Shale Shaker Selection
Shale shaker screen capacity is a strong function of solids loading. i.e., penetrate rate and mud plastic viscosity as well as the design of the shaker itself.
In the 17.5 in. surface hole is typically drilled to a depth of 4000 ft to 5000 ft. Bit hydraulics and hole cleaning considerations dictate a flow rate of approximately 1000gpm. The rig being analyzed comes equipped with two high speed shale shakers capable of running fine mesh screens. Will the rig’s shakers be capable of processing this flow rate over 150 mesh screens? If not, what additional equipment is required to accomplish this?
This problem was set up on the simulator to determine the capacity of the rig’s shakers considering cuttings load at maximum rate of penetration and associated mud viscosity. It is predicted that with 150 mesh screens approximately 108gpm of fluid will be lost over the end of the screens per shaker at a mud flow rate of 1000gpm. Further tests indicate that by adding two similar brand shakers the entire flow rate can be handled with two-thirds screen coverage on all four shakers. A sensitivity analysis to mud viscosity showed that even if the plastic viscosity increased from ii to 19 cps that the four shakers would be adequate.
Hydrocyclone and Pump Design
Designing desander, desilter, centrifugal pump and motor configurations is a complicated process. It is possible by conventional means to calculate feed head available at the input manifold of a bank of cones given the exact geometry of the suction lines, basic information about the pump and motor and the mud properties. This sometimes can be a time consuming exercise. What cannot be calculated by conventional means however is how flow rate and feed head ultimately affect the D50 cut-point, solids removal rate, and liquid discharge rate. Considering the same hole as in the above example it was first desired to check if the given equipment could adequately process the mud. It was desired to process 100% of the mud flow rate and reprocess 20% through the desilter. Underflow from the desilter is routed to the centrifuge suction tank. It was evident from simulations that an 1100 RPM motor could not provide adequate head to the desilter, regardless of impeller size for the given pump. It was found that by switching to a 1750 RPM motor that the pump did provide the proper head at the device manifold. ESD predicted performance of the rig’s desilter with the initial rig equipment indicates with a 13 psi pressure at the manifold that the D50 cut point was 49 microns at a flow rate of 888gpm. Fig. 1 shows the ESD depiction of the initial operating conditions of the desilter.
By changing the motor the manifold pressure is raised to 36 psi. Fig. 2 is the ESD status display of the performance of the 5 in. cones. With the increase in pressure and flow rate there is a reduction in the median cut point from 49 to 33 microns. In addition the solids removal rate is increased from 13 Lbs/min to 30 lbs/min. Table 1 is a summary of the initial and optimum operating condition for the desilter. Other parametric studies are possible such as the response in cut point to varying solids content and mud viscosity.
|Initial Rig Conditions||ESD Optimized Conditions|
|Motor Power||50 hp||75 hp|
|Manifold Pressure||13 Psi||36 Psi|
The ESD enables its operator to optimize the performance of centrifuges. Design variables are: number of centrifuges, centrifuge routing, dilution rate, feed flow rate, and pool depth.
The operator can compare the cost of centrifuges with the benefits of using centrifuges in order to choose the optimum number of centrifuges. The costs associated with centrifuges are rental fees and power consumption. The benefits of centrifuges are higher solids separation efficiencies and dryer solids than obtained from hydrocyclone and shale shakers provide. Thus, centrifuges increase penetration rate by removing fine solids, and centrifuges reduce mud disposal costs by discarding relatively dry solids. The operator may vary the ESD’S number of centrifuges while observing penetration rates and discarded mud volumes in order to determine the optimum number of centrifuges to use.
The operator may route mud to a centrifuge from a compartment, or the operator may route a hydrocyclone’s underflow to a centrifuge. The first alternative has the possibility to maximize the total solids removed from the system and increase penetration rate, while the second alternative will result in less liquid mud discarded from the system. The ESD can provide the operator data to decide which alternative is best.
An increase in flow rate decreases the retention time of mud inside the centrifuge. The decrease in retention time increases the solids cut point size. An increase in flow rate, however, increases the total solids fed into the centrifuge and may result in a net increase in solids separated. A number of factors, especially solids distribution and mud viscosity, determine whether a flow rate increase improves the centrifuge performance. The operator can vary the flow rate while observing the solids separation rate on the simulator graphics screen to determine the optimum centrifuge flow rate.
The ESD was used to optimize the centrifuge dilution rate and pool depth on the rig previously described. The design criteria were to maximize the rate of solids separation. A previous simulation had already determined that 155gpm was the optimum mud feed rate. Starting at zero, the dilution rate was increased in increments of 20 to 35gpm. Data from the simulation produced the performance curve presented by Fig. 3. The performance curve indicates that 90gpm is the optimum dilution rate for this scenario.
Next, holding all other variables constant, the optimum pool depth was determined. Low, medium, and high pool depth settings yielded solids separation rates of 276, 360, and 360 lbs/hr respectively. Thus, the medium and high pool depth settings were determined to be optimum. The above examples of centrifuge operation on the ESD are greatly simplified. In fact, the centrifuge optimization process is iterative due to the complex interactions of pool depth, dilution rate, feed rate, etc. The ESD, however, enables the user to test hundreds of parameter permutations in several hours.