Patent Publication Number: US-2016222751-A1

Title: Self-Lubricating Seal Element for Rotating Control Device

Description:
TECHNICAL FIELD 
     This disclosure relates generally to a seal element for a rotating control device (RCD) used in rotary drilling systems, and particularly to a self-lubricating seal element for the RCD. 
     BACKGROUND 
     During drilling, an earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated to form a wellbore by rotating the drill bit, such as by rotating the drill string and/or rotating the drill bit relative to the drill string using a downhole motor. During this process erratic pressures and uncontrolled flow known as formation “kick” pressure surges can emanate from a well reservoir, potentially causing a catastrophic blowout. Because formation kicks are unpredictable and would otherwise result in disaster, flow control devices known as blowout preventers (“BOPs”) are required on most wells drilled today. BOPs are often installed redundantly in stacks, and are used to seal, control and monitor oil and gas wells. 
     One common type of BOP is an annular blowout preventer. Annular BOPs are configured to seal the annular space between the drill string and the wellbore annulus. Annular BOPs are typically generally toroidal in shape and are configured to seal around a variety of drill string sizes, or alternatively around non-cylindrical objects such as a polygon-shaped Kelly drive. Drill strings formed of drill pipes connected by larger-diameter connectors can be threaded through an annular BOP. Annular BOPs are designed to maintain a seal around a stationary drill string. Rotating the drill string through an annular BOP would rapidly wear it out, causing the annular BOP to be less capable of sealing the well. 
     Closed annulus drilling operations include managed pressure drilling, underbalanced drilling, mud cap drilling, air drilling and mist drilling. A rotating control device (RCD), which may alternatively be referred to as a rotating drilling device, rotating drilling head, rotating flow diverter, pressure control device and rotating annular, may be located on top of the BOP stack, and is used to close the annulus while allowing rotation and reciprocation of the drill string in above hydrostatic pressure conditions within the closed annulus. During this type of drilling the wellbore/closed annulus is held at pressures that are well above atmospheric. The RCD forms a seal between the wellbore and the drill pipe so that the drill string can move vertically and rotationally without the loss of well pressure while continuing with all normal subterranean drilling operations including but not limited to drilling ahead, reaming, back reaming, tripping drill pipe, stripping drill pipe, rotating drill pipe and sliding drill pipe. 
     The key component in the RCD, which allows for the separation of high and low pressure regions, is the RCD seal element. The RCD seal element is comprised of a core and an elastomeric body. The core is molded into the upstream end of the elastomeric body and is used to fasten the element to the RCD. Cores can be made in many shapes and sizes and fabricated from many materials. For example, an RCD core can be made from steel and is referred to as a cage. 
     A drill string of varying diameter is passed through the center of an RCD seal element. RCD seal elements are currently made so that the inside diameter of the RCD seal element is smaller than the smallest outside diameter of any part of the drill string passing through it for the wellbore section to be drilled. As the various parts of the drill string move longitudinally through the interior of the stripper rubber, a seal is continuously maintained. 
     RCD seal elements seal around rough and irregular surfaces such as those found on a drill string and are subjected to conditions where strength and resistance to wear are very important characteristics. However, RCD seal elements often have a short life expectancy, especially when they are used in wells that have high wellbore and/or applied annulus pressures. Loads exerted on the outside of the element body by the high pressure region of the well cause the element to deform and press against the drill pipe. High frictional loads result from the pipe being stripped through the element as it is deformed against the drill pipe. High pressures in the well can accelerate RCD seal element failure. Common modes of RCD seal element failure include side wall blow through, vertical and horizontal cracking and chunking away of the interior region of the seal element body also known as “nibbing”. 
     Conventional prior art seal elements in rotating control devices (RCDs) tend to split or experience chunking when encountering harsh loading conditions due to poor tear resistance. Further, over time the seal element may become worn and may become unable to substantially deform to provide a seal around the drill string. Consequently, the seal element must be replaced, which may lead to down time during drilling operations that can be costly to a drilling operator. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. 
         FIG. 1  is an example drilling system with a rotating control device (RCD). 
