| Deep drilling is the most direct way for people to understand the Earth and to establish a passage in deep crust. Meanwhile, there is a significant imbalance between supply and demand of resources because of the depletion of shallow resources, leading to Imperativeness of deep drilling. The classification criteria of boreholes in geological exploration is that borehole depth between 0 and 300 m is called a shallow hole, that between 300 and 600 m is called a medium-shallow hole, that between 600 and 1200 m is called a medium-deep hole, that between 1200 and 2500 m is called a deep hole, and that above 2500 m is called a super deep hole. Wire line coring process costs less auxiliary time, gets high cores quality, and is efficiency, as well as it is preferred to because of its advantages by drilling workers, the mud problem is prominent in wire line coring process, which is a restriction of the development of the process. In this paper, drilling fluid technologies are discussed and studied of small diameter wire line coring in deep drilling.The scientific drilling hole in Tibet Luobusha chromites mining area is the first of the seven boreholes in the "Continental Scientific Drilling Site Selection and Pilot Holes ". Its main goals are to reveal the deep geological structure and the extension and distribution of rock mass, to evaluate the prospect of Luobusha chromites and the chromites in ultramafic rock in Brahmaputra suture zone, and to find out the genesis and structure background of Luobusha rock and of the ultramafic rock zone it exists in. The ultrabasic rock of Luobusha in Tibet alone the Yarlung Zangbo River by the valley of the distribution strictly controlled by the tectonic zone of the Yarlung Zangbo River. Late in the digenetic and digenetic have suffered intense tectonic deformation, formed a series of complex tectonic deformation. It directly expressed as the development of regional deposits, complex formation. The upper parts of the gravel sediment diameter, which is poor cementation, the lower ultrabasic rock crushing and serious alteration (serpentinization), impacting the drilling with poor core recovery, and existing uninterrupted problems of mud leakage, drilling difficulties, collapse in the whole holes.Drilling in the formation of geological structural development and rock broken, hole stabilization is the key. Mainly researched from two aspects of hole stabilization problems:the stress analysis around the hole wall and analysis of physical and chemical properties of drilling fluids. As the pressure for drilling the rock wall is greater than hydrostatic pressure, the hole wall is likely to damage or collapse, which can accurately calculate the reasonable density of the drilling fluid under hydrostatic equilibrium conditions. In addition, the drilling fluid on the pore wall rock hydration, hole erosion caused by too large velocity in annulus, and the pressure generated by raise or down the drilling tools are likely to undermine the stability of the hole wall. According to the leakage characteristics of formation, the channel intensity values, and Strength loss will be divided into 3 classes and 12 subclasses, and the way of plugging along with the change in classification. Relationship model about drilling fluid viscosity and the thickness of strata corresponding breaking was established for Luobusha Stratum, and the extreme conditions what use drilling fluid to protect holes was proposed preliminarily.From the early research 70s research and development the drilling technology of wire line coring to now, the drilling technology of wire line coring was widely used in core drilling. When drilling the rock of the hardness level is 6 to 9, drilling is deep and long bit life; core drilling can best play to their strengths. However, deep drilling small diameter wire line coring technical problems of drilling fluid technology is more prominent, including drill pipe wall is easy scaling, the annular clearance is too small, poor drilling fluid lubrication, large circulation resistance, poor carrying powder capacity, and high filtration, do not well in protected the hole wall and so on. Solving these problems need to improve drilling fluid performance. Through the establishment of the circulating pressure loss model derived:use lower penetration rate, strengthen the control of solid content, using clay free drilling fluid to control the drill pipe scaling; appropriate to increase the diameter of drill bit (raise the annular clearance 1mm, can reduce 0.5 times of the circulation pressure); Mixed with O-fluid lubricant in drilling fluid, PVA can reduced 0.6 times of lubrication coefficient; Mixed with PAC, XC and other good shear thinning properties of macromolecular polymers, PVA can improve the ratio of dynamic shear force of the drilling fluid and the capacity of portability powder; Mixed with SPNH and FT-1, PVA can reduce the filtration under 8ml.According to the Strata and the field drilling fluid of Luobusa in Tibet, PVA solid free drilling fluid was researched, which was low viscosity, low-cut, lubrication properties, strong inhibition ability and portable rock ability, with a shear thinning, and preferably resolve some of the issues in the drilling process, and ensure the drilling safe and improve drilling efficiency. Through a large number of field and laboratory experiments, PVA-clay free drilling fluid formulation is as follows:2.5%PVA+0.05%PAC+3%SPNH+0.5% O-lubricant. It has the following advantages:(1) Good rheology behavior:drilling fluid has low viscosity and low shear force, suitable core drilling small annular gap will help improve the penetration rate, reduced circulate resistance;(2) Significant effect of protection wall:compare with the usual low-solid mud by soaking experiment, after immersed in PVA-clay free drilling fluid 24h,the core is not deformation or expansion, and then put it into clear water 7d, it still no deformation, no collapse;(3) Excellent inhibitive ability:through the shale scrolling recovery test and expansion of high temperature and pressure experiments, summarized that PVA-clay free drilling fluid can be well suppressed high clay content, cementation loosed rock swelling and dispersion;(4) Small filtration and excellent lubrication:valchovite mainly used to decrease the filtration effect of drilling fluid, and plays a significant role; but it also more excellent in lubrication, and it has been able to meet the general requirements. When drilling deeper, larger circulate resistance, lubrication requirements need stronger, you can add O-type lubricant, it can significantly enhance the lubricating effect;(5) Good ability to carry meal:the drilling fluid through own development capacity of drilling fluid residue suspended test equipment, test PVA-clay free drilling fluid residue suspended effect is not better than low-solid mud, but on clay free drilling fluid terms, PVA-clay free drilling fluid has high ratio of dynamic shear force, can be better hanging row of diamond drill bit grinding down the rock powder, clean the bottom of the hole.
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