SYSTEMS THEORY: LOGGING-WHILE-DRILLING (LWD)
SYSTEMS THEORY: LOGGING-WHILE-DRILLING (LWD)
The emergence of new engineering technology of well logging sensors placed into drill collars has stimulated a revolution in the field of logging and drilling. It provides the drilling party highly comprehensive real time information, as well as the geologist who is also able to evaluate the formations while invasion activity is ongoing. This new technology has altered logging hardware used in the interpretation methodologies applied and overall economics of the drilling and logging. Furthermore, it is critical that this has altered the roles assumed by professionals such as geologists, drillers, log analysts and petrophysicists. This report provides an in-depth analysis of the Prospector Logging While Drilling system in terms of operability, effectiveness and validity in delivering the needed service for drilling parties.
The Prospector Logging While Drilling system provides a variety of measurements namely neutrons, density, and spectral gamma rays from the various tools which are built ton the drilling collars. This system can be used simultaneously with the Anadrill’s Measurements –while drilling system with access to Anadrill’s Mud Pulse telemetry as well as the Advisor surface system (American Society of Mechanical Engineers, 2005, 43). This system involves complex engineering process to enable integration of real time drilling and logging data as well as capacity for wireline measurement data collection. It is critical to understand that this technology has been faced by extensive commercialization.
Further and future developments will be reliant on the industry demand as well as the continued endeavor in research and development of new engineering technologies in drilling and logging fields. This new technology is bound to enhance the level of competition in drilling and logging operations and services. This is largely because of the availability of real-time analytics for geologists and engineers, which is anticipated to contribute to improved information availability (Guo & Liu, 2011, 21).
The activity of real time logging during drilling is a typical challenge experience in engineering. The use of two highly complex technologies under harsh circumstances presents the challenge for engineers. The wireline logging tool endures singing occasional jolts during movement into and out of the hole in intermittent phases during data collection.
In essence the wire line logging tool used in this system should ensure the constant jolt in the back and forth movement fro the hole during measurement collection process. The LWD tools usually perform exceptionally well under adverse conditions that are marked by sudden and brief shocks of an estimated 1000g per millisecond (g-ms)1 in vertical, lateral and torsional locations during the life of the bit run which may be well over 100 hours (Rehm, 2008, 19). The LWD tools may work for a similar number of hours to the wire line tools in a range of two years. These tools should survive under harsh conditions and provide reliable information even if the bottom hole assembly is in a phase of resonance. In addition, these tools should also survive the overly erosive effects associated with mud burdened cuttings as well as sand (National Research Council (U.S.), 2011, 59).
The wireline tool undertakes communication through a cable, which moves all the information. The status of the LWD tool is understood to be complicated. This is because real time data is only communicated through mud-pulse telemetry that has a relatively lower bandwidth and bit rate level when compared to cable telemetry. This is an estimated 3-bits a second in the Anadrill’s MWD-XL3 as compared to the wire-line, which holds 100kilobits per second. This is a limitation towards the real-time movement of information capacity such that non-essential information is stored in down-hole memory, which is evaluated when the LWD tools move to the surface (Rehm, 2012, 26). Oil drilling parties should decide, before undertaking logging activities, the necessary data it requires in real-time.
The wireline engineer is tasked with logging at an optimal rate of sampling in regard to the borehole conditions and tool suite. The data is sampled at incremental levels in the depth of the borehole. In addition, the LWD tools usually sample the data at regular time intervals. The rate of logging at the surface level is only changeable when the tools reach the surface. The sampling interval rate when compared to the death results in the ratio of the time sample interval as well as the drill bit rate of penetration (ROP) used (American Society of Mechanical Engineers, 2005, 29). Therefore, the LWD usually require a means of accommodation of changes in the survey speeds used. Furthermore, real-time information sampling levels are limited by the presence of mud telemetry.
The wireline tool operates individuals inside the borehole. It is accommodated only in the diameter of the borehole, mud properties and deviation. Te LWD tools should be fitting for insertion into the drill collar and permit the uniform movement of fluids. In addition, they are designed in a manner that the wear points are rapidly and cost effectively replaced in the event of wear and tear. These tools should be versatile to function effective in the various bottom-hole assemblies activities and makeup-combinations in drilling activities (Gulf Publishing Company & American Association of Drilling Engineers, 1999, 46).
To meet the various challenges associated with real time logging, engineers developed to tools, which can be combined with Anadrill’s MWD system. In one output, the well site geologist and drilling engineer are able to have the MWD information as well as six additional formation measurements using the two new tools namely the Compensated Density Neutron (CDN) and Compensated Dual Resistivity (CDR) (National Research Council (U.S.), 2011, 67). The information collected from these two new tools is critical towards provision of reliable lithology information or logs. In addition, it enables the estimation of the associated formation pore pressure as well as the rock mechanical properties.
