Carbon dioxide pump, pumping system, and method of controlling the same

[0002] This application is a continuation-in-part of U.S. application Ser. No. 09/296,683 entitled “Carbon Dioxide Pump and Pumping System” filed on Apr. 20, 1999, which issued as U.S. Pat. No. 6,224,355 on May 1, 2001.

[0003] This invention generally relates to a pump and pumping system, particularly for carbon dioxide, and more particularly to a pump and pumping system for injecting dense phase carbon dioxide into an oil or gas reservoir. This invention can also be used to pump greenhouse gases such as carbon dioxide, methane, nitrous oxide, or chlorofluorocarbons that can exist in a dense phase and dispose of the gases underground or underwater as applicable. In addition, this invention can be used to pump any gas that can exist in a dense phase. Finally, the invention also relates to a method of controlling the pump using a variable speed drive in conjunction with an on-line gas chromatograph to maintain a constant carbon dioxide injection rate.

[0004] Contrary to popular belief, the United States still has enormous oil resources. Right now, more than 135 years after the birth of the U.S. oil industry, the nation has twice as much oil remaining in its reservoirs as it has so far produced. For every barrel of oil produced to date, two barrels have been left behind. The U.S. oil industry has produced almost 160 billion barrels, but some 350 billion barrels remain as the target of improved oil technologies. Most of this remaining oil, however, is hard to produce. Locked in complex geologic structures, bypassed by conventional technologies, or simply beyond the capability of today's recovery processes, this oil remains elusive.

[0005] There are a number of enhanced oil recovery systems available for increasing production from oil and gas reservoirs. These include steam injection, carbon dioxide and/or nitrogen gas flooding, waterflooding with chemicals such as polymers, surfactants, and alkalines, and the use of microbes to produce gases or chemicals underground that increase the mobility of remaining oil. One of the more popular enhanced recovery systems in the greater Permian Basin area of West Texas and Southeast New Mexico is carbon dioxide (“CO2”) flooding. Carbon dioxide flooding has proven to be among the most promising enhanced oil recovery methods for the United States because it takes advantage of plentiful, naturally-occurring carbon dioxide. When CO2 is injected into a reservoir above its minimum miscibility pressure (a miscible flood), the gas acts as a solvent. The CO2 picks up lighter hydrocarbon components, swelling the total volume of oil and reducing the viscosity of the oil so that it flows more easily. When a field has already been waterflooded, a tertiary CO2 flood will normally provide incremental recovery of about 8% to about 16% of the original oil in place. When CO2 is used instead of waterflood for secondary recovery, the field can produce up to about 40% of the original oil in place.

[0006] Usually, CO2 flooding involves the use of CO2 at existing pipeline pressures, and then injecting the CO2 into the field. When existing pipeline pressures are not high enough to inject the CO2 into the reservoir, the CO2 pressure is boosted with a CO2 pump. Existing dense phase CO2 booster pumping technology uses multistage centrifugal or reciprocating pumps with expensive, double mechanical seals in conjunction with high pressure seal oil systems and seal oil cooling systems. This type of prior art pumping system is typically custom built and housed within large support buildings. Consequently, this type of pumping system is costly, uses large amounts of space, is overly complicated, requires considerable maintenance, and is very time consuming to repair or replace.

[0007] In addition, those involved in carbon dioxide flooding of a field have attempted to adjust or control CO2 injection rates to account for changes in field conditions and changes in the composition of the CO2 being injected into the field. For example, prior art efforts to control CO2 injection rates include floating the CO2 injection pressure on the pipeline pressure, the CO2 recovery plant pressure, or the CO2 re-injection compressor discharge pressure (for sour gas re-injection where no CO2 recovery plant is available). CO2 injection volumes are controlled using a control valve located at a manifold (for a radial system design) or at the wellhead. The limitation for this type of system is that if the gas composition of the CO2 stream becomes less dense due to a rise in gas stream temperature or gas stream composition, CO2 injection can cease altogether and ultimately lower reservoir processing rates.

