Martin P. Mintchev

Accuracy limitations of FOG-based continuous measurement-while-drilling surveying instruments for horizontal wells

Horizontal drilling processes in the oil industry utilize directional measurement-while-drilling (MWD) instruments to provide real-time monitoring of the position and the orientation of the bottom hole assembly (BHA). It has been reported that a single fiber optic gyroscope (FOG) can be incorporated with three-axis accelerometers to provide real-time MWD surveying of horizontal wells. However, the long-term performance and the accuracy limitations of this FOG-based instrumentation system have not been discussed. This article aims at describing a methodology for quantitative long-term analysis of the various surveying errors while drilling the near-vertical sections of the well. It also offers some techniques to enhance the long-term surveying accuracy in an experimental model of the FOG-based downhole-surveying instrument. The surveying errors are optimally estimated by the Kalman filtering techniques, and their long-term analysis is based on studying the corresponding mean square estimation errors. In order to limit the long-term growth of the surveying errors, we suggest improving the velocity computation provided by the FOG-based system either by continuous velocity update or by zero velocity update at some predetermined surveying stations. These techniques have significantly limited the long-term growth of the position errors (less than 100 m over a more than 2-h experiment). Moreover, the errors associated with the BHA orientation components were kept at less than 1/spl deg/. Suggested methodology significantly improved the surveying accuracy in an experimental model of the FOG-based MWD surveying system.

Measurement-while-drilling surveying of highly inclined and horizontal well sections utilizing single-axis gyro sensing system

In the oil industry, when developing a plan for drilling horizontal wells, measurement-while-drilling (MWD) surveying becomes one of the important prerequisites for the successful completion of the drilling process. MWD surveying of horizontal wells determines the position and the orientation of the bottom hole assembly (BHA) in real-time during the drilling operation. The BHA orientation is determined by its inclination from the vertical direction as well as its azimuth. The present MWD surveying system incorporates three-axis accelerometers and three-axis magnetometers mounted in three mutually orthogonal directions. This magnetic surveying system suffers from the deviation of the magnetic field measurements due to the massive amount of steel around the drilling rig. A new method utilizing the fibre optic gyroscopes (FOG) technology was suggested to replace the magnetic surveying system. It was reported that a single FOG mounted inside the bearing assembly with its sensitive axis along the tool spin axis could be incorporated with three-axis accelerometers to continuously survey the near-vertical well section. This study aims at suggesting a surveying methodology for highly inclined and horizontal well sections utilizing FOG sensors. At each surveying station, the intrinsic uncertainties of the surveying sensors and the other vibration-induced noise are reduced using a transversal finite impulse response filter. The inclination is then computed by processing the accelerometer measurements, while the gyro measurement is utilized to determine the azimuth. In addition, optimal estimation techniques based on Kalman filtering are used to improve the azimuth accuracy and to limit the effect of the drift of the surveying sensors over time. This study indicates that gyroscopic surveying utilizing the FOG technology can be a reliable solution for MWD surveying of horizontal wells.

FOG-based navigation in downhole environment during horizontal drilling utilizing a complete inertial measurement unit: directional measurement-while-drilling surveying

Presently used surveying sensors in directional drilling processes include accelerometers and magnetometers arranged in three orthogonal directions. The magnetometers in these setups are negatively affected by external magnetic interferences induced by various sources. Therefore, expensive, heavy and lengthy protective nonmagnetic collars need to be installed. Fiber-optic gyroscopes (FOGs) in an inertial navigation setup have been proposed as an alternative to magnetometer-based downhole surveying. The present study explored the feasibility of utilizing a FOG-based tactical-grade inertial measurement unit (IMU) as a complete surveying sensor for measurement-while-drilling (MWD) processes downhole. Alignment and real-time navigation under laboratory conditions were demonstrated. Analysis of vibrations and temperature as possible factors limiting the accuracy of the navigation process was performed. Severe vibration effects were reduced using software techniques, and a shock-absorbing housing was suggested. The temperature range of the IMU is limited by the optical components of the device, but dynamic temperature changes within this range did not present a major problem. A downhole sub design demonstrated that the actual integration of the IMU requires only minor changes in the presently used drilling tools. The utilization of a tactical-grade IMU eliminates the necessity of nonmagnetic collars, which results in lower costs and improved accuracy.

