Density measuring apparatus containing a densimeter and a method of using the same in a pipeline

A densimeter contains an inlet pipe section, a second pipe section joined to the downstream end of the inlet pipe section, and an outlet pipe section joined to the downstream end of the second pipe section. The inlet and outlet pipe sections are constrained by a support structure, while flexible couplings allow the second section of pipe to undergo a limited amount of radial motion relative to the adjacent pipe sections. The weight of the second pipe section is supported by a weight measuring unit, which continuously measures the weight of the second pipe section as fluid flows through the densimeter. One preferred embodiment of the invention uses the densimeter to measure the density of a proppant in a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations.

FIELD OF THE INVENTION

The present invention relates generally to a density measuring apparatus for measuring the density of a fluid flowing continuously through a pipeline, and a method of using the same. In a preferred embodiment, the density measuring apparatus of the invention is used to measure the density of a proppant slurry in a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations.

BACKGROUND OF THE INVENTION

Fracturing and propping an oil well is a well known process in which fluid, generally water or oil, is pumped into an oil well at high flow rates (typically 200 to 5000 gallons per minute) and high pressures to hydraulically fracture the underlying oil bearing formation. The fluid is combined with any number of chemicals to produce certain fluid properties. Generally the fluid is mixed with certain polymers to increase its viscosity and allow it to transport a proppant into the fracture created. The fluid is further designed to lose viscosity once it is in the fracture allowing it to leave the porous proppant in the fracture to provide a path for the oil to flow back to the well bore.

To achieve the best performance from the fluid, the various components of the fluid must be mixed together in the proper proportions. One way to verify the composition of the fluid is to measure its density. Because the various components have different densities, the density of the fluid will vary according to the composition of the fluid. It is desirable to measure the density of the fluid before the fluid leaves the blender system, so that the composition of the fluid can be corrected before the fluid is pumped into the well.

Several different types of densimeters are currently used to measure the density of the fluid in the blender system. Some densimeters known in the art use a nuclear gauge which sends a stream of gamma rays through the flowing fluid and determines the density based on the amount of radiation scattered by the fluid. These nuclear densimeters have certain drawbacks, including safety concerns that are always present when working with radioactive materials, and a time lag associated with each density measurement. Also, nuclear densimeters measure the density of only a localized area of the fluid. This is a disadvantage when using a fluid such as a sand slurry, that may not have uniform consistency. When using a nuclear densimeter with such a fluid, the densimeter may measure a localized change in density due to a small pocket of denser material that is not thoroughly mixed into the surrounding fluid. In that case, it is impossible to know whether the changed density is due to a localized concentration of material or a systemic problem with the fluid composition.

Another method of measuring fluid density involves diverting a portion of the fluid flow into a separate U-tube. The U-tube is weighed, along with its contents. The density of the fluid is then calculated, based on the fluid volume contained in the U-tube. While eliminating the safety concerns associated with nuclear densimeters, U-tube densimeters can still provide misleading results when used with inconsistently mixed fluids. Furthermore, the additional tubing needed to construct the U-tube presents a disadvantage, especially in an oilfield environment.

It is the object of the present invention to provide an improved density measuring apparatus which is capable of accurately measuring the density of a fluid flowing in a pipeline and which is not nuclear based. More specifically, it is an object of the present invention to provide a densimeter that can measure the density of a sufficiently large sample of the fluid such that the effect of any local inconsistencies in the fluid composition on the density measurement is minimized. Additionally, it is an object of the present invention to provide a densimeter having a simple, durable structure. It is also an object of the invention to provide a densimeter which produces a density measurement with no appreciable time delay, so that any irregularities in the fluid composition can be identified and corrected immediately.

Further, it is the object of the present invention to provide a method of using the density measuring apparatus to measure the density of a fluid flowing in a pipeline. More specifically, it is an object of the present invention to provide a method of measuring the density of a fluid of unknown density flowing in a closed pipeline system, such as a cementing unit or a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations.