[0008] In the past, positive displacement plunger pumps have been used to pressurize the dense phase CO2 up to the surface pressure necessary to overcome the lower hydrostatic head caused by the less dense CO2 due to gas impurities and higher temperatures. However, positive displacement pumps are difficult to keep from leaking around the plungers, require much more maintenance than a centrifugal pump, and are not as reliable as centrifugal pumps (run time between maintenance and repair periods are much less than with centrifugal pumps). Where centrifugal pumps were used to boost CO2 injection pressure, the pumps ran at 60 hertz and used a control valve downstream of the pump to adjust the capacity of the pump. This method is inefficient since the medium being pumped is pressurized to a high pressure and is then reduced to a lower pressure across the control valve—resulting in high energy costs per unit volume, reduced reliability of the pump operating away from the best efficiency point on the pump curve, and the initial cost and on-going maintenance of the control valve.

[0009] For the foregoing reasons, there is a need for an improved CO2 pump and pumping system and a method of controlling the same.

[0010] The present invention is directed to a pump and pumping system for injecting dense phase carbon dioxide into an oil or gas reservoir and for injecting dense phase greenhouse gases such as carbon dioxide, methane, nitrous oxide, or chlorofluorocarbons into a reservoir or underwater.

[0011] In one aspect, the invention comprises a downhole electric pump, a motor, and a casing in which the pump and motor are encased or reside, for boosting carbon dioxide or any other dense phase gas. The apparatus has a power source and at least one power lead that connects between the motor and the power source. In another aspect, the invention comprises a method for pumping dense phase gas that involves inserting the casing within an existing piping system by connecting the discharge flange and inlet flange of the piping system with the opposing flanges of the casing. The gas that is pumped by the pump and motor within the casing may be dense phase carbon dioxide gas or any dense phase greenhouse gas.

[0012] Among other advantages, the present invention does not require the use of expensive mechanical seals or their associated seal oil systems, does not require large support buildings, and can be maintained, repaired, or replaced easily and inexpensively.

[0013] The present invention is also directed to a method of controlling the pump and pumping system using a variable speed drive in conjunction with an on-line gas chromatograph to maintain a constant carbon dioxide injection rate. With such control, the CO2 injection rate into the wellbore will remain constant with varying gas compositions, varying surface or bottom hole temperatures, and varying bottom hole pressures. This will result in optimized CO2 flood sweep efficiency and reservoir processing rates.

[0014] The foregoing has outlined rather broadly the features and technical advantages of the present invention so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily used as a basis for modifying or designing other structures, systems, methods, or algorithms for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

[0015] The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention, and, together with the description, serve to explain the principles of the invention. In the drawings:

[0016] FIG. 1 shows a front view of a pumping system of the present invention in its operating environment;

[0017] FIG. 2 shows a cross section of a front view of a pumping system of the present invention illustrating the basic components;

[0018] FIG. 3 shows a detailed cross section front view of a portion of a pumping system of the present invention;

[0019] FIG. 4 shows a suction pressure transmitter, CO2 pump, a pump motor, a volumetric flow meter, a gas chromatograph, a programmable logic controller, and a variable speed drive of the present invention;

[0020] FIG. 5 provides a flow chart of a surface pressure adjustment algorithm;

[0021] FIGS. 6-9 are look-up tables for determining density and specific heat of CO2 gas for varied pressures, temperatures, and gas compositions; and

[0022] FIGS. 10-17 illustrate the change in surface injection pressure as a function of injection rate for varied gas compositions, with changes in the surface temperature and the bottom hole pressure.

[0023] It is to be noted that the drawings illustrate only typical embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention will admit to other equally effective embodiments.

[0024] The present invention relates to a pump and pumping system for injecting dense phase or supercritical carbon dioxide into an oil or gas reservoir. In general, the system involves the use of a downhole electric submersible pump with an appropriate motor encased in a piece of pipe or casing that is placed in an existing surface CO2 pipeline or piping system. Among other advantages, the use of an electric submersible pump does not require the use of mechanical seals or their associated seal oil systems. And, as discussed later, the present invention is directed to a method of controlling the pump and pumping system using a variable speed drive in conjunction with an on-line gas chromatograph to maintain a constant carbon dioxide injection rate. With such control, the CO2 injection rate into the wellbore will remain constant with varying gas compositions, varying surface or bottom hole temperatures, and varying bottom hole pressures. This will result in optimized CO2 flood sweep efficiency and reservoir processing rates.