         FIG. 2  is a partial cross-sectional view of an example RCD with dual seal elements. 
         FIG. 3A  is a cross sectional view of an example RCD with a single seal element;  FIG. 3B  is a cross sectional view of the example RCD of  FIG. 3A  without housing; and  FIG. 3C  is a side view of the example seal element of  FIG. 3B . 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to an example seal element for a rotating control device (RCD). The seal element has self-lubricating properties and can create a seal between the drill pipe passed through the RCD and the interior of the wellbore below the RCD. In some embodiments, a lubrication medium can be provided to the seal element or packer/drill pipe interface by the incorporation of lubricating component additives such as, but not limited to, polarized graphite, to be embedded in the seal element or packer at the molding stage of the manufacturing process. The lubricating components can be subsequently released in operation as the interface is worn to reduce the coefficient of friction between the seal element or packer and drill pipe (or other tubular) thus reducing seal element or packer wear and providing extended operable life. As a result, drilling operations can be extended with reduced seal element degradation. Decreased seal element or packer wear leads to greater operational efficiency on site. With reduced wear, seal elements or packers are replaced less frequently thus saving considerable drilling rig lost time for replacing worn seal elements or packers. 
     This disclosure also relates to a method of improving the material properties of the elastomeric RCD seal element by introducing a self-lubricating material into the elastomer. In some implementations, the self-lubricating concept focuses on inclusion of solid-state lubricants into the elastomer formulation. As a RCD elastomer seal element undergoes wear during normal operations, it would be advantageous to have solid state lubricants incorporated into the material that would be deployed continuously in small doses as wear occurs. During the preparation of the elastomer raw material, self-lubricating components can be added so that the performance characteristics of the finished element are altered. RCD with self-lubricating seal elements can have reduced friction coefficient, improved resistance to wear and extended elongation. 
     Often RCD seal element life is short which can result in frequent element replacement during drilling operations. It is well-known that rig time can be very expensive, especially when drilling operations are performed in deep water. Typical deep water daily rig costs can range between $400,000 and $900,000 a day. If an RCD seal element can last for drilling a complete borehole section, the approximate two hours rig time for an element change out equates to a rig downtime saving of $33,000 to $75,000. Improving element life with an element with improved life and durability according to this disclosure will reduce to costs. This cost saving will be achieved by fewer elements being required to complete an operation, as well as saving in much more costly rig down time. Improving element life will also result in a reduction of nonproductive time for the rig since the rig must be shut down each time an element is changed out. 
       FIG. 1  illustrates an example drilling system configured to perform closed annulus drilling operations. During closed annulus drilling operations, also referred to as managed pressure drilling, underbalanced drilling, mud cap drilling, air drilling and mist drilling, the annulus of the drill string is closed off using a device referred to as a rotating control device (RCD), also commonly known as a rotating drilling device, a rotating drilling head, a rotating flow diverter, pressure control device or a rotating annular. The principle sealing mechanism of the RCD, referred to as a seal element (or a packer, stripper element, or stripper rubber), seals around the drill string, thus, closing the annulus around the drill string. During drilling operations, the seal element may experience wear that degrades the seal provided by the seal element. In order to minimize costly down time for the drilling system when replacing the seal element, lubricating components may be added in the seal element to lubricate the seal element and reduce wear, degradation and vibration associated with the seal element. 
     Drilling system  100  may include drilling unit  102 , drill string  104 , rotating control device (RCD)  106 , sliding joint  108 , and riser assembly  110 . Drilling unit  102  may be any type of drilling system configured to perform drilling operations. Although  FIG. 1  illustrates the use of RCD  106  from a floating drilling unit, those skilled in the art will understand that RCD  106  can be deployed from any type of onshore or offshore drilling unit including, but not limited to, semi submersible, drill ship, jack up, production platform, tension leg platform and land drilling units. In some implementations, including, but not limited to, land drilling units and jack up drilling units, a surface blowout preventer (BOP) stack may be incorporated into the drilling system. In these embodiments, RCD  106  may be coupled to a drilling annular incorporated in the BOP stack, an operations annular added to the BOP stack and drilling annular, or directly coupled to the BOP stack. In other implementations, RCD  106  may be coupled directly to a wellhead or casing head for drilling operations prior to the BOP stack being installed. 
