Abstract:
A well tester adapted to select among members of an oil well family to provide a purged and clean vessel to receive samples of well production that include gas and oil, and by selective use of production pressure and lease water under pressure, allow for the separation and separate measure of the oil and gas, and optionally to provide for improved sampling of production fluids with high gas/oil ratios.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This is a continuation in part of applicants&#39; copending U.S. patent application Ser. No. 09/340,517 filed Jun. 28, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    Apparatus and method for obtaining representative samples of the fluids produced by an oil well, and measuring the amounts of oil, water and gas in the sample.  
         BACKGROUND OF THE INVENTION  
         [0003]    The need for accurate and timely oil well production data is critical. The extremely erosive and harsh environments in which well pumping systems operate create constant deterioration of the equipment, often resulting in rapid failure. To maintain production rates in the face of this situation, well operators invest substantial resources in well maintenance. For wells using steam drives and waterfloods, well servicing is frequently the highest non-energy cost.  
           [0004]    Knowledge of the production performance of an individual well on a current basis is the most important tool for maximizing production from the well and optimizing reservoir management. Because of the well-known advantages of prompt, accurate and low-cost cost performance data, numerous testing systems have been devised. Despite extensive previous efforts, there still remain many unsolved disadvantages, resulting in uncertainly of accuracy of the data, and long testing periods because the sampling procedures were very slow themselves.  
           [0005]    Early pioneers in production of oil from wells as early as about 1900 began to measure the performance of their individual wells. The early efforts were little more than collecting well production for a day or so, and measuring the gross output and the relative amounts of water and oil with a dip stick or tape measure. That method is still widely used, but is not truly sufficient when optimum and most economical production is the objective.  
           [0006]    Open tank systems can indeed accurately measure the output of low production rate wells, but they take a very long time to collect the sample. Worse, production conditions can vary widely during that long period. A true production sample representative of a short preselected period of time can not be obtained with this practice.  
           [0007]    Beginning in the 1960&#39;s this procedure was improved by providing tall, vertical, closed separators and using various mechanical devices to measure the levels and read them out. These largely succeeded because of the advent of pollution control rules, which adversely affected the earlier open tank samplers rather than because of any inherent sampling superiority. The closed samplers simply produced less pollution.  
           [0008]    Beginning in the 1980&#39;s improved sensing devices became available, and their use improved the accuracy of the procedures but still their collection times were slow, and they did not provide for suitable purging between tests, leading to contaminated samples. Also the potential for measurement errors in low flow rate wells was and is much higher than it should be. Additional problems reside in the complexity and inaccuracies of the more modern sensors and measuring devices, especially at slow flow rates. Because of their sophistication, the initial costs and the costs of maintenance and operation of these newer systems are much higher than they should be.  
           [0009]    Especially in periods of low oil prices at the well, it is essential not only to minimize operation and maintenance costs, but also to maximize production both of the well and of its field. Dollars are very scarce in times of low oil prices, and any reduction of costs is not only welcomed but may contribute to the decision to keep a well or field in operation rather than to shut it down.  
           [0010]    It is an object of this invention to provide sampling apparatus and method that can obtain a sample which is suitably small to reflect fluid produced in a relatively short time, even from low production rate wells. A single sampler has the capability to service a substantial number of wells, often up to  60  wells, and to provide frequent samples from them. Importantly, the lines leading to it are fully purged of fluid remaining from previous tests as is the apparatus itself so that the sample is truly representative of well production at a very specific time.  
           [0011]    This apparatus can be operated manually. In practice, programs to cause the sequential operation of the valves will be provided. Such programs form no necessary part of the invention, and can readily be devised by persons skilled in the controls art  
           [0012]    This apparatus can be automated to perform its method on a programmed basis without supervision, and can also be programmed to alert the owner to any departure of a well&#39;s performance from previous samplings, thereby alerting the operator to potential problems in a particular well.  
