WO2015101859A1

WO2015101859A1 - Methods for improved plunger lift operation

Info

Publication number: WO2015101859A1
Application number: PCT/IB2014/066887
Authority: WO
Inventors: Niket KAISARE; Arun Gupta; Nareshkumar NANDOLA
Original assignee: Abb Technology Ltd.
Priority date: 2013-12-31
Filing date: 2014-12-14
Publication date: 2015-07-09
Also published as: IN2013CH06209A

Abstract

In various aspects, the invention provides a system and methods to improve plunger lift operations, by providing improved estimates for at least one of: a liquid level in a well reservoir at any level, the position of the plunger at any given time, and threshold pressure value and threshold flow rate to make more informed decisions to open, close the valve, and maintain the productivity of the well. The improved estimations include obtaining dynamic operating variables by use of existing surface measurements, and based on historical data and mathematical models developed on the basis of the historical data. This in turn also enables the estimation of other values related to plunger lift operations more accurately.

Description

METHODS FOR IMPROVED PLUNGER LIFT OPERATION

TECHNICAL FIELD

[0001] The invention relates to improved methods of operation of plunger lifts based on surface measurements, and more specifically to a system for controlling plunger operations and improved methods to predict optimum open and close valve times, estimating liquid levels at a given time accurately, and obtaining plunger position at a given time accurately.

BACKGROUND

[0002] The primary focus of a plunger lift system is production of natural gas and oil from a liquid-loaded well. Over time, the reservoir pressure depletes and liquids (water, oil and hydrocarbon condensates) accumulate at the bottom of the well, restricting the flow of gas from the reservoir formation. A plunger lift system is used to remove these liquids and maintain low well bottom pressures. FIG. 1 shows a schematic of an exemplary plunger lift system used in gas/oil wells. A plunger lift system consists of the following components: an outer tube called casing (100); an inner tube called tubing (102), which is connected to the sales line (via 112); a control valve (110) that can be opened or closed to allow the well to flow or shut-in; a plunger (104) that can move up and down the tubing; perforations (114) in the casing (100) are provided to allow flow of fluids from the reservoir / formation (116); a controller (130) is provided to collect measurements, determine control actions and communicate data with a SCADA. Some exemplary sensors that are connected to the controller (130) shown in FIG. 1 include: casing pressure (120); tubing pressure (122); line pressure (124); flow rate (128) and a sensor called arrival sensor (126) to detect arrival of the plunger in the catcher and record the arrival time. The valve is used to control the movement of the plunger. When the valve is opened, the plunger is intended to eventually come to rest in the catcher/lubricator (108) located at the well-head. When the valve is closed, the plunger falls and eventually comes to rest at the bottom seat (106). [0003] The control valve that connects the tubing to the sales line is initially shut, allowing the gas pressure to build up within the well-bore. The plunger is at the well-bottom and the liquid in the tubing collect on top of the plunger. At a particular time, the control valve is opened, tubing gas discharges to the sales line, and the plunger is lifted from its initial bottom position to the top of the well by the force of the gas accumulated in the annulus between casing and tubing. The plunger lifts up the liquids to the top of the well; it finally reaches a "catcher" and stays there as long as the control valve is kept open. After certain after-flow period, during which time the gas accumulated in the casing as well as gas from the reservoir formation is produced at the sales line, the control valve is closed. [0004] FIG. 2 shows a typical plunger cycle. A typical plunger lift cycle consists of the following steps: At certain point, an operator or controller decides to close the valve (200). The cycle starts when the valve is put in the closed position. The plunger starts to descend from the catcher towards the well bottom. It takes some amount of time (220) to reach the bottom spring (indicated by block 210). Once the plunger is at the bottom spring, it stays there as long as the well is shut-in. The valve is kept shut for additional build-up time (222). In the total amount of time of shut-in (220 as well as 222), the tubing and casing pressures increase. After the build-up period (indicated by 222), the valve is opened (202). Gas starts to flow and the plunger ascends with liquids (224). After some time has elapsed (224), the liquid slug arrives at the surface (212); after additional time (226), the plunger arrives in the catcher (214). The time taken for the plunger to reach the catcher after the valve is opened (224 as well as 226) is measured and recorded. This is called the plunger arrival time. After additional period of time, called after-flow (indicated by 228), the valve is closed again, and the cycle is repeated.