     Drilling unit  102  may include rig floor  112  that is supported by several support structures (not expressly shown). Rotary table  114  may be located above rig floor  112  and may be coupled to drill string  104  in order to facilitate the drilling of a wellbore using a drill bit (not expressly shown) coupled to the opposite end of drill string  104 . Drill string  104  may include several sections of tubular members with connecters at each end (e.g. drill pipe with connectors known in the art as “tool joints”) that communicate drilling fluid from drilling unit  102  and provide torque to the drill bit. 
     In the illustrated example, the drilling fluid may be circulated back to drilling unit  102  through riser assembly  110 . In other implementations, such as a land drilling unit, the drilling fluid may be circulated through the wellbore or a casing included in the wellbore. Additionally, various cables  116  may couple RCD  106 , slip joint  108 , and riser assembly  110  to equipment on drilling unit  102 . 
     In the illustrated example, drill string  104  may extend from drilling unit  102  through riser assembly  110  and into a subsea wellbore (not expressly shown) formed in the ocean floor. An upper portion of RCD  106  may be coupled to drilling unit  102  by an above RCD riser, tie back riser or telescoping joint, where the upper end of the riser or joint may be coupled to a drilling unit diverter housing (not expressly shown). A seal element or packer (not expressly shown) may be located within the body of RCD  106  and may be removed or inserted with the aid of latch assembly  103  integral, either internally or externally, to RCD  106 . In some implementations, latch assembly  103  may include a hydraulic clamp that can be remotely controlled from drilling unit  102 . A lower portion of RCD  106  may be coupled to sliding joint  108 . In one example implementation, sliding joint  108  may be a telescoping joint that includes an inner barrel and an outer barrel that move relative to each other in order to allow offshore platform  102  to move during drilling operations without breaking drill string  104  and/or riser assembly  110 . In other implementations, sliding joint  108  may be a multi-part sliding joint. Sliding joint  108  may be coupled to riser assembly  110 , which provides a temporary extension of a subsea wellbore (not expressly shown) to offshore drilling unit  102 . 
       FIG. 2  illustrates a partial cross-sectional view of the example RCD  106  in  FIG. 1 . RCD  106  may be used to seal annulus  202  formed radially between body  204  of RCD  106  and drill string  104  positioned within body  204 . RCD  106  may allow drill string  104  to rotate and enter and exit the wellbore while maintaining pressure in annulus  202 . In the illustrated example, bearing assembly  206  may be located in bearing assembly housing  208 . Seal element  210  may be positioned within body  204  of RCD  106  by a mandrel (not expressly shown) connected to bearing assembly  206  such that seal element  210  may rotate with drill string  104 . In other implementations, RCD  106  may not include bearing assembly  206  such that seal element  210  remains stationary while drill string  104  rotates within RCD  106 . Latch assembly  103  may be used to secure and release bearing assembly  206  and seal element  210  relative to body  204 . 
     Seal element  210  may form a seal around drill string  104  (e.g., drill pipe and tool joints) to close annulus  202  and maintain pressure in annulus  202  during drilling operations. In the illustrated example of  FIG. 2 , RCD  106  includes dual seal elements  210 . The two seal elements  210  can have the same size, configuration, or property; or the two seal elements  210  can be different. For example, one or both of the seal elements be a self-lubricating seal element that includes self-lubricating components in the seal material. The two seal elements  210  may include the same type of self-lubricating components with the same concentration, or the two seal elements  210  can include different self-lubricating components with different concentrations. The self-lubricating components can be added into the seal elements  210  based on specific applications or system requirements to optimize the performance and operable life of the whole RCD  106 . 
