1.Collecting Data for the Capture Analysis
A sample set of each of the three process streams should be obtained, sealed, and labeled for identification. The size of each sample should be 50–100 ml. For each set, the time between catching each of the samples should be as brief as possible.
The required laboratory work consists of determining the weight percentage of suspended solids in the samples. With water-based samples, the simplest method is to determine the weight of the sample using a precise analytical balance, remove the water by dehydration at 200 F in an oven, and weigh the remaining solids. Correction factors should be determined and applied in cases in which the base liquid contains more than 10,000 ppm salt, or emulsified oil.
Determining Percentage of Suspended Solids in Water-Base Samples
For water-base samples, dehydration ovens are the most convenient heating devices for removing the liquid from the sample. Retorts are required for oil-based samples. Regardless of the dehydration method used, the data must be evaluated on the basis of weight, not volume.
Determining Percentage of Suspended Solids in Oil-Based Samples: Unweighted Fluids
For fluids that do not contain barite or another high specific-gravity (SG) weighting agent, the procedure outlined in the preceding section completes the laboratory work. The quantitative determination of the effects of the solids-removal process are then obtained using the capture calculation and by simply multiplying the volume fraction of the solids in the discharge stream by the rate at which solids enter in the feed stream. The mass flow rate of solids in the discharged stream can be expressed in dry tons per hour or in other units.
APPLYING THE CAPTURE CALCULATION
On drilling mud applications for both centrifuges and hydrocyclones, the discarded stream can be either the heavy phase or the light phase. Care must be taken to apply the equation correctly for the two different cases. It must be understood that the precision of the calculation of the rate of solids separation is dependent on the accuracy of the feed rate determination and that this is difficult to measure. Even though circumstances may require imprecise measurement, or estimation, of the feed rate, useful and meaningful results are obtained with this procedure.
1.Case 1: Discarded Solids Report to Underflow
In applications in which the underflow is discarded, consideration of the feed rate, together with the percentage of capture as determined with the capture equation (see Section 14.1), permits the calculation of the rate at which solids are being removed.
2.Case 2: Discarded Solids Report to Overflow
If the discarded solids exit with the light phase, then the percentage of capture can be determined by subtracting the capture calculated with equation i from the solids in the feed stream. In applications in which the overflow is discarded, this calculation is used, together with the feed rate, to determine the rate of solids separation.
3.Characterizing Removed Solids
Separated solids are characterized on two bases: their SG and their particle size.
Weight Material and Low-Gravity Solids
A dried and weighed sample of the separated solids can be added to a measured volume of water, and the average SG of the solids can be determined from the increase in volume and weight. The percentages of weight material and low-gravity solids can then be determined using the following equations.
%weight material = (ASG−2.6)⁄(SGWM−2.6)
%low-gravity solids = total solids weight material
ASG¼average solids gravity
SGWM¼specific gravity of weight material
In order to obtain meaningful and repeatable results, a high degree of precision is required in obtaining these data. The use of a pressurized pycnometer is recommended.
The primary function of centrifugal processing of oil well drilling fluids is the removal of viscosity-building fines. Removal of these particles limits the need for dilution. Given the undeniable influence of average particle size on drilling-fluid quality, it is recommended that occasional particle size analyses be used to monitor the concentration of colloids and near-colloids and ensure that their concentration does not become excessive.
USE OF TEST RESULTS
This procedure provides a means of monitoring the concentration of low-gravity solids in the feed and the discarded material. Inasmuch as we can easily remain aware of the concentration of desirable lowgravity solids by simple record keeping, it also provides a means of monitoring the volume of drilled solids in the mud and in the discarded material.
Experience in an area, coupled with the knowledge that penetration rates tend to decrease while torque, drag, and the likelihood of sticking the drill string increase with increasing concentrations of fines, can provide area-specific guidelines concerning tolerable concentrations of these colloidal and near-colloidal particles. The monitoring of particle size helps determine when the removal of fines by centrifuging is desirable.
The economics of discarding the underflow of centrifuges used for solids reduction with unweighted muds can be evaluated by comparing the cost of the solids removal with the cost of the dilution required by the incorporation—rather than removal—of the separated solids, and the differences in waste disposal costs.
The effect of centrifuging upon the mud cost can be determined by calculating the volume of dilution that would have been required to compensate for the incorporation of the separated solids, and multiplying it by the unit cost of the fluid. Inasmuch as all dilution adds directly to waste volume, the cost of disposing of the dilution volume must be added to the cost of preparing it.
Hydrocyclone use is not ordinarily recommended with weighted fluids because high solids content interferes with their operation, and weight material is concentrated in the underflow and discarded.
Traditionally, centrifuging has been used with weighted fluids to reduce dilution requirements and barite consumption. Comparison of the cost of the centrifuging with the value of the barite recovered from the discarded fluid is often used as a measure of its economic effectiveness. While this is a valid, and important, basis for evaluation, the fact that drilling-fluid quality tends to be much better when centrifuges are used can be of much greater economic importance.
The effect of centrifuging on waste volume must be considered also. Dilution volume at the centrifuge feed and the disposal of the liquid in the overflow are obvious factors. Less obvious, but of greater importance, is the fact that the disposal of the colloids and near-colloids discarded with the liquid reduces dilution requirements and therefore mud cost and the volume of waste generated.
A rough approximation of the waste volume reduction achieved by discarding centrifuge overflow can be arrived at by calculating the volume of solids that are being discarded, assuming that they are all colloids or smaller and troublesome ultra-fines, and calculating the dilution that would be required to maintain their concentration at 5% by volume if they were not separated. This dilution, 19 times the volume of the separated solids, can be taken as an approximation of the reduction in dilution.
An evaluation of the economics of centrifuging must include the cost of the preparation of the mud used for dilution, the reduction in waste volume, and the value of the barite returned to the system via the underflow.
Nontraditional use of centrifuges with weighted fluids should be evaluated by comparing the benefits with the value of desirable materials discarded in the process.
COLLECTION AND USE OF SUPPLEMENTARY
In addition to the samples and feed rate data, a fully completed copy of the most recent Mud Report and the following information should be obtained at the well site:
. Drilling fluid composition and unit cost
. Density of feed, overflow, and underflow
. Funnel viscosity of feed and overflow
This information can be helpful in economic analyses and in evaluation of the operation of the centrifuge(s).