A three-stage solids control system （Figure 1.）with shale shaker, desander (desilter) and centrifuge was employed for drilling diamond. A complete desanding and desilting cleaning system consisting of MG4 linear motion shale shaker. APCS240 (10 in.) cyclone desander and APCN200 desilter from Aipu solids control Company was employed. A LW500 × 1250-N low speed decanter centrifuge with large discharge and a LW500G × 1250-N high speed decanter centrifuge with large discharge were equipped, aiming at the fine cuttings produced by diamond drilling. Besides, LZCQ/3 deaerator was employed.
In diamond core drilling, the screen cloth of shale shaker screen in solids control system was of 170 meshes (91 μm), through the outlet of the shale shaker screen only a very small part of coarse cuttings could be separated out from the drilling fluid, while most of cutting chips flowed through the shale shaker screen into mud tank, and small part deposited in triangular tank which could be then separated out by desander and desilter. Most of the suspended cutting chips could be separated out by perpetual working of the low speed centrifuge, and fine cutting chips could be eliminated by interrupted working of the high speed centrifuge. Almost all the cuttings and fine chips could be separated out by this solids control system and the sand content of drilling fluid could be maintained at about 0.03 %, thus the properties of drilling fluid could be guaranteed over a long period of time.
The grain distribution of the cuttings could be separated out by high and low speed decanter centrifuge can be found in Table 1. The average volume grain size of the cutting chips separated out by low speed centrifuge was 50.53 μm. According to the quantity of separated cuttings it could be known that 4.83–104.32 μm was the main distribution range of the cuttings grain size produced by diamond core drilling. The average volume grain size of the cutting chips separated out by high speed centrifuge was 20.80 μm. The grain diameter of the cutting chips separated out by high speed centrifuge was mainly distributed between 3.8 and 47.21 μm. So high speed centrifuge could eliminate extremely fine cutting chips, beneficial to decreasing the shearing force of drilling fluid.
|Table 1 Cuttings grain size separated out by centrifuge|
|Sample||Size range (μm)||Average volume grain size (μm)||Specific surface area (m2/g)||Average surface area grain size (μm)|
|D (0.1)||D (0.5)||D (0.9)|
|Separated by low speed decanter centrifuge||4.83||27.03||104.32||50.53||0.52||11.51|
|Separated by high speed decanter centrifuge||3.80||13.87||47.21||20.80||0.70||8.60|
In diamond drilling, the produced cutting chips are very fine and usually smaller than 200 meshes (70 μm), so the maintenance of drilling fluid is very important. Though the quantity of the cutting chips is not much, the accumulated cutting chips disperse in drilling fluid can affect rheological property of the drilling fluid and thus causing the vicious circle of increase of cutting chips quantity, increase of solids content, increase of shear force, cuttings are hard to be separated out and then shear force increases even more, then finally lead to an abandonment of the drilling fluid. By using a combined solids control system with desander, desilter and centrifuge for drilling diamond Well, solids content in drilling fluid was strictly controlled, tiny cutting chips was eliminated in time, so a continual stability of drilling fluid performance was guaranteed which was not only propitious to the control of drilling parameters but also benefit for stability of well wall. In the whole drilling progress, no drilling fluid was replaced due to weak bearing capacity to solids (all drilling fluid replacement only happened after well cementation as drilling fluid was contaminated with cement slurry), by employing the solids control techniques, high quality circulating medium was supplied for core drilling and for down hole power engine.