       FIG. 3A  is a cross section view of another example RCD  300 . RCD  300  can be used as the example RCD  106  in  FIGS. 1 and 2  or RCD  300  can be used in another manner. While the example RCD  106  in  FIG. 2  includes dual seal elements  210 , RCD  300  includes a single seal element  305 . Seal element  305  is located within the body or housing  304  of RCD  300 . Latch assembly  360  (e.g., a hydraulic clamp) secures RCD seal element  305  inside the housing  304  and facilitates installation, removal, or replacement of seal element  305 . RCD seal element  305  acts as a passive seal that maintains a constant barrier between the atmosphere above and wellbore below. Drill string  310  extends from a drilling rig (not shown) through the seal element  305  and into the wellbore (not shown) RCD seal element  305  seals around the drill string  310  (or other tubular used to convey a drilling, or completion, or well fracturing, or other Bottom Hole Assembly (BHA)) thus “closing” the annulus. In some implementations, the RCD seal element rotates with the drill string, and in some other implementations, the RCD seal element remains stationary while the drill string rotates within. 
     A drill string typically includes multiple tubular members commonly known as joints of drill pipe connected by threaded connections located on both ends of the drill pipes. Although the threaded connections (referred to in the art as “tool joints”) may be flush with outer diameter of the drill pipes, they generally have a wider outer diameter. As illustrated, drill string  310  is formed of a long string of threaded pipes  303  joined together with tool joints  315 . Drill string  310  can pass through seal element  305  with rotation, reciprocation, or both. In some implementations, more reciprocation can be involved during drill operations than rotation. Seal element  305  can accommodate the change of the outer diameter of drill string  310 , for example, via expansion and relaxation. For example, as shown in  FIG. 3A , there is spacing  365  between the outer surface of seal element  305  and the inner surface of RCD body  304  and the seal element can expand outwards to let through tool joints  315  of a wider outer diameter. A seal element can accommodate both a rotating drill sting and a non-rotating drill string with tool joint drill string through the bore of the seal element. In the illustrated example in  FIG. 3A , drill pipe  303  is passing through the bore in seal element  305  while tool joint  315  is about to pass through seal element  305 . While much of this description has discussed drill strings with drilling BHA&#39;s being run through RCD&#39;s those skilled in the art will recognize that other types of strings may be run under closed annulus pressure and be sealed against by the RCD and its various types of sealing elements. Other types of string include but are not limited to completion strings containing production tubing and completion devices, well fracturing drill strings comprising drill pipe or production tubulars and downhole packer equipment, gravel pack strings comprising drill pipe or production tubing and gravel pack equipment and casing strings or liners. 
       FIG. 3B  is a cross section view  350  of the example rotating control device (RCD)  300  without RCD housing  304 . As illustrated, tool joint  315  passed through the bore in seal element  305  defined by inner surface  306  of seal element  305  and inner surface  306  seals against drill string  310 . Tool joints  315  have an outer diameter  316  that is larger than the outer diameter  311  of drill pipes  303 . As drill string  310  is longitudinally translated through the wellbore and the RCD  300 , the RCD seal element  305  squeezes against an outer surface of the drill string  310 , thereby sealing the wellbore. In particular, the inner diameter of the RCD seal element  305  is smaller than the outer diameter of the items passed through (e.g., drill pipes, tool joints) to ensure sealing. 
       FIG. 3C  is a side view of RCD seal element  305  in  FIG. 3B . RCD seal element  305  has a base end  320  and a nose end  330 . The base end  320  is typically attached to a mandrel (not shown) running through the center of the bearing assembly, however it could also be attached to a stripper housing that does not include a bearing. The mandrel is attached to the bearing housing via two sets of bearings. The element is then screwed onto the mandrel or bolted to the mandrel; this allows the element to rotate with the drill string during drilling operations. For example, holes  321  are provided for set screws to lock the element to the mandrel once the element has been threaded onto the mandrel. There are multiple other techniques used to mount the RCD seal element to the RCD. This disclosure shall not be limited to this style of core but rather encompass all styles of core. 
     The nose end  330  has an inner diameter  334  that is smaller than the inner diameter of the base end  320  to provide a tight seal against the drill string  310 . The outer diameter  322  of the base end  320  may be larger than the outer diameter  332  of the nose end  330 . Similarly the inner diameter  324  of the base end  320  may be larger than the inner diameter  334  of the nose end  330 . 