[0025] A. Pump and Pumping System

[0026] FIG. 1 shows a front view of a pumping system 10 of the present invention in its operating environment. The pumping system 10 includes a pipe or casing 20 that is placed in-line on an existing pipeline shown in part by inlet piping 70 and discharge piping 60. The casing 20 has flanges 22 and 24 that are connected to the discharge piping flange 62 and the inlet piping flange 74, respectively, with a plurality of nuts and bolts. The casing and flanges may be manufactured of any suitable material capable of withstanding the operating parameters of the invention, such as metals, fiberglass, plastics, or advanced composites of epoxy resins reinforced with continuous high-strength, low-density fibers with a thermoplastic lining. The pumping system 10 may be isolated by one or more valves (not shown) upstream or downstream of the piping 70 and 60.

[0027] Although not required, the casing 20 can be supported by one or more beams, here 31 and 33, planted securely in the ground. The casing 20 is fastened to the beams 31 and 33 by u-shaped bars 35 and brackets 37 that encircle the casing 20. The u-shaped bars 35 and brackets 37 may be tightened around the casing 20 by using nuts on the threaded ends of the bars, or by any other fastener means known in the art. Although FIG. 1 shows beam 31 being longer than beam 33 resulting in a slightly-off horizontal orientation (about 15 degrees off the horizon), the pumping system or casing orientation may be horizontal, vertical, or any direction between to fit the existing pipeline or pipeline system.

[0028] There is shown in FIG. 2 a cross section front view of a pumping system 10 illustrating the basic components of an apparatus of the present invention. The pumping system 10 comprises the pipe or casing 20, with flanges 22 and 24, a pump 40, and a motor 50.

[0029] The casing 20 may be any suitable dimension needed to encase the pump 40 and motor 50 of interest. More specifically, the casing 20 may be any suitable dimension as long as the velocity of the CO2 passing over the outside of the motor is sufficient to absorb the heat rejected from the motor (from about 1.0 to about 7.0 feet per second). In the example discussed below, the casing 20 has an inside diameter of about 5.5 inches and a length of about 10 feet. If desired, the casing 20 may have a plurality of centralizers 25 for centering the pump 40 and motor 50 within its internal diameter. Depending upon its construction, the casing 20 is capable of connecting with any size of inlet or discharge piping. In the example discussed below, the casing 20 is connected with a 6-inch pipe that serves as the inlet piping 70 and a 2-inch pipe that serves as the discharge piping 60. The flanges 24 and 74 are ANSI 900 Class raised-face welded neck flanges. The flanges 22 and 62 are ANSI 900 Class ring type joint welded neck flanges. The flanges 22, 24, 62, and 74 can be either slip-on, socket welded, lapped, or welded neck flanges. The ring type joint welded neck flanges were used on the discharge end of the pump casing 22 and the pump discharge end connection 62 to aid in centering the pump and motor assembly. Those skilled in the art will recognize that the pumps can also use ANSI 600 or ANSI 1500 flanges for suction and discharge flanges as well. For higher pressures, API flanges can be used. The limiting factor in the design is the pressure collapse resistance of the motor housing.

[0030] The pump 40 may be any suitable downhole electric submersible pump of sufficient size to create the pressures of interest in the casing 20 selected. While such pumps are typically used downhole for pumping water, oil, and other fluids, the pump 40 of the present invention is used on the surface for pumping dense phase CO2. In contrast to other pumps such as centrifugal, turbine, reciprocal, or gear pumps that include single, tandem, double, or even triple mechanical seal arrangements, a downhole electric submersible pump lacks these expensive seals. In the example discussed below, the pump 40 has 35 stages, can generate approximately 1300 barrels per day (approximately 3,250 thousand cubic feet of dense phase CO2 depending on the purity and temperature of the CO2) at 270 psi of boost, and has an outer diameter of about 4.00 inches and a length of about 4.30 feet.

