Measurement While Drilling System Research Articles

Anti-Vibration Method for the Near-Bit Measurement While Drilling of Pneumatic Down-the-Hole Hammer Drilling

Pneumatic down-the-hole (DTH) hammer drilling technology has been used extensively in the fields of heat reservoir exploitation and geological exploration owing to its advantages of high efficiency and low pollution. However, the vibration near the bit is up to 40 g while DTH hammer drilling, which significantly affects the performance and longevity of the near-bit measurement while drilling (MWD). To enhance the environmental adaptability of the near-bit MWD in pneumatic DTH operations, a design method for a vibration-damping system based on the parameter optimization of a non-dominated sorting genetic algorithm II (NSGA-II) is proposed in this study. First, the whole structure of the near-bit MWD is designed, including the MWD sub-shell, sensors, measurement circuits, batteries, and connecting structures (the circuit unit). Secondly, this study analyzes the vibration characteristics of the pneumatic DTH hammer near the bit. According to the damping structure, the vibration response model for the circuit unit and the damping model are established. Thirdly, NSGA-II is employed to optimize the parameters of the damping model in terms of the low-frequency, high-intensity vibration characteristics near the bit in pneumatic DTH operations, thereby devising a damping scheme tailored to the unique conditions of DTH hammer drilling. Finally, vibration experiments were conducted to verify the effectiveness of the vibration-damping device. The experimental results indicate that within the vibration frequency range of 5–20 Hz and vibration level of 10–40 g, the peak attenuation rate of the circuit unit is more than 86.446%, and the improvement rate of the vibration stability of the system is more than 75.214%; the anti-vibration performance of the near-bit MWD system in DTH hammer drilling is improved remarkably. This study provides strong technical support for the stability of MWD equipment under such special working conditions. It has broad engineering application prospects.

Calibration of accelerometer and magnetometer sensors of MWD systems in directional drilling application

Measurement while drilling (MWD) tools include accelerometers, magnetometers and temperature sensors. The accuracy of these sensors are highly important in measuring inclination and Azimuth angle parameters in a directional well. These factors can increase costs when drilling a directional well. During the past years, many efforts have been made to increase the accuracy of these tools. Accuracy of accelerometer and magnetometer sensors are influenced by scale, bias and non-orthogonal errors. Considering that MWD systems work in a wide range of temperatures (for example 0 °C–150 °C), the temperature parameter should also be added to the factors affecting the errors. Here, after making a sample of MWD tool, a method to calibrate the tool using mathematical model based on total field calibration algorithm, least square method and the transfer matrix is presented. Calibrated parameters at ambient temperature are calculated with acceptable speed. Then, a method for calculating temperature coefficients in the desired range is provided. Compared to the case where temperature coefficients are not considered, the obtained results show a significantly increased accuracy of Inclination and Azimuth parameters through the use of these coefficients.

Physics-Based Observers for Measurement-While-Drilling System in Down-the-Hole Drills

Measurement While Drilling (MWD) is a technology for assessing rock mass conditions by collecting and analyzing data of mechanical drilling variables while the system operates. Nowadays, typical MWD systems rely on physical sensors directly installed on the drill rig. Sensors used in this context must be designed and conditioned for operating in harsh conditions, imposing trade-offs between the complexity, cost, and reliability of the measurement system. This paper presents a methodology for integrating physics-based observers into an MWD system as an alternative to complement or replace traditional physical sensors. The proposed observers leverage mathematical models of the drill’s electrical motor and its interaction with dynamic loads to estimate the bit speed and torque in a Down-the-Hole rig using current and voltage measurements taken from the motor power line. Experiments using data collected from four test samples with different rock strengths show a consistent correlation between the rate of penetration and specific energy derived from the observed drilling variables with the ones obtained from standardized tests of uniaxial compressive strength. The simplicity of the setup and results validate the feasibility of the proposed approach to be evaluated as an alternative to reduce the complexity and increase the reliability of MWD systems.

