Abstract:
A method and mobile system for cleaning dirty gas from a newly stimulated gas well. The entire system is supported on a trailer or other mobile support so that it can be driven from well site to well site for short-term, post-stimulation use only. The system comprises a gas separator, such as a membrane separator. The system also includes a pretreatment assembly for preparing the gas for the gas separator. The pretreatment assembly may include separators, a heater, a guard vessel and a polishing filter. A chiller or heat exchanger cools the treated gas to a marketable temperature. A generator and a hydraulics plant provide power to the system. Each mobile system will be designed to treat gases with widely different operating conditions varying from well to well.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and devices for cleaning “dirty” gas from a recently stimulated gas well. 
     BACKGROUND OF THE INVENTION 
     When a gas well is stimulated, the initial raw gas emitted at the wellhead is a mixture of natural gas, other hydrocarbons and contaminates, such as hydrogen sulfide (H 2 S), water (H 2 O) and carbon dioxide (CO 2 ). This so-called “dirty” gas may also contain particulate matter, such as sand and particles of drilling fluids. 
     Each pipeline company has its own set of natural gas quality specifications that delivered gas must meet. For example, pipelines typically limit CO 2  content due to its corrosive properties. A common maximum for CO 2  content in delivered is gas is one to three percent (1–3%) by volume, while raw gas from a recently stimulated well may exceed 30 percent. Similarly, hydrogen sulfide is corrosive and is hazardous to humans if inhaled, so it is also subject to restrictions (typically ≦4 ppm) by pipeline companies. In addition, delivered gas specifications typically limit water vapor content. 
     Because of these common contaminants, raw gas produced immediately after stimulation rarely meets typical delivered gas specifications. However, in most wells, the gas stream will soon become pipeline quality if the well is flared or vented for a brief period. For example, most gas wells begin producing marketable gas after 3–5 days of flaring. The dirty gas usually is vented to the atmosphere until sensors show reduced and acceptable levels of contaminants. 
     Flaring has a detrimental effect on the environment because it releases the contaminants into the air. In addition, flaring wastes a significant amount of natural gas and other hydrocarbons. For example, when raw gas containing five percent (5%) contaminants is flared, ninety-five percent (95%) of the flared product is good natural gas that is wasted. Treatment and sale of the dirty gas would significantly increase profits to operators, tax revenues to the states, and payments to the royalty owners. 
     Permanent and semi-permanent treatment systems have been used at well sites where the deep gas is not pipeline quality and permanent treatment of the produced gas is required. Alternately, produced gas from such wells has been shipped to processing facilities remote from the well. Neither of these options is economically feasible for cleaning the gas produced after a stimulation procedure in a well where only the initial post-stimulation gas is unmarketable. 
     The present invention provides a mobile gas separation system suitable for temporary use at the well site of a recently stimulated gas well. The system is adequate to serve wells with a range of different requirements. The method of this invention permits the sequential use of the gas separation system for short periods of time at a number of wells, each having different capacities and requirements. Accordingly, the method and system of this invention provide a temporary, on-site cleaning of dirty gas, preserving this valuable natural resource, protecting the environment, and maximizing revenues to the state, the operators and the royalty owners. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a mobile gas separator system for temporary use at the well site of a natural gas well following stimulation procedures. The system comprises a mobile support adapted to be parked temporarily at the well site. A gas separator is mounted on the mobile support and adapted to remove selected contaminants from dirty natural gas to produce marketable gas. A pretreatment assembly is also mounted on the mobile support. The pretreatment assembly is adapted to receive dirty gas from the gas well, to prepare the dirty gas for the gas separator, and to conduct the prepared dirty gas to the gas separator. 
     The present invention further comprises a method for processing dirty natural gas to produce marketable gas. The method comprises conducting dirty natural gas from a first gas well following stimulation to a mobile gas separation system at the well site of the first gas well. Then, the dirty gas is processed in the gas separation system to produce marketable gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the method and system of the present invention constructed in accordance with one preferred embodiment of the present invention. 
         FIG. 2  is a detailed schematic illustration of a preferred mobile gas separator system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the drawings in general and to  FIG. 1  in particular, there is shown therein a mobile gas separator system constructed in accordance with one preferred embodiment of the present invention and designated generally by the reference numeral  10 . The system is designed for temporary, short-term use at a gas well immediately after stimulation, when the initial raw gas is dirty or heavily contaminated. In particular, the system  10  is designed to be moved from well to well. In this way, a small mobile system is available to operators to treat and sell the initial post-stimulation gas instead of flaring or venting it to the atmosphere. 
     As used herein, “immediately after stimulation” and similar expressions refer to the period following completion of a stimulation procedure during which the well is producing gas that is not marketable. As used herein, “marketable gas” refers to natural gas that meets pipeline company delivered gas standards. 
