Abstract:
During fracking processes, fluid is injected into the injection wells to cause micro-fractures in the shale. Contact between shale and water causes the development of micro-fractures. Given the deep location of the injection wells, the water is under high pressure that can build up over time and could potentially cause tremors. Based upon experiments on Pierre shale, it has been determined that the appearance of micro-fractures in shale begin with the saturation of capillaries, followed by ionic and diffusive transport of water into the shale clays. Using this discovery, a method for predicting the post-fracking pressure build-up in shale is disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/355,363, filed Jun. 28, 2016, titled the “Method of Predicting Post-Fracking Pressure Build-UP in Shale Using Chemo-Physical Modeling.” 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       REFERENCE TO A “SEQUENCE LISTING”, A TABLE, OR COMPUTER PROGRAM 
       [0003]    Not applicable. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0004]    The drawings constitute a part of the specification and include exemplary examples of the METHOD FOR PREDICTING POST-FRACKING PRESSURE BUILD-UP IN SHALE, which may take the form of multiple embodiments. It is to be understood that in some instances, various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. Therefore, the drawings may not be to scale. 
           [0005]      FIG. 1  depicts the first and second hydration shells for a sodium ion. The image demonstrates that there is an existence of an ionic bond (shown by the dashed lines in  FIG. 1 ) between the sodium and oxygen due to the presence of hydration shells. 
           [0006]      FIG. 2  is a plot of the change in Gibbs free energy versus the Hayatdavoudi Hydration Index (“HHI”) for different clay families. The Y-axis (which represents the estimated change in Gibbs free energy in Kcal/mole) has been increased by a scale of 1000. 
           [0007]      FIG. 3  is the plot f the Gibbs free energy versus the HHI, plotted as a function of new HHI values. 
           [0008]      FIG. 4( a )  is a plot of bottomhole pressure versus time for a standard hydraulic fracture. 
           [0009]      FIG. 4( b )  is a plot of bottomhole pressure versus time that demonstrates the post-fracture pressure build-up in Pierre Shale as an effect of Gibbs free energy due to the ionic action indicated on the dotted line. This figure shows the effect of Gibbs free energy as a source of pressure built up in the shale mass that could only come about with the excess oxygen in the clay monolayer. This figure is not to scale. 
           [0010]      FIG. 5  is a diagram depicting the post-fracking build-up of pressure process within the shale. 
           [0011]      FIG. 6  is an example Montmorillonite cell structure. 
           [0012]      FIG. 7  provides a table that shows the excess oxygen, new HHI, and Gibbs free energy for each increasing number of water molecules. 
           [0013]      FIG. 8  provides a chart of the post-fracturing process diagram with additional methods of shale characterization. 
       
    
    
