During the last decades, non-conventional solutions for more economical well bore drilling have been investigated. Big research efforts and budgets have been spent on alternative drilling methods; such as laser drilling or jet assisted drilling. Simplified assumptions predicted a big future for such technologies. However, to get a better and more realistic picture of the feasibility of such unconventional ways to drill rock, the ability to apply them in field scale applications needs to be questioned. In this paper a holistic energy approach to evaluate drilling alternatives is proposed. To evaluate the energy balance of the drilling system the amount of energy generated, e.g by the rig at surface, needs to be put in comparison with all system energy consumption to lead to an effective energy available to destroy rock at the bottom of the hole. To test this concept a small scale laboratory setup is used to compare different drilling methods under similar conditions. This paper presents a systematic approach using an energy concept to compare alternative drilling methods on a laboratory scale. In this first approach laser assisted spallation and conventional drilling were compared. As a by-product, also the optimum mode for spallation drilling, continuous versus pulse wave, will be presented.
Vibrations are caused by bit and drill string interaction with formations under certain drilling conditions. They are affected by different parameters such as weight on bit, rotary speed, mud properties, BHA and bit design as well as by the mechanical properties of the formations. During the actual drilling process the bit interacts with different formation layers whereby each of those layers usually have different mechanical properties. Vibrations are also indirectly affected by the formations since weight on bit and rotary speed are usually optimized against changing formations (drilling optimization process). Therefore it can be concluded that for optimized drilling reduction of vibrations is one of the challenges.A fully automated laboratory scale drilling rig, the CDC miniRig, has been used to conduct experimental tests. A three component vibration sensor sub attached to drill string records drill string vibrations and an additional sensor system records the drilling parameters. Uniform concrete cubes with different mechanical properties were built. Those cubes as well as a homogeneous sandstone cube were drilled with different ranges of weight on bit and bit rotary speed. The mechanical properties of all cubes were measured prior to the experiments. During all experiments, drilling parameters and the vibration data were recorded. Based on analyses of the data in the time and the frequency domain, linear and non-linear models were built. For this purpose the interrelations of sandstone and concrete mechanical properties, drilling parameters and vibration data were modeled by neural networks. Application of sophisticated attribute selection methods showed that vibration data in both, time-and frequency domain, have a major impact in modeling the rate of penetration.
Drilling programs continue to push into new and more complicated environments. As a result, accurate measurements of drilling data in real time are becoming more critical by means of minimizing the risks as well as the costs. An ultrasonic caliper sensor is a key measurement for determining the borehole diameter in MWD and LWD tools. Important is the use of ultrasonic caliper tools to offer a method for calculating borehole volumes, on the final bit run, the sensor collects data while tripping out of the hole for determining borehole size and furthermore required cement volumes. Real-time applications of ultrasonic caliper measurements also strongly support the early detection of borehole instability.This paper describes the experiments related to the accuracy of the ultrasonic sensor measurements for estimation of the wellbore diameter. A fully automated test robot has been designed and tests have been performed in different fluids and geometrical conditions. That test robot allows emulating vertical as well as lateral movements of a sensor head in an artificial wellbore which can be run with different fluids. The results of the tests were compared and the weak points and problems of the sensor for detecting the echoes were determined. Numerical simulation of the ultrasonic measurements and comparison of the simulated results to the recorded data gives estimates about the accuracy dependency to different drilling conditions. Tests with different decentralized positions of the ultrasonic caliper tool inside the wellbore give estimates about the accuracy dependence with respect to the decentralization of the tool. Finally measurements were performed in wellbores with geometrical anomalies like washouts and squeezing formations. It is shown that such anomalies can be detected in an appropriate accuracy if circle fitting methods like the Kasa method in combination with robust error models are applied.
Drill string vibration and shock loads are known as destructive loads while drilling and are the reason for tool failure, lost time and reduction in rate of penetration. Vibrations of drill strings can be effected by bit and bottom hole assembly design, interaction of bit/formation and drilling parameters. To manage vibration, however, weight on bit and rotary speed are the only means that can be changed by the driller while making a hole. Therefore it has been always tried to define an optimum range for drilling parameters as key components of the vibration reduction and the rate of penetration management process. A fully automated laboratory scale drilling rig (CDC miniRig) and vibration sensor sub are used to monitor and record drilling parameters such as weight on bit, rotary speed and vibration of drill string among others. Based on different ranges of weight on bit and rotary speed, drilling data and vibration readings are analyzed and the effects of drilling parameters due to vibrations are better understood. Parameter ranges based on the experimental results leading to minimum vibration and optimum rate of penetration are presented.
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