[0031] The motor 50 may be any suitable motor of sufficient horsepower and voltage to drive the pump of interest in the casing selected. Depending upon the circumstances of use, the motor may range from about 1 to about 300 horsepower and from about 100 to about 2500 volts. In the example discussed below, the motor 50 is a 10 horsepower, 460-volt unit, with an outer diameter of about 3.75 inches and a length of about 3.85 feet.

[0032] The elastomeric components for the o-rings and seals for the pump 40 and motor 50 should be carefully selected to avoid explosive decompression problems. These problems arise when dense phase CO2 permeates the elastomers at high pressure. When the pressure is suddenly reduced, the CO2 cannot escape the elastomer before it flashes to vapor. The net result is explosive decompression. The o-rings used in the pump are a fluoroelastomer (AFLAS) material with a Shore A durometer hardness of 80±5 made by Seals Eastern, Inc. of Red Bank, N.J. The elastomers in the motor are standard materials by Franklin Electric of Bluffton, Ind. If desired, the elastomers in the motor can also be made of AFLAS by Seals Eastern, Inc. The motor leads are made of a nitrile rubber (Paracril) outer jacket with an ethylene propylene (EPDM) core and are also furnished by Franklin Electric.

[0033] The motor 50 has one or more power leads 82 that stretch from the motor 50 through a port 86 in the flange 62 to a power source. The power leads 82 can be protected with several types and combinations of armor, such as a hollow bar shown as 84. Any power lead suitable for the operating conditions of interest may be used. In the example discussed below, the motor 50 has three individual power leads that are about 12.50 feet long that run within a mini mandrel high-pressure pass-through 88 to the power source. A ground 89 may also be used.

[0034] Turning now to FIG. 3, there is shown a detailed cross section front view of a portion of a pumping system of the present invention. The electrical pass through shown as 86 and 88, also known as the potting assembly or mini mandrel, is used to seal around the individual power leads and prevent leakage of the dense phase CO2 in the pump casing to the atmosphere. The electrical pass through consists of a metal housing made of carbon steel or alloys such as 304 stainless steel or 316 stainless steel. The part of the pass through shown as 86 can be threaded or welded to the discharge flange of the pump shown as 62. The part of the pass through assembly shown as 88 is attached to 86 by means of a coupling nut with a female thread which attaches to a male thread on 86. An o-ring and pipe threads prevent leakage of the dense phase CO2 between electrical pass through parts 86 and 88. In addition, leakage of the dense phase CO2 around the power leads 82 is prevented with a combination of elastomer boots and a two-part epoxy which is impervious to CO2 permeation. The electrical pass through is an engineered product provided by EFT Systems, Inc. of Houston, Tex.

[0035] The carbon dioxide used as a secondary or tertiary recovery method in the present invention may be obtained from CO2 supply contracts, recovered from the field, or purified from a natural gas stream. For example, the movement of modest amounts of CO2 from the principal sources to the project site can be done by either bulk truck or rail tank car shipment. These commercial shipments are typically made at 300 psig and 0 degrees F. (liquid). Pipelines are used for larger volumes usually operating above the critical pressure of CO2. As a rule of thumb, to effectively and economically CO2 flood a field, the field should be large enough to have original oil in place of more than five million barrels, and have more than 10 producing wells. The field should also be in an area with an existing infrastructure of CO2 source fields and distribution pipelines. The CO2 injection pressure required for the reservoir is a site variable since miscibility pressure can range from about 1100 psig to about 5000 psig. This pressure varies depending upon the depth of the reservoir and the type of rock, among other things. For low pressures, injection can take place directly from the pipeline, while for higher pressures, the pressure must be increased on site.

[0036] In the example discussed below, the pipeline pressure of the carbon dioxide source ran from about 1750 psig to about 1910 psig, depending upon the CO2 recovery plant discharge pressure and CO2 usage from other pipeline customers. Moreover, because of the attributes of the field, it was necessary to boost the pressure of the carbon dioxide before injecting it into the field, to a pressure of about 2160 psig. Of course, an analysis of the well logs, core samples, maps, production data, well completion history, waterflood injection history, and other data available for the field may dictate other operating pressures.

