Publications

Our charter at Sandia National Laboratories is to develop technology to reduce the development cost of geothermal drilling. Due to their aggressive penetration rate performance, Polycrystalline Diamond Compact (PDC) bits are of particular interest for this application and they have recently been demonstrated to be capable of drilling hard-rock formations characteristic of geothermal reservoirs. Additionally, oil and gas operators are increasingly forced to extend their drilling targets to include these hard-rock formations as our fossil energy reserves dwindle. However, PDC bits are particularly susceptible to impact-type damage due to the onset of drilling vibrations that can cause bit failure. Bit vibration produces an undulated surface in the rock that in turn produces a time-variant force that feeds back into the vibration of the bit and drillstring. While there is considerable debate in the drilling community regarding the relative significance of the various types of vibrations, self-induced vibrations do occur and can be mathematically predicted if the drill bit, drillstring, and rock type are not correctly matched. One way to alleviate this problem is to insert a vibration absorber into the drillstring. Given the broad range of parameters contributing to bit vibrations, any damper installed in the drillstring should be controllable to give it more dynamic range. We have experimentally demonstrated that a controllable damper can introduce stability in PDC bits drilling hard rock typical of geothermal formations.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Typical laboratory testing of Polycrystalline Diamond Compact (PDC) bits is performed on relatively rigid setups. Even in hard rock, PDC bits exhibit reasonable life using such testing schemes. Unfortunately, field experience indicates otherwise. In this paper, the authors show that introducing compliance in testing setups provides better simulation of actual field conditions. Using such a scheme, they show that chatter can be severe even in softer rock, such as sandstone, and very destructive to the cutters in hard rock, such as sierra white granite.

Polycrystalline diamond compact (PDC) bits have yet to be routinely applied to drilling the hard-rock formations characteristic of geothermal reservoirs. Most geothermal production wells are currently drilled with tungsten-carbide-insert roller-cone bits. PDC bits have significantly improved penetration rates and bit life beyond roller-cone bits in the oil and gas industry where soft to medium-hard rock types are encountered. If PDC bits could be used to double current penetration rates in hard rock geothermal well-drilling costs could be reduced by 15 percent or more. PDC bits exhibit reasonable life in hard-rock wear testing using the relatively rigid setups typical of laboratory testing. Unfortunately, field experience indicates otherwise. The prevailing mode of failure encountered by PDC bits returning from hard-rock formations in the field is catastrophic, presumably due to impact loading. These failures usually occur in advance of any appreciable wear that might dictate cutter replacement. Self-induced bit vibration, or ''chatter'', is one of the mechanisms that may be responsible for impact damage to PDC cutters in hard-rock drilling. Chatter is more severe in hard-rock formations since they induce significant dynamic loading on the cutter elements. Chatter is a phenomenon whereby the drillstring becomes dynamically unstable and excessive sustained vibrations occur. Unlike forced vibration, the force (i.e., weight on bit) that drives self-induced vibration is coupled with the response it produces. Many of the chatter principles derived in the machine tool industry are applicable to drilling. It is a simple matter to make changes to a machine tool to study the chatter phenomenon. This is not the case with drilling. Chatter occurs in field drilling due to the flexibility of the drillstring. Hence, laboratory setups must be made compliant to observe chatter.