           [0013]    It is an object of this invention to provide with minimal effort a clear and receptive tester for sequential tests for individual of a group of wells, and to provide a simplified arrangement to determine gas proportions.  
           [0014]    In addition wells whose gas/oil ratio is very high involve special problems in measuring the gas of the product, which it is one object of this invention to overcome.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    Sampling apparatus according to this invention includes a vessel having a bottom wall, a top wall and a vertically extending sidewall which form a sample chamber.  
           [0016]    A water inlet passes through the bottom wall. A test fluid exit port passes through the top wall. A test fluid line opens into the vessel at a mid-elevation. A test fluid inlet valve controls flow from a test fluid supply line to the test fluid line. A purge line opens into the vessel at a lower elevation.  
           [0017]    A sample exit sensor senses flow from the sample exit port, and a purge control valve controls flow from the purge line.  
           [0018]    A return line returns fluid from the vessel to a point of use. A flow line interconnects the purge line and the sample exit line at their intersection downstream from their respective valves. A selector valve is placed between said intersection and the fluid exit valve.  
           [0019]    A gas discharge line is connected between said fluid exit line and said return line. A gas control valve and a gas flow meter are placed in the gas discharge line.  
           [0020]    A flow meter device is placed in said return line.  
           [0021]    A test fluid by-pass line interconnects the test fluid supply line to the return line. A by pass valve selectively controls flow through the test fluid inlet line and through the return line.  
           [0022]    By suitable manipulation of the aforesaid valves along with the availability of lease water under pressure, test fluid under well pressure, and a suitable return to the system, a unique sampling process can be accomplished.  
           [0023]    This system utilizes water from a separate water supply often called “lease water”. This water is used and discarded as part of the procedure. This is not water from the samples.  
           [0024]    A procedure according to this invention begins with the vessel full of water. First, the contents of the lines leading to the vessel wherever they may have come from will be forced into the vessel for a measured period of time. The pressure is that of the well production fluid. The water expelled by this incoming fluid passes through the flow meter. From this is calculated how long a time it will take for the flow from this well to completely form a suitable sample, or how large the sample will be in a selected period of time. This places produced material in the chamber.  
           [0025]    The vessel next is closed and its contents permitted to settle (gas on top, water on the bottom, oil in between). During this time, test fluid from the well continues to purge the supply line through the by-pass.  
           [0026]    Next gas is measured and purged. The lease water drives the fluids upwardly until liquid is sensed at a sensor (the supply line purge continuing). The gas flow through the gas flow meter measuring the gas content.  
           [0027]    Oil is next purged from the vessel by further supply of lease water thereby driving oil out through the exit port. This driving action will continue until water has been sensed at the sensor for a given length of time, usually one minute. Thus, all of the piping downstream of the vessel will be purged.  
           [0028]    Now a production sample is collected by opening the sample inlet valve and the water exit valve. The sample fluid will drive the purge water from the vessel and occupy the vessel by a known amount. After that time the vessel is closed and the sample water, gas and oil separate.  
           [0029]    If desired for high oil/gas samples, a gas vent line is provided which relieves the vessel from excessive amounts of gas to enable a sufficient sample of oil to be obtained.  
           [0030]    The above and other features of this invention will be fully understood from the following detailed description and the accompanying drawings, in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    [0031]FIG. 1 shows the presently-preferred embodiment of a tester in its repose condition, coupled to a group of producing wells, shown principally in schematic notation;  
         [0032]    FIGS.  2 - 5  schematically show successive events in the process of this invention; and  
         [0033]    [0033]FIG. 6 shows a modified embodiment of the apparatus of FIG. 1. It provides for more precise analysis of the sample when the gas/oil ratio is very high. It utilizes the construction already described in FIG. 1, and bears the same corresponding numbers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    A well tester  20  is shown in FIG. 1. It includes a vessel  21  which has a bottom wall  22 , a top vessel wall  23  and a peripheral sidewall  24 . It is a closed vessel with a substantial vertical dimension compared to its diameter. A frusto-conical chamber top wall  25  reduces the cross-section of sample chamber  26  at its top. It connects to an exit port  27  through the vessel top wall.  