[0005] FIG. 3 is a plot of typical trends of the measured values of casing pressure

(300), tubing pressure (302) and flow rate (304). The area under the flow rate curve, indicated by the shaded region 306 represents the net productivity of gas (in terms of thousands of standard cubic feet, Mcf) in the current cycle. This is used to calculate the daily production rate in Mcfd. The various events during a plunger cycle (denoted in FIG. 2 as 200, 210, 202, 214 and 200) are indicated at the bottom of the time axis (X-axis) in FIG. 3. The times 320, 322, 324 and 326 correspond to the plunger fall time (220 in FIG. 2), build-up time (222 in FIG. 2), plunger rise time (224 and 226 in FIG. 2) and after-flow time (228 in FIG. 2), respectively.

[0006] The decision on valve open and valve close on the sales line valve controls the plunger lift system. Current industrial practice for valve open and close is pre- decided to fixed time for which valve remains in either open or closed state. In prior art some field studies have been used to determine a minimum set of casing pressure conditions at which the valve should be opened. Similarly, a criterion for valve close based on critical flow rate through the tubing has been proposed. These methods presents a good understanding of physical behavior of well, however, are oversimplified to address practical challenges. Methods of valve opening based on plunger arrival time are presented in US patent 5785123 and 6241014B1. However, thumb rules for plunger arrival times are provided by manufacturer which need not represent optimal plunger operation. Methods to determine optimal valve closing conditions are presented in US patents US2007/0012442A1 , US2009/0200020A1 , 6883606B2. The presented methods talks about closing the valve based on difference in casing and tubing pressure OR tubing and line pressure, whereas the valve opening remains either fixed time based or arrival time based. Another method of operation is to use load ratio to determine valve open conditions.

[0007] Methods for plunger lift operation presented in literature are most commonly based on fixed time parameters which do not account current well conditions and are not reactive. Some pressure and flow based methods use real time measurements however are heuristic based and does not accommodate reservoir flow and pressure conditions, due to lack of down-hole measurements. These methods may work for some of the wells while can be counterproductive for some others. There exists a need of systematic method which considers the well and reservoir conditions in real time feedback control for determining optimal valve open and close conditions.

[0008] US4392782 and US-5094102 describes some methods for liquid level prediction that involves the use of special down-hole sensors. Due to reliability and cost issues, down-hole sensors are typically not used. Therefore, for measuring liquid level at the bottom of the well an acoustic gun, which generates high frequency pressure signals and measures the reflections, is proposed in US5285388. In US6634426 use of the acoustic gun to identify the location of plunger in the well is also disclosed. A smart plunger, generating acoustic signals while traveling in well, can also be used to estimate liquid level and plunger location in a well as described in US7819189. All the above methods use special equipment: down-hole sensors, acoustic gun or smart plunger to measure the liquid level in the well. There is also a dire need in the art to determine liquid level in a facile and accurate manner without the use of expensive extraneous equipment.

