Background

Liquid gel concentrates (usually a slurry of guar, suspending agent and dispersing agent in diesel) have been used in oil and gas fracturing services for more than 15 years. The use of concentrates to producing fracturing gels gave the user a better quality gel and eliminated the need to premix gel before the job.

Introduction

Serva’s gel mixing system provides all the advantages of liquid gel concentrates but eliminates the need for making a gel concentrate, the storage tanks, carrier fluids and required chemicals. Thus, our system eliminates the need to make and store a gel concentrate and reduces fracturing fluid costs.

Reasons for using Serva’s Gel Mixing System

• Continuously mix all guar powders to produce a quality, lump free fracturing fluid
• Allows use of relatively small hydration tanks to obtain almost fully hydrated gel (due to high energy and progressive dilution)
• Lower fracturing fluid cost (due to elimination of diesel and chemicals)
• Allows remote location of bulk guar storage (due to the strong vacuum of our mixer)

Principle features

• Variable venturi mixer
• Dynamic diffuser
• Progressive dilution
• High energy mixing and dilution

How the system works

Bulk guar powder is volumetrically metered and then conveyed by vacuum to Serva’s automatic variable venturi mixer that efficiently wets all gel powders. The mixer discharges into a dynamic diffuser for quickly removing the air from the fluid. The diffuser discharges into a series of hydration tanks that feature a progressive dilution system starting with a concentrate gel fluid and ending with the desired gel concentration. This system maximizes the hydration time without allowing the gel to become so viscous that it is not easily pumped or diluted. High shear mixing and agitation of the fluid between the hydration tanks helps to increase the hydration rate and assure uniformity of the fluid. Progressive dilution of a concentrate gel in the hydration tanks increases residence time of the gel in the tanks and thus results in longer hydration time in the limited tank space available. As a result, the system is able to continuously produce gel that is almost fully hydrated by the time it is transferred to the fracturing blender without the need for an increase in the volume of the hydration tanks.

Components Feature Discussion

Our “variable venturi mixer” produces a gel that is free of lumps, stays clean (no fouling) and creates a strong vacuum for guar powder transfer to the mixer. It is also designed to provide optimum performance in a wide range of operating rates (approx. 10/1 turn-down).

The dynamic diffuser effectively removes air from the gel fluid (air enters with the dry bulk) and keeps the diffuser relatively empty even after stopping the process.

The progressive dilution process allows for the maximum hydration of the fracturing fluid in a given hydration tank volume. This feature allows the hydration tank system to be small enough to provide easy transportability to the job site (depending on the throughput rate required, it can be either truck or trailer mounted). The job plan will not require separate hydration tanks on location.

An intensive initial mixing, dilution mixing and agitation process increases the rate of hydration.

Progressive Dilution Discussion

Below is a comparison between a gel created employing the progressive dilution of our system and a gel created according to current mixing practice. In both cases, the feed rate into tank no. 1 is 67.2 lbs/min of guar powder diluted as shown below. Also, in both cases the output produced is forty (40) barrel per minute (bpm) or 1,680 gallons per minute (gpm) gel fluid at a final concentration of forty (40) lbs guar/1000 gal.

For simplification of the examples presented above, the hydration tanks are all shown as equal in size. Hydration tanks do not need to be equal sizes and the dilution amount for each tank does not need to be the same. Individual tank volumes can be adjusted in size to optimize the process. However, the total dilution throughout the process should be the same to create the end-desired concentration. Although equal dilution amounts makes the illustration easier, the dilution process can be tailored to optimize the hydration time without exceeding practical limits in gel fluid viscosity. Thus, since hydration proceeds most rapidly at first and since the first tank has the highest concentration, the tank throughput rate must designed so that the viscosity does not get too high. The same is true for the other intermediate tanks.

Thus, as the foregoing example illustrates, progressive dilution of gel allows the hydration time of guar gel to be increased by more than double without changing the capacity of the tanks used for hydration. In more than doubling the hydration time using existing tank capacity, and by employing centrifugal high sheer pumps and static mixers between the tanks that are used for hydration, the normal hydration rate is increased. Thus, the system produces gel that is more fully hydrated than can be achieved with other gel mixing and hydration systems currently used in the industry.

Tables 1-5 illustrate method for controlling our system while changing throughput rate. The method maintains the concentration in each tank constant even though rate is changing. In order to limit the viscosity in individual upstream tanks, the tank compartment levels are also reduced. With this approach, almost constant output viscosity is realized while limiting the maximum viscosity in any of the hydration tanks to less than 88 cp with throughput rates of 20 – 50 bpm and 97 cp for 15 bpm. Figures 1 – 5 are for 50 bpm, 40 bpm, 30 bpm, 20 bpm and 15 bpm, respectively.

Table 1 shows an example of an initial system with a constant 50 bpm throughput at a guar concentration 35 lb/100 gal of water. This example utilizes four dilution tanks with each tank having a capacity of 40 barrels. The guar feed rate for this concentration is 73.5 lb/min, and the estimated 100% hydration viscosity for the resulting mixture is 33 cp. With this example, an approximate 93% hydration is achieved for an estimated final viscosity of 30.6 cp.

Tables 2, 3 and 4 show the same system as illustrated in Figure 1 when the throughput has been reduced to 40 bpm, 30 bpm and 20 bpm, respectively. In these cases, the individual tank volumes are reduced to 40 bbl, 30 bbl and 20 bbl, respectively. With this approach, the maximum individual tank viscosity is held constant (i.e. residence time in the individual tanks is maintained constant by controlling tank level proportionately to throughput rate.

Table 5 illustrates the same system with a 15 bpm throughput rate but the tank compartment volumes have been maintained at 20 bbl each. Therefore, the original total hydration time is increased and accordingly, the maximum viscosity increases in the compartments containing a concentrate. The original concentration is maintained in all dilution tanks despite the reduction in throughput.

The control, as illustrated in all the figures, is designed so that the original respective concentration in each compartment is maintained while changing mixing rates. It is accomplished by proportionally changing the dilution in all of the dilution tanks simultaneously whenever there is a change in the throughput.