[0037] The CO2 pumping system of the present invention has several advantages over prior art pumping systems such as cost, simplicity and ease of operation, availability and delivery, environmental, safety, and ease of installation. For example, the installed cost of the CO2 booster pump is approximately $200,000 less per installation versus typical industry installations. The self-contained pump and motor assembly does not require maintenance or special training for operating personnel. Likewise, all components used for the CO2 pump can be standard “off the shelf” items that do not have a long lead time for delivery. The type of CO2 booster pump can be ready for delivery two weeks after the receipt of the order versus 26 weeks or more for typical industry installations. Replacement of the entire pump assembly can be done in 48 hours or less. Besides dense phase carbon dioxide, this type of pump can be used for other environmentally sensitive liquids, as well as any gas that can exist in a dense phase, due to the lack of external seals. In addition, this pump can be used in higher pressures than other pumps without seals, namely magnetic drive pumps. The booster pump is also safe. Due to the lack of external seals, the danger posed by leaking seals is eliminated. The time required to install this type of pump is considerably less than that of competing technologies since it can be bolted between a set of flanges and does not require on-site precision alignment. Other pumping systems need to have the motor and pump aligned on-site for reliable operations. Another advantage is that the pump can be installed vertically and does not take up as much space as competing technologies.

[0038] B. Example

[0039] The following example is a description of a field use of the pumping system of the present invention. It is offered for purposes of instruction to those skilled in the art and should not be considered as a limitation on the scope of the described invention as claimed.

[0040] The carbon dioxide pump of the present invention was used at Altura Energy Ltd.'s (a joint venture between Amoco Corporation and Shell Oil Company) South Wasson Clearfork Unit in Gaines County, Tex. Altura Energy needed to boost the pressure on a CO2 return line from a nearby natural gas plant to be able to re-inject the CO2 into the field to enhance recovery.

[0041] The composition of the source CO2 was as follows:

[0042] Other characteristics of the source CO2, the existing pump suction parameters, and other physical properties germane to this example are as follows:

[0043] a) Pump inlet pressure (psig)=1835

[0044] b) CO2 temperature range (deg F.)=from about +40(winter) to about +80(summer)

[0045] c) CO2 density (lb/cu. ft)=see following tables

[0046] d) Specific gravity @ 60 deg F.=0.7522

[0047] e) Molecular Mass=41.190

[0048] f) Flow rate=3.5 million standard cubic feet per day CO2 mixture described above (converts to approximately 1,225.barrels per day)

[0049] The thermodynamic properties of the source CO2 at different pressures is as follows:

[0050] Thermodynamic Properties at P=1800.000 psia

[0051] Composition: N2 0.0048 CO2 0.8787 C1 0.0819 C2 0.0335 C3 0.0002 IC4 0.0002 C4 0.00007

[0052] Thermodynamic Properties at P=1850.0000 psia

[0053] Composition: N2 0.0048 CO2 0.8787 C1 0.0819 C2 0.0335 C3 0.0002 IC4 0.0002 C4 0.0007

[0054] Thermodynamic Properties at P=1900.0000 psia

[0055] Composition: N2 0.0048 CO2 0.8787 C1 0.0819 C2 0.0335 C3 0.0002 IC4 0.0002 C4 0.0007

[0056] Thermodynamic Properties at P=2100.0000 psia

[0057] Composition: N2 0.0048 CO2 0.8787 C1 0.0819 C2 0.0335 C3 0.0002 IC4 0.0002 C4 0.0007