         [0035]    A water supply port  30  enters the sample chamber through the bottom wall. A water valve  31  is placed in a water line  32  which interconnects a supply  77  of lease water to the water supply port.  
         [0036]    A test fluid line  35  enters the vessel, most conveniently through bottom  22 . Its discharge end  36  is at a mid-elevation in the sample chamber. It could instead enter through the side wall, but the disclosed arrangement is much more convenient.  
         [0037]    A test fluid inlet valve  38  interconnects the test fluid line  35  to sampling line  39 . A by-pass line  40  interconnects a return line  41  and the sampling line  39 . A by-pass valve  42  is disposed in the by-pass line. It will ordinarily be activated by an actuator  43  which sets valves  38  and  42  in alternate flow conditions.  
         [0038]    A unidirectional check valve  44  is plumbed into return line  41  just upstream from the junction  44   a  of the by-pass line  40  and return line  41 . The permissible flow through valve  44  is only from return line  41 .  
         [0039]    A purge line  45  extends from an inlet opening  46  near the bottom of the chamber, out of the chamber, conveniently through the top wall. A purge valve  47  is placed in purge line  45 .  
         [0040]    A pressure transducer  50  or other pressure measurement device senses the pressure inside the chamber.  
         [0041]    An exit line  55  exists from the exit port  27 . A liquid sensing probe  56  in the exit line will detect the interface or presence of a liquid, so it is sensitive to a gas/liquid interface or of a water/liquid interface when one passes the probe.  
         [0042]    Exit line  55  branches at intersection  57  to a gas discharge line  58  and to a sample exit line  59 . A selector valve  60  in line  59  is upstream of an intersection  61  of the sample exit line  59 , return line  41 , and purge line  45 . A flow meter  62  measures the rate of flow of liquid through return line  41 .  
         [0043]    A gas vent valve  65  in gas vent line  58  controls flow through line  58  to return line  41 . A gas flow meter  66  is fitted in gas vent line  58  downstream from gas vent valve  65 .  
         [0044]    As shown in FIG. 1, this tester is plumbed into a system which selectively provides flow from a selected well. A central station  70  is connected by collector lines  71 ,  72 ,  73  to individual wells  74 ,  75 ,  76 . This valve will select the well, and through sampling line  39  will send product from a well being sampled to the tester. A family line  80  sends all production from wells not being sampled to a sump  81 .  
         [0045]    Lease water is supplied from a source  77 . Return line  41  will return the output from the tester to sump  81  or to some other receptacle or system, along with production fluid from any other wells not in this family. To this sump are added the production through family line  80  from the wells in this family that are not being sampled at the time.  
         [0046]    Later it will be appreciated that the sampled fluids and all production fluids from other wells are all returned to the sump along with what lease water is used in the testing. Lease water is used and disposed of the same as water in the sampled fluids. Sump  81  may also be regarded as a dehydration plant in which water is removed from product oil.  
         [0047]    Controls for this system will customarily be programmed, but could instead be manually operated. The details of an automatic control are not an essential part of this invention and may readily be devised by a person skilled in the controls art. Care must be taken in the lines where flow and pressure and measured, to be certain that the velocities are such as to prevent stratification, and to permit collection of a sample in a suitably short time.  
         [0048]    A unidirectional check valve  44  is plumbed into return line  41  upstream from junction  44   a  of the by-pass line  40  return line  41 . The permitted flow through valve  44  is toward junction  44   a.    
         [0049]    [0049]FIG. 1 illustrates the construction and operation of one embodiment of the invention. FIG. 6 shows a construction which is generally identical to FIG. 1, except that it includes modifications which make it even more useful for testing production of production from wells whose gas/oil ratio is quite high, for example, ratios as high as 600:1 and 1,600:1.  