BRIEF DESCRIPTION

[0009] In one aspect, the invention provides a system for operating a plunger lift. The system includes multiple sensors to obtain the surface measurements related to the plunger lift operation; an operational measurement module to receive the surface measurements; and a plunger reference module to receive historical data and mathematical model for the plunger lift operation. The system further includes a plunger processor module configured for estimating dynamic values for a plurality of plunger operation variables from the surface measurements, historical data and mathematical model. The plunger operation variables comprise dynamic values for one or more of plunger location, plunger arrival time, slug volume, well bottom pressure value, gas flow rate, a threshold pressure value, a pressure correction factor, a threshold flow rate value, a correction factor for Turner flow rate, a corrected Turner flow rate, a liquid level, and net production rate. A plunger controller module is configured to use the dynamic values for the plurality of plunger operation variables to determine an optimal open valve condition, an optimal close valve condition, an optimal flow condition, where the optimal open valve condition, the optimal close valve condition, and the optimal flow condition are executed as controller commands by the plunger controller module to yield improved plunger lift operation. A remote terminal unit is provided to receive the controller commands to operate the plunger lift. [0010] In another aspect, the invention provides a method for controlling plunger lift using historical data, mathematical models developed on the basis of historical data and surface measurements for which sensors and actuators are already available in most existing plunger lift operations. The method comprises obtaining plunger location versus time relationship and dynamic slug volume measurement based on the surface measurements and the mathematical model. Subsequently, the method involves estimating the plunger location versus time relationship followed by estimating well bottom pressure based on the mathematical model and obtaining dynamic gas flow rate based on the well bottom pressure. Then, using the estimations, a threshold pressure value is estimated, which forms the basis of opening the valve. The method further comprises estimating a correction factor for critical flow rate such as Turner flow rate based on the mathematical model to obtain a corrected threshold flow rate which is then used to assess whether to close the valve. The method also includes estimating liquid level in the plunger lift by obtaining an instantaneous well bottom pressure based on at least one historical data, which is used to estimate an amount of liquid entering the well; and finally estimating an instantaneous liquid level at annulus and an instantaneous liquid level inside tubing. The method further includes estimating plunger location at a given time in a plunger lift based on surface measurements, by using, mass of slug, liquid slug height, and coefficient of friction based on the surface measurements for more than one point between the well top and bottom at the given time; and estimating a velocity and location of the plunger based on the mass of plunger, the mass of slug, and the coefficient of friction. DRAWINGS [0011] These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: [0012] FIG. 1 is a schematic representation of an exemplary plunger lift installation;

[0013] FIG. 2 is a schematic of a typical plunger-lift cycle; [0014] FIG. 3 is a plot of typical trends of measured operating parameters in a single exemplary cycle;

[0015] FIG. 6 provides a schematic of an exemplary well comprising a plunger illustrating plunger location;

[0016] FIG. 5 is a graphical representation for flow rate and pressure for multiple past n exemplary cycles;

[0017] FIG. 6 provides a schematic of an exemplary well comprising a plunger illustrating liquid level during plunger operation; and

[0018] FIG. 7 shows a block diagrammatic representation of a system of the invention.

DETAILED DESCRIPTION

[0019] As used herein and in the claims, the singular forms "a, " "an," and "the" include the plural reference unless the context clearly indicates otherwise.

[0020] As noted herein, the invention provides a system and methods for improved plunger lift operations.

[0021] In one aspect, the invention provides a method for controlling plunger lift operation. The method for controlling plunger lift comprises obtaining surface measurements related to the plunger lift. Exemplary surface measurements include at least one of, but not limited to casing pressure, tubing pressure, line pressure, flow rate, plunger arrival time, or combinations thereof. These measurements are made available from sensors and actuators that are present in the well where the plunger lift is being operated. The surface measurements are also stored in a suitable storage medium along with other identification tags associated with the measurement. Exemplary identification tags include time of measurement, other relevant factors, performance of the well etc. The surface measurements are also used to derive other estimated parameters based on simple calculations. Such estimated parameters and the calculations to derive them are known to one skilled in the art. [0022] The method includes providing a mathematical model based on at least one historical data. Several models for the surface measurements and the estimated parameters are known in the art, and the choice of suitable model for a particular situation can be made by one skilled in the art without undue experimentation.

[0023] The method includes obtaining plunger location at any given time point may be determined as follows: estimating mass of plunger, mass of slug, liquid slug height, and coefficient of friction based on the surface measurements for more than one point between the lower plunger height and the upper plunger height at the given time. This is achieved by considering force balance on the plunger, which in turn provides position and velocity predictions throughout the well. At any given time during plunger motion, the force balance on the plunger yields (see FIG. 4):

In the above equation, and ^iat are pressures at the top and bottom of the plunger, ^t ^*^∑ is the weight of plunger and liquid slug, ^K is the factional coefficient and " is the plunger velocity.

[0024] During plunger rise, the mass is given by ^mi>*s^{— m}s» Pi^*^ and friction coefficient is given by ^{κ ~ ώ} Pit " ^5miJ, where ^L'∑ is the slug height. During plunger fall, the friction coefficient is a parameter; and ^m?÷s ^{— πίρ} the mass of plunger since there is no liquid slug on top of the plunger.

