Overcoming Hard Rock Drilling Challenges

Abstract

Abstract Drilling through hard rock has been a limiting factor to economically develop significant hydrocarbon reserves in several areas. The high cost to explore and develop an oil/gas accumulation is the key factor to be overcome. This paper describes a Joint Industry Project (JIP) that has been started to select and field test the most promising technologies under development that will allow a significant reduction on well cost. Problems faced while drilling hard rocks are not only restricted to slow penetration rate, but also the non-productive time associated to drill string failure, wellbores out of gauge, stuck pipe associated to block instability that falls into the well, and fluid losses due to fractures. Introduction Drilling through hard rocks is most often very costly due to low penetration rates and several related aspects such as, wellbore instabilities, drill string failures, excessive trips required, among others. A JIP was created to select several technologies with potential to improve hard rock drilling performance and the associated problems. The main goal of this JIP is to provide the industry with technological solutions that ultimately can reduce the well costs. The focus when drilling through hard rocks has been to increase the rate of penetration. However, experience has showed that by eliminating the non-productive time associated with high vibration levels, for example, can contribute significantly in turning the development of an oil field in such environments a profitable endeavor. Not only technologies to improve the rate of penetration, but also ways of reducing the vibration levels have been mapped in this project, and will be field tested. The paper initially describes the JIP, the steps and main goals of the project. Then, it presents the most promising technologies selected to be field tested: fluid hammer, jet-assisted drilling, vibration reducers, and a new bit design. The field tests have been planned taking into consideration the risk associated to each technology, as well as the risk involved in the well itself. Minimization of the risk is considered the key issue to successfully field test any technology still under development. Finally, the paper presents the most important results obtained so far. Available Technologies Several technologies1 can be used to improve hard rock drilling performance and solve associated problems. This paper briefly presents them. However, not all of them can be considered ready for field use yet. a) Percussion Hammer Percussion means impact, collision or vibratory shock. Since the early 1900s tools with this concept has been used. In the 1960s happened a increase in the use of these tools, but as long as the drilling industry were not convinced about its effectiveness, utilization decreased during the 1970s. Nowadays, due to the increasing difficulties in some applications, like hard rock or horizontal drilling, the oil industry is reconsidering its use. The poor performance of conventional hammers in mud system with high proportion of solids and abrasives has led to the development of some hydraulic hammer drilling systems. In some instances, wellbore instability and water influx require a hydraulic hammer. In these two cases air hammer is not technically feasible. b) Thrusters The thrusters are hydraulic shock absorbers used to minimize downhole vibrations, reducing bit and drill string failures, improving wellbore stability and increasing the rate of penetration.