Key technologies of Well GH-05 drilling for hot dry rock in Gonghe Basin

Abstract:

Well GH-05 is an efficiency-enhancing well deployed for the hot dry rock pilot production project in the Gonghe Basin， Qinghai Province. It is a dual-target directional well with a designed depth of 4000 m. Based on the construction experience of adjacent wells， the running depth of the surface casing and technical casing was increased， the risk of block falling and sticking in the long open hole section was reduced， and the cementing quality of the production casing was improved. In addition， a shock-absorbing thruster and a drilling jar were added in the bottom hole assembly， which effectively reduced the adverse effects of severe vibrations on downhole tools. The service life of the motor was increased by 35%， and no accidents happened such as cone dropout， bearing failure， and drill string breakage， achieving safe drilling. Based on the theory of specific energy of rock fragmentation， taking both the rock fragmentation efficiency and roller cone bit life into account， meeting the upward return of cuttings， for the borehole diameter of ?311.2 and ?215.9 mm， the weights on bit were selected to be 203~291 and 131~186 kN respectively， the rotational speeds were 120~180 and 180~270 r/min respectively， and the displacements were 43.3~51.4 and 33~38.5 L/s respectively. Furthermore， strengthening the drilling parameters increased the rate of penetration （ROP） in the granite formation by 34%~47%， and the drilling cycle was shortened by over 20%. It has created the drilling records of 207.51 m with a ?311.2 mm bit and 183.42 m with a ?215.9 mm bit in the granite formation， basically achieving the economic indicators of “ROP of 4 m/h and bit life of 200 m”. Finally， a composite organic salt drilling fluid system with a temperature resistance of 220 °C was developed and applied， reducing the near-wellbore pollution in the geothermal reservoir， effectively reducing the directional sticking pressure， and cutting the drilling fluid maintenance costs by over 40%.