     Turning now to  FIG. 2 , the system  10  comprises a mobile support adapted to be moved from well site to well site and to be parked temporarily at each site preferably near the well head  14  ( FIG. 1 ). As used herein, “at the well site” means in the general vicinity of the well head or in a nearby location associated with the well. In the case of adjacent wells, “at the well site” may include a single location near to all the adjacent wells. 
     This mobile support preferably takes the form of one or more trailers  16  and  18  that can be removably connected to trucks or other vehicles (not shown). In this way, the system  10  can be transported easily from well to well. In one preferred embodiment, the entire system is supported on two equally-sized trailers  16  and  18  that can be parked side by side adjacent the wellhead  14 . 
     A gas separator is mounted on the mobile support. The gas separator is adapted to remove selected contaminants from the dirty gas to produce marketable gas. The contaminants to be removed may vary depending on the characteristics of the gas produced. Accordingly, the type of gas separator may vary as well. As explained herein, typical major contaminants of raw gas produced immediately after stimulation include carbon dioxide, hydrogen sulfide and water. Accordingly, an ideal gas separator for this application is a membrane separator  20 . More preferably, the membrane separator  20  is adapted to selectively reduce the content of carbon dioxide, hydrogen sulfide and water in the gas being treated. Most preferably, the membrane separator  20  comprises cellulose acetate polymer membrane modules. Suitable membrane separators are available from Natco Group, Inc. (Houston, Tex.), UOP L.L.C. (Des Plaines, Ill.), and Kvaerner Process Systems US, Inc. (Houston, Tex.). 
     Optimal functions and durability of the preferred membrane separator  20  depends on the condition of the raw gas introduced into the separator. For example, permeation characteristics of the cellulose acetate membranes can be adversely affected by liquid water, glycol, amine, lubricating oil, and other heavy hydrocarbon liquids in the gas. In addition, permeability of a given molecule is affected by feed gas pressure, feed gas temperature and concentration of the molecule in the feed gas. Thus, the dirty gas should be heated and pressurized to a prescribed range. For these purposes, the system  10  preferably will include a pretreatment assembly  24  mounted on the trailers  16  and  18 . 
     The pretreatment assembly  24  is adapted to receive the dirty gas from the wellhead  14  of gas well (not shown). Usually, the operator will provide a sand separator  26 , a production unit  28  and a frac tank  30  at the well site, and the system will simply receive dirty gas from the production unit in a known manner through a conduit  32 . The conduit  32  to the pretreatment assembly  24  typically will be equipped with an isolation (ball) valve  34  and a throttle valve  36 . This throttle valve  36  allows the dirty gas feed to the system  10  to be maintained at a constant flow and pressure. The isolation valve  34  is a shutdown and safeguard. Conduits  38  and  40  are provided to the frac tank  30  for oil and water, respectively. 
     The pretreatment assembly  24  is adapted to prepare the dirty gas for the membrane separator  20 . To that end, the pretreatment assembly  24  preferably includes a first separator  44  adapted to remove selected contaminants from the dirty gas. More preferably, the first separator  44  is a sand separator adapted to remove sand and other particulate matter from the dirty gas passing through it. 
     The pretreatment assembly  24  preferably includes a second separator  46  connected to the first separator  44  by a conduit  48 . The second separator  46  is adapted to remove small oil and water aerosols from the dirty gas. Most preferably, the second separator  46  is a two-chamber coalescing filter separator designed to agglomerate and capture about ninety-nine percent (99%) of small oil and water aerosols greater than 0.3 microns. This filter may be equipped with two independent level controls, two level gauges, and two automatic liquid level control valves. 
     The pretreatment assembly  24  preferably also includes a heater, such as a water bath heater  50 , adapted to adjust the temperature of the dirty gas to a temperature suitable for the membrane separator  20 . In most instances, this temperature range will be from about 125 degrees to 120 degrees Fahrenheit. A conduit  52  connects the separator  46  to the heater  50 . A conduit  54  connects the production unit  28  to circulate dirty pretreatment gas to fuel the heater  50 . An isolation valve  56  may be included in this conduit. Thus, the heater  50  initially can be fueled by the dirty gas until sweetened gas from the system  10  is available, as explained hereafter. 
     The pretreatment assembly  24  preferably also includes a guard vessel  60  adapted to remove oil and glycol vapors from the dirty gas. Preferably, the guard vessel comprises an activated carbon adsorbent. The guard vessel  60  receives warm dirty gas from the heater  50  through the conduit  62 . The conduit  62  and other conduits downstream of the heater  50  should be insulated to minimize heat loss. 
