Shale wellbore instability is a complex difficulty encountered during drilling all over the world. And the most important factor to determine shale wellbore stability is the distribution and expansion of micro-cracks in hard brittle shale. The usual ways to avoid collapse of shale wellbore mainly include increasing the inhibition of drilling fluid filtrate, improving the quality of drilling fluid cake and sealing the pores and micro-cracks near the wellbore surface. However, the shale wellbore collapsing difficulty was not solved effectively in field, which mainly because that the effective sealing is difficult to achieved in the case of crack width is unknown. Furthermore the drilling fluid cake is more difficult to be formed on shale sections because of the filtration rate is very low compared to sand sections under the same differential pressure. A kind of nanomaterial has been applied to achieve a sealing film rapidly, improve the sealing strength of the cake and reduce the mud cake permeability, which have been verified through the use of sound wave propagation speed, the pressure transmission experiment and cake strength. The technology had also been used in Well Exp1, which exhibited a smooth borehole and the borehole enlargement rate was below the wells in the same block.

At present, Ordos Daniudi gas field is developed with the gas of Shan-1 and He-1 reservoirs, whose lithologies mainly are tight sands. The reservoirs have severely potential formation damages, induced by mechanisms such as strong watersensitivity, water-blocking and solid slugging with small particles. The solid-free brine completion fluids are usually used in Daniudi gas field, but it had little positive effect on the formation damage relieving, sometimes even aggravated formation damage. This article has developed a new bio-enzymatic completion fluid. The laboratory test results showed that the completion fluid can thoroughly clean out the mud cakes, evenly relieve the pore slugging damage, and greatly reduce the formation water-sensitivity and water-blocking damages. It is also environment-acceptable without any toxic materials. The completion fluid has been applied in 15 horizontal wells in Ordos Daniudi gas field. The results of applications showed that the skin factors of all wells obviously decrease, especially in Well DX-1, the skin factor decreases to -2.3, and the Shan-1 formation has firstly got a good production without any stimulation treatments. The productions of all the wells are more than double times than those of former adjacent wells.

The Well Shengke-1 is one of ultra-deep wells with total depth of 7026m, bottom hole temperature of 235 ℃ and more than pressure of 100MPa. Ultra-deep well drilling fluid faces many serious challenges. Severe engineering problems would arise if the drilling fluid is not treated properly and even the success of the well is determined by the drilling fluid. The difficulties including complex formation, long barefoot borehole, and ultra-high temperature were confronted. The stratums of φ215.9mm hole included section KongDian 1 and section KongDian 2, in which the clay minerals in upper shale stratum consist of mainly mixed layer of inordinate illite and montmorillonite. Therefore, the troublesome conditions could probably occur during drilling. The open-hole interval was as long as about 3000m, and pressure indices of formations varied enormously. In upper stratum, after high pressure brine formation had been drilled at 4598m, the density of drilling fluid increased to 1.83g/cm 3 . Meanwhile, the pressure index of under-stratum was about 1.30g/cm 3 , so lost circulations had happened many times in high differential pressure. The high temperature of the drilling would induce complicated reaction between the clay and drilling fluid so as to slough of the clay. During drilling yield point and gel strength increased quickly with the effect of high temperature. The Poly-sulfonate plugging and anti-sloughing (5155~4598m), high density poly-sulfonate non-invasive (4598~5800m), and ultra-high temperature and super high density drilling fluid( 5800~7026m) had been used in 215.9 mm interval of the well, respectively. The drilling fluid kept stationary for a long time during completion which caused drilling fluid thickening, dehydration and mud cake thickening. The solid-free anti-high temperature completion fluid was used to guarantee the success of logging and casing. It was proved in the field application of Shengke1 that the drilling fluids exhibited a series of advantages, including excellent rheology behavior, good thermal stability and low filtration rate. It was easy to treat, formulate and maintain, and could deal with the salt and brine contamination. It had been stable under 235℃ of the BHT in Shengke1, the highest temperature encountered in China at that time, and guaranteed the well to be completed successfully.The drilling fluid formulation prepared through lab selection and compatibility experiment includes: 3% bentonite + 0.2% KPAM + 0.1% PAC-141 + 1% emulsifier + 2% SR-1 + 3% SD-102 + 4% KFT + 3% DYWD + 6% lubricant + 2% glycol.High density poly-sulfonate noninvasive drilling fluid(4598~5800m). High temperature (180~200 ℃, at the bottom) andsloughing characterized this interval. The drilling fluid provided glycol anti-sloughing additives, silicon-fluorine stabilizer to enhance the inhibity, humic acid and sulfonate resin filtrate reducers to control the filtrate, noninvasive additives to plug shale microcrack effectively and reduce the filtrate into shale. The drilling fluid system, with strong inhibity, good ...