[0025] Subsequently, the method for obtaining the plunger location at a given time involves estimating a velocity and location of the plunger based on the mass of plunger, the mass of slug, the liquid slug height, and the coefficient of friction.

This is achieved by generating a set of ordinary differential equations starting

h = fr = H

at location ^{11 *"} (during plunger rise), or " (for plunger fall):

[0026] The set of ordinary differential equations provided in equation (6) may be solved using standard numerical techniques. In this manner, the method provides a facile manner to obtain location and velocity of the plunger at any time. Further, the time instant when plunger hits liquid at the bottom of well can be estimated by solving for the position of plunger during its fall.

[0027] Thus, in another aspect, the invention provides a method for obtaining the plunger location at any given time with a fair degree of accuracy in a well.

[0028] The method for controlling plunger lift then comprises estimating a dynamic value for slug volume based on the surface measurements using historical data and mathematical models, which is more accurate than the standard de-fault value used in prior art systems.

[0029] Subsequently, a dynamic value for well bottom pressure may be calculated based on the mathematical model. Using the dynamic value for well bottom pressure, dynamic gas flow rate can also be estimated that increases the accuracy.

[0030] Subsequently, a threshold pressure value is estimated based on plunger rise velocity, and slug volume as given in equation 7 and 8, which is then used to make decisions to open and close the control valve. It may be noted here that after the valve is closed, the controller determines the amount of time the plunger takes to reach the bottom. During the build-up stage, the controller takes a decision to open the control valve.

[0031] In the equations given below, ^s , " f™ and ^{~ rs&1!i} represent improved more dynamic values as compared to the ones calculated using the equations given in prior art. :

(7)

¾ = (Pw + (fW + + Pane) I

After the valve is opened, the controller waits for the plunger to arrive in the catcher. Only during the after-flow stage, the controller takes a decision to close the control valve.

The method provided herein also includes estimating a correction factor for critical flow rate. In the exemplary method a Turner flow rate is used as critical flow rate. . Turner rate is given by the following equations:

(67 - α,ύί Ζγ Σ)*

(9)

And the Turner flow rate is given by

Qrwae-i ^A4^ifi (10)

The decision to close the valve is based on when the measured flow rate falls below this threshold value (i.e. corrected Turner flow rate as in equation (11)).

The correction factor for Turner flow rate is a variable ^τ that modifies the above calculated value to obtain corrected Turner flow rate:

The calculation of ^βτ is based on the gas production rates calculated in the past ^fi cycles (see FIG. 5). The values of gas production rates in the past ^s cycles, and the corresponding values of ^ar are stored as lProdiil Prodm.- , Pro<l(n)} _and ί«_τ(1),«_Γ(2), _f

_Based on these past values, the value of increment, ^Uffr, is computed using steepest gradient method. The next value is then calculated as: ^{¾r ~ s}r^¾.^) + _such ^ maximizes overall daily production. As already noted, the decision to close the valve is based on whether the value of measured flow rate falls below the corrected Turner flow rate value. Note that the Turner flow rate used in the exemplary method is one of the way to calculate critical flow rate. Thus, instead of Turner flow rate any other equation for critical flow calculation can also be used in similar manner to obtain threshold flow rate (given by equation 11).

[0034] In this manner, an improved control of the plunger lift by performing at least one of opening the valve based on the threshold pressure value or closing the valve based on the corrected Turner flow rate is achieved. Thus, the productivity of the well and increased output yields may be achieved using the method of the invention as described below. Further, the life of the plunger lift may also be increased in this manner.

[0035] For productivity and increased output yields, pressure threshold is obtained based on pressure threshold of the past cycles as follows:

□□□□□□□□ ^^threshold^ⁿ+¹'^:ilcycle ⁼ ^threshold) n^thcycle

[0036] The calculation of ΔΡ is based on the pressure threshold and plunger rise velocity in the past n cycles. The values of pressure threshold in the past n cycles, and the corresponding values of plunger rise velocity are stored as

C½^ra ccyyccllee] i (14)

Based on these past values, qualitative and/or quantitative relation (e.g. line fit, polynomial fit, statistical model, etc.) between threshold pressure and plunger rise velocity is obtained. This relation is then used to obtain value of increment, ΔΡ. Note that the value of ΔΡ can be negative or positive or 0. The resulting pressure threshold will take plunger rise velocity (or plunger rise time) within expected range. Moreover, any of the following can be considered as pressure threshold: Casing Pressure, Line Pressure, Tubing Pressure and a combination thereof.