         [0050]    The following sequence of testing steps will be understood from the sequence of drawings in FIGS.  2 - 5 , all relating to the configuration of FIG. 1.  
         [0051]    Vessel and Piping Purge Step (FIG. 2)  
         [0052]    Assuming the tester is making a series of tests from a sequence of wells, this is the first step following the previous test or the startup of a fresh system. Its purpose is to assure that the system is cleared of substances from the previous test, and that the instrumentation will be free from fouling by residues from the previous test. It does this by introducing water into the system to displace such residues.  
         [0053]    For this purpose water, shown in horizontal dashed lines, is introduced into the vessel through port  30  by opening valve  31 . It provides water at a selected flow velocity, preferably about 12 feet per second as measured across liquid sensing probe  56 . Valve  60  is opened, valve  65  is closed, and valve  47  is closed. Flow is though flow meter  62 . It washes the instrument clean of any oil which had not previously been removed.  
         [0054]    After a preselected time, valve  60  will be closed and valve  47  opened to purge the purge line  45 .  
         [0055]    After another preselected period of time sufficient to clear purge line  45 , purge valve  47  will close and gas vent valve  65  will be opened. This will purge gas vent line  58 . These time periods are generally very short, so short that they may seem to be almost simultaneous. However, they are sequential steps.  
         [0056]    This step leaves only the test fluid line not purged. However, because oil and gas will rise in water, its residual contents will have generally risen into the tank, to be displaced by water. The system will therefore have been entirely purged and is now filled with water.  
         [0057]    Pre-test Sampling Line Purge Step  
         [0058]    The sampling line  39  from selector valve  70  will contain production fluids from a previous test. Of course these are not to be passed through a vessel prepared for, or currently engaged in a successive test. Therefore, either before or after the procedure shown in FIG. 2, valve  38  will be closed and valve  43  opened by actuator  43 . These settings are alternative, one of the valves being open and the other closed. Check valve  44  prevents back flow into the system.  
         [0059]    Production fluids pass through the open by pass  40  until about 6 to 8 times the volume of sampling line  39  have flowed through the by-pass. This assures that the well sample to be tested will not be contaminated by fluids from the previously tested well.  
         [0060]    Gross Rate Measurement and Gas and Oil Collection Step (FIG. 3  
         [0061]    Valves  31 ,  60  and  65  are closed. Valve  38  is opened to introduce produced fluids into vessel  26 . Valve  47  is open. Now purge water from the previous steps flows through flow meter  62  (a Coriolis meter). This meter accurately measures the gross production rate from the well, even though it is measuring water flow rather than flow of other fluids. No produced fluids are displaced.  
         [0062]    Also, a collection of oil and gas is being obtained in proportion to the gross flow rate input, which is known. This will continue until a predetermined (or preselected) percentage of the vessel volume has been displaced, at which time actuator  43  reverses the settings of valves  38  and  42 , so that no more production fluids are sent to the vessel. Instead they will flow through by-pass line  40  to the sump. Therefore production from the selected well is never interrupted. Its production rejoins that of its family wells in the sump. Ultimately, so do the tested fluids. The tester is now ready for further measurement steps.  
         [0063]    Settle Step  
         [0064]    The condition reached in the foregoing step is allowed to persist. The sample is completely held in the system by the closed valves, while production fluids continue to flow through the by-pass. This settle step is intended to give the sample time to separate into its phases, water on the bottom, oil in the middle, and gas on top. In some slow-flowing wells, where the tester may be responding sequentially to wells having varying flow rates, the previous gross rate measurement and collection step may have to be repeated enough times to obtain an oil sample of suitable size. The gross production will, of course, have been measured for each repetition. In whatever event, the sample to be tested will now have settled into its phases, and measurements can proceed.  
