Patent Publication Number: US-8985213-B2

Title: Micro proppants for far field stimulation

Description:
BACKGROUND 
     In certain low permeability formations, such as shale, hydraulic fracturing stimulation is necessary to effectively produce fluids from the formation. A hydraulic fracturing stimulation in shale and similar formations not only forms primary fractures in the near field around the well bore, but also forms induced, dendritic fractures in the far field extending from the primary fractures. These induced, dendritic fractures are generally formed at the tip and edges of the primary fractures, and extend outwardly in a branching tree like manner from the primary fractures. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic of a fracturing system for a well. 
         FIG. 2  is a schematic side view of a well system during a fracture treatment. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     As mentioned above, in certain low permeability formations, hydraulic fracturing stimulation forms primary fractures in the near field around the well bore and induced, dendritic fractures in the far field. The dendritic fractures are generally formed at the tip and edge of the primary fractures, and extend outwardly in a branching tree like manner. Because these secondary, dendritic fractures can extend transversely to the trajectory of the primary fractures, they reach and link natural fractures both in and adjacent to the trajectory of the primary fractures. As such, they reach a larger portion of the naturally occurring fracture network, and link the natural fractures back to the primary fractures and to the well. Shale, coal and many other low permeability formations, for example formations having a permeability of approximately 1 millidarcy (mD) or less, are known to fracture in this manner. 
     The concepts herein encompass propping the induced, dendritic fractures and, in certain instances, the linked natural fractures, to potentially improve recovery from the formation. The induced, dendritic fractures are small. Typical proppants used in hydraulic fracturing stimulation, in the range of 100 to 12 mesh (149-1680 μm), cannot invade the dendritic fractures, and therefore, will not prop or keep the dendritic fractures open when hydraulic pressure from the fracturing treatment is withdrawn. Thus, micro proppants smaller than 100 mesh (149 μm), and in certain instances equal to or smaller than 200 mesh (74 μm), 230 mesh (63 μm) or even 325 mesh (44 μm), are used to prop open these induced, dendritic fractures. In certain instances, the size of the micro proppant can be selected in relation to the size of the dendritic fractures to be propped, such that the particle size is less than the transverse dimension of the dendritic fracture when held open under fracturing pressure. 
       FIG. 1  is one example of a fracture stimulation system  10  in accordance with the concepts herein. In certain instances, the system  10  includes a fracturing gel producing apparatus  20 , a fluid source  30 , a proppant source  40 , and a pump and blender system  50  and resides at a surface well  60  site. In certain instances, the gel producing apparatus  20  combines a gel pre-cursor with fluid (e.g., liquid or substantially liquid) from fluid source  30 , to produce a hydrated fracturing gel that is used as a fracturing fluid. The hydrated fracturing gel can be a gel for ready use in a fracture stimulation treatment of the well  60  or a gel concentrate to which additional fluid is added prior to use in a fracture stimulation of the well  60 . In other instances, the fracturing gel producing apparatus  20  can be omitted and the fracturing fluid sourced directly from the fluid source  30 . In certain instances, the fracturing fluid can include water, a hydrocarbon fluid, a polymer gel, foam, air, wet gases and/or other fluids. 
     The proppant source  40  can include a pre-made proppant for combination with the fracturing fluid and/or, as discussed in more detail below, the proppant source  40  can include a source of proppant pre-cursor. The proppant pre-cursor is a composition that generates the proppant after being combined with the fracturing fluid and/or while downhole (i.e., in the well bore and/or in the fractures of the subterranean zone). In certain instances, the proppant source  40  can additionally include a source of an activator for the proppant pre-cursor that activates the proppant pre-cursor to generate the proppant. 
     The system may also include various other additives  70  to alter the properties of the mixture. For example, the other additives  70  can be included to reduce pumping friction, to reduce or eliminate the mixture&#39;s reaction to the geological formation in which the well is formed, to operate as surfactants and/or to serve other functions. 