Alternately, the pressure threshold may be calculated using a pressure correction factor as follows .PthresholdXi+i^thcycle ⁼ ^ⁿ+¹^^FG>*)_n+_l ^thcycle (15)

OR

(Pthreshold)ri+l^thcycle ~

Pline) _n+1th_cy_Ci_e (16) where P_FGi is derived from using equation (8) or from prior art based calculations. Equation (15) is used when "Casing Pressure " is considered as pressure threshold, while Equation (16) is used when "Casing Pressure-Line Pressure" is considered as pressure threshold.

[0037] The net production rate as described in equation (17a) below and values for the past n cycles are stored. Again, as described above, the value of increment

Δ" from a steepest gradient calculation is used to calculate the new value of ^F. Alternatively, P can be calculated using past pressure threshold and past Foss & Gaul pressure (Foss and Gaul pressure is calculated by using equation (8) or equation from prior art) . □□□□

Tetai gas produced interne T

Net production = ^:

^Γ ( 1 7a )

threshold) _nth _Cy_Ci_e

(^pFC,<.)„th_cyt._ie

(17)□□□□ [0038] Thus in an alternate embodiment, the pressure threshold may be calculated using weighted sum of pressure threshold obtain in equations 12, and 15or 16, which can be given as P_iferesiiS55i . = y?tfew*ifl ⁺ C^{1 ~} Y ^reshs ^ , where ^ is a tuning parameter between 0 and 1.

[0039] Alternately, in another implementation, the output conditions include net production rate and arrival time for the plunger and manipulated inputs consist of ^ατ and AP D .

[0040] In yet another implementation, the output conditions include net production rraate and arrival time for the plunger and manipulated inputs consist of ^T and β

[0041] In yet another implementation, the output conditions include net production rate and arrival time for the plunger and manipulated inputs consist of α_τ,β, γ and ΔΡ

[0042] The measurement or prediction of liquid level and plunger location in the plunger operated well is critical for operation of plunger lift system. The current practices use special equipment to measure liquid level, which is often costly to install and require high maintenance. In the absence of such specialized equipment, after closing of the valve, the liquid accumulated in the well is not measured. Amount of liquid present in the well is a critical factor in operating plunger lift system. There is a requirement for a cost effective way to predict the plunger location and liquid level in well using only the typical measurements available at well surface.

[0043] Fig. 6 shows schematic of the system used for the calculations given below for liquid level determination. The casing and tubing are connected at the bottom of the tubing. The pressure measured at this level is common and depends on the conditions in tubing and casing as: = P_S(M - Us + Pi a + ft _{( 14)} [0044] In the above equations, ^ is the well depth, ® is gravitational acceleration, rc^iPi _{are sur}f_aCe pressures in casing and tubing, and ¾' ^t<!i are gas and liquid p densities, respectively. There are three unknowns in the above equation, ^{* ¾r}*^f - well bottom pressure, and - liquid levels in the annulus and tubing, respectively.

[0045] The prior art methods for estimating liquid levels only use the current values

_Gf 'cr' ^f Hence, they typically make assumption that there is no liquid in the annulus. Consequently, a simple expression relating liquid level in tubing is obtained as: ^{Li ~} ^ ^{* t}) Pⁱ£ J The above expression is at best approximate, at worst incorrect.