     An RCD seal element may be a molded device that is often made from of an elastic material which is flexible enough to deform to fit around and seal the varying diameters. Seal element material may include but not be limited to natural rubber, nitrile rubber, nitrile, butyl or hydrogenated nitrile, urethane, polyurethane, fluorocarbon, perflurocarbon, propylene, neoprene, hydrin, for example, and depends on the type of drilling operation. For instance, RCD seal element  305  of the present disclosure can be made from an elastomer  370  and is flexible enough to deform to fit around and seal the varying diameters of drill pipe  310  (e.g., diameters  311  and  316  shown in  FIG. 3B ), for example, during reciprocation of drill string  310 . 
     During drilling operations, seal element (e.g., seal element  210  or  305 ) and the bearings (not expressly shown) of bearing assembly (e.g., bearing assembly  206 ) may experience wear due to rotation and reciprocation of drill string (e.g., drill string  104  or  310 ). To alter the performance characteristics of various RCD seal element body materials, the addition of self-lubricating component of many kinds and sizes may be used. Self-lubricating components may include, but are not limited to, polarized graphite, calcium stearate, flurons, PTFE solid powder, graphene/few-layered graphene (e.g., 1 to ˜12 atomic layers of SP2 carbon), graphene oxide (e.g., chemically exfoliated and functionalized graphite layers), hexagonal boron nitride (h-BN, e.g., same structure as graphite but with alternating B and N atoms with improved oxidation resistance at any temperature (e.g., at high temperatures above 200° C.)), intermediate compositions (e.g., boron-doped graphene and graphite, nitrogen-doped graphene and graphite, and carbon-doped h-BN/B&amp;N co-doped graphene), multi-walled carbon nanotubes where the break-down product is fragments of graphene/few-layered graphene, or a combination thereof. Other compositions with layered structures such as chalcogenides (MoS2, WS2, NbS2, TaS2, VS2, ReS2, MoSe2, WSe2) could also be utilized as a lubricating phase within the elastomer material. The self-lubricating components can be fibers, particles, nanotubes, or in other forms. The self-lubricating components may be of varying deniers, lengths, diameters, sizes, shapes, or other properties. For example, a self-lubricating component may include fibers of uniform length and varying denier or a self-lubricating component may include particles of uniform shape and varying size. Other combinations are permissible. 
     The materials would be envisioned to impart lubricity to the contact areas of the tool as well as improved mechanical and thermal stability and thermal conductivity to elastomers. The materials could be incorporated as solid powders or slurries during the elastomer compounding process, incorporated as a dispersion or solution in a liquid state during compounding or crosslinking, or incorporated in another manner. 
     As shown in  FIG. 3B , self-lubricating components  375  can be added to the elastomer raw material  370  to form a resultant composite material  380  for RCD seal element  305 . This composite material  380  can be comprised of both uniformly distributed and non-uniformly distributed self-lubricating components. Self-lubricating components  375  can be randomly oriented, or may be non-randomly oriented (i.e., oriented radially, oriented longitudinally, or oriented at some other angle or combination of angles). 
     The concentration of self-lubricating components  375  within the elastomer material  370  can be varied to alter the properties of the composite material  380 , allowing for the customization of element material properties. For example, an RCD seal element may be molded with an elastomer that has a uniform concentration of self-lubricating components throughout. Any component concentration is permissible. In some implementations, the concentration of self-lubricating components (e.g., polarized graphite) can be in a range of approximately 7% to 25% by volume, for example, depending on an application and environment the seal element or packer will be exposed to. As the seal element or packer is worn away at the seal element or packer/drill pipe (or other tubular) interface, flakes of the self-lubricating components are released lubricating the seal interface and reducing wear. 
