Abstract:
A method of determining the suitability of a surfactant for use in a formation can include sampling water in the formation, providing at least two surfactants, and mixing each of the surfactants with the formation water to form surfactant/water samples. The method can further include determining the solubility of each surfactant with the formation water, comparing the solubility of each surfactant with the other surfactant, and assigning a solubility performance value for each surfactant based on its solubility in the formation water sample compared to the other surfactant.

Description:
BACKGROUND 
     1. Field 
     The disclosure relates generally to the field of fracturing fluids used in fracturing subterranean formations during hydrocarbon recovery. More specifically the disclosure relates to methods for selecting surfactants used in fracturing fluids. 
     2. Background Art 
     Hydraulic fracturing is a formation stimulation technique used to create additional permeability in a producing formation to increase the flow of hydrocarbons toward the wellbore. Typically, during a hydraulic fracturing operation, a high hydraulic pressure is used to fracture the subterranean formation, creating cracks that facilitate the increased flow of hydrocarbons. Often, proppants are used to keep cracks open that are created during the fracturing operation. 
     Fracturing fluids include a number of components and are most often water-based. These components typically include acids, biocides, breakers, corrosion inhibitors, friction reducers, gels, iron control chemicals, oxygen scavengers, surfactants and scale inhibitors. 
     Conventional selection for selecting a surfactant typically focuses on one or two attributes of the surfactant. In particular for unconventional oil and gas plays, efficacy of the surfactant chosen for hydraulic fracturing depends on a number of factors, including formation characteristics, oil types, reservoir temperature, and the other elements of the fracturing fluid. 
     What is needed is a method of determining the efficacy of a surfactant for a fracturing fluid for a particular use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying FIGS. It is emphasized that, in accordance with the stand practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily reduced for clarity of discussion. 
         FIG. 1  is an example of a model of dynamic surface tension measurements as a function of time. according to one or more aspects of the present disclosure; 
         FIG. 2  is a graph of diffusion coefficients of various surfactants in accordance with Example 2. 
         FIG. 3  is a graph comparing the phase separation rates of a non-emulsifying surfactant with a weakly emulsifying surfactant in accordance with Example 3. 
         FIG. 4  illustrates the droplet size distributions of a non-emulsifying surfactant with a weakly emulsifying surfactant in accordance with Example 3. 
         FIG. 5  is a production table illustrating the field example. 
     
    
    