[0046] To overcome this deficiency and error, the invention provides a method for estimating the liquid level in the plunger lift using surface measurements only. The method comprises obtaining an instantaneous well bottom pressure based on at least one historical data, which is more accurate and relevant to the current situation. Specifically, the amount of liquid entering the well is given by:

[0047] Then, the total amount of liquid in the well-bore at any time is estimated using the equation: + = j£ ¾,m<*> di^" ₍₁₇₎

Solving Eq. (14), (15) and (17) simultaneously for the entire history from time to ^" gives us the two liquid levels ("^{ai 4}) and the well bottom ( ^w-^f * at time

. In this manner, an instantaneous liquid level at annulus and an instantaneous liquid level inside tubing are estimated in an accurate and real- time manner. This provides greater control in the entire plunger lift operation thus improving productivity. Further, to improve the prediction of liquid level in the reservoir in the previous cycle, the time instant when plunger hits liquid at the bottom of well as explained in relation with Eq. 5 is used. The mass ' "^* in Eq. (5) depends j

on the size, , of the liquid slug, which can also be used to improve the accuracy in the prediction of the liquid level in the reservoir. This is related to the amount of liquid in the tubing just at the time when the valve was opened.

Thus, the value of can be adjusted to match the measured plunger arrival time. This will yield an accurate (dynamic) value of at ^"°-^B'S!S.

Using the methods described herein, performance of plunger lift and the well may be improved significantly with very little extra additional cost that does not involve any additional expensive equipment, wherein the methods utilize the surface measurements such as casing pressure, tubing pressure, sales line pressure, gas flow rate and plunger arrival time, which are available in real time. Some of advantages enabled through the methods described herein include, improved prediction of liquid level without any direct measurement of liquid level, greater accuracy in the prediction of plunger location without any direct measurement of plunger location, a better predictability of plunger reaching the bottom of the well, and improved prediction in well bottom pressure.

Based on the methods provided herein, a system for operating a plunger lift system by manipulating (open/close) of sales line valve may be implemented in a plunger lift operation such that the decision for opening and closing of sales line valve is optimally decided in a real time controller (e.g. remote terminal unit) using surface level measurements.

The invention also enables the estimation of key parameters that affect the net production of gas and liquids from a particular well, and determine the conditions for valve operation that maximize the daily hydrocarbon production rate.

In another embodiment, a feedback control is provided to update the well parameters online. [0053] The invention further enables use of a dynamic model to estimate the key parameters of a well operation based on the history of measurements of surface casing, tubing and line pressures; and flow rate. This assists in determining the pressure threshold for valve open condition to ensure plunger arrival at the surface with the accumulated liquid. It further allows for determination of the pressure and flow rate conditions for valve close operation that maximizes the daily production from the well.

[0054] In another aspect, the invention provides a control system to implement the methods described herein. FIG. 7 shows a block diagrammatic representation 400 of a control system of the invention that can be implemented in a plunger lift operation. Numeral 500 represents an exemplary well and plunger lift, such as the one shown in FIG. 1. Multiple sensors as shown by block 410 are used to obtain the surface measurements related to the plunger lift operation. Exemplary measurements include, but not limited to, the gas flow rate; pressure values measured in the tubing, the casing and the sales-line; the arrival time taken for plunger to reach the surface after the valve has been opened. An operational measurement module 412 receives the surface measurements from the sensors. A plunger reference module 416 is provided to receive historical data and mathematical model for the plunger lift operation.

[0055] A plunger processor module 416 receives the outputs of operational measurement module 412 and plunger reference module 416 at pre-determined time-intervals and is configured for estimating dynamic values for a plurality of plunger operation variables from the surface measurements, historical data and mathematical model, where the plunger operation variables include one or more of dynamic plunger location, dynamic plunger arrival time, dynamic slug volume, dynamic casing pressure value, dynamic well bottom pressure value, dynamic gas flow rate, a threshold pressure value, a pressure correction factor, a correction factor for Turner flow rate, a corrected Turner flow rate, a dynamic liquid level, an instantaneous plunger location, and net production rate, as already described herein above. As described herein the surface pressures are used to estimate the well energy, and the gas flow rate is used to compute productivity.

A plunger controller module 418 is configured to use the dynamic values for the plunger operation variables as obtained from the plunger processor module 416 to determine an optimal open valve condition, an optimal close valve condition, an optimal flow condition, where the optimal open valve condition, the optimal close valve condition, and the optimal flow condition are executed as controller commands by the plunger controller module to yield improved plunger lift operation. A remote terminal unit (RTU) is used in a non-limiting exemplary embodiment to receive the controller commands to operate the plunger lift.

It would be understood by one skilled in the art that one or more modules as described herein may be also be integrated as a functional component and these modules are configured on a computer processor or as integrated chips.

