Drilling fluid is the circulating fluid used in drilling process, a mixture of liquid, solid and chemical agents. It is commonly known as mud and also called the blood of drilling, playing an important role in the whole drilling process and accounting for 7%~10% of total drilling cost. That’s why treatment of harmful solids and effective recycling seem so particularly important. Initially, it is necessary to look into composition characteristics of solids in drilling fluid and its influence on well drilling, so as to remove harmful solids economically and effectively for further recycling. Solids generated in drilling process mainly include mixing clay, weighting substance, cuttings and other solid substance. They are divided into clay, cuttings, weighting substance and solid substance in treating agent according to their sources in drilling fluid.According to the modes of action of them and their ions in water, low density solids further fall into active solids and inert solids.
Active solids are easily hydrated and can react with other substances in liquid. This kind of solids has high surface activity and specific surface area with relatively active electrochemical property. Clay particles in mud, clay and ions and clay and polymer are closely bonded. Function of these solids is to regulate drilling fluid properties and they are called useful solids. Without surface charge and electrochemical reaction, inert solids do not react to the change of surroundings, including sand, flint, limestone, dolomite, some shale and a mixture of various minerals. They accumulate together, compressing active solids and coagulate them, which increases viscosity and causes change in drilling fluid properties. These solids are useless in drilling fluid and are called useless solids. When particle size of such solids is over 15μm, they abrade the circulating equipment and are thus called harmful solids. Cuttings are major harmful solids in drilling fluid and affect physical properties of drilling fluid during the whole drilling process, increasing density, viscosity, yield point, water loss, mud cake, abrasiveness, glutinousness and flow resistance of drilling fluid. During drilling process, cuttings may even damage oil and gas reservoirs, reduce ROP, increase rotary table torque and cause blockage during tripping, even resulting in lost circulation, blowout and other complex downhole problems. Besides, cuttings in drilling fluid seriously abrade the circulating system.
The key point for achieving drilling fluid recycling is to effectively treat the harmful solids contained within. Solids control system can effectively achieve such a goal. In the system, shale shaker can remove harmful solids with diameter ≥74μm, desander can remove those with diameter of 44~74μm, desilter can remove those with diameter of 15~44μm (high efficiency cleaner is a kind of equipment integrating both desander and desilter and can remove harmful solids of 15~74μm) and centrifuge can remove those with diameter ≥2μm. Mud density may not be too high during normal operation to reach its performance requirements, so the density must be reduced to a proper value, and solids with diameter less than 15μm need to be treated by centrifuge. If barite is required to be recycled while treating harmful solids, a high speed centrifuge is necessary.
Thus it can be seen that drilling fluid can be recycled through the above sequential treatment by solids control system, which greatly saves drilling cost and enhances drilling profit. Xi’an KOSUN Machinery, Co., Ltd. has been dedicated to research and design of solids control system and equipment since 1992. KOSUN has possessed fairly mature technical system through over 20 years of experience and is capable of designing reasonable and effective solids control system solutions according to customers’ requirements. KOSUN solids control equipment (shale shaker, vacuum degasser, desander, desilter, high efficiency cleaner, centrifuge, etc.) and solids control system sell well on domestic and foreign markets.