An Attitude Improvement Method of FOG-Based Measurement-While-Drilling Utilizing Backtracking Navigation Algorithm

Directional drilling utilizes the measurement-while-drilling (MWD) to provide attitude for drilling bit. A traditional MWD system, consisting of magnetometers and accelerometers, relies on expensive and huge nonmagnetic drill collar (NMDC) to shield from magnetic interference to obtain accurate attitude angles. A fiber optic gyroscope-based MWD (FOG-based MWD) that is based on inertial navigation is an alternative survey scheme. But, random vibration environment of underground introduces high-amplitude random noise to the FOGs. Besides, the auxiliary measuring equipment [such as global position system (GPS)] will not be available in underground environments, which will bring massive attitude error under pure inertial navigation. Therefore, combined with the engineering application conditions, this article proposes a modified attitude precision improvement approach of the FOG-based MWD. First, an autoregressive moving average (ARMA) error model for FOG is established to restrain the random vibration environment interference of underground. Second, combined with the FOG error model, a continuous backtracking navigation error model is developed to improve the azimuth accuracy. Moreover, the continuous backtracking algorithm frame is designed to estimate the attitude. Finally, a semi-physical experiment and a drilling test verify that the error model has an obvious effect on reducing the amplitude of FOG random noise. The proposed backtracking algorithm make the azimuth angle achieve a high-level estimation in short time. The result of the drilling test shows that the azimuth has an improvement of 48.79%, as compared with the traditional zero velocity update (ZUPT).

Study of Downhole Lateral Force Measurement Modelling and Devices in Petroleum Exploration

Measurement while drilling (MWD) technology is important for obtaining downhole parameters. As an essential parameter in drilling engineering, lateral force provides a powerful reference basis for judging the downhole drilling direction. Although stress measurement technology has matured, research on downhole lateral forces, especially near-bit lateral forces during the drilling process in petroleum exploration, is lacking. Based on the force analysis of a short measurement circuit, the lateral force measurement value of a drill bit and centralizer was converted to a drill string’s radial-bending-force measurement. Two perpendicular lateral force components were measured using strain gauge technology, a lateral force measurement theory model was established, and a set of MWD systems was designed according to the model. The systems’ function was verified through laboratory and field tests, and the field-test data were successfully obtained in the field application. The test results showed that the tested MWD system had acceptable accuracy, stability, and reliability and had the application potential to measure lateral force in the drilling industry. This article provides a new idea to study lateral force while drilling, which is of great significance to oil-drilling exploration and development.

A Novel Attitude Measurement While Drilling System Based on Single-Axis Fiber Optic Gyroscope

Aiming at the inability to achieve high-precision real-time magnetic heading angle estimation under single degree of freedom rotation, a novel attitude measurement while drilling (MWD) system is designed, which consists of a single-axis fiber optic gyroscope (FOG), a tri-axial micro-electromechanical system (MEMS) accelerometer, and a tri-axial magnetometer. The magnetometer bias calibration model based on uniform rotation is established and verification experiments are carried out. The uniform rotation experiment shows that six calibration results fluctuate within 5% of the mean. A real-time estimation model of the magnetic heading angle under random rotation is established, of which the theoretical error is less than 0.38°. Three real-time estimation experiments of magnetic heading angle are carried out, and the overall estimation error did not exceed 0.35°, and the average estimation error did not exceed 0.3°. Under continuous rotation of large angle, the bias estimation of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$X$ </tex-math></inline-formula> -axis and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Y$ </tex-math></inline-formula> -axis magnetometers can be achieved. The results show that the estimated value of the bias of the magnetometer under dynamic conditions can provide a reference for the initial magnetic heading angle measurement under static conditions. The experimental results prove the effectiveness of the proposed method, which provides a reference for the further improvement of the attitude MWD system.

Prospects of Applying MWD Technology for Quality Management of Drilling and Blasting Operations at Mining Enterprises

This paper presents a review of measurement while drilling (MWD) technology as applied to the mining industry, describes its development path, provides a global review of literature on this topic, and outlines further trends of development for research on MWD application in drilling and blasting (D&amp;B) operations at mining enterprises. The current review serves as a starting point for anyone interested in the research or application of MWD technology in Mining and Construction. In the paper, the authors examine major works of researchers in this area, describe current state of the art, and propose a way to improve MWD for drilling equipments. The paper contains examples of technology application in various processes, associated with drilling and mining operations, describes approaches and problems of MWD system utilization, revealed in the course of data collection and analysis of drilling processes. The study also presents a summary of existing approaches in the area of data validation and verification, applied up to the present day to cope with the problems of global MWD use in Mining and Construction. The authors outline future areas of study which are of interest and deserve the attention of the scientific community and researchers working on the development of MWD technology.

