A field application of the economic and performance analysis model for a closed-loop system implementation. A district had realized substantial benefits through more effective solids control in earlier years and was interested in 1986 in analyzing the economics of further improvements in solids control. The economic and performance analysis programs were run to predict the costs, potential savings, and recommend a suitable solids control system. It was a nine-well (2-vertical and 7-directional, 4300 ft average depth) infill drilling program within the city limits, which imposed certain constraints such as minimal or no reserve pits with all the extra mud and wastes hauled for off-location disposal.
The 12-1/4 in. (31.1 cm) surface hole for a typical well was to be drilled (and cased) to 1600 ft (487.7 m) in 2 days, using water gel mud with initial mud weight of 8.6 lbm/gal (1030.5 kg/ma) and the ending mud weight of 9.4 lbm/gal (1126.4 kg/m3), corresponding to a 6% by volume maximum permissible drilled solids. After setting 8-5/8 in. (21.9 cm) casing at 1600 ft (487.7 m), 7-7/8 in. (20 cm) hole was to be drilled to a depth of 4300 ft (310.64 m), using 10 lbm/gal (1198.26 kg/ma) brine mud and an ending mud weight of 10.3 lbm/gal (1234.2 kg/m3) with drilled solids less than 3% by volume, and rig released after 12 days from the spud. The approximate mud addition/dilution cost for the surface and production hole was $1.00 and $2.00 per bbl (0.159 m3), respectively. The solids disposal cost which includes dump truck plus disposal costs, and the liquid disposal cost that includes vacuum trucking and disposal for the area were $5.40 and $3.00 per bbl, respectively. The initial and the final mud volume in surface tanks was to be kept at 360 bbls (57.24 rna), and 10% washout for the surface and 30% for the production section of the hole were assumed.
The output summary of the economic analysis program is given in Table 1. Assuming reasonable removal equipment rental costs, the program predicts the mud addition/dilution, waste disposal and total costs including rental costs for the surface (S); production (P) section and complete (C) well in terms of % drilled solids removal (0%-to-closed loop condition). The analysis of data in Table 1 leads to many interesting results. For example, we can easily compute from the last two columns the gross and net savings in mud addition and waste disposal costs alone, as the drill solids removal percentage is increased from 0% to the closed-loop condition. For the production section of the hole, these costs are shown in Fig. 0 which clearly illustrates the potential savings with more drilled solids removal despite additional costs for solids control. These costs and savings would obviously be different for different drilling conditions and are mainly dictated by the dilution, disposal and solids control costs in a particular area.
Based on the leverage indicated by the economic analysis, two closed-loop system designs were recommended, one using only mechanical equipment and the other using, in addition, chemical flocculation before centrifuging (Mud Processor). The second system was chosen to study the effectiveness bf chemical flocculation and to see if its cost can be justified. The mechanical system was selected on the basis of availability and cost. It mainly consisted of a high-performance shaker and banks of 4 in. (10.16 cm) and 2 in. (5.08 cm) hydrocyclones that were essentially used as concentrators with their underflow processed by a high-g centrifuge.
The performance analysis program was run to predict the removal efficiency of each piece of the selected removal equipment and the overall efficiency of the selected mechanical solids control system. Fig. 1 shows the grade efficiencies obtained from the models in the performance analysis program for the shaker (Layered 100 screen), 4 in. (10.16 cm), 2 in. (5.08 cm) hydrocyclones and for the high-speed centrifuge that were used in the system. It is interesting to note that if each device could process the total fluid volume, the last device providing the finest separation decides the total system efficiency. In other words, if one has a centrifuge that can process the rig circulation rate without overloading or solids plugging, the other coarse separation devices are not necessary. However, cascading of various devices is normally required to prevent overloading or plugging of the later devices, which also improves the separation efficiency of these later devices. The economics and performance of each case has to be analyzed separately.
The log normal particle-size distribution, assumed for the surface hole (x = 8 micron, ρg= 4), is shown in Fig. 2 as the shaker feed distribution. The drilled solids distribution for feed streams of the two different sizes of hydrocyclones and for the effluent stream of the 2 in. hydrocyclone, as predicted by the performance analysis model are also shown 1n Fig. 5. For comparison, the actual distribution measured from a field sample of a 2 in. hydrocyclone is shown as dotted curve in Fig. 2. The performance analysis program predicted an overall system removal efficiency of about 60% for the surface hole with shaker contributing 5%, each of the two sets of hydrocyclone about 20%, and centrifuge another 15% toward the solids removal from the active system. For the production section, the corresponding overall removal efficiency was predicted around 80% for an assumed distribution and configuration. The required solids removal efficiencies for ideal closed-loop implementation, i.e., 81% for the surface section and 90% for the production section of the hole as indicated by the economic model, could not be realized with the equipment that was selected based on field constraints.
The dilution and disposal volumes were closely monitored during drilling of these wells and over 350 samples were collected from the feed, discard and effluent streams of various solids control devices at three different depths. The discussion of field monitoring and laboratory analysis techniques is beyond the scope of this paper, and the detailed results of this study are too voluminous to be included in this paper. Briefly stated, the analysis of the data generally confirmed the predicted values for the removal efficiency of each of the devices and the system. It must, however, be pointed out that there was considerable variation in these values from one well (or depth) to another. According to theory, it is not difficult to derive from material balance equations the expressions that are necessary to evaluate the performance of any solids removal device. However, problems arise because of the difficulty in collecting representative samples of the three streams, improper measurements and retort analysis, and because of the inherent limitations of various measuring instruments.
The overall solids removal efficiency was also estimated from the dilution volumes and the economic analysis program. For example, the monitored average dilution volume was 900 bbls (143 m3 ) for the surface hole and 1100 bbls (174.9 m3 ) for the production section of the hole. Using Table 1 or Eq. 3 or 4, these dilution volumes correspond to about 67% and 80% drilled solids removal for surface and production sections, respectively. As regards the comparative savings, the corresponding dilution volumes in the 1984-85 drilling for two sections of the hole were 1350 bbls (214.6 m3 ) and 2300 bbls (365.7 m3 ), which correspond to 46% and 67% solids removal, respectively, according to the results of economic analysis program for the earlier wells. From Table 1, the gross savings over 1984-85 for two sections of the hole work out to approximately $1500 and $6000, respectively. Taking into account the additional costs of $1000 for renting additional solids control equipment, the net savings should be about $6500, which agree with the actual savings in these costs. In addition, the lost circulation and differential sticking problems experienced in earlier drilling were considerably reduced which contributed to an average three-days per well reduction in rig days. Thus, the total savings even for these shallow wells are considerable.