SUMMARY OF THE INVENTION

The present invention provides an apparatus capable of continuously measuring the density of a fluid flowing through a pipeline. The apparatus of the present invention comprises an inlet pipe section, a second pipe section joined to the downstream end of the inlet pipe section, and an outlet pipe section joined to the downstream end of the second pipe section. The inlet and outlet pipe sections are constrained by a support structure as known in the art, but the flexible couplings allow the second section of pipe to undergo a limited amount of radial motion relative to the adjacent pipe sections. The weight of the second pipe section is supported by a weight measuring unit, such as a load cell, which continuously measures the weight of the second pipe section as fluid flows through that pipe section. In this way, the entire second pipe section functions as a densimeter.

For a pipe section having a known empty weight and a known fluid flow volume, the density of the fluid can be easily calculated based on the weight measured by the weight measuring unit. Because the sample size includes all of the fluid in the second pipe section, localized inconsistencies in the mixture are not reflected in the density measurement. Furthermore, the structure of the density measuring apparatus is simple and durable, making it easy to install and maintain in a variety of applications. Also, the weight measuring unit measures the weight of the second pipe section continuously, eliminating the time delay.

In one specific embodiment described herein, the apparatus of the invention is used to measure the density of a proppant in a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations. The discharge manifold of the blender system is fitted with a flexible coupling on each end, permitting it a limited range of motion independent of the pipe sections immediately upstream and downstream. A load cell measures the weight of the discharge manifold during fluid flow.

The invention also encompasses an improved method for measuring the density of a fluid flowing through a closed pipeline system. According to the method of the invention, the densimeter is first calibrated by weighing the densimeter while a fluid of known density flows through the pipeline. Next, a fluid of unknown density is introduced into the pipeline. As the fluid flows through the densimeter, the combined weight of the densimeter and the fluid is measured. From this information, the density of the fluid is calculated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-3have similar elements that are similarly numbered and will be described in conjunction with each other. A pipeline carries a fluid flow in the direction indicated by the arrows inFIG. 1. A densimeter12is installed in the pipeline, the densimeter12comprising an inlet section14having a downstream end16, a second section18having an upstream end20and a downstream end22, and an outlet section24having an upstream end26. A first flexible coupling28joins the inlet section downstream end16to the second section upstream end20. A second flexible coupling30joins the second section downstream end22to the outlet section upstream end26. The flexible coupling may be any type of flexible connector known in the art, including, without limitation, elastomeric couplings or flexible metal couplings. A preferred embodiment uses part number 208-008 mechanical joint couplings manufactured by John L. Schultz, Ltd.

An inlet support32carries the weight of the inlet section14, while an outlet support34carries the weight of the outlet section24. A weight measuring unit36positioned underneath the second section18carries the weight of the second section18. The weight measuring unit36also measures the weight of the second section18as fluid flows through the second section18. In a preferred embodiment, the weight measuring unit36is a compression-type load cell, such as the LCH series manufactured by Omega Engineering. However, the densimeter will function equally well with any other type of weight measuring unit known to the art, such as a hydraulic gauge. The weight measuring unit36is supported by a bracket38.

FIG. 4illustrates an alternate embodiment of the density measuring apparatus. In this alternate embodiment, weight measuring unit40is positioned above, rather than below, the second section18. Weight measuring unit40is attached to a bracket42or other support structure located above the pipeline. In one embodiment, weight measuring unit40is a tension-type load cell, such as the LCH series manufactured by Omega Engineering. As discussed above, other types of weight measuring units may be used without departing from the scope of the invention. Weight measuring unit40is attached to second section18by a support rod44. The support rod44, which carries the weight of the second section18, may be connected to second section18by any means known to the art.

As can be best seen inFIG. 3, the densimeter12has an outer surface46and an inner surface48. The inner surface48defines a bore50through which fluid can flow. As shown inFIG. 3, the bore50has a circular shape as is commonly known in the art. In a preferred embodiment of the present invention, inlet section14, second section18, and outlet section24all have the same cross-sectional shape, which matches that of the pipeline generally. This cross-sectional shape is uniform and does not vary along the length of the densimeter12. Although the densimeter of the preferred embodiment has a uniform, circular bore, the densimeter would function equally well with any type of cross-section known to the art, such as a non-circular cross-section, an open U-tube that carries the entire flow volume, or a cross-sectional shape that varies along the length of the densimeter12. The density measuring apparatus of the present invention can therefore be adapted for use with any type of pipeline known to the art.