Communication Distance Test of Pulse MWD System for Mining Mud

Aiming at the problem of limited communication distance when the current wireline measurement system is applied in directional drilling to realize the advanced detection and control of mine disaster factors, a mine mud pulse measurement-while-drilling (MWD) system has been developed. However, there is no means to verify and detect the communication distance of this new system in production, acceptance and testing. Based on the communication principle of mud pulse MWD system, this paper establishes a communication data model, analyzes the factors affecting the communication distance of this system, and finally, through building a test platform, tests the communication effect of the mine mud pulse MWD system by simulating the communication distance of 3000 meters. The results show that the pulse MWD system can achieve communication while drilling of 3000 meters or longer.

Experimental Study on Comprehensive Real‐Time Methods to Determine Geological Condition of Rock Mass along the Boreholes while Drilling in Underground Coal Mines

The geological condition is essential for mining design and disaster control in underground coal mines. The present research focuses on the real‐time assessment method on rock mass condition during drilling boreholes. In situ comprehensive experiments were carried out using three methods, which are measurement while drilling (MWD) system, vibration measurement while drilling (VMWD) system, and borehole camera detecting system. In the MWD system, the operating parameters of the drilling machine were recorded, and a dimensionless index Id based on the collected parameters was adopted to assess the geological condition along the borehole. The results show that the state of rock mass can be well classified using the MWD system for both the cross‐layer and in‐seam boreholes. In the VMWD system, the vibration of the drilling bit was monitored, and the signal was analyzed in both time domain and frequency domain. The results indicate that the rock mass condition can be quantitatively evaluated using the mean square value of the signal and qualitatively estimated using the energy of the spectrum. In the borehole camera system, the photos of the rock mass along the borehole could be well captured, and the identified rock mass condition was used to verify the results of the MWD and VMWD systems. Comprehensive compassion between the results from the three systems shows that all the methods can give valuable information for the geological condition, and the outcomes of the different methods are generally comparable. For practical purposes, the advantages of the involved three detecting systems are discussed.

Research on Drilling Bit Positioning Strategy Based on SINS MWD System

Accurate measurement of drill bit position and direction is the main technology to realize automation and intelligence in oil drilling field. In recent years, with the rapid development of complex drilling technologies such as high deviation wells, directional wells and horizontal wells, higher requirements have been put forward for measurement technology, especially for real-time monitoring of bit attitude and position during the drilling process. Therefore, for the past few years, the measurement while drilling (MWD) system has been widely recognized and developed rapidly in precision targeted drilling. among them, strap-down inertial navigation system (SINS) consisting of accelerometer and gyroscope is the key of down-hole measurement while drilling. To solve the problem of accumulated errors in SINS, in this paper, a positioning error correction method based on kinematic constraint-aided (KC) SINS zero velocity updated (ZUPT) model is proposed. Firstly, based on the acceleration and angular velocity information measured by SINS, empirical mode decomposition (EMD) and wavelet de-noising reconstruction are performed for MWD signals. Secondly, the static detection model of the drill bit is established by using the reconstructed signal. Thirdly, using drilling technology to analyze the motion attitude of the bit, the KC model of the down-hole bit is established. By analysis the alternating effect of the KC model and the ZUPT model in the process of the bit movement and stop, the ZUPT model of the SINS is established. Finally, experimental verification is performed by building a drilling platform. The experimental results show that the maximum positioning error of the proposed positioning model is 0.15 m within 300 s. Comparing with a single KC model and a single ZUPT model, the bit positioning accuracy is improved to 92.6%, which effectively suppresses the original cumulative error, and verifies the feasibility of the proposed method.