[0058] The carbon dioxide pump used in this example was a D1400, 35 stages, downhole electric submersible pump manufactured by REDA (a Camco International Company) of Midland, Tex. The pump motor was an FN4 Series, 10 hp, 460V, high temperature, stripper motor for severe duty service manufactured by Franklin Electric Company, Inc. of Bluffton, Ind. The pump and motor were modified to include a brass bushing pump base, bearing material made of graphalloy, and the addition of a thermoplastic centralizer just downstream of the pump intake. The centralizer reduced the unsupported length of the pump and motor assembly from about 8 feet to about 4 feet. The centralizer also reduced the deflection at the coupling between the pump and the motor that in turn extends the life of the seals and bearings on the pump and motor. The potting assembly for the pump motor was specially designed by EFT Systems, Inc. of Houston, Tex. to withstand CO2 permeation. B&M Tool Company of Midland, Tex. manufactured the CO2 booster discharge end connection 62. The end connection was machined out of 4130 material. The pump casing was fabricated by JPN Service Company of Denver City, Tex. and included a 6-inch Schedule 120 A106 Grade B seamless pipe for the outer casing, a 6-inch normalized A105 forged steel raised face welded neck flange with a Schedule 120 bore for the suction flange, and a 6-inch normalized A105 forged steel ring type joint welded neck flange with a Schedule 120 bore for the discharge flange.

[0059] The CO2 pump was tested with recycled CO2 at a pressure of 1910 psig at a suction and discharge pressure varying between 2060 psig and 2200 psig. The pump delivered approximately 1300 barrels of CO2 per day for 344 days. The total measured pump throughput for the 344-day runtime was 1.09 billion cubic feet of dense phase CO2.

[0060] C. Method of Controlling

[0061] The present invention also relates to a method of controlling the pump and pumping system using a variable speed drive in conjunction with an on-line gas chromatograph and flow meter to maintain a constant carbon dioxide injection rate, based on moles, volume, or weight, for varying gas compositions, ambient conditions, downhole temperatures, downhole pressures, and CO2 supply pressures to the pump suction. With such control, the CO2 injection rate into the wellbore will remain constant despite varying gas compositions, varying surface or bottom hole temperatures, varying bottom hole pressures, and varying surface injection pressures. This will result in optimized CO2 flood sweep efficiency and reservoir processing rates.

[0062] Turning now to FIG. 4, there is shown a CO2 pump 40, a pump motor 50, a volumetric flow meter 100, a gas chromatograph 110, a programmable logic controller (PLC) 120, and a variable speed drive or variable frequency drive 130.

[0063] In the preferred embodiment, the carbon dioxide pump 40 is a J350N, 29 stages, downhole electric submersible pump manufactured by REDA (a Camco International Company) of Midland, Tex. The pump motor 50 is an FN4 Series, 250 hp, 460 volts, high temperature, stripper motor for severe duty service manufactured by Franklin Electric Company, Inc. of Bluffton, Ind. The pump 40 and motor 50 are modified to include a brass bushing pump base, bearing material made of graphalloy, and the addition of a thermoplastic centralizer just downstream of the pump intake. Those skilled in the art appreciate that other pumps and pump motors with different specifications may be suitable for the present invention.

[0064] In the preferred embodiment, the flow meter 100 is an ABB Wedge, Model WMP06*20LE10A001 meter, with 6-inch Schedule 160 buttweld ends, supplied by Rawson & Co., Inc. of Houston, Tex. The flow meter or wedge meter 100 may be operated in either horizontal or vertical planes. The flow meter 100 also has a Rosemount Model 3095 multivariable transmitter 102 with a separate remote temperature detector (RTD) 104. As such, the flow meter 100 has multiple 4-20 mA dc outputs (such as pressure P, differential pressure dP, and flow temperature T). Other flow meters are known in the art and can be used in the present invention, subject to such considerations as line size, flange types, process piping temperature ranges, and the need for straight pipe runs up and down stream from the flow meter.

[0065] In the preferred embodiment, the gas chromatograph 110 is a Rosemount Analytical Model GCX Transmitter Process gas chromatograph with integral sample handling system to measure nitrogen, carbon dioxide, methane, ethane, and C3+ in the process stream. The Model GCX gas chromatograph features a fully modular design with components housed inside a compact, lightweight, field-mountable explosion-proof case. Depending upon the installation site, the gas chromatograph 110 may be enclosed and provided with a thermostatically-controlled heater or air-conditioner to keep the chromatograph within operational temperature ranges. The Model GCX has up to six isolated 4-20 milliamp DC trend outputs (such as density, Z compressibility factor, and percent CO2, among others). Those skilled in the art will recognize that other on-line gas chromatographs may be suitable in the present invention.