Observability Analysis for INS Alignment in Horizontal Drilling

Contemporary surveying in measurement-while-drilling (MWD) processes incorporates measurements from three-axes accelerometers and magnetometers. Unfortunately, magnetometer-related problems limit the navigation performance of this technique. The introduction of fiber-optic-gyroscope (FOG)-based inertial navigation system (INS) in MWD aims at overcoming these limitations. However, drifts in the measurements provided by the INS might be prohibitive for the long-term utilization of this modern navigation-method downhole. One of the main obstacles precluding the elimination of these measurement drifts is the limited observability of the azimuth angle state provided by the INS. This paper explores the feasibility of utilizing a FOG-based tactical-grade inertial measurement unit (IMU) as a complete surveying sensor for a MWD processes downhole by implementing an innovative in-drilling alignment (IDA) procedure. During IDA, the IMU is exposed to controlled dynamics that excites azimuth-related states. This allows better and faster alignment that can reduce long-term navigation drifts, thus improving the overall accuracy in INS-based MWD processes. It is suggested that one take advantage of the longitudinal space available in the drilling-pipe system and impose controlled motion on the IMU to excite its states and increase its observability. Theoretical simulations and analytical approximations exploring the IDA idea have shown reduction in the steady-state azimuth-error variance and in the time required to achieve convergence with the increase of the acceleration-controlled motion. Several practical aspects of implementing this approach are evaluated and compared.

Experimental Feasibility of the In-Drilling Alignment Method for Inertial Navigation in Measurement-While-Drilling

Conventional methods in horizontal drilling processes incorporate magnetic surveying techniques for determining the position and attitude of the bottom-hole assembly (BHA). This results in an increased weight of the drilling assembly, higher cost due to the use of nonmagnetic drill collars necessary for shielding the magnetometers, and significant errors in the position of the drilling bit. A novel inertial navigation system (INS)-based technique has been previously proposed as an alternative to magnetometer-based downhole surveying. Previous studies have shown theoretically that an adaptive-filter-based in-drilling alignment (IDA) fine alignment method successfully limits the error growth associated with INS. This study aims at examining IDAs practical feasibility and, specifically, its ability to estimate the azimuth angle. Experimental testing of the IDA method was conducted under laboratory conditions with an apparatus that can easily be adopted for downhole conditions. The experimental results demonstrate that the IDA-estimated azimuth is more precise, compared with the one estimated by conventional magnetic surveying systems. The high accuracy and implementation simplicity of the proposed INS-based surveying system render it a preferred method for future horizontal drilling operations.

Computer simulation of the effect of changing abdominal thickness on the electrogastrogram

The effect of different abdominal thickness on cutaneous recordings of gastric electrical activity (GEA) known as electrogastrograms (EGG) have not been adequately studied. The stomach was represented as a truncated conoid in spherical system of coordinates. Gastric electrical field was modelled using previously described methodology. Electrical voltages were calculated in simulated standard cutaneous recordings. The effect of increased thickness of the abdominal layers was quantitatively examined. Changes of the thickness of the abdominal layers significantly affected signal-to-noise ratio of EGG. When the critical abdominal thickness of 7 cm was exceeded, EGGs were quantified as abnormal although the internal GEA was normal. Computer modelling indicated that changeable abdominal thickness caused by the abdominal layers separating the stomach from the recording electrodes significantly influence the EGG recordings even if the layers are homogeneous.

Improving geometric accuracy in the presence of susceptibility difference artifacts produced by metallic implants in magnetic resonance imaging

Geometric and intensity distortions due to the presence of metallic implants in magnetic resonance imaging impede the full exploitation of this advanced imaging modality. The aim of this study is to provide a method for (a) quantifying and (b)reducing the implant distortions in patient images. Initially, a set of reference images (without distortion) was obtained by imaging a custom-designed three-dimensional grid phantom. Corresponding test images (containing the distortion) were acquired with the same imaging parameters, after positioning a specific metallic implant in the grid phantom. After determining: 1) the nonrecoverable; 2) the distorted, but recoverable; and 3) the unaffected areas, a point-based thin-plate spline image registration algorithm was employed to align the reference and test images. The calculated transformation functions utilized to align the image pairs described the implant distortions and could therefore be used to correct any other images containing the same distortions. The results demonstrate successful correction of grid phantom images with a metallic implant. Furthermore, the calculated correction was applied to porcine thigh images bearing the same metallic implant, simulating a patient environment. Qualitative and quantitative assessments of the proposed correction method are included.