     FIELD OF THE INVENTION 
       [0014]    The subject matter of the present invention generally relates to the field of hydraulic fracturing. More specifically, this invention is related to the determination of internal forces and pressures within the bottomhole of a well after fracking, which can assist fracking professionals in maximizing production. 
       BACKGROUND OF THE INVENTION 
       [0015]    Novel oilfield technologies such as horizontal drilling and hydraulic fracturing have allowed producers to generate a tremendous amount of hydrocarbon from tight, ultra-low permeability source rock such as shale and similar formations. The process of fracking involves the high-pressure injection of fracking fluid—which is typically a mixture of water, sand, and other additives—into a wellbore to create cracks in the deep rock formations. The grains of sand or other hydraulic fracturing proppants hold these micro-fractures open, allowing natural gas, petroleum, and brine to flow more freely. More often than not, the wells begin producing immediately after fracking. 
         [0016]    At the beginning of the well&#39;s production, there is a period of high production rate, also known as “flash production.” After that time, oil and gas production levels rapidly decline. Consequently, tracking the process of the formation of micro-fractures in the shale is important to maximize production. 
       SUMMARY OF THE INVENTION 
       [0017]    Based upon experiments performed on Pierre shale, it has been determined that the appearance of micro-fractures in shale begin with the saturation of the capillaries, followed by ionic and diffusive transport of water into the shale clays. Based on this discovery, a method for predicting the post-fracking pressure build-up in shale, which consequently increases gas and liquid production in post-fractured shale, is disclosed. In this method, the post-fractured shale is saturated with fluid, and then the hydraulic and chemical potential of the fluid and bond dissociation energy of the shale is determined. The bond dissociation energy is then used to find the Gibbs free energy. By then monitoring the additional pore pressure and increase in micro-fractures, and increase in gas or liquid production is obtained from the fracking. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The subject matter of the present invention is described with specificity herein to meet statutory requirements. However, the description itself is not intended to necessarily limit the scope of the claims. Rather, the claimed subject matter might be embodied in other ways to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. 
         [0019]    Furthermore, the described features, structures, or characteristics may be combined in any suitable manner into one or more embodiments. 
         [0020]    When performing fracking, contact between a fluid, such as water, and the shale causes fractures in the shale, often because the shale is hit with fluid at a high pressure. Contact between the shale and fluid results in the development of micro-fractures of a size that is typically possible using standard hydraulic fracturing. Based upon experiments performed on Pierre shale, it has been determined that the appearance of micro-fractures begin in two stages. The first stage is the saturation of capillaries under hydraulic potential. The second stage is by ionic and diffusive transport of fluid into the shale clays. 
         [0021]    During these two stages, the fluid&#39;s properties (pH, Eh, and temperature) are monitored using a pH/ATC (Automatic Temperature Compensation) meter. The activity coefficients of the ions and the saturation indices of various potential minerals in the shale are also calculated using a software package that is known to those having ordinary skill in the art. An X-ray diffraction, SEM/EDS (EDAX), was carried out for the examination of minerals in shale. Methods of performing this X-ray diffraction are also known to those having ordinary skill in the art. Additionally, using an Inductively Coupled Plasma-Mass Spectroscopy (“ICP-MS”), the concentration of ions present in the fracking fluid, the fluid&#39;s conductivity, and the fluid&#39;s resistivity are recorded. 
         [0022]    Once the capillaries in the shale are saturated, micro-fractures are made in the shale as a result of the conversion of ionic activity or exchange to excess pore pressure that did not exist prior to fracking. Contrary to standard fracking mechanisms, the micro-fractures initiated in this manner are not induced through hydraulically induced fracturing. This results in lowering the effective stress in the capillaries of shale mass, thus initiating the onset of micro-fractures at saturated sites. This indicates that the propagation and areal extent of micro-fractures may be influenced by time, which has not previously been considered in existing macro/micro-fracture models. 
         [0023]    Fluid adsorption/absorption in the shale capillaries can now be explained as hydration energy of the clays that are part of shale. This hydration energy can be defined as Gibbs free energy, which is directly proportional to the Hayatdavoudi Hydration Index (“HHI”). Additionally, it has been determined during this testing that the number of oxygen molecules inside of the cell structures of clays as well as those contained in the clay monolayer which contributed to the Gibbs free energy and creation of micro-fractures. The creation of the micro-fractures results in sustained gas and liquid production. 
         [0024]    The water to be used to create the micro-fractures in shale is dependent upon the type of clay distribution, the clay cell structure, the type of ions inside the cells, their concentration, activity coefficients of those ions and saturation indices of their potential minerals.  FIG. 2  provides a plot representation of the change in Gibbs free energy versus the HHI for different clay families where fracking is often applied. These values are also plotted in  FIG. 3 , which shows the plots of Gibbs free energy versus the HHI instead plotted as a function of the new HHI values. 
         [0025]      FIGS. 4( a ) and 4( b )  shows the effect of Gibbs free energy as the source of pressure build up in the shale mass which could only have come about with the excess oxygen in the clay monolayer.  FIG. 4( a )  provides the plot of the bottomhole pressure in the shale mass for a standard hydraulic fracture.  FIG. 4( b )  provides the plot of the bottomhole pressure versus time that demonstrates the post-fracture pressure build-up in Pierre Shale as an effect of Gibbs free energy due to the ionic action indicated on the dotted line. This figure shows the effect of Gibbs free energy as a source of pressure built up in the shale mass that could only come about with the excess oxygen in the clay monolayer. 
         [0026]    Energy is the most fundamental characteristic of any system. In order to account for the excess oxygen in the clay monolayer, it is important to consider the bond dissociation energies (“BDE”) of the clay structure. An example cell structure of montmorillonite is shown in  FIG. 6 , which is known in the art. The C-axis spacing is defined as the distance between the Aluminum atoms of two unit cells. To estimate the excess oxygen contributed to the monolayer by water contained in the clays, it is important to consider both cell units: 
         [0000]      Excess  O =((BDE) Na—O )cell structure/BDE) Si—O    
         [0027]    As shown in  FIG. 1 , the number of water molecules in the first and second hydration shells for a sodium ion range from 4 to 8. Due to the hydration shells for both unit cells, sodium forms an ionic bond with the oxygen from the water, thus giving rise to excess oxygen into the main cell structure. Consequently, as represented in the below equation, the previous formula for excess oxygen can be rewritten in the following new form: 
         [0000]      Excess  O =2(BDE Na—O)     cell structure   )+(BDE Na—O ) monolayer water ))/(BDE Si—O ) 
         [0000]    Accordingly, for one molecule of water in the monolayer: 
         [0000]      Excess  O =2(64.5+64.5)/191.1 
         [0000]      Excess O=1.351 
         [0000]    In the above calculation, 64.5 kcal/mol is the bond dissociation energy of Na—O bond and 191.1 kcal/mol is the bond dissociation energy of the Si—O bond. 
         [0028]    For Smectite—the original HHI was O/OH=20/4=5. However, the new HHI is equal to (original oxygen+excess oxygen)/OH=(20+1.351)/4=5.337. The Gibbs free energy with the new formula, considering excess oxygen: 
         [0000]      Δ G=R Tln (HHI)= R Tln (O/OH)
 
         [0000]      Δ G=[ 1.987×10 −3 (kcal/mol.K)]×[298.13 K]×ln(5.337)
 
         [0000]      ΔG=0.992 kcal/mol
 
         [0029]      FIG. 7  provides a table that shows the excess oxygen, new HHI, and Gibbs free energy for each increasing number of water molecules in Smectite. The Gibbs free energy can be adjusted using pure freshwater without additives all the way to adjusted formulation of additives, given that there must always be a differential ionic distribution, no matter how small, between the fracturing water and shale water to induce ion exchange and the propagation of micro fractures. 
         [0030]    The post-fracturing process then leads to the pressure build up due to chemical potential according to the process chart seen in  FIG. 5 . 
         [0031]    Consequently, a method for increasing the gas or liquid production is disclosed. The user would first saturate the capillaries in the post-fractured shale with fluid. In the preferred embodiment, that fluid is water. In additional embodiments, that fluid comprises water and additional additives, such as sand. After determining the hydraulic and chemical potentials of the fluid and the bond dissociation energy of the post-fractured shale to determine the Gibbs free energy, the initiation of additional pore pressure in the saturated post-fractured shale capillaries can be monitored. 
         [0032]    As pore pressure increases in the capillaries, additional micro-fractures are created in the post-fractured shale, which allows increased gas or liquid to escape the shale and be collected by the user.