         [0065]    Gas Measure Step (FIG. 4)  
         [0066]    There are two procedures available to measure the gas. The first procedure is preferred, but if the expense of a gas flow meter  66  cannot be justified, the second procedure is available, which does not need it.  
         [0067]    In the first procedure shown in FIG. 4, lease water is supplied under pressure to the vessel by opening water supply valve  31 . Gas vent valve  65  is opened, and the volume of gas is measured by gas flow meter  66 . This will continue until probe  56  senses oil instead of gas because the interface between gas and oil has arrived at the probe. Then valve  65  is closed and valve  60  is opened. This will conclude the gas measure step.  
         [0068]    The second mode for gas measurement, which does not require the use of gas flow meter  66  relies on the application of Henry&#39;s Law of compressibility. In this step, the vessel pressure is known at the start. Lease water is admitted into the vessel through valve  31  to raise the pressure by a known amount, perhaps 50 psi. Then valve  31  will be closed.  
         [0069]    Valve  47  will now be opened releasing water through purge line  45  which flows through flow meter  62  to return the vessel to its previous pressure, at which time valve  47  will close. Valve  60  was always closed. The volume of water measured by flow meter  62  is equated to gas volume, and thereby the gas content is measured.  
         [0070]    Gas Purge Step  
         [0071]    This step inherently follow the gas measurement step. The gas will have been purged from the vessel, and the flow of fluid will now be through valve  60 , valve  65  being closed.  
         [0072]    Oil Measure Step (FIG. 5)  
         [0073]    Now the flow of fluid which is oil (the gas having been purged), passes through valve  60  and flow meter  62 , impelled by lease water admitted to the vessel through valve  31 . It continues until probe  56  senses water-the interface between oil and water. The volume which has passed the flow meter is known. To it is added the known volume of the lines ahead of it. The sum is the oil volume of the sample.  
         [0074]    The reader will now recognize that he has been in this cycle before. The gas and oil of the sample have been measured, and all or most of the system is filled with water. Then the procedure is repeated, starting with step 1, except this time for a different one of the wells.  
         [0075]    A comparison of FIGS. 1 and 6 shows in FIG. 6 the addition of a gas discharge line  70  from junction  71  between lines  57  and  58  to a junction  72  downstream from gas vent valve  65 . Junction  72  is between gas vent valve  65  and gas flow meter  66 . A gas discharge valve  73  is fitted in line  70 . Gas discharge valve  73  is preferably a smaller valve than gas vent valve  65 , perhaps a ½ inch valve compared to a 2 inch valve, to provide for lesser flow rates. A level sensor  75  is placed in the vessel, and is sensitive to the gas/liquid interface.  
         [0076]    A sensor circuit  76  responds to the sensor. It will sense the presence or absence of a sufficient sample by responding to the gas/oil interface  77 . A suitable example of a sensor is shown as a float mounted to a pivoted arm, whose position is responded to by the sensor circuit. Alternatively, a rod (not shown), electronically or electronically responsive to the location of oil along its vertical length can be used, and its output utilized by the sensor circuit.  
         [0077]    Some wells produce vastly larger quantities of gas than oil. Ratios of 1000:1 are common. In such situations, in order to collect a suitable and sufficient oil sample, very large volumes of gas must be passed and measured. These gas volumes emanate from the oil and rise to the top. When the vessel is filling, the gas vent valve will accommodate to the rapid flow of gas, which is measured by gas flow meter  66 .  
         [0078]    When the oil level finally reaches its desired level, as detected by the sensor, a lesser flow rate will often be preferred. In this event, gas vent valve  65  will be closed, and the smaller gas discharge valve  73  will be opened. Gas passing through either or both of these valves will be measured by flow meter  66 . In whatever event, until a sufficient volume of oil is collected, the large volumes of gas will more speedily be removed so the oil can be collected more quickly.  
         [0079]    This invention is not to be limited by the embodiments shown in the drawings and described in the description, which are given by way of example and not of limitation, but only in accordance with the scope of the appended claims.