Do increased electrogastrographic frequencies always correspond to internal tachygastria

This study was undertaken to investigate the possible origin of some cutaneously recorded higher frequency electro-gastrographic signals. Computer modeling of gastric electrical uncoupling was performed using previously described conoidal dipole model. Cutaneous electrogastrograms were simulated after uncoupling was introduced. In separate, real-life experiments, 6 pairs of bipolar electrodes were inserted into the gastric wall (3 anterior, 3 posterior) of 15 anesthetized dogs at laparotomy to record 6 channels of internal gastric electrical activity (GEA). Eight-channel bipolar cutaneous electrogastrography (EGG) was simultaneously recorded. Three separate 1/2-hr recordings were made from each dog in the basal state and after each of two circumferential euts of all gastric muscle. Distal stomach was surgically divided into three equal-sized areas, each with an electrode pair in its anterior and posterior walls. Gastric electrical activity and EGG were digitized, bandpass-filtered and analyzed in frequency domain using the fast Hartley transform. The phenomena of tachygastria and tachyarrhythmia were quantitatively compared in internal and cutaneous recordings. Computer modeling indicated that it is possible to record cutaneous “tachygastric” or “tachyarrhythmic” signals without them being present internally. Real experiments on dogs showed higher percentage of tachyarrhythmia in EGG than in the internal signal in the basal state. After the first circumferential cut, the periods of tachygastria and tachyarrhythmia increased, with EGG signals showing again a higher percentage. This tendency persisted after the second circumferential cut. A similar pattern was observed when monitoring the percentage of uncorresponding tachygastrias/ tachyarrhythmias. Our findings indicate that gastric electrical uncoupling can be another possible reason for abnormal frequency characteristics of EGG. Not all cutaneously recognized tachygastrias and/or tachyarrhythmias could be related to objectively existing internal tachygastric events.

New technique for reducing the angle random walk at the output of fiber optic gyroscopes during alignment processes of inertial navigation systems

Aboelmagd NoureldinDave Irvine-HallidayUniversity of CalgaryDepartment of Electrical and ComputerEngineeringAlberta, Canada T2N 1N4Herb TablerInternational Downhole Equipment, Ltd.Edmonton, Alberta, CanadaMartin P. MintchevUniversity of CalgaryDepartment of Electrical and ComputerEngineeringAlberta, Canada T2N 1N4E-mail: mintchev@enel.ucalgary.caAbstract. Angle random walk (ARW) is the noise component at the out-put of a fiber optic gyroscope (FOG) and it affects the FOG short-termaccuracy. Practical applications of FOGs inside an inertial navigationsystem necessitate monitoring the Earth’s rotation rate component alongthe FOG sensitive axis to determine the initial attitude of the movingobject. The ARW increases the measurement uncertainty, thus affectingthe overall accuracy. We introduce a new filtering approach that signifi-cantly reduces the ARW at the FOG output to a level that can ensure anaccurate measurement of the Earth rotation rate. The filtering approachemploys the forward linear prediction (FLP) technique to design a tapdelay line filter and a new criterion, based on controlling the step sizeparameter, is introduced to ensure the fastest possible convergence ofthe adaptive algorithm, while keeping the minimal possible mean squareerror. The proposed FLP filter of 300 tap weights is capable of reducingthe ARW from 4.66 to 0.0694deg/h(

Self-Stabilizing Colonic Capsule Endoscopy: Pilot Study of Acute Canine Models

Video capsule endoscopy (VCE) is a noninvasive method for examining the gastrointestinal tract which has been successful in small intestine studies. Recently, VCE has been attempted in the colon. However, the capsule often tumbles in the wider colonic lumen, resulting in missed regions. Self-stabilizing VCE is a novel method to visualize the colon without tumbling. The aim of the present study was to comparatively quantify the effect of stabilization of a commercially available nonmodified capsule endoscope (CE) MiroCam and its modified self-stabilizing version in acute canine experiments. Two customized MiroCam CEs were reduced in volume at the nonimaging back-end to allow the attachment of a self-expanding, biocompatible stabilizing device. Four mongrel dogs underwent laparotomy and exteriorization of a 15-cm segment of the proximal descending colon. A single CE, either self-stabilizing or nonmodified was inserted through an incision into the lumen of the colon followed by phar macologically induced colonic peristalsis. The inserted capsule was propelled distally through the colon and expelled naturally through the anus. Novel signal processing method was developed to quantify the video stabilization based on camera tracking a pre determined target point (locale). The average locale trajectory, the average radius movement of the locale, and the maximum rate of change of the locale for sequential images were significantly lower for the stabilized capsules compared to the nonstabilized ones (p <; 0.05). The feasibility of self-stabilized capsule endoscopy has been demonstrated in acute canine experiments.