A new fracturing sand mud (slurry) mixing system has been designed. High sand concentration slurries using a high viscosity non-Newtonian gel can be mixed at low or high rates. A full size mud mixing system was built and tested with fracturing gel and sand. This system includes automatic agitation control, new baffle and mud agitator design excepts, and new methods for rapid material wetting. The new mixing system has been utilized on new stimulation blending equipment.
Mixing sand slurries to essentially homogeneous uniformity for fracturing is a problem in the oil service industry. Problems typically encountered when mixing high concentration slurries at high rates are air entrainment in the fluid, inadequate solids wetting, and dispersion of solids. Various mixing method shave been tried by service companies with varying degrees of success. As a part of the development of new fracturing equipment new sand slurry mixing system has been designed. High sand concentration slurries using a high viscosity non-Newtonian gel can be mixed at low or high rates. Conventional mud agitator designs typically will not perform the mixing job at high sand concentrations, because as the sand concentration increases the slurry becomes more viscous. Sand concentration in the oil service industry is defined as the mass of solids per unit volume of fluid.
The specific objectives of this research (1) were to obtain the information necessary for the design of a full scale fracturing sand slurry mixing system and (2) to design a mud mixing system to produce a homogeneous, high sand concentration slurry.
Review of Literature
Mixer performance is typically expressed in terms of fluid velocity generated, total pumping capacity of the impeller, total flow in the tank or in terms of an evaluated criterion. If the application is relatively simple and easily analyzed, a detailed examination of complicated fluid mechanics in a mixing tank will be unnecessary. When the process is complex, a thorough examination of shear rates and tank turbulence may be required. Sometimes it is possible to express a complex mixing process in terms of bulk velocity and pumping-capacity relationships. Significant progress has been made in organizing and presenting mixing design concepts and procedures. These procedures and techniques were utilized in this work to make preliminary estimates of mixing system specifications.
Classification of Problem
Mixing fracturing sand slurries can be define as: the mixing of solids and liquids into a pseudo-homogeneous mass that is more or less stable. A well mixed slurry using fracturing gel should have some life before noticeable sand settling occurs. These fracturing gels are normally very viscous and non-Newtonian in nature. They are shear thinning fluids, a property which makes the design of the mixing tank and agitator system extremely critical.
The mixing task to be performed is to mix fracturing sand and gel into a uniform suspension. Agitation within the tank must be intense enough to create the required fluid turbulence to complete the mixing job in the required time. The process is a continuous one with flow rates up to 75bbl/min. A larger mixing tank increases the fluid residence time in the tank, thus increasing the time available to do the mixing job. The volume of the tank is constrained by its installation on mobile equipment. The volume was chosen to be as large as possible to accommodate a mixing tank whose diameter was approximately equal to its fluid depth and still fit within the constraints of mobile equipment. The mixing tank design volume used in this work was 9bbl. Residence time in the tank at this volume and design flow rates ranged from 60 seconds at 9 bbl/min to 7.2 seconds at 75bbl/min. the time available to perform a mixing task has a considerable effect on mixer power requirements. As mixing time decreases the input power required will increase for a constant process result. This mixing task is further complicated because most fracturing sand slurries are high viscosity, non-Newtonian and shear sensitive. Mixing shear sensitive fluid normally requires a large, slow speed agitation system to provide uniform slurry movement from top to bottom of the mixing tank. The diameter of the slurries agitator must be large enough in relation to the mixing tank diameter to provide approximately the same slurry velocity throughout the mixing tank. A more uniform shear rate within the tank is the result, thus minimizing dead spots or low turbulence areas.
The difficulty of solids wetting is another important design consideration. Fracturing sand is typically very difficult to wet with high viscosity fracturing gels. Sand and geo to be mixed must enter the mixing tank at approximately the same place and be subjected to intense turbulent mixing to prevent formation of sand agglomerates. The sand input rate into the mixing tank increases with throughout rate or sand concentration. As the amount of sand to be wetted increases, intensity of agitation must also increase to complete the sand wetting process and achieve a constant process result. As the intensity of agitation increases the input power required will increase.