A Fault-Tolerant Integrated Borehole Trajectory Location Method Based on Geomagnetism/IMU of MWD

In the oil industry, directional drilling technology plays a significant role in oil-gas exploration. The key to success of directional drilling technology is locate the borehole accurately using measurement while drilling instruments. Measurement While Drilling(MWD) technology has the problems of data loss and gross error in complex down hole environment. Aiming at the problems that gross error and data missed consequences the accuracy of positioning calculation seriously, this paper presents a method of fault-tolerant integrated borehole trajectory location based on geomagnetism/IMU of MWD system. Firstly, the geomagnetic survey system is established to locate the borehole position based on tri-axis fluxgate and tri-axis accelerometer. The Inertial Measurement Unit(IMU) calculates the borehole trajectory at the same time. Then, the fault-tolerant judgment mechanism is introduced to discriminate and evaluate data loss and gross error of measurement parameters. Furthermore, Kalman Filter algorithm is implied to construct fault-tolerant integrated borehole trajectory positioning system. Finally, the simulation experiment is performed on the drilling experimental platform. The experimental result shows that the fault-tolerant integrated borehole trajectory positioning system can detect data loss and gross error effectively. In addition, the location precision of the fault-tolerant integrated positioning system can be maintained within 1 meter when the time of data missed and gross error state lasts for 6min respectively. From the simulation experiments, comparing with the position calculation of pure geomagnetism, and the method of integrated MEMS and geomagnetism, the precision of the fault-tolerant integrated location calculation is improved by 68.7% and 62.8% respectively.

A Low-Cost Calibration Strategy for Measurement-While-Drilling System

The measurement-while-drilling (MWD) system is mainly used to draw borehole trajectory in a coal mine. However, its measurement error directly demonstrates a deviation between the drawing locus and the actual path. Thus, it is imperative to effectively compensate the error in advance. In this study, the traditional-integrated error model is adopted and a low-cost calibration strategy, namely an incomplete calibration strategy, is proposed. In this strategy, the ellipsoid-fitting method is quoted to determine the combined bias and orthogonalize the coordinate frame of the triaxial accelerometer, and a new plane-fitting method is exploited to align the orthogonal accelerometer coordinate frame with the body coordinate frame of the MWD system; meanwhile, the theoretical basis is also stated with the simulation verification of MATLAB. Besides this, the scalar product constant method is finally employed to acquire the error model of the triaxial magnetometer on the condition of the previously calibrated accelerometer coordinate system. In the end, the newly designed calibration strategy was experimentally tested with the MWD prototype developed by ourselves. The results reveal that the strategy can not only be implemented without the help of other expensive auxiliary equipment but also has good performance in attitude reconstruction

Investigation of tribotechnical and corrosion behaviour of material for light-alloy drill pipes

Special aluminum alloys appear to be promising materials for manufacture of high-strength light-alloy drill pipes (HSLADP) that can be used in areas with a severe climate and challenging geology. The effect of using light-alloy drill pipes (LADP) depends directly on the properties of the aluminum alloys from which such pipes are made. As the wells become deeper and horizontal wellbores get longer, use of LADPs becomes more relevant. Since light-alloy pipes are 2.8 times softer than steel pipes, LADPs offer the same performance as steel drill pipes of the lowest strength grade even in the case of rotary drilling. The materials from which such pipes are made have a number of unique advantages: extra light weight in the drill mud, allowing the coefficient of sliding friction between the pipe surface and the borehole wall to be reduced; high corrosion resistance in aggressive media with A high concentration of hydrogen sulfide and carbon dioxide; and high magnetic inductive capacity that allows LADPs to be used as a housing for MWD (measurement while drilling) and LWD (logging while drilling) telemetry systems during well-drilling operations. This study suggests methods for industrial production of submicrocrystalline (SMC) structure in aluminum alloys with the help of severe plastic deformation. Through the example of model aluminum-lithium alloys 1420 (Al-Mg-Li-Zr) and 1460 (Al-Сu-Li-Zr), the researchers demonstrate that SMC structure helps significantly increase resistance to wear and reduce the rate of corrosion depending on the pH value. The research team also states that severe plastic deformation methods may be used to develop highly promising technologies for manufacture of high-strength LADPs with advanced strain-stress properties for use during operations in the Arctic.

