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.
The measurement of actual wellbore shape in real-time can be considered as one of the key components to detect problems such as borehole instability. Abnormal wellbore shape will allow drawing conclusions on the stress field and the impact on other sensors' responses as part of LWD measurements. Accurate wellbore caliper measurements in realtime will also allow to determine cement volume requirements with less measurement time.
In order to get a quantitative assessment of measurement accuracy, an experimental evaluation of the performance of ultrasonic measurements to determine wellbore geometry while drilling has been performed. A fully automated test robot has been designed and experiments were performed in different drilling fluids and wellbore geometries. That test robot allows emulating vertical as well as lateral movements of a sensor head in an artificial wellbore filled with different types of fluids.
Experiments were performed in relation to the radial position of the ultrasonic caliper tool inside the wellbore including different objects inside the borehole to simulate latches or dog legs.
The results were compared and the limitations and arising problems for this type of measurement were identified. The wellbore profiles were measured with different axial and rotational survey speeds. The results allowed the quantification of the errors caused by different wave paths and were compared to the actual borehole geometry. In addition, numerical models were applied for the simulation of the measurements; again the results were compared to the actual ultrasonic measurement.
Nowadays ultrasonic caliper tools which are mounted on MWD and LWD gather information about wellbore shape and size are used by several disciplines in petroleum engineering. Petrophysicists are using the results for environmental corrections and quality control of log data, and geologists use it for the evaluation of mechanical formation properties and breakout and fracture orientation determination among others.
Recognition of variations in borehole shape in real time drilling allow the drilling engineer to actuate appropriate counteractions to avoid costly failures, or to implement alterations in the drilling practices to optimize the shape of the borehole and thus improve the drilling efficiency. The received data help the driller to make proper decisions such as reaming a critical zone, changing the flow rate to reduce erosion or modifying the string rotation speed to reduce vibrations.
Real time application of ultrasonic caliper tools also provides a method for calculating borehole volumes. On a final bit run for instance, the sensor may collect the caliper data during trip-out for the estimation of cement volumes (Maranuk et. al. 1997). Other applications of ultrasonic caliper tools might include real time casing wear and borehole stability detection, evaluation of borehole cleaning and determination of tight spots or formation ledges (Maranuk et. al. 1997). This paper is continuing the tasks performed in a previous paper of the author (Elahifar et al. 2012).
However, even with the modern technology improvements, ultrasonic caliper cannot provide absolute reliable information for all drilling environments and conditions. There are several operational factors such as mud density, borehole wall roughness and tool centralization and position in the borehole among others. They all have to be considered or optimized in order to get accurate caliper data. In addition, for data processing tasks like noise reduction and the conversion of pulse-echo travel times into distances, robust algorithm need to be implemented.
This paper will also discuss the application of reference sensors to measure the speed of sound of drilling fluid. Those
special sensors - a transmitter with opposite receiver - support the actual ultrasonic caliper to determine the borehole
diameters more accurate.
A drill bit creates a hole in subsurface formations by applying weight on bit and rotary speed. The bit is the first segment of the bottom hole assembly (BHA) and the drill string which contacts formation. Today it is possible to monitor drilling parameters such as weight on bit, rotary speed of drill string, azimuthal and inclination angles of the well using downhole sensors. An additional set of parameters recorded and monitored downhole near the bit is the vibration of the drill string as a consequence of the interaction between bit and different formations drilled. Dependent on formation characteristics such as the uniaxial compressive strength of rock, axial vibration is expected to change when the bit contacts formation with changing mechanical properties. Therefore, in case of using drill bits with similar characteristics, recognizing different layers of formation in real-time is expected to be possible if the axial vibration is monitored.
Experiments, using a fully automated laboratory scale drilling rig, the CDC miniRig, were performed. A vibration sensor sub is attached to the drill string above the bit. Concrete cubes with uniform strength as well as layered bonded cubes with different strength were drilled using different ranges of weight on bit and rotational speed. Higher order frequency moments were calculated and used for evaluation of the drilled concrete cubes. The cubes were built with different compressive strength and quantified by uniaxial compressive strength measurements prior to the test. The results of the experiments showed good correspondence with the expected behavior and allowed to differentiate the individual layers of concrete of different strength. The means to identify layers of rock based on vibration analysis are described in this paper. Necessary next steps to translate these results to more complex rocks are presented.
Drill string vibrations are always known as unavoidable destructive loads which lead to harmful downhole fatigue failures, severe bottom hole assembly and bit wear and cause wellbore instability. When the drill bit as the pioneer segment of the bottom hole assembly impact the formation, energy is generated. The energy generated at the bit translates to the drill string as axial, torsional and lateral vibration as the main types of downhole vibration.
Nowadays downhole sensors can monitor drilling parameters such as weight on bit, rotary speed of drill string, azimuthal and inclination angles of the well. Vibrations can be measured at the surface and downhole. However surface measurements do not provide the full picture of downhole environment. Downhole vibrations are measured by accelerometers near the drill bit. Measurements of the vibrations can provide valuable information about downhole conditions and even characteristics of the formation drilled. Therefore, vibration should be fully studied both in laboratory scale and real rigs.
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.