     Alternatively, an RCD seal element may be molded with an elastomer material that has a non-uniform concentration of self-lubricating components along the length (i.e., along a longitudinal or axial axis) of the RCD seal element. For example, an RCD seal element can have a higher concentration of self-lubricating components at its base  320  and a lower concentration of self-lubricating components at its nose  330 . Any combination of component concentration is permissible. In some instances, more than two concentrations (i.e., three different self-lubricating component concentrations) can be used. For example, a seal element can have a region with high concentrations of self-lubricating components, a region with moderate concentrations of self-lubricating components and a region with low concentrations of self-lubricating components. In some implementations, in a varying self-lubricating component concentration RCD seal element, the self-lubricating component concentration at different regions can be selected to optimize performance of the different regions of the RCD seal element. As an example, a particular region (e.g., the nose end  330 ) may have a higher self-lubricating component concentration if the particular region is subject to more friction and wear than other regions. Additional or different configurations associated with the self-lubricating components can be designed. 
     To fabricate an RCD seal element of the present disclosure, one or more raw elastomer materials are prepared. Once prepared, the elastomer is molded around a core to form a complete RCD seal element. The element can be made from cast polyurethane, which uses a mold with a core. The core is used to form the inside diameter (“ID”) of the element. The RCD seal element has a steel cage or core molded into its base. RCD seal elements can be molded using a single elastomer with a uniform self-lubricating component concentration, or using multiple combinations of elastomers with various self-lubricating component concentrations, or no self-lubricating components at all. For example, an element may be molded with a high self-lubricating component concentration region at its base which transitions into a region of low self-lubricating component concentration in its middle which transitions into a region of no self-lubricating component concentration at its nose. Likewise, elements may be molded with various combinations of elastomer with the same amount of self-lubricating component concentration. For example, an element may be molded with a region of low durometer elastomer and a region of high durometer elastomer, both with equal amounts of self-lubricating components. Any combination of elastomer and self-lubricating components is permissible. 
     In some implementations, the base elastomeric material in the elastomer to mold an RCD seal element can be selected from a group of natural rubber, nitrile rubber, hydrogenated nitrite, urethane, polyurethane, fluorocarbon, perflurocarbon, propylene, neoprene, hydrin, or a combination thereof in some instances, the base elastomeric material can constitute approximately from 50% to 99% by volume in the composite elastomer. In some instances, the base material in the elastomer being used to mold an RCD seal element is primarily polyurethane. Polyurethane may be used in any combination with natural rubber, nitrile, or butyl. Polyurethane is a flexible elastomer that can be stretched over the changing outer diameter of drill pipe and tool joints. To form an RCD seal element of the current disclosure, the polyurethane is cast by pouring polyurethane in a liquid state into a mold. 
     To create a self-lubricating RCD seal element, self-lubricating components (e.g., polarized graphite) are mixed into the liquid state elastomer (e.g., polyurethane). The mixture is poured into the mold. Heat and time are then applied to allow the material to set by heating in a curing oven. The formed seal element has a longitudinal bore, through which both a rotating drill sting and a non-rotating drill string with tool joint thereon can pass. In some instances, the seal element is fabricated such that the self-lubricating compound is (evenly) distributed throughout the entire seal element. In some instances, the seal element can be fabricated such that the self-lubricating compound is only distributed in the wall section adjacent to the drill pipe. In some instances, the seal element can be fabricated such that the mixture of the self-lubricating component and the liquid elastomer is adjacent to at least an inner circumferential surface of the longitudinal bore of the seal element. The mixture of the self-lubricating component and the liquid elastomer can fill a portion of the seal element extending outward radially from an upper end of the longitudinal bore to a lower end of the longitudinal bore of the seal element. As a specific example, the seal element can have at least a portion adjacent to and within  2  centimeter radially of an inner circumferential surface of the bore that contains the self-lubricating component and wherein the mixture of the self-lubricating component and the liquid elastomer fills a portion of the seal element extending outward radially from an upper end of the longitudinal bore to a lower end of the longitudinal bore of the seal element. The distribution of the self-lubricating components can be fabricated in another manner. 
     In one example implementation, self-lubricating components are added to the liquid elastomer and the mixture poured into the mold results in a uniform distribution of self-lubricating components with random orientation. 
     In another example implementation, the self-lubricating components are longitudinally suspended from the top of the mold so that they hang down throughout the length of the element running parallel to the central axis of the element. When the mold is filled the elastomer will fill in around the suspended self-lubricating components and cure with the self-lubricating components inside of the element. 