Guest Editorial: Innovation Will Drive Shale Survival

Guest editorial That which does not kill us, makes us stronger.—Friedrich Nietzsche, 1889 The petroleum industry has seen up and down cycles. In the past, they have been driven by politics, such as in the case of the Arab oil embargo, or supply and demand imbalances, driven primarily by unrest and recessions. The creation of OPEC dampened the latter. But this time it is different. Shale, a new, abundant source of oil and gas has more than halved oil prices. It has kept prices low for nearly 3 years, and threatens to do so for many more. This threat will be realized if the industry can be profitable at sustained prices below $40/bbl. History informs us that this is likely. The last long-lived drop in activity roughly spanned the decade from the mid-1980s to the mid-1990s. The rig count was more than halved from 1984 levels for well over a decade. This trailed the oil price drop, which nearly halved from 1980 to 1985, and stayed that low for over a decade. For the industry to survive, oil companies needed lower production costs and the service companies still needed to make a profit at low activity levels. This is precisely the situation today in shale oil and gas. On that occasion, the industry responded with innovation in technology and, to a lesser degree, in business models. This too will happen today. Horizontal Wells and More Horizontal wells, possibly the single greatest productivity improvement innovation in the business, came into their own during that earlier downturn. They had been known for decades, but this period produced additional key enablers for economic construction. Desktop 3-D seismic interpretation allowed for placement in the most productive portions of reservoirs. Polycrystalline diamond composite (PDC) bits improved drilling rates. Measurement while drilling (MWD) gave crucial position information on the fly. Later, MWD systems allowed formation evaluation, which was prohibitive with conventional logging methods in hole angles greater than 60 degrees. At this point, drillers knew where to be, and where they were, in 3-D and geologic space.

A Long-Term Performance Enhancement Method for FOG-Based Measurement While Drilling.

In the oil industry, the measurement-while-drilling (MWD) systems are usually used to provide the real-time position and orientation of the bottom hole assembly (BHA) during drilling. However, the present MWD systems based on magnetic surveying technology can barely ensure good performance because of magnetic interference phenomena. In this paper, a MWD surveying system based on a fiber optic gyroscope (FOG) was developed to replace the magnetic surveying system. To accommodate the size of the downhole drilling conditions, a new design method is adopted. In order to realize long-term and high position precision and orientation surveying, an integrated surveying algorithm is proposed based on inertial navigation system (INS) and drilling features. In addition, the FOG-based MWD error model is built and the drilling features are analyzed. The state-space system model and the observation updates model of the Kalman filter are built. To validate the availability and utility of the algorithm, the semi-physical simulation is conducted under laboratory conditions. The results comparison with the traditional algorithms show that the errors were suppressed and the measurement precision of the proposed algorithm is better than the traditional ones. In addition, the proposed method uses a lot less time than the zero velocity update (ZUPT) method.

Design and Algorithm Verification of a Gyroscope-Based Inertial Navigation System for Small-Diameter Spaces in Multilateral Horizontal Drilling Applications

In the recent years horizontal drilling (HD) has become increasingly important in oil and gas exploration because it can increase the production per well and can effectively rework old and marginal vertical wells. The key element of successful HD is accurate navigation of the drill bit with advanced measurement-while-drilling (MWD) tools. The size of the MWD tools is not significantly restricted in vertical wells because there is enough space for their installation in traditional well drilling, but the diameter of devices for HD must be restricted to less than 30 mm for some applications, such as lateral drilling from existing horizontal wells. Therefore, it is essential to design miniature devices for lateral HD applications. Additionally, magnetometers in traditional MWD devices are easily susceptible to complex downhole interferences, and gyroscopes have been previously suggested as the best avenue to replace magnetometers for azimuth measurements. The aim of this paper is to propose a miniature gyroscope-based MWD system which is referred to as miniature gyroscope-based while drilling (MGWD) system. A prototype of such MGWD system is proposed. The device consists of a two-axis gyroscope and a three-axis accelerometer. Miniaturization design approaches for MGWD are proposed. In addition, MGWD data collection software is designed to provide real-time data display and navigation algorithm verification. A fourth-order autoregressive (AR) model is introduced for stochastic noise modeling of the gyroscope and the accelerometer data. Zero velocity and position are injected into a Kalman filter as a system reference to update system states, which can effectively improve the state observability of the MGWD system and decrease estimation errors. Nevertheless, the azimuth of the proposed MGWD system is not observable in the Kalman filter, and reliable azimuth estimation remains a problem.

