This invention relates generally to apparatus and methods for mixing at least two substances, such as dry cement and water. This invention relates more particularly, but not by way of limitation, to an apparatus for producing a cement slurry at a well site and to a method of performing a cement job on a well so that a cement slurry is made and placed in the well.
After the bore of an oil or gas well has been drilled, typically a tubular string, referred to as casing, is lowered and secured in the bore to prevent the bore from collapsing and to allow one or more individual zones in the geological formation or formations penetrated by the bore to be perforated so that oil or gas from only such zone or zones flows to the mouth of the well. Such casing is typically secured in the well bore by cement which is mixed at the surface, pumped down the open center of the casing string and back up the annulus which exists between the outer diameter of the casing and the inner diameter of the well bore. Typically a displacement fluid, such as water, is pumped behind the cement to push the cement to the desired location.
The mixture of cement to be used at a particular well usually needs to have particular characteristics which make the mixture, referred to as a slurry, suitable for the downhole environment where it is to be used. For example, from one well to another, there can be differences in downhole pressures, temperatures and geological formations which call for different types of cement slurries. Through laboratory tests and actual field experience, a desired type of cement slurry, typically defined at least in part by its desired density, is selected for a particular job.
Once the desired type of cement slurry has been selected, it must be accurately produced at the well location. If it is not, adverse consequences can result. During the mixing process, slurry density has typically been controlled with the amount of water. Insufficient water in the slurry can result in too high density and, for example, insufficient volume of slurry being placed in the hole. Also, the completeness of the mixing process can affect the final properties of the slurry. A poorly mixed slurry can produce an inadequate bond between the casing and the well bore. Still another example of the desirability of correctly mixing a selected cement slurry is that additives, such as fluid loss materials and retarders, when used, need to be distributed evenly throughout the slurry to prevent the slurry from prematurely setting up. This requires there to be sufficient mixing energy in the slurry mixing process. More generally, it is desirable to obtain a consistent, homogeneous slurry by means of the mixing process. This should be done quickly so that monitored samples of the slurry are representative of the larger volume and so that dry and wet materials are completely or thoroughly combined to obtain the desired slurry.
The foregoing objectives have been known and attempts have been made to try to meet them with continuous mixing systems. In general, these systems initially mix dry cement and water through an inlet mixer which outputs into a tub in which one or more agitators agitates the resulting blend of materials. The process is continuous, with slurry which exceeds the volume of the tub flowing over a weir into an adjacent tub which may also be agitated and from which slurry is pumped down into the well bore. Such systems typically also include some type of recirculation from one or the other of the tubs back into the inlet mixer and the first tub to provide an averaging effect as well as possibly some mixing energy. One or more densimeters are typically used in the systems to monitor density (this is the means the operator uses to determine cement/water ratio), the primary characteristic which is used to determine the nature of the cement slurry. Through this process density averaging occurs in the mixtures in the tubs, with the goal being a slurry having a density within an acceptable tolerance of a desired density. Although more than one densimeter may be used in one or more of these prior systems, there is the need for an improved system wherein multiple recirculations and multiple densimeters responsive to the recirculations are used to enable faster density control.
Despite these continuous mixing systems having significant utility, the oil and gas industry today is seeking systems which provide better mixing than such continuous mixing systems have been able to achieve. It has been observed that in some prior systems the inlet mixer configuration provides inadequate mixing and causes, rather than reduces, air entrainment. Excess air entrainment can adversely affect density measurements which in turn affect control systems and thus resultant slurry properties. Inadequate mixing can also allow "dusting" (escape of unmixed dry cement from the mixer). Other shortcomings of at least some prior continuous mixing systems include the necessity of controlling multiple mixing water valves, and in at least one type of system, one of such valves chokes the water source pressure upstream of where mixing occurs so that much of the mixing energy is lost. At least one prior system includes a primary water inlet valve which has an adjustable conical space that can become clogged by debris in the water.
To try to overcome at least some of the shortcomings of continuous mixing systems alone, batch mixers have been used in combination with continuous mixers. These batch mixers are basically larger volume tubs which provide better averaging of the slurry so that at least better density control may result and possibly better additive distribution. For example, a continuous mixer having a capacity of five to eight barrels may be used to produce a blend which is pumped into fifty-barrel batch mixing tanks.
Although such batch mixing systems may provide some advantages over smaller continuous mixing systems, the batch mixing systems also have shortcomings. In a batch system, the total job volume is typically made before the job starts; therefore, several batch tanks/mixers need to be on location to hold the pre-mixed volume. This requires much equipment and personnel and takes considerable space at the well site.
In view of the aforementioned shortcomings of the continuous or hybrid continuous/batch mixing systems, there is the need for a mixing system which provides the desired fluid property averaging and which permits rapid changes of the desired property to be obtained. It is desirable to obtain such a mixing system in a way which efficiently uses equipment, personnel and space at the well site. Another desirable feature of such an improved system is for it to have additional or better applied mixing energy because there is a desire in the industry to try to have mixing energies which approach the API laboratory mixing energies at which proposed slurries are developed and tested.
Another aspect of prior systems is the use of water or other displacement fluid from displacement tanks for accurately determining how much fluid is pumped behind the cement to place it at a desired location in the well. These displacement tanks are carried on prior mixing system vehicles which typically do not have enough extra space or weight capacities to accommodate a number of mixing tubs. For example, a prior system includes a vehicle on which are mounted a five-barrel mixing tank and two ten-barrel displacement tanks. This vehicle does not have enough room and weight allowance for additional twenty-barrel averaging tanks. Therefore, there is the need for a mixing system which uses the displacement tanks both as averaging containers and as displacement tanks. To permit this without contaminating the displacement fluid (if that would be undesirable), there is also the need for "on-the-fly" washing of the tanks between their averaging and displacement/measurement usages.
In summary, there is the need for an improved mixing system, including both apparatus and method, which provides fast density control while providing fluid process averaging of one or more desired properties (e.g., density). Such a system should also permit the magnitudes of desired properties to be changed quickly. Such a system preferably has increased or better applied mixing energy and can be implemented with existing displacement tanks used both as mixing containers and as displacement tanks.