Abstract:Geothermal energy is the only base load power among the major renewable energy sources. Hot dry rock （HDR） is a high temperature rock mass buried in the deep earth without connate water or with a little water， which can form an enhanced geothermal system reservoir through hydraulic stimulation， and a considerable amount of geothermal energy can be extracted. There are two main technological challenges associated with the development of HDR geothermal resource， i.e.， very hard rock， such as granite or basalt limits the rate of penetration （ROP）， and the temperature of the drilling system exceeds the operational limits of electronic tools like measurement while drilling （MWD） and steerable rotary tools. In recent years， the Utah FORGE project funded by the U.S. Department of Energy （DOE） provided an opportunity to test new drill bit technology and optimize drilling procedures to deliver a step change in ROP performance and drilling cost reduction. In this paper， the deployment of geothermal drilling technology R&D in the recent development plan of DOE Geothermal Technology Office is reviewed， the application of the redesign workflows based on physical limits in the drilling practice of the Utah project is selective analyzed， and the main areas covered by the multiple plans and projects funded by DOE in terms of geothermal drilling technology as well as the lessons learned from the Utah project in exploring and practicing drilling operation optimization are summarized.

Abstract:Polycrystalline diamond bits have the widest application in oil and gas drilling and deep formation drilling. Polycrystalline diamond compact （PDC） consists of a polycrystalline diamond layer and a cemented carbide substrate. Due to the difference in the coefficient of thermal expansion between the polycrystalline diamond layer and the tungsten carbide matrix， there is a huge residual stress inside the PDC after sintering and cooling. To solve the problem of reducing the residual stresses inside the PDC， this paper reports a preparation technology for gradient structure diamond compact. Through the process of fused deposition modelling and sintering （FDMS）， the technical route for the preparation of gradient structure diamond compact was determined； the micro-morphological features of the prepared gradient structure diamond compact and the distribution of the residual stresses were analysed； and the indoor drilling tests were carried out to verify the drilling performance of the gradient structure diamond compact. The results show that the gradient structure diamond compact prepared by the FDMS has a compressive stress of 1.4GPa at the interface between the polycrystalline diamond layer and the gradient layer， which significantly improves the interlayer bonding ability and effectively reduces the internal residual stress， and improves the drilling efficiency by about 36% and the service life is longer compared with that of the conventional compact.

Abstract:In oil and gas， unconventional oil and gas， and scientific drilling， drilling efficiency in sandy mudstone， and interbedded sand and mudstone is low， and the bit service life is short. Through the staggered arrangement of tooth shaped PDC and circular PDC， a new high-efficiency and impact-resistance PDC core bit was developed and tested in Lucaogou Formation of shale in Well 36-4 of Jimusaer Oilfield， Xinjiang. The results showed that the penetration rate of the new type of high-efficiency and impact-resistance PDC core bit was 2.4 to 3.1 times higher than that of other bits， and good social and economic benefits have been achieved.

Abstract:Great progress has been made in the research and efforts on diamond bits used in hard and compact, strong abrasive and mudstone formations. In hard and compact formation, by adding rare earth and selfsharpening materials in bit matrix and with low pressure bit structure design, the drilling speed can be up to 1.5-2.0m/h in hard and compact slipping formation with drillability less than grade 11 and the bit service life reaches 30-300m; in hard and abrasive formation, with the ultra high diamond working layer and optimized hydraulic structure design, ultrafine prealloyed powder and ultra wearresistant matrix performance, the POP and bit service life are increased by 20%-70% and 1.4-10 times respectively; in hard and compact mudstone formation, by the study on sharpteeth PDC bit and Baras bit, the drilling efficiency is increased by about 50%. For deep hard rock drilling in oil and gas field, compared with cone bit, NR826 series of impregnated diamond bit is used with screw drill, the ROP and bit service life are increased by 18%-85% and 5-8 times respectively. In addition, good progress has also been made in the research on PDC bit for offshore deep drilling in China.

Abstract:PDC bits have greatly changed the oil and gas drilling since their introduction in 1973. But PDC bits grew slowly in early about 15 years. Then their growth sped up obviously for innovations and improvements, such as Layered diamond table、NPI patents、Cobalt leaching、 Anti-whirl、 Hydraulics design, Computer Modeling etc. At present，PDC bits drill 90% of worldwide footage. In recent years, NOV has launched 4 types of Helios PDC cutters, which could increase the thermal stability、abrasion resistance、 impact toughness and steerability of PDC bits. Those bits are suitable for drilling hard、abrasive formations, as well as some difficult-to-drill formations with high ROP and durability. Applications were successful in Norway、Middle East、Australia、US Texas etc. ONYX 360° rolling PDC cutter，launched by Schlumberger, which keeps the cutter’s edge sharp and relatively cool, prolonging cutter life and overall bit durability. Applications in many oil and gas field showed that the average ROP of the bits with rolling PDC cutters increased more 40% compared with the bits installed by fixed cutters. The rolling cutter’s bits also increased footage 40%～70% compared with the fixed cutter’s bits.

Abstract:Residual thermal stress of functionally graded polycrystalline diamond compact was analyzed by the finite element method with ANSYS software. It was showed that the axial tensile stress in functionally graded polycrystalline diamond compact was reduced from 698MPa to 283 MPa compared with two layer polycrystalline diamond compact, radial tensile stress was reduced from 1770 MPa to 880 MPa, and shear stress was reduced from 653 MPa reduced down to 368 MPa．In addition, the stress distribution is improved in the interface of functionally graded polycrystalline diamond compact, and bond strength are greatly increased.