Horizontal Well Applications-Canadian Achievements

Abstract Spectacular! It is the only word that can be used to describe the rapid growth in the advancement of horizontal well technology that has taken place over the past 14 years in the petroleum industry. This is the most exciting development that this industry has known. The history of horizontal drilling in North America, although brief, has been one filled with dramatic developments. Nearly 95% of all the horizontal wells drilled in the world are in Canada and the United States. The development of horizontal well technology in the US brought revolutionary advances to the drilling sector such as the development of poly-crystalline compact bits, downhole drilling motors, top-drive drilling rigs, logging-while-drilling, geo-monitoring with steerable drill systems, and re-entries from existing vertically cased wells with coiled tubing. These achievements were accomplished mainly in the Austin Chalk of Texas (1989) and the Bakken shales of North Dakota (1991). These tools brought a higher drilling rate, lower drilling cost, reduced rig time, better interpretation of the drilling bit position, and faster geological descriptions. In Canada, on the other hand, because of the larger variety of target reservoirs and geological settings, the synergistic interdisciplinary approach created unique solutions with significant qualitative improvements in implementation. The first Canadian well was drilled in 1978 and over 14,700 wells have been licensed to date. We never dreamt ten years ago that drilling would use tools which provide real time data about the reservoir with the aid of LWD (logging-while-drilling) or MWD (measurement-while-drilling) systems. Today, we use a rotary closed loop drilling system, which combines the performance of continuous rotary drilling with precise placement, all made possible by LWD sensors and automated downhole guidance systems; this enables the driller to find the right path in the reservoir. On the production side, there are a variety of multilateral well junctions in order to assure the functionality of multilateral horizontal wells with regard to the access for cleaning, sands/solids barrier, and isolation. Sperry Sun introduced the lateral tie back system (LTBS) that allows multilateral branches to be drilled and cased from a single horizontal wellbore, followed by innovative production techniques based on progressing cavity pumps. This new solution brought high handling efficiency and considerable savings in production costs. Canadian Western Natural Gas drilled the first underbalanced well with natural gas (not with air or nitrogen) in a gas storage, low pressure, and water sensitive reservoir. The use of natural gas, although it involves unprecedented high safety standards, it allows, at the same time, to minimize the formation damage that is essential in order to maintain demanding deliverability requirements. In low permeability gas reservoirs, several projects have been promising in reservoirs that could not have been exploited otherwise. In naturally fractured reservoirs, in the Midale carbonate beds in Weyburn, PanCanadian created an innovative solution by placing the wells parallel to the fractures rather than perpendicular to them, as had been routinely practised. The intent was to avoid water being drawn upwards from fractures that extend both vertically and laterally.

Newly Applied BHA Elements Contribute to Mainstreaming of Coiled-Tubing Drilling Applications

Summary Coiled-tubing drilling (CTD) is coming into the mainstream in a number of locations and applications, and the availability of improved bottomhole assembly (BHA) elements is a significant contributor in this regard. This paper describes BHA elements, including a hydraulic orienter and measurement-while-drilling (MWD) system, recently used by Arco in CTD operations; examines their capabilities; and provides examples in which these tools were used successfully. The BHA orienter is a newly engineered, simple, hydraulically actuated device. This paper examines its performance attributes affecting overall rate of penetration (ROP) and directional control of the BHA, including its actuation speed and its ability to change the BHA orientation while in a medium-radius curve. The MWD system represents a new application of an existing reliable system, with attributes that are especially important in the CTD environment. Modifications related to downsizing are examined, as well as relevant performance attributes including steering-data-update rate; real-time gamma log quality; mud-pulse-detection capability on coil in a range of drilling fluids; and reliability. The BHA is also looked at from an overall system perspective, including its integration with the coiled tubing (CT) and other equipment associated with the CTD operation. As of the writing of Ref. 1, Arco Alaska Inc. and THUMS Long Beach Co. (a wholly owned subsidiary of Arco) had completed 16 CTD re-entry sidetrack jobs in Alaska and Long Beach, California, with these BHA elements. This paper summarizes overall performance and reliability from these jobs. Additionally, specific cases are examined in greater detail, with these BHA elements contributing to successful CTD window milling, medium-radius curves, and laterals. (As of this publication, Arco companies have performed 47 such jobs with these BHA elements; however, this paper focuses on the first 16 jobs.)

