutions) more knowledge about the surface energy distributions in complex shale samples can be gained.  FIG. 5  is a flow chart showing steps for combining the technique of  FIG. 3  with other experimental configurations, according to some embodiments. In step  510 , the floatation test as shown in  FIG. 3  is carried out for a number of fluids. In step  512  the results of the floatation tests are used in combination with a conventional slopes analysis which is described more fully in the &#39;463 patent application. In a conventional Washburn slopes analysis, it has been customary to determine the contact angle of an unknown fluid by comparing its performance in an imbibition test with that of a baseline fluid, or calibration fluid, whose contact angle is believed to be known. Often, the assumption is made that the baseline fluid wets the porous solid perfectly; this is equivalent to assuming that the fluid forms a contact angle of 0 degrees. Using the results of the floatation test, a suitable fluid can be selected as this baseline fluid. 
         [0046]    In the &#39;463 patent application, a modified Washburn imbibition measurement of the advancing contact angle is also described. According to some embodiments, in step  514 , the floatation test results from step  510  are combined with the modified Washburn approach described in the &#39;463 patent application. In particular, technique is very useful for characterizing formation material that have contact angles of 90° or greater when exposed to pure water, since in such cases a conventional imbibition test cannot be carried out. 
         [0047]    As can be seen in  FIG. 2 , that lowering the surface tension of the fluid to a value less than 24 dynes/cm provides near perfect wetting. Such a fluid should provide an advancing contact angle of very nearly 0°. According to some embodiments, this result can be compared to the value determined using our method thereby providing further opportunity to calibrate the methods described in the &#39;463 patent application. 
         [0048]    In step  516 , the floatation test results are combined with methods described in commonly-assigned Unites States patent application Entitled “Method to Characterize Underground Formation” filed simultaneously with the present application (hereinafter “the Receding Contact Angle” patent application”), incorporated herein by reference, describes a centrifuge method to measure the drainage contact angle. According to some embodiments, this method of the “Receding Contact Angle” patent application is enhanced by using an analysis method as shown in  FIG. 3 . The equation: 
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         [0000]    shows how one may determine the product of the surface tension and the cosine of the receding contact angle. A series of tests can be performed where the surface tension of the test fluid is varied. Normally, the surface tension of the test fluid will remain unchanged during a test, but a sample can be readily obtained for a measurement of surface tension, thereby allowing for a determination of average contact angle as a function of surface tension. 
         [0049]    It should be noted that although the embodiments have been described with respect to recovery of hydrocarbon from a source formation, according to some embodiments techniques described herein are also applied to a source that is obtained via mining operations, e.g., surface mining or subsurface mining, especially in the case of coal seams (coalbed methane). For example, material obtained from surface mining could be treated with fluid to recover or remove hydrocarbon from the material. 
         [0050]    While the invention is described through the above exemplary embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while the preferred embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures. Accordingly, the invention should not be viewed as limited except by the scope and spirit of the appended claims.