Figure 1: Formation Parameters
|Correlation||CDR||Dual resistivities (Rps&Rad) and Gamma ray (total AP)|
|Porosity||CDN||Epithermal neutron compensated spectral gamma and gamma density|
|Shale Volume||CDR||Spectral gamma rat (total AP and U, K, Th)
Computed gamma rays
Logging in Real-Time
Real-time logging provides a variety of benefits irrespective of telemetering of data through stored down-hole or mud. These benefits include:
- Insurance logging is provided whereby there is guarantee of data recovery, even in the event that the well is lost or there is impossibility of logging through wireline.
- Real-time location of casing and coring points. This eliminates the incidences of drilling over the interval that is to be cored or cased and preventing early tripping before the setting of casing. In addition it also eliminates the estimation of potential of bad hole conditions.
- Precision location of the existing seismic reflectors becomes possible during the drilling process. It also eliminates the incidences of unnecessary drilling and improves the stratigraphic mapping and more so the well-to-well correlation which aids location of the needed target depths as well as control of the depth of the borehole.
- Early stage reconnaissance becomes a possibility for potential pay and gas zone locations.
- Rt determination during invasion phase, whereby the dynamic Rt is present
- Enhanced statistical accuracy levels of the nuclear measurements whereby ROP is an estimated 50feet (15) meters in an hour or less.
Logging while drilling technologies and services are usually expensive when compared o other wireline services. They will contribute to the overall cost effectiveness of drilling and subsequent successful well completion. In addition critical design and engineering challenges that may face new logging while drilling technologies have been achieved. Some of the logs recorded by the system under specific conditions usually meet the wireline standards while at the same time surviving the harsh conditions of drilling activity. The system faces full-scale commercialization to establish its suitability and sustainability to drilling and logging activities.
A number of companies have already embarked in the use of full suite LWD and wireline services within same wells. A number of field studies are reliable indicators that the costs associated with LWD services are offset by the savings made on minimal incidences of blowouts, loss of wells, circulation, cement and attached mud weight which reduces side tracking activities.
It is appropriate that the combination of MWD and LWD measurements holds specific benefits namely:
- The assessment of rock mechanical properties is enabled , which provides better boreholes and effective drilling practices given that curate information is used to make effective and accurate decisions in bit changes and drilling speed to be used.
- LWD and MWD combined measurements provide enhanced pore pressure estimates. This permits continued real-time adjustments in the mud weight and identification of the mud gain and loss. This enables minimization of mud overbalance incidences, which contributes to the enhanced rate of core recovery, mud economics, and rate of penetration. In mud weights which are beyond 11pounds a gallon (1lb/gal/0.1g/cm3), this increases the costs significantly. Additionally, in drilled wells that have oil-base muds, the CDR measurements are critical towards the determination of pore pressure given that the short-normal resistivity tool work in conductive muds.
- It also provides for enhanced assessment of gas logs
- LWD lithology measures and indicators enable improvement in Mechanical Efficiency Log (MEL) data collected which is indicative of drilling incidences such as the loss f bit cone. In addition, it provides for improvements in Stick Pipe Indicator (SPIN) data, which indicates the possibility of sticking by the bottom hole. The LWD measurements are able to identity the location and reasons for stickiness.
- Furthermore, it enables effective assessment of the stability of the borehole, bit selection, mud engineering for subsequent determination of the optimal pump rates and various casing points.
- Lithology identification is also possible. This aids the well-to-well correlation as well as identification of the facies boundaries.
- Mud log integration is enabled, with cuttings and CDN and CDR data availed on the log presentation and single database.
American Society of Mechanical Engineers, 2005, Drilling fluids processing handbook, Elsevier, Amsterdam.
Chin, WC, 2014, Measurement while drilling (MWD): Signal analysis, optimization, and design, Scrivener Publishing/Wiley, New Jersey.
Guo, B., & Liu, G., 2011, Applied drilling circulation systems: Hydraulics, calculations, and models, Gulf Professional Pub, Burlington.
Gulf Publishing Company & American Association of Drilling Engineers, 1999, Shale shakers and drilling fluid systems: Techniques and technology for improving solids control management, Butterworth-Heinemann, Woburn.
Leonov, EG, & Isaev, VI, 2010, Applied hydroaeromechanics in oil and gas drilling, Wiley Hoboken.
National Research Council (U.S.), 2011, Scientific ocean drilling: Accomplishments and challenges, National Academies Press, Washington.
National Energy Technology Laboratory (U.S.), David, G, Robert, R, Robert, S, John, F, Technology International Incorporated, 2008, Advanced Seismic While Drilling System, United States Dept. of Energy, Washington.
Rehm, B, 2008, Managed pressure drilling, Gulf, Pub Houston.
Rehm, B, 2012, Underbalanced drilling: Limits and extremes, Gulf Publishing Company, Houston.
Sandia National Laboratories., United States., Williams, C. V., Lockwood, G. J., Normann, R. A., Bishop, L. B., Floran, R. J. 1995, Environmental Measurement-While-Drilling system for real-time field screening of contaminants, United States Dept. of Energy, Washington.
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