Resistivity Measurements at the Bit Provide Real-Time Formation Evaluation Before Invasion

Real-time formation evaluation with today's conventional horizontal drilling techniques is limited by the distance between the bit and resistivity measurements. Logging-while-drilling (LWD) sensors reach the formation long before wireline measurements, and so generally view it before wellbore degradation, but not before some invasion has occurred. Rapid invasion, called spurt, may mask true resistivity in some formations. The solution to this problem is to relocate logging measurements to the bit itself. A new LWD resistivity tool improves and simplifies formation evaluation by allowing geologists to visualize and log the formation around the wellbore before mud invasion or wellbore damage has occurred. The tool is normally run as a near-bit stabilizer on a rotary bottomhole assembly or just above the motor in a steerable assembly. It makes five formation evaluation resistivity measurements and an azimuthal gamma ray measurement. One resistivity measurement uses the bit as part of the measuring electrode to provide the earliest possible indication of a change in formation resistivity. This real-time correlation capability leads to rig time savings during searches for casing or coring points, in effect allowing immediate "geostopping." The top of a reservoir can be cored without danger of drilling up the interval first. At-the-bit resistivity also allows the fastest detection of pore pressure anomalies. Four additional resistivity measurements are high-resolution focused electrode resistivities. One measurement uses a ring electrode to make an azimuthally averaged resistivity that is accurate up to 20,000 ohm-m. The other three use button electrodes to make azimuthal resistivity measurements that scan the borehole as the tool rotates. Together the ring and three button electrodes give four depths of investigation with identical vertical resolutions. Conventional 2-MHz LWD resistivity measurements are limited to environments favoring induction-type settings; that is, resistive mud (i.e., fresh or oil-based mud) and conductive rock. The solution to this problem is a high-resolution, focused electrode resistivity that provides accurate Rt measurements for the first time in "laterolog" environments of high formation resistivities and salty mud. An example is carbonates, where 2-MHz tools are inherently less accurate. This high resistivity measurement has excellent vertical resolution, allowing evaluation of beds as thin as 3 in. and detection of beds as thin as 1 in. Multiple depths of investigation of the button measurements, in combination with the focused electrode resistivity measurement, allow evaluation of permeable beds from early-time invasion detection. The array of three button electrodes provides azimuthal resistivities that scan the borehole as the tool rotates. These measurements form the basis for advanced processing and interpretation of full-bore resistivity images (see Fig. 1), introducing the potential for fracture detection, dip determination, and improved formation evaluation and characterization. The technical leap that allows measurements to be made at the bit is a wireless telemetry system that sends data from sensors near the bit to the measurement-while-drilling (MWD) tool up to 200 ft. behind the bit, bypassing the intervening drilling tools. Data sent to the MWD system are then transmitted to the surface in real time using mud-pulse telemetry. Resistivity at the bit is measured by attaching the tool directly to the bit and driving an alternating electric current down the collar, out through the bit, and into the formation. The current returns to the drillpipe and drill collars above the transmitter. In water-based mud, returning current is conducted from the bit, through the mud, into the formation, and back to the bottomhole assembly. In oil-based mud, which is an insulator, current returns through the inevitable but intermittent contact of the collars and stabilizers with the borehole wall, leading to a qualitative indication of resistivity. Formation resistivity is obtained by measuring the amount of current flowing into the formation from the bit and normalizing it to the transmitter voltage. Although drilling with at-the-bit measurements is still in its infancy, petrophysical measurements near the bit provide numerous benefits for geologists, petrophysicists, and drillers and extend the range of conditions under which it is possible to measure formation resistivity accurately while drilling. SI Metric Conversion Factors ftx3.048* E-01=m in.x2.54* E+00=cm *Conversion factor is exact.

MWD: A Case Study in Applying New Technology in Norwegian Block 34/10

Summary Developments in measurement while drilling (MWD) technology have led to the successful operation of a multisensor, negative-pulse telemetry system in the Norwegian sector of the North Sea. This paper describes the MWD system and discusses its application on three wells drilled in Block 34/10 and its advantages over a more conventional drilling program. Particular attention is paid to Well 34/10-20, where the simultaneous drilling, logging, and hole-opening of the 26-in. [66-cm] tophole section was performed for the first time. We believe that as multisensor MWD systems become more reliable, substantial savings in rig time and increased drilling control and rig safety will result.