     In another example longitudinal channels running from the base (top) to the nose (bottom) and running parallel to the central axis of the element are left in the base material in the initial molding process. The channels are later filled with the selected self-lubricating compound in a second molding process. 
     In a further example implementation, the self-lubricating components are connected to the mold core and extended to the mold shell. This would orient the self-lubricating components in a radial direction. Again the mold would be filled and the elastomer allowed to cure. 
     Another example implementation involves filling the mold with the liquid elastomer and then inserting the self-lubricating components into the liquid with an insertion tool. In some instances, the elastomer (e.g., polyurethane) is a highly viscous fluid when it is poured into the mold, a self-lubricating component could be inserted and once released it would stay in the location it was deposited. Self-lubricating components could be inserted in any orientation and concentration desired. 
     Concentration and placement of the self-lubricating component in elastomers containing polyurethane can be carefully controlled, thus allowing regions of the element to be targeted with more self-lubricating components and other regions to be given very little or no self-lubricating components. To create an element with targeted regions of self-lubricating component concentrations, multiple batches of liquid elastomer with different amount of self-lubricating components are mixed. When filling the RCD seal element cast, the appropriate mixture would be used to fill the portion of the cast that is being target for a specific self-lubricating component concentration. For example, after placing a first elastomer material having a first concentration of self-lubricating components, a second elastomer material having a second concentration of self-lubricating components can be placed into the mold. 
     Although the above mentioned examples are described as having two separate portions, wherein each separate portion has a different self-lubricating component concentration, it is also within the scope of the present disclosure for the at least two elastomer materials to partially mix. For example, approximately a 0.5″-1″ region of mixing can exist between layers. In some implementations, the region of mixing can be about 0.25″ to about 0.5″. Alternatively, the region that experiences mixing could be increased. 
     As an example use of a self-lubricating RCD seal element in rotary drilling systems, the self-lubricating seal element is held inside the RCD. The RCD is positioned at an upper proximal end of a wellbore, for example, as shown in  FIG. 1 . RCD has a housing configured to receive a seal element molded from elastomer and a self-lubricating component mixed into at least a portion of the elastomer. In some implementations, the outer surface of a seal element does not need to conform with the inner surface of the RCD housing so that there is room for expansion and relaxation of the seal element to accommodate drill strings with different sizes. 
     A drill pipe can be stabbed (passed) through a bore extending axially through the seal element. The bore is sized to seal against and allow passage through the bore of an outside diameter of the drill pipe and a tool joint having a larger outside diameter in the housing of the RCD. In some instances, a drill string can be a tapered drill string (e.g., smaller outside diameter (“OD”) pipe on the bottom, larger OD pipe on the top). The RCD can be only required to seal on the larger pipe, not the entire string. In particular, when the drill pipe enters the RCD, the inner surface of the seal element seals against the drill pipe and deforms the inner diameter of the RCD seal element to fit over the larger diameter of the drill pipe. An interfacial seal is created which is capable of separating the high pressure region of the wellbore from the atmospheric pressure region of the rig floor. The seal element can maintain a pressure seal between the seal element and the drill pipe wherein a pressure in the wellbore below the RCD is greater than the ambient pressure outside the RCD. While attached, the drill pipe penetrating the RCD seal element is capable of vertical motion as well as rotational motion. The RCD seal element is also able to expand to fit over tool joints as new sections of drill pipe are added to the drill string. The drill string comprised of multiple joints of drill pipe can pass through the bore of the seal element. In some implementations, the drill string can be rotated and the pressure seal can be maintained while passing the rotating drill string through the bore of the seal element. During this process, the self-lubricating seal element releases self-lubricating components and lubricates the contact surfaces between the drill string and the seal element. Friction and wear can be reduced and the durability of the seal element can be improved. 
     In some implementations, although the tubular passing through the RCD and being sealed against is primarily the drill string, the tubular could also be a completion string comprising production tubing, fracturing string comprising drill pipe or production tubing, gravel pack string comprising drill pipe or production tubing, casing string or liner, or another type. In this disclosure, the term “string” is used to encompass all possible types of tubulars that can be passed through the RCD. 
     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.