     To ensure that the gas entering the guard vessel  60  is warmed to the desired temperature, the system  10  may be provided with a recirculating assembly. The recirculating assembly preferably takes the form of a recirculating valve  64  and temperature sensor  66  connected in series in the conduit  62  between the heater and guard vessel  60 . The fluid leaving the heater through the conduit  62  will be diverted through the three-way valve  64  to the conduit  52  through a crossover conduit  68  until the sensor  66  senses that the fluid has acquired the desired temperature. When the gas leaving the heater  50  has reached the desired temperature, it will redirected to the guard vessel  60 . 
     Still further, the preferred pretreatment assembly  24  includes a polishing filter  70  connected to the guard vessel  60  by a conduit  72 . The polishing filter  70  is designed to remove additional aerosols and fine particulate matter from the dirty gas. Pre-treated gas from the polishing filter is fed to the membrane separator  20  through the conduit  74 . A gas analyzer  76 , preferably providing a BTU reading, may be included in the conduit  74  to verify the condition of the pretreated dirty gas before it enters the membrane separator  20 . 
     Conduits, indicated collectively at  78 , allow carbon dioxide and methane removed from the gas by the membrane separator  20  to be flared to the atmosphere, vented to the frac tank  30  by a conduit (not shown), or collected and treated further. For example, the carbon dioxide can be liquefied and recycled. 
     Sweetened gas produced by the membrane separator  20  exits the separator through a conduit  80  and is conducted to a heat exchanger  82 . The heat exchanger  82 , or chiller, cools the treated gas to a pipeline-acceptable or marketable temperature range, usually about 65–70 degrees Fahrenheit. The chilled gas is directed to the sales connection through the conduit  84 , which connects to the conduit  80  by means of a four-way valve  86 . An adjustable choke valve  88  may be included in the sales conduit  84  to maintain the pressure of the treated gas directed to the sales pipeline at a marketable level. A gas analyzer  90  of any suitable type may be included in the sales line. Preferably, the gas analyzer will provide BTU reading. 
     The preferred heat exchanger  82  is electrically operated. To provide power to the heat exchanger  82 , the system  10  preferably includes its own generator  92 , which may be mounted on the trailer  18 . The generator is electrically connected to the heat exchanger  82  by a suitable connector (not shown). The generator, then, preferably is powered by natural gas. Initially, dirty gas from the production unit  28  is used to fuel the generator  92  through the conduit  94 . Once the system begins producing clean or “sweetened: (post-treatment) gas from the membrane separator  20 , the sweetened gas is used to fuel the generator  92  through the conduit  96 . Sweetened gas may also be circulated through the conduit  96  to fuel the heater  50 . An isolation valve  98  may be included in the conduit  96 . 
     In the preferred design of the system  10 , the valves are operated hydraulically. Thus, the system  10  preferably includes its own hydraulic plant mounted on the trailer  18  to supply hydraulic power to the system. The hydraulic plant  100  is electrically connected to the generator  92  by conductors not shown, and is fluidly connected to the various valves and other components by conduits, not shown. 
     Controls for the various components in the system conveniently may be enclosed in a control room  102 . Preferably, the control room  102  is enclosed. If space permits, a storage area  104  may also be provided on the trailer  18 . 
     Returning to  FIG. 1 , where a pipeline is not available, the treated or “clean” gas can be liquefied and placed in containers. To that end, the system  10  may further include a liquification unit  106 . The unit will remove any remaining water in the clean gas and convert it to a liquid phase. The liquid gas can then be placed in containers  108  that can be stored on site until a pipeline becomes available, at which time the liquefied natural gas can be restored to a gaseous state and sold. Alternately, the storage containers  108  can be transported for sale or use elsewhere. 
     In accordance with the method of the present invention, a first gas well is selected. The selected gas well preferably will have recently undergone a stimulation treatment and will be producing dirty gas. Following the stimulation procedure, the dirty natural gas from the first gas well is conducted to a mobile gas separation system at the well site. Preferably, the mobile gas separation system is similar to the system described above. Next, the dirty gas is processed in the gas separation system to produce marketable gas for subsequent sale to a pipeline company. 
     In the preferred practice of this method, the pre-processed dirty gas coming from the well is intermittently tested to determine its marketability. This testing is carried out with conventional equipment according to known procedures and is not described in detail herein. Once the pre-processed gas is determined to be marketable, then the processing of the gas is terminated. The mobile system now can be removed from the first well site and transported to a second gas well in need of temporary gas processing in accordance with the method of this invention. 
     Various additional features will suggest themselves to those skilled in this field. For example, a low pressure alarm would be advantageous as it would alert the operator of the system to a leak or other problems that require operator intervention. 
     Changes can be made in the combination and arrangement of the various parts and elements described herein without departing from the spirit and scope of the invention as defined in the following claims.