Increasing effective viscosity in the mixing tank, as sand concentration increases, also adds difficulty to the mixing task. As the effective viscosity increase, the intensity of agitation must also increase to keep the mixing process turbulent. The size of the mixing tank was determined from mobile equipment considerations and constraints, diameters of the agitators were made as large as possible because of shear is not as important as the uniformity of shear. With the mixing tank size and slurries agitator size fixed the task was to experimentally determine the following.
- Number and type of slurries agitators.
- Best location for incoming sand and fracturing geo to enter the mixing tank to enhance initial sand wetting.
- Slurries agitator speed and input requirement for the range of throughout flow-rates and sand concentrations.
BENCH SCALE TESTS
A bench scale mixing tank approximately half scale was built to determine initial design criteria. All bench scale tests were done using 20/40 mesh sand and fracturing fluid containing 40lb HPG/1,000 gal. Figure 1 is a picture of this mixing tank. Figure 2 is general schematic of the final mixing tank and agitator system, which is the result of intensive testing with bench scale mixing tank. The bottom agitator (mud agitator, slurries agitator) is an axial flow type (flow is through the center) that prevents sand settling in the bottom of the tub.
The top agitator is a non-conventional type that flows radically outward as well as flowing downward through the middle. Extensions on all the blades are pitched to cause upward flow. This extension provides flow velocity to increase the turbulence at the top of the tub. The sand and clean liquid enter there and initial sand wetting takes place. If the slurry is not turbulent, dry sand stacks up, leading to a complete sand out of the mixing system.
Although baffles are necessary to prevent high sand concentration slurry from just spinning in the tub, when solids baffles are used, sand settles out around them, increasing the probability of mixing system sand out. Expanded metal baffles were used to provide enough resistance to force primarily top to bottom flow within the tank.
Figure 3 is a plot of sand concentration vs time. This plot is an example of the type data collected with the bench scale system. It is at a flow rate of 5bbl/min and shows that a sand concentration of approximately 21lb/gal was achieved for over 3 min.
FULL SCALE MUD MIXING SYSTEM
Dimensional analysis indicates that scale-up methods can use the results of small scale mixing tests and duplicate the fluid behavior that is necessary to achieve equipment process results in large scale equipment. Geometric, kinematic, and dynamic similarity cannot always be achieved at the same time.
In this mixing system geometric similarity was used to scale up the geometric parts. Various lengths within the system were scaled up by a fixed ratio. The mud agitator speed was then adjusted on the large scale system to achieve the desired process result. An automatic agitator speed control system was incorporated into the design of the new blending equipment on which this mixing system is utilized. The control system increases the agitator speed as the sand concentration increases and as the throughout flow rate increases in an attempt to keep the process result the same.
A full scale mixing system was built, tested, then utilized as a part of new automatic remote controlled blending equipment (figure 4). Figure 2 and 5 are schematic of the mixing system showing the mixing tank, mud agitator (slurry agitator), sand and fluid entrance points, and slurry exit point.
Data collected during full scale testing are shown in figures 6 though 10. All full testing used 20/40 mesh sand and fracturing fluid containing 40lb HPG/100gals. These figures show sand concentration of 21 lb/gal was achieved at a flow-rate of 10bbl/min. Figure 7 show a stepped increased in sand concentration up to 18lb/gal. Figure 8 shows a continuous increase in sand concentration to 18lb/gal then holding 18lb/gal for 90 second. Figure 9 shows a continuous run to a sand concentration of 19lb/gal. Figure 10 is for a test at a slurry rate of 50bbl/min and sand concentration ramped up to 8lb/gal.
- A reliable mixing system for fracturing sand slurries has been bult. This system will mix sand concentrations of up to 22lb/gal.
- Existing information and scaling coupled with proper test work was utilized to design this fracturing sand slurry mixing system.