     The pump and blender system  50  receives the fracturing fluid and combines it with other components, including proppant or proppant pre-cursor (and in some instances, the activator) from the proppant source  40  and/or additional fluid from the additives  70 . The resulting mixture may be pumped down the well  60  under pressure to fracture stimulate a subterranean zone (i.e., produce fractures), for example to enhance production of resources from the zone. In instances using an activator, the activator can be combined with the proppant pre-cursor at the pump and blender system  50  and/or injected down the well  60  at another time. Notably, in certain instances, different sources of fluids are valved to the pumping and blender system  50  so that the pumping and blender system  50  can source from one, some or all of the difference sources of fluid at a given time. Thus, for example, the pumping and blender system  50  can provide just fracturing fluid into the well at some times, just proppant pre-cursor and/or activator at other times, and combinations of the fluids at yet other times. 
       FIG. 2  shows the well  60  during a fracture treatment of a subterranean zone of interest  102  surrounding a well bore  104 . The subterranean zone  102  can include one or more subterranean formations or a portion of a subterranean formation. 
     The well bore  104  extends from a terranean surface  106 , and the fracturing fluid  108  is applied to the subterranean zone  102  surrounding the horizontal portion of the well bore. Although shown as vertical deviating to horizontal, the well bore  104  may include horizontal, vertical, slant, curved, and other types of well bore geometries and orientations, and the fracturing treatment may be applied to a subterranean zone surrounding any portion of the well bore. The well bore  104  can include a casing  110  that is cemented or otherwise secured to the well bore wall. The well bore  104  can be uncased or include uncased sections. Perforations can be formed in the casing  110  to allow fracturing fluids and/or other materials to flow into the subterranean zone  102 . In cased wells, perforations can be formed using shape charges, a perforating gun, hydrojetting and/or other tools. 
     The well is shown with a work string  112  depending from the surface  106  into the well bore  104 . The pump and blender system  60  is coupled a work string  112  to communicate the fracturing fluid  108  into the well bore  104 . The working string  112  may include coiled tubing, jointed pipe, and/or other structures that communicate fluid through the well bore  104 . The working string  112  can include flow control devices  223  (e.g., bypass valves, ports, and or other tools or well devices) that control a flow of fluid from the interior of the working string  112  into the subterranean zone  102 . For example, the working string  112  may include ports adjacent the well bore wall to communicate the fracturing fluid  108  directly into the subterranean zone  102 , and/or the working string  112  may include ports that are spaced apart from the well bore wall to communicate the fracturing fluid  108  into an annulus in the well bore between the working string  112  and the well bore wall. 
     The working string  112  and/or the well bore  104  includes one or more sets of packers  114  that seal the annulus between the working string  112  and well bore  104  to define an interval of the well bore  104  into which the fracturing fluid  108  will be pumped.  FIG. 2  shows two packers  114 , one defining an uphole boundary of the interval and one defining the downhole end of the interval. 
     The rock matrix of the subterranean zone  102  is of a type that when fractured, produces both a primary fracture  116  in the near field and secondary, induced, dendritic fractures  118  in the far field. The secondary fractures  118  have propagated from or near the ends and edges of the primary fracture  116 . In certain instances, the subterranean zone  102  is a low permeability zone having a permeability of 1 mD or less. For example, the subterranean zone  102  can be shale. In certain instances, the rock matrix of the subterranean zone  102  may include cleating or natural fractures (i.e., those that existed prior to, and were not caused by, a fracture treatment). The natural fractures tend to run generally in a direction that is parallel to the primary fracture  116 . The secondary fractures  118  run in many directions including directions non-parallel and, in certain instances, perpendicular to the direction of the primary fracture  116 . As a result, the secondary fracture  118  can cross, and thereby link, the natural fractures to the primary fracture  116 . 
