Agitating – Having interrelated feed and discharge means – Recirculating from and to mixing chamber
Reexamination Certificate
2001-11-01
2004-06-15
Cooley, Charles E. (Department: 1723)
Agitating
Having interrelated feed and discharge means
Recirculating from and to mixing chamber
C366S160200, C366S173200, C366S178200, C366S178300
Reexamination Certificate
active
06749330
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high efficiency, high energy slurry mixer used primarily to mix oil field cement in a recirculating system for cementing the casing in oil and gas wells. Specifically, the present invention employs continuous recirculation via a central recirculation line and via annular recirculation jets that are spaced alternately with annular adjustable mix water jets.
2. Description of the Related Art
Utilization of cement within oil wells, particularly in the cementing of casing therein, has been under development since the early 1900's. Two of the purposes of placing cement into the annular space between the casing and the formation are to support the casing within the well, and to seal off undesirable formation fluids.
Casing is typically secured in the well bore by the cement which is mixed at the surface, then pumped down the open center of the casing string and thence back up the annular space which exists between the outer diameter of the casing and the inner diameter of the oil well bore. A displacement fluid, such as drilling mud, is pumped behind the cement to push the cement to the desired location. In many oil and gas well applications it is often necessary to provide cement mixers which will rapidly prepare large quantities of material to be pumped into the well by a batch or continuous process until a sufficient predetermined quantity has been applied.
In either case, the process usually begins with the material being pre-prepared by dry blending and then adding water at the well site. Batch mixing is one form of system to obtain a satisfactory slurry, but batch mixing requires an initial outlay of a large amount of equipment, people, and space. In offshore operations, space and weight capacity are expensive. Batch mixers use valuable space and add to rig weight. Typically, large tanks with rotary paddle type mixers, although being able to adequately perform the mixing operations, have not been efficient in term of space, numbers of people required or equipment costs where large volumes of mixing must be done at the well site.
The quality of the cement slurry placement process involves the completeness of the mixing process and the pumping rate which can affect the bond between the casing and the well bore. The completeness of the mixing process depends on the efficiency of wetting all the dry bulk particles. The pumping rate affects the bond by having an important effect on mud displacement efficiency. There are many other factors that affect the quality of the cementing process.
Many types of cement mixers have been known in the prior art. For example, jet-type mixers and vortex mixers such as those disclosed in U.S. Pat. Nos. 3,201,093 and 3,741,533 have been used with considerable success but have not necessarily been successful in continuously mixing cement slurries. Such jet or eductor type mixers worked reasonably well when slurry designs were simple. With the more enhanced slurry designs of today, the jet mixer cannot adequately mix these slurries. Early type mixers generally had centrally located water jets, while later models added a recirculation flow in combination with the central water jets.
Continuous recirculation mixers were developed to overcome some of the deficiencies of the jet type and batch mixers. These systems mix dry cement and water in an inlet mixer, the output going to a tank for agitation with excess slurry flowing over a weir to an averaging tank, which may be agitated, thence pumped into the well. Typically, a portion of the mixed slurry was recirculated from the mixing tank and directed back into a modified jet mixer. Thus, newly delivered dry bulk cement was wetted both by water and recirculated cement. This provided additional mixing energy that enabled satisfactory mixing. These type mixers were first introduced during the early 1970's. Since that time, cement slurry design has evolved into the use of more complex slurries that earlier continuous mixing systems are unable to mix satisfactorily. Thixotropic slurries with very low “free water” requirements have evolved for the deep, high temperature, high pressure gas wells. It seems as though the industry is constantly testing the ability of mixers by developing even more difficult to mix slurries.
Although prior inventions have taught use of centrally located recirculation jets, or alternately, annularly located recirculation jets, none of the prior art teaches or suggests the desirability of providing both centrally located and annularly located recirculation jets. The present invention incorporates this arrangement and adds mix water jets located between the discrete annularly located recirculation jets so that flows from the annular recirculation jet and the water jets overlap each other. With this arrangement, recirculation rate in the present mixer is independent of slurry design or mixing rate, but is only dependent upon recirculating pump capacity and mixer design.
One of applicant's previous inventions, as taught in U.S. Pat. No. 5,046,855, provided a combination of annular water jets and recirculation jets with no centrally located jets. Another of applicant's previous inventions, as taught in U.S. Pat. No. 5,571,281, included annular recirculation jets and a centrally located water jet. The short coming of the U.S. Pat. No. 5,571,281 mixer was when mixing slurries that had a low water requirement, i.e. small number of gallons of water per sack of cement, or when the mixer was used in a batch mode, there is insufficient energy to effectively wet all the incoming dry bulk cement. The present invention addresses this problem by having both centrally located and annularly located recirculation jets which operate all the time and provide good mixing regardless of slurry design or operation in continuous or batch modes.
Prior art mixers, including both the U.S. Pat. No. 5,046,855 mixer and the U.S. Pat. No. 5,571,281 mixer, utilize discrete annular recirculation jets, i.e. the former having two and the latter having four. The use of discrete jets is practical, but allows the potential bulk cement to by pass the jets, and thus discharge without becoming wet. The present invention addresses this problem by having the mix water jets located between the discrete recirculation jets. These mix water jets overlap the discrete recirculation jets, thus providing 100% coverage of the flow path of the dry bulk cement.
Also, the U.S. Pat. No. 5,046,855 mixer also suffered from discrete mix water jets. That design included six sets of three jets each, for a total of eighteen jets. These jets opened consecutively as increasing water rate was required. Coverage was good when all jets were open, but when only the first set of jets was operational due to low mix rate or low water requirement or both, coverage was poor and mixing quality suffered.
The U.S. Pat. No. 5,571,281 invention provided a continuous circumferential and diverging flow pattern for mix water which worked well when relatively high water rates were required but provided little mixing energy when low water requirements existed or when batch mixing. The present invention provides good coverage and mixing energy regardless of mix water requirement or while batch mixing.
The prior U.S. Pat. No. 5,571,281 mixer provides “baffled” annular space through which recirculated flow passes. This design is unnecessarily restrictive to the recirculation flow and has a low coefficient of discharge. The flow path of the present invention for annular recirculated flow is more streamlined and thus has a higher coefficient of discharge. With a higher coefficient of discharge, this means that pressure head is converted to velocity more efficiently and therefore provides the same mixing energy with less input horsepower.
One object of the present invention is to improve the mixing capabilities as compared to prior art mixers. The present invention will provide more effective and efficient mixing over a wide range of conditions, including both batch
Cooley Charles E.
McKay Molly D.
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