This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 217718, “Thermal Effects on Drilling Performance in Hot Dry Rock,” by Aleksandr Vetsak, SPE, Eavor Technologies; Laurent Gerbaud, Mines Paris Tech; and Roman J. Shor, SPE, University of Calgary, et al. The paper has not been peer reviewed.
High-temperature geothermal resources at a distance from tectonic boundaries or geologic hotspots can be accessed by the drilling of deep wells through significant sections of basement rock. This paper evaluates the effect of rapid cooling on the rock-cutting process and incorporates this effect into a drilling-performance optimization approach.
Construction of deeper wells for geothermal heat involves drilling into the basement of the continental crust. Accessing geothermal resources in impermeable igneous or metamorphic rock is referred to as drilling hot dry rock (HDR).
HDR, characterized by low permeability, minimal porosity, and negligible initial water saturation, maintains high temperatures through conductive heat exchange. Accessed from the surface by conventional rotary-drilling technologies, HDR fails in compression under axial loading of roller-cone drill bits and fails in compression shear under axial and tangential loading of polycrystalline-diamond cutters (PDC) in fixed-cutter drill bits.
During HDR drilling, circulated drilling fluid exits through the bottomhole assembly (BHA) into the annulus. If the static formation temperature is higher than the temperature rating of the BHA, then the colder drilling fluid is used to cool down and keep the BHA within its temperature limit. When such colder fluid exits the drill bit, it cools down the rock surface in front of the bit and around the BHA. Combined with rotary drilling, the circulation of the colder drilling fluid creates a temperature difference at the bit/rock interaction surface by cooling the thin layer of hot rock in front of the drill bit in milliseconds, later to be removed by rotary cutting. Thermal conductivity affects heating or cooling rates. High angular velocities of the drill bit and low depth of cut per revolution characterize rapid cooling of HDR. The complete paper analyzes drilling-performance variations in HDR associated with a combination of rotary drilling and rapid cooling in front of the drill bit.
Equipment and Materials.
The vertical-drilling-test simulator consists of a drill shaft with a swivel for drilling-fluid injection at one end and a screw-in drill bit at the other. A direct-current electric motor rotates the drillstring, and two hydraulic cylinders apply weight on bit (WOB). Hydraulic pressure controls confining pressure, and drilling-fluid pressure is exerted by the circulating fluid in the active mud system.
The full-scale drilling system accommodates a wide range of parameters: WOB up to 20 tons, rotational speed up to 1,000 rev/min, and flow rates up to 610 L/min. Standard tests involve constant WOB or constant rate of penetration (ROP) with a full-size drill bit, up to 216 mm in diameter. Data for each test are collected and recorded at a sampling rate of 200 Hz.