     The fracturing treatment may be performed in one or more stages, where different amounts, sizes, and/or concentrations of proppant (including micro as well as larger proppant) or, in some stages, no proppant is provided into the fractures  116 ,  118 . For example, in certain instances, the fractures  116 ,  118  can be initiated with a fracturing fluid containing little or no proppant, then subsequent stages can provide the proppant to the fractures  116 ,  118  in a manner that fills and props both the secondary fractures  118  and primary fractures  116  open. Given the small size of the dendritic, secondary fractures  118 , one or more of the stages may introduce a micro proppant such that the particle size is less than the transverse dimension of the fracture when held open under fracturing pressure. In certain instances, the micro proppant is smaller than 100 mesh (149 μm), and in certain instances equal to or smaller than 200 mesh (74 μm), 230 mesh (63 μm) or even 325 mesh (44 μm). The stages provide proppant such that the secondary fractures  118  are propped by the micro proppant. Notably, the proppant is provided into the subterranean zone  102  at a concentration equal to or less than the critical bridging concentration of the micro proppant in the subterranean zone  102 . In certain instances, the stages can additional provide proppant of larger than micro proppant to prop the primary fractures  116 . The stages can be arranged to provide the proppant and micro proppant intermixed and/or some stages can provide substantially just micro proppant and other stages can provide just larger proppant. 
     The proppant source can provide proppant and/or proppant pre-cursor to the fracturing fluid. In the instance of proppant pre-cursor, the proppant can subsequently be generated in the fracturing fluid. For example, the proppant can be generated in the fracturing fluid at the surface and/or in the well bore  104 , and in certain instances, in the primary fractures  116  and/or secondary fractures  118  of the subterranean zone  102 . The proppant can take many forms, as described below. Notably, although many examples of micro proppant are discussed below as capable of being formed downhole, it is also within the concepts herein to pre-form these micro proppants at the surface and provide them as proppant to the fracturing fluid or to form them in the fracturing fluid at the surface prior to pumping the fracturing fluid into the well bore  104 . 
     In certain instances, micro proppant in the form of silicate particulate can be generated downhole (i.e., in the well bore  104  and/or in the fractures of the subterranean zone  102 ) by providing a proppant pre-cursor of organic silicate at neutral pH into the well bore  104  along with the fracturing fluid. In certain instances, the organic silicate can be tetraethylorthosilicate (TEOS) and/or other organic silicates. Once in the well bore  104 , the pH of the fracturing fluid is changed to either basic or acidic to hydrolyze the organic silicate. The pH can be changed by introducing an activator such as by injecting an acid or base fluid into the well bore  104 , by injecting a slow dissolving pH changing material with the fracturing fluid, and/or in another manner. On hydrolysis, the organic silicate will form a gel which will eventually turn into small particles. The concentration of organic silicate in the fracturing fluid drives the particle size, and concentrations can be selected to produce micro proppant. Notably, micro proppant can be generated in this manner in situations where oil is used for the fracturing fluid (e.g. gas wells and/or other types of wells). For example, the organic silicate can be emulsified to form a microemulsion in the oil fracturing fluid. On contacting with formation water and changing the pH, the organic silicate will hydrolyze and will generate micro proppant. 
     In certain instances, micro proppant in the form of alumina particles can be generated downhole by providing a proppant pre-cursor of organic acid aluminoxane into the well bore  104  along with the fracturing fluid. The organic acid aluminoxane will hydrolyze slowly to generate alumina particles as micro proppant. The aluminoxane can be tailored to hydrolyze fast or slow depending on the requirements of the fracture treatment, and can be tailored to promote formation of the micro proppant in the secondary fractures  118 . 