The calculations as described in the method of the invention to obtain more accurate values of various parameters such as liquid levels, plunger location at a given time which are the dynamic values of the operational variables, improved Turner flow rate, and other estimations, are carried out in a remote terminal unit (RTU) or a programming logic controller (PLC) or an equivalent on-field controller. In another embodiment, the measured data (surface measurements) are communicated by the RTU to a central computer or a supervisory control and data acquisition (SCADA) system and the plunger controller module is integrated with the central computer/SCADA, and the results are communicated back to the RTU and implemented on the shale gas well.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

We Claim:

1. A system for operating a plunger lift in the liquid loaded well, the system comprising: a plurality of sensors to obtain the surface measurements related to the plunger lift operation; an operational measurement module to receive the surface measurements from the plurality of sensors; a plunger reference module configured to receive historical data and mathematical model for the plunger lift operation; a plunger processor module configured for estimating dynamic values for a plurality of plunger operation variables from the surface measurements, historical data and mathematical model, wherein the plunger operation variables comprise one or more of plunger location, plunger arrival time, slug volume, well bottom pressure value, gas flow rate, a threshold pressure value, a pressure correction factor, a correction factor for critical flow rate, a threshold flow rate, a liquid level, and a net production rate; a plunger controller module configured to use the dynamic values for the plurality of plunger operation variables to determine an optimal open valve condition, and an optimal close valve condition, wherein the optimal open valve condition and the optimal close valve condition, are executed as controller commands by the plunger controller module to yield improved plunger lift operation; and a remote terminal unit to receive the controller commands to operate the plunger lift, wherein the operational measurement module, the plunger processor module, the plunger a plunger controller module are configured on a computer processor.

2. The system of claim 1 wherein the optimal open valve condition is based on the threshold pressure value and the optimal close valve condition is based on the threshold flow rate.

3. The system of claim 2 wherein the pressure threshold value and threshold flow rate and liquid level are based on the surface measurements comprising at least one of a casing pressure, line pressure, tubing pressure, flow rate, plunger arrival time or a combination thereof.

4. The system of claim 1 wherein the threshold flow rate is based on the correction factor for the critical flow rate.

5. A method for controlling a plunger lift in a liquid loaded well, the method comprising: providing a mathematical model based on at least one historical data for a plunger lift operation; obtaining surface measurements related to the plunger lift operation; estimating plunger location at a given time in a plunger lift; estimating well bottom pressure based on the plunger location and obtaining gas flow rate based on the well bottom pressure; estimating a dynamic value for slug volume based on the surface measurements; estimating a threshold pressure value based on the dynamic casing pressure, the well bottom pressure, gas flow rate, and slug volume; estimating a correction factor for critical flow rate based on the mathematical model; estimating a threshold flow rate using the correction factor for the critical flow rate. ; estimating liquid level in a plunger lift by estimating an instantaneous liquid level at annulus and an instantaneous liquid level inside a tubing; and controlling the plunger lift by performing at least one of opening a valve based on the threshold pressure value or closing the valve based on the threshold flow rate, using the liquid level for optimal plunger lift operation.

6. The method of claim 5 further comprising estimating a pressure correction factor based on the mathematical model to be applied to obtain a new dynamic threshold pressure.

7. The method of claim 5 wherein the surface measurements comprise at least one of: casing pressure, tubing pressure, line pressure, flow rate, arrival time, or combinations thereof.

8. The method of claim 5 further comprising maximizing daily production for the liquid loaded well by opening the valve by using the threshold pressure value and by closing the valve by using the threshold flow rate.

9. The method of claim 5 wherein estimating liquid level in a plunger lift comprises obtain an instantaneous well bottom pressure based on at least one historical data.

10. The method of claim 5 wherein the estimating plunger location at a given time in a plunger lift, comprises estimating mass of liquid slug, liquid slug height, and coefficient of friction based on the surface measurements for more than one point between a lower plunger height and a upper plunger height at the given time; estimating a velocity and location of the plunger based on the mass of plunger, the mass of slug, the liquid slug height, and the coefficient of friction; and generating a time taken for the plunger to reach the lower plunger height.