     SUMMARY 
     The methods described herein relate generally to the field of gas and oil production. In particular, methods of selecting surfactants for fracturing fluids are described. 
     In one embodiment of the present disclosure. 
     DETAILED DESCRIPTION 
     The disclosure below is not limited to the embodiments, versions or examples described, which are included to enable a person having ordinary skill in the art to make and use the disclosed subject matter when the information in this patent is combined with available information and technology. 
     Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition skilled persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing. Further, unless otherwise specified, all compounds described herein may be substituted or unsubstituted and the listing of compounds includes derivatives thereof. 
     Further, various ranges and/or numerical limitations may be expressly stated below. It should be recognized that unless stated otherwise, it is intended that endpoints are to be interchangeable. Further, any ranges include iterative ranges of like magnitude falling within the expressly stated ranges or limitations. For example, if the detailed description recites a range of from 1 to 5, that range includes all iterative ranges within that range including, for instance, 1.3-2.7 or 4.9-4.95. 
     The present disclosure describes a number of tests that may be performed to select a particular surfactant for a fracturing fluid. In one embodiment, all of the tests are used. In other embodiments, select tests may be performed. These tests may be performed in any order and the order described below is non-limiting. 
     The tests include: 
     1. Water solubility—A surfactant may be tested to if that surfactant is soluble in water. A water solubility test may assist in selecting a surfactant to determine if that surfactant is able to travel with the leading edge of a water front and reach the interior of the rock formation; 
     2. Emulsion tendency—A visual inspected of oil and water containing surfactants may be performed and emulsion droplet size and zeta potential may be determined to understand the tendency of a surfactant to create aweak or transient oil in water emulsion; 
     3. Interfacial surface tension measurements between hydrocarbon and surfactant solutions. 
     4. Wettability-spontaneous imbibitions of surfactants into the rock formation 
     5. Oil recovery—In the oil recovery test, crushed formation cores or drill cuttings may be saturated with crude oil from the same formation and the surfactant solution is passed through the cores that are packed in a glass column. Effluents are collected and oil recovery by individual surfactants may be quantified. 
     6. Adsorption to proppants—during fracturing operations, some surfactant molecules can be adsorbed onto the proppant surface and never reach the interior of the reservoir. This test is to quantify how much surfactant molecules are lost to proppants. 
     Each of the tests above will now be specified in greater detail. 
     1. Water Solubility 
     Surfactants that are soluble or dispersible in water may more easily reach the interior of the formation. Because of the surface tension gradient or the Gibbs-Marangoni effect, where surfactants diffuse from the areas of low surface tensions to those of high surface tensions, surfactants can remain at the tip of the water front and further penetrate the formation. In this test, fresh or source water from the formation is typically used and different concentrations of the chosen surfactant is added to the water. If the surfactant is soluble or dispersible, the water typically remains clear or slightly cloudy. If insoluble, the water typically turns turbid or opaque. In certain cases where the surfactant is insoluble, precipitates may be found. In addition to the traditional hydrophile-lipophile balance numbers (HLB) that may be used for water free of high concentrations of divalent ions (hard water) and salts (brine), where surfactants having a HLB less than 4 remain insoluble, a turbidity meter may be used to monitor the cloudiness or turbidity of the solutions. 
     2. Emulsion Tendency 
     Shale plays often have low porosity and ultra-low permeability. In some situations, the permeability may be in the nanodarcies or millidarcies ranges. Consequently, the flow path for oil molecules to migrate from the interior of the reservoirs to the artificial propped fractures created in a hydraulic fracturing process may be confined and/or limited. In certain embodiments, a surfactant may be needed to minimize the formation damage induced by large quantities of water and enhance the oil and gas production. Traditionally, a non-emulsifying surfactant is used so that less oil/water emulsion is generated. However, in one embodiment of the present disclosure, a weak emulsifying surfactant may be used to enhance the formation production. 
     In one embodiment of the present disclosure, an emulsification test is used to quantify the phase separation rates and emulsion droplet size distribution by monitoring the emulsion with dynamic light scattering measurements. Surfactants may then be screened to remove surfactants that may separate too quickly, have a droplet size larger than 10 microns and a loose distribution, resulting in possible poor field production. 
     3. Interfacial Surface Tension 
     Dynamic and static surface tension are two physical properties of the surfactants that typically determine the surface tension between air/gas and surfactant solutions. Whether it is at air/liquid or solid/liquid interface, surfactants travel to the interface from the bulk of the solution. The speed with which the surfactants travel plays a significant role in processes where new interfaces are generated. 
     Dynamic surface tension measurements may record surface tension reduction as a function of time. Dynamic surface tension may relate to processes such as foaming, bubble dynamics, solubilization and detergency, emulsion droplet size and thin film stability. Without being bound by theory, it is believed that as time elapses, there is sufficient time available for more surfactant molecules to travel to and accumulate at the interface. Those molecules may pack tightly at the interface and hence lower the surface tensions between two immiscible phases. A typical measurement is illustrated in  FIG. 1  by using a bubble pressure tensiometer such as Kruss BP100. An additional measurement related to this effect may be whether the movement of surfactants to the interface is dominated by diffusion. A characteristic time may be determined and correlated back to the diffusion coefficient of the surfactant. 
     Lowering interfacial surface tensions (IFT) between surfactant solutions and crude oil or condensate allows mobilization of the oil globules inside the pore space. IFT is typically directly proportional to emulsion droplet size, i.e. the lower IFT, the lower the emulsion droplet size. The oil emulsion droplets typically must be preferably less than 10 microns that they can escape from the tiny pore space in the shale formation. IFT is measured by using a ring or plate method with Kruss K100. To qualify a surfactant, it must lower IFT to preferably lower than 20 mN/m 
     4. Wettability/Capillary Pressure 
     During a hydraulic fracturing operation, millions of gallons of water may be pumped into the shale formation. Because of the ultralow permeability and nanometer-sized pores in the shale, water tends to display high capillary pressure and imbibe into the pores. If the formation pressure is lower than the capillary pressure of invaded water, the water can get stuck, plug the pores and the oil or gas cannot flow out when the well is put on production. In the presence of surfactants, the high capillary pressure of invaded water may be reduced and the water can be readily returned together with oil and gas, thereby reducing formation damage/plugging and enhancing production. 
     5. Oil Recovery 
     Shale core plugs obtained from thousands of feet below the ground are typically of ultralow permeability and contain oil globules that are deeply trapped inside the pore space in the formation. It may not be feasible to pump the surfactant solution directly through a shale core plug because large differential pressure is required. In addition, the oil recovered from shale core plugs is typically so little that the results are not reproducible. With the oil recovery test of the present disclosure, it is possible to differentiate the oil extraction capabilities by various surfactants and surfactant blends. 
     Adsorption to Proppants 
     Proppants including sand or ceramic are usually pumped together with surfactants. Some surfactants tend to adsorb onto the proppant surface and hence do not reach the interior of formation. The adsorbed surfactants may be considered to be lost and might not contribute to oil and gas production. The adsorption of surfactants onto the proppants can be evaluated to account for the loss. The loss to proppants can serve as a reference for comparing various surfactants. 
     EXAMPLES 
     Example 1 
     Water Solubility 
     99.9 gram of source water was poured into a glass bottle. 0.1 grams of a linear ethoxyated alcohol surfactant was added to the source water. The bottle was shaken by hand by hand for 30 seconds and the solution was allowed to stand overnight. A turbidity meter was used to measure the turbidity of the solution. If the value is higher than 20%, preferably 40%, then the surfactant is verified to be soluble or dispersible. 
     Example 2 
     Dynamic Surface Tension 
     100 grams of 1000 parts per million (ppm) of the surfactant in source water was prepared to form a surfactant solution. 70 grams of surfactant solution was added to the measuring container in a bubble pressure tensiometer (e.g. Kruss BP100) 
     Start the measurement and record the surface tension as a function of surface age time 
     The characteristic time τd and the equilibrium surface tension γeq were determined from the data fitting of the curves, following the equation below: 
     