[0066] In general, gas chromatographs are used to measure various gas species in a gas sample. A portion of the gas sample is received at an inlet of the gas chromatograph. The gas sample is moved through a column which has an interior that is lined with one of any number of known materials, depending on the particular application or gas chromatograph being used. The column separates the larger and smaller molecules in the gas sample. Thus, the gas sample exits the column in such a manner that the first gas species out of the column is the one with the smallest and lightest molecules, while the last species is the one with the largest and heaviest molecules. The gas exiting the column is directed to a detector which detects the various gas species in the sample, as they exit the column. The detector, in turn, provides an output signal indicative of those gas species.

[0067] In the preferred embodiment, the programmable logic controller 120 is a Simatic S7-300 mini PLC manufactured by Siemens AG. It is a modular rail-mounted system with a CPU 318-2DP, 512KB system, STEP 7 programming software, analog input 8 ch 9/12/14 bits signal module, SM332 analog output 4 ch 12 bits signal module, PROFIBUS-DP open bus communications interface, ET200M distributed I/O system, Simatic S5/S7 bus connections, and OP17 human-machine interface. Other PLC systems known in the art, such as those manufactured by Allen-Bradley, Mitsubishi, Telemecanique, AEG Modicon, and the like, with different CPU performance requirements, signal modules for digital and analog input and output, function modules for high-speed counting, positioning and position control, and communication modules, may be used in the present invention. Generally speaking, a PLC is a (micro)processor or a system of (micro)processors for the execution of a specific task, for example the monitoring and control of a process. Such a process may be the control of a machine as well as of a complex of collaborating installations. For correct operation, it is necessary to execute a number of tasks while responding to signals originating from the process or system to be controlled and to control parameters from, for example a user console.

[0068] As its name implies, the variable speed drive 130 varies the rotational velocity of the motor 50 driving the pump 40 in accordance with a commanded speed N produced by the microprocessor controller 120. In the preferred embodiment, the variable speed drive 130 is a Toshiba G3 Series adjustable speed drive with true torque control, IGM/IGBT transistors, and high current capability. Here, the speed drive 130 has a maximum horsepower of 250 hp, 460 volts, 3520 rpm, service factor of 1.15, and maximum amps of 340. As before, those skilled in the art will recognize that other variable speed drives having different horsepowers, voltages, rpm's, service factors, and amperages may be suitable in the present invention.

[0069] In operation, the on-line gas chromatograph 110 takes a gas sample off the CO2 pump discharge line 60 approximately every 15 minutes (about 4 times per hour) and supplies five (5) 4-20 milliamp outputs to the programmable logic controller 120:

[0070] 1. Density;

[0071] 2. Z compressibility factor;

[0072] 3. Carbon dioxide (mole %);

[0073] 4. Sum of non-carbon dioxide components (mole %); and

[0074] 5. Sum of hydrocarbon components (mole %).

[0075] If desired, the gas samples may be taken less frequently, such as every 60 minutes (1 time per hour) or every 30 minutes (2 times per hour), or more frequently, such as every 12 minutes (5 times per hour) or every 6 minutes (10 times per hour), or even randomly, depending upon the requirements of the operator.

[0076] In addition to the inputs sent by the gas chromatograph 110, the PLC 120 also receives inputs from a downhole pressure sensor 142 and a downhole temperature sensor 144, or from manual inputs based on prior downhole pressure and temperature surveys. Because these inputs are less variable, the downhole pressure and temperature are typically checked quarterly (every 3 months) or less frequently (every 4 months).

[0077] The programmable logic controller 120 also receives the following 4-20 milliamp inputs from a multivariable transmitter 102 mounted on the differential pressure flow measuring device 100 (such as a wedge meter, orifice meter, mass flow meter, or V-cone meter):

[0078] 1. Static pressure P;

[0079] 2. Differential pressure dP; and

[0080] 3. Flowing Temperature T.