The operation of the density measuring apparatus of the invention will now be described in detail. In operation, the densimeter12is initially empty of fluid. While the densimeter12is empty, the weight measuring unit36is calibrated to produce a reading of zero. After the weight measuring unit36is calibrated, a fluid of known density, such as water, is pumped through the densimeter12at a known flow rate. The weight measuring unit36is again calibrated to produce a reading of zero, while the flow rate of water through the densimeter12is maintained at a constant level. After the weight measuring unit36is calibrated to zero with a constant flow of water through the densimeter12, other components, such as proppants or cement powder, are introduced into the water in desired proportions, resulting in a slurry. The slurry components may be any type known in the art, having a density different from that of water. Due to the different densities of the slurry components, the density of the resulting slurry will vary depending on the amount of each component present in the slurry.

The slurry flows through the pipeline in the direction indicated by the arrows inFIG. 1. The slurry flows first through the densimeter inlet section14. When the slurry reaches the downstream end16of the inlet section14, the slurry passes into the second section18. The slurry continues to flow from the upstream end20of the second section18to the downstream end22of the second section18. When the slurry reaches the downstream end22of the second section18, the slurry passes into the outlet section24. The slurry enters the upstream end26of the outlet section24, flows through the outlet section24, and continues through the pipeline.

Inlet support32and outlet support34constrain the inlet section14and outlet section24from movement in the vertical direction during slurry flow. The first flexible coupling28and the second flexible coupling30allow the second section18to undergo a limited amount of radial movement relative to the inlet section14and outlet section24during slurry flow.

Although the weight measuring unit36carries the entire weight of the second section18and the slurry that flows therethrough in the preferred embodiment, other configurations are possible. For example, a portion of the weight of the second section18may be carried by the inlet section14, by the outlet section24, or by any other support structure as is commonly known. Other possible configurations may also include using multiple weight measuring units; for example, one weight measuring unit may be positioned near the upstream end20of the second section18with a second weight measuring unit (not shown) positioned near the downstream end22of the second section18. The density measuring apparatus of the present invention will work equally well in any of these configurations, once the weight measuring unit36is calibrated as described above.

Continuing with the description of the preferred embodiment, the weight of the second section18will change as the density of the slurry changes. For example, in the case of a slurry containing a proppant having a density greater than that of water, the density of the slurry will increase as the amount of proppant in the slurry increases. As the density of the slurry increases, the weight of the second section18will also increase. The flexible couplings28,30enable the second section18to move in a radial direction independently from the inlet section14and outlet section24. This independent movement of the second section18in turn enables the weight measuring unit36to register the changing weight of the second section18. The weight measuring unit36is connected to a control unit (not shown) to continuously meter the weight of the second section18and, based on the weight of the second section18, to calculate the density of the fluid. The control unit may be of the type disclosed in U.S. Pat. No. 6,007,227, incorporated herein by reference.

As can be seen inFIG. 5, a further preferred embodiment of the invention uses the densimeter to measure the density of a proppant in a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations. In this preferred embodiment, flexible couplings28,30join the discharge manifold52of a blender truck54to the immediately adjacent upstream section of pipe56and downstream section of pipe58. The discharge manifold52therefore acts as the densimeter second section18in the above discussion. A weight measuring unit60is installed on the blender truck54underneath the discharge manifold52to continuously measure the weight of the discharge manifold52. Alternatively, weight measuring unit60can be installed on the blender truck54above the discharge manifold52.

The weight measuring unit60is calibrated as described above, by first measuring the empty weight of the discharge manifold52, zeroing out the weight measuring unit60, flowing a continuous stream of water through the discharge manifold52, and zeroing out the weight measuring unit60again. Once the weight measuring unit60is calibrated, the blender tub62begins mixing a proppant into the water stream to create a slurry. The slurry leaves the blender tub62and flows through the discharge manifold52, at which point it is discharged from the blender truck54. As the slurry flows through the system, weight measuring unit60continuously measures the weight of the discharge manifold52containing the slurry. Weight changes in the discharge manifold52can indicate problems with the slurry composition; for example, if the proppant flow is obstructed, the slurry will contain too little proppant, resulting in a lower density slurry.

While the invention has been described with reference to a specific illustrative embodiment for use in a blender system, the present invention will also be useful in a wide range of applications wherein it is desirable to determine the properties of a variable-density substance. Therefore, the preferred embodiment showing the densimeter of the present invention used in a blender system is understood to be illustrative and not limiting.