       
         
           
             
               Δγ 
               ⁡ 
               
                 ( 
                 
                   t 
                   ⪢ 
                   
                     τ 
                     d 
                   
                 
                 ) 
               
             
             ≅ 
             
               
                 Δγ 
                 eq 
               
               ⁡ 
               
                 ( 
                 
                   1 
                   - 
                   
                     
                       
                         τ 
                         d 
                       
                       t 
                     
                   
                 
                 ) 
               
             
           
         
       
     
     The diffusion coefficient was determined from the molecular size a and the volume fraction φ b  of the surfactant: 
     
       
         
           
             D 
             ≅ 
             
               
                 1 
                 
                   ϕ 
                   b 
                   2 
                 
               
               ⁢ 
               
                 
                   a 
                   z 
                 
                 
                   πτ 
                   d 
                 
               
             
           
         
       
     
     The diffusion coefficients of various surfactants were compared under different field conditions and the faster surfactants are selected as shown in  FIG. 2 . 
     Example 3 
     Emulsion Tendency 
     Equal volumes of 1000 ppm surfactant solution and crude oil were combined in a quartz tube (note that condensate may also be used). The tube was shaken with a mechanical shaker. A high speed blender may also be used. The tube was immediately placed in a dynamic scattering device such as LumiSizer or Turbiscan. The data was collected for two hours. The phase separation rates or instability index of emulsions were then calculated from the slopes of the curves by
 
ΔTransmission or BackScattering= f (Time).
 