[0081] If desired, the programmable logic controller 120 can receive a 4-20 milliamp input from a pressure transmitter 146 mounted on the suction piping 150. This pressure transmitter 146 is a safety device that will ensure that the CO2 pump 40 does not operate above the operating limits of the pump 40, the discharge piping 60, or the reservoir frac pressure. In addition, the pressure transmitter 146 protects the pump 40 from low suction pressure that could cause the pump to reach shut off head, also known as flow condition.

[0082] Other manual inputs for the programmable logic controller 120 are specific for a particular well and include the following:

[0083] 1. Well depth;

[0084] 2. Dense phase gas injection rate;

[0085] 3. Tubing internal diameter, length, thickness, and associated heat transfer coefficients;

[0086] 4. Tubing internal and/or external coating thickness and associated heat transfer coefficient;

[0087] 5. Well casing internal diameter, length, thickness, and associated heat transfer coefficients;

[0088] 6. Well casing internal and/or external coating thickness and associated heat transfer coefficient; and

[0089] 7. Thermal conductivity of the earth and thermal diffusivity of the earth.

[0090] Turning now to FIG. 5, there is shown a flow chart of the surface pressure adjustment algorithm used in the present invention.

[0091] The first step of the algorithm is to calculate a temperature profile. The PLC 120 takes some of the noted inputs discussed previously and puts them into a wellbore heat transmission algorithm developed by Ramey (1962) to calculate a non-linear temperature profile of the dense phase gas stream as it is injected down the tubing string.

[0092] The second step of the algorithm is to calculate temperature rise due to friction using classical fluid dynamics. If the work lost due to friction and the resulting heat rise is significant, the temperature profile in the first step is adjusted, otherwise friction is ignored.

[0093] The third step of the algorithm is to calculate the pressure profile down the wellbore.

[0094] As noted in the fourth step, the NIST 14 Mixture Property Database algorithm, available from the National Institute of Standards & Technology in Gaithersburg, Md. at their website of <www.nist.gov>, is utilized to calculate density and specific heat of the gas for varied pressures, temperatures, and gas compositions. Alternatively, the PLC 120 can use a reference look-up table to interpolate bottom hole density and specific heat. Sample look-up tables are provided in FIGS. 6-9, where the gas composition varies depending upon the source, such as pipeline CO2, recycle CO2, off spec CO2, and pure CO2.

[0095] In the fifth step of the algorithm, the pressure is then integrated down the wellbore, adjusting the surface injection pressure to achieve the desired injection bottom hole pressure.

[0096] Returning to FIG. 4, the PLC 120 will output a 4-20 milliamp signal depending upon the calculated surface injection pressure to the variable speed drive 130. The speed drive 130 will use the input in a Proportional-Integral-Derivative (PID) loop to vary the frequency of the motor 50 driving the CO2 pump 40, causing the pump discharge pressure to vary. As is appreciated to those skilled in the art, the CO2 pump discharge rate and discharge pressure and head vary according to the affinity laws:

[0097] 1. Capacity (Q) varies directly as the speed (N);

[0098] 2. Head (H) varies as the square of the speed; and

[0099] 3. BHP varies as the cube of the speed.

[0100] Several charts, FIGS. 10-17, showing the change in surface injection pressure as a function of injection rate for varied gas compositions, with changes in the surface temperature and the bottom hole pressure, are provided for purposes of illustrating the principles of this invention.

[0101] The advantage of this system versus existing systems is that CO2 injection rate into the wellbore will remain constant with varying gas compositions, varying surface or bottom hole temperatures, and varying bottom hole pressures. Using the present invention of a pump combined with a variable speed drive and gas chromatograph, the pump will run at or very close to the best efficiency point by maintaining a constant discharge pressure or by maintaining a constant rate (based on the affinity laws). This will result in optimized CO2 flood sweep efficiency and reservoir processing rate.

[0102] Although the present invention and its advantages have been described in considerable detail, it should be understood that various changes, substitutions, and alterations can be made herein to the pump, pumping system, and method of controlling the same without departing from the spirit and scope of the invention as defined by the appended claims.