     In certain instances, micro proppant in the form of calcium carbonate (CaCO 3 ) and barium sulfate (BaSO 4 ) can be generated downhole. For example, CaCO 3  can be generated by providing a proppant pre-cursor of calcium oxide (CaO) into the well bore  104  along with the fracturing fluid in a very low concentration, and then additionally and/or subsequently providing an activator of an aqueous fluid containing carbon dioxide (CO 2 ) into the well bore  104 . The CaO will react with water to form Ca(OH) 2  which in turn reacts with the CO 2  to form CaCO 3  and precipitate as micro proppant. To prevent aggregation of particles, surfactant can be added to the fracturing fluid or in connection with the activator. In another example, BaSO 4  can be generated by providing a proppant pre-cursor of barium carbonate (BaCO 3 ) in the fracturing fluid in a very low concentration, and additionally and/or subsequently providing an activator of aqueous sulfuric acid (H 2 SO 4 ) into the well bore  104 . The resulting reaction will form the BaSO 4  which will precipitate as micro proppant suspended in the solution. 
     In certain instances, micro proppant in the form of a polymer can be generated downhole. The micro proppant can be generated by free radical polymerization of a monomer with a cross linker. For example, a monomer along with a crosslinker is emulsified in water and provided as a proppant pre-cursor into the well bore  104  along with the fracturing fluid and/or emulsified directly in the fracturing fluid. The emulsification can be performed with a surfactant. Polymerization of the monomer is initiated downhole by heat of the subterranean zone  102  and/or by an activator that is included in the microemulsion to form micro proppant. 
     In one example, styrene along with small amount (1-3%) of 4-vinylstyrene can be emulsified in water and/or the fracturing fluid with the aid of a surfactant to form a microemulsion. Oil soluble azo-initiators are included in the emulsion to start polymerization of styrene as the temperature increases, such as due to heat of the subterranean zone  102 , to generate micro proppant. The amount of crosslinker in the emulsion determines the hardness, and thus the hardness of the micro proppant can be tailored for various pressure ranges. 
     Another way to form the micro proppant is by forming thermosetting particles downhole. In one example, furfural is emulsified in water and provided as a proppant pre-cursor into the well bore  104  along with the fracturing fluid and/or emulsified directly in the fracturing fluid. The emulsification can be performed with a surfactant. An acid as an activator can be introduced downhole by injecting an acid fluid into the well bore  104 , by injecting a slow dissolving acid generating material with the fracturing fluid or separately, and/or in another manner. The acid will initiate formation of furan resin particles as micro proppant. The introduction of the acid fluid can be delayed or the rate at which the dissolving material forms acid can be selected to delay the reaction to facilitate generating the micro proppant in the secondary fractures  118 . 
     In another example, epoxy resin can be emulsified in water and provided as a proppant pre-cursor into the well bore  104  along with the fracturing fluid and/or emulsified directly in the fracturing fluid. A hardener (e.g., amine and/or another hardener) can also be emulsified in the water or fracturing fluid. The epoxy will harden downhole due to heat from the subterranean zone  102  and form micro proppant. The hardener can be selected based on its rate of reaction to delay the reaction to facilitate generating the micro proppant in the secondary fractures  118 . 
     In certain instances, the micro proppant can be pre-formed, for example, in a manufacturing facility and provided as proppant to the fracturing fluid. The micro proppant can be organic or inorganic in nature and can be synthesized by known methods. In certain instances, organic proppant can be created by spray drying polymeric materials. In certain instances, inorganic proppant can be created in solution by precipitation and/or another method. In one example, fly ash can be used as micro proppant. Notably, the fly ash can be non-reactive or substantially non-reactive to the constituents of the downhole environment. In another example, the micro proppant can be pre-manufactured bubbles or microspheres, such as made from glass, ceramic, polymer and/or another material. 
     In certain instances, the fracturing fluid can contain water and natural and synthetic polymers, where the polymers are selected to deposit in the secondary fractures  118  as micro proppant to harden and behave like particles. The polymers can be tailored to act as micro proppant in the fracture after the fractures have been formed, as well as not substantially degrade with heat or moisture. In one example, the fracturing fluid can contain cellulosic whiskers. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other embodiments are within the scope of the following claims.