     Those values are chosen as a reference to compare the efficiency of surfactants.  FIG. 3  compares the phase separation rates of a non-emulsifying surfactant (surf  1 - a  linear ethoxylated alcohol) and a weakly emulsifying one (surf  2 - a  linear ethoxylated sulfate).  FIG. 4  illustrates the droplet size distributions between surf  1  and surf 2 . 
     Example 4 
     Oil Recovery 
     Shale core plugs from different depths of the wells were crushed to 80˜100 mesh or 149˜177 microns to expose the large surfaces in the shale. The crushed core was then saturated with the crude oil from the production well at the formation temperature for an extended period of time. The saturated core was then filtered and dried in a thermal oven 
     The saturated core was packed into a glass column and a surfactant solution of 1000 ppm is pumped through the column a few times at a fixed flow rate. The effluent was collected at the exit of the column and the oil recovery was calculated for each pass, by using infrared spectroscopy. As shown in Table 2, surf  2  has superior oil extraction capability than surf 1 . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Oil Recovery, 
                   
                   
                 Third 
                   
                 Residual 
               
               
                 % 
                 First Pass 
                 Second Pass 
                 Pass 
                 Fourth Pass 
                 Oil 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Surf1 
                 5 
                 5 
                 3 
                 4 
                 83 
               
               
                 Surf2 
                 10 
                 20 
                 8 
                 12 
                 50 
               
               
                   
               
             
          
         
       
     
     Example 5 
     Capillary Pressure 
     3 grams of crushed shale core were loaded into a powder cell and connected to a force transducer. The powder cell was slowly brought to contact to a surfactant solution of 1000 ppm. The weight gain of the powder cell is recorded as a function of time. The square of weight gain is plotted against the time as illustrated by  FIG. 5 . The slopes of the plots are used to compare the capillary pressure. Typically, the smaller the slopes are, the lower the capillary pressure. It is evident that surf 2  enables lower capillary pressure than surf 1 . 
     Example 6 
     Adsorption to Proppants 
     10 g 100 mesh proppants were added to 100 grams of 1000 ppm surfactant solutions. The solutions were shaken in a mechanical shaker and heated at the formation temperature for two hours. The surfactant solutions containing the proppants were filtered and the proppants removed. The residual surfactant amount is determined by either surface tension, titration or UV-Vis spectroscopy. Surface tension measurement is preferred. The surface tension is directly proportional to the surfactant residual. A higher surface tension corresponds to a lower residual. The surfactant with the lowest surface tension and thereby the highest residual amount was selected. 
     Example 7 
     Performance Index 
     The above-mentioned tests need not be performed in the same order. A performance index can be assigned to a surfactant, based on the scores from each single test, 10 being the best and 1 being the poorest, respectively. Typically, a surfactant is selected and recommended for field applications if its performance index exceeds 35. The performance indices for surf 1  and surf  2  are indicated below. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Properties 
                 Surf1 
                 Surf2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Water solubility 
                 10 
                 10 
               
               
                 Dynamic surface tension 
                 5 
                 8 
               
               
                 Interfacial surface tension 
                 6 
                 8 
               
               
                 Emulsion tendency 
                 1 
                 8 
               
               
                 Oil recovery 
                 4 
                 8 
               
               
                 Capillar pressure 
                 5 
                 7 
               
               
                 Adsorption to proppants 
                 4 
                 6 
               
               
                 Total score 
                 35 
                 55 
               
               
                   
               
             
          
         
       
     
     Field Example 
     A surfactant (surf  2 ) selected from the above-mentioned test matrix was evaluated in a shale formation in South Texas. Initial results shown in  FIG. 5  after the first 45 days suggest that the selected surfactant has increased the oil production by 25%, gas production by 50%, as compared to other wells using the previous surfactant.