In-Field Referencing Research Articles

Combining Magnetic and Gyroscopic Surveys Provides the Best Possible Accuracy

Summary A survey program is designed for every well drilled to meet the well objective of penetrating the target reservoir and avoiding a collision with nearby offset wells. The selection of the wellbore survey tools within the survey program is limited in number and accuracy by the current surveying technologies available in the industry. This article demonstrates how a higher level of accuracy can be achieved to meet challenging well objectives when the accuracy of the most accurate wellbore surveying tools and technologies taken individually is insufficient. This high level of wellbore positioning accuracy is achieved by combining two independent wellbore positions of the same wellbore trajectory. The first wellbore position is calculated using the latest technology of magnetic measurement-while-drilling (MWD) definitive dynamic surveys (DDS). The accuracy of the MWD DDS can be further improved by minimizing error sources such as misalignment of the survey package from the borehole, drillstring magnetic interference, the use of localized geomagnetic reference, using high-accuracy accelerometer sensors, and a high-accuracy gravity reference. Furthermore, the MWD DDS inclination accuracy is improved using an independent inclination measurement from the rotary steerable system. A first wellbore position is calculated from the magnetic MWD DDS after applying in-field referencing (IFR), multistation analysis (MSA), bottomhole assembly (BHA), sag correction (SAG), and dual-inclination (DI) corrections to improve both azimuth and inclination accuracy. A second wellbore position is calculated using gyro-MWD (GWD) technology. The results and comparisons of multiple combined survey runs are presented. The highest accuracy of wellbore positioning had been proved in this successful case study by penetrating a very small reservoir target on an extended-reach well that was unfeasible using either the most accurate enhanced MWD DDS or GWD technology individually. The presented case study shows how the wellbore objectives of penetrating a very small reservoir target had been confirmed by logging-while-drilling images and the reservoir mapping interpretation of the client subsurface team. This gave a high-accuracy wellbore position during drilling and provided higher confidence in wellbore placement to maximize reservoir production without colliding with nearby offset wells. Wellbore survey accuracy limits a borehole’s lateral and true vertical depth (TVD) spacing, constraining reservoir production in those sections. In the top and intermediate sections, wellbore survey accuracy limits how close the wellbore can be drilled to other offset wells due to collision concerns. This directly impacts the complexity of the directional work and the cost per section. Combining independent wellbore surveys unlocks the potential to improve the wellbore positioning accuracy significantly. It demonstrates the highest wellbore positioning accuracy that can be achieved to date compared with the latest magnetic MWD surveys after correcting all known errors or compared with GWD.

The Combination of Solid-State Gyroscopic and Magnetic Surveys Provides Improved Magnetic-Survey Data and Enhanced Survey Quality Control

Summary The use of advanced solid-state gyroscopic sensors has become both a viable and practical option for high-accuracy wellbore placement, with the potential to outperform traditional mechanical gyroscopic systems. In this paper, we describe how the contributions of the new gyroscope technology are causing service providers to reconsider current survey practices, and to examine how the new gyroscopic-survey tools can best be used for wellbore surveying and real-time wellbore placement. The simultaneous application of multiple survey tools, largely made possible as a result of the unique attributes of solid-state gyroscopic sensors (including small size and significant power reduction), has clear benefits in terms of enhanced well placement, reliability, and the detection of gross errors in the survey process. Further advantages accrue through the combination of different but complementary survey methods. In this paper, we focus mainly on the benefits of combining gyroscopic and magnetic measurements to reduce or remove the known errors related to the Earth's magnetic field to which magnetic-survey systems are susceptible: errors in total magnetic field, declination, and dip angle. In this context, the use of statistical estimation techniques depending on the performance models of the survey systems used is described. For post-drilling surveys (using drop-survey tools or wireline-conveyed tools, for example), post-run analysis of the data using least-squares estimation techniques is appropriate. Alternative methods capable of achieving real-time data correction during drilling are also described, and results are presented to demonstrate the potential for enhanced magnetic-survey performance. The principles described can be used when running basic magnetic measurement-while-drilling (MWD) systems and for systems that use field correction methods, such as the various in-field referencing (IFR) techniques, that are frequently used. The proposed methodology is of particular benefit in the former case, allowing enhanced magnetic surveying to be achieved without the need for expensive and complex magnetic-field correction procedures. The potential also exists either to identify or to correct possible errors in the IFR data when such methods are used. The particular strength of the methods described in this paper is in the estimation and correction of errors in the Earth's magnetic-field parameters while drilling, or after a section of the well has been drilled. Currently, all the magnetic models are dependent on magnetic measurements alone and at the surface or in space. Although there are techniques to estimate the downhole values, such techniques are difficult to quality control (QC) or validate. These updated magnetic-field data allow further sections of the same well, or additional wells in the same region, to be drilled using MWD alone, without the need to use field referencing techniques to achieve accurate well placement. The consequent rig-time and cost savings are expected to be significant.

Dynamic Error Analysis of Measurement While Drilling Using Variable Geomagnetic In-Field Referencing

Summary The safe and economical determination of a wellbore trajectory in directional drilling is traditionally achieved by measurement-while-drilling (MWD) methods, which implement magnetic north-seeking sensor packages. Inaccuracies in the determination of well path arise because of random and systematic errors in the measurements of the sensors. Multistation analysis (MSA) and magnetic in-field referencing (IFR) have already demonstrated the potential to decrease the effects of errors because of magnetization of drillstring components along with variable errors caused by irregularities in the magnetization of crustal rocks in the vicinity of wells. Advanced MSA methodologies divide a borehole into several sections and use the average reference values of the total magnetic field, declination, and dip angle for analysis of errors in each section. Our investigations indicate that the variable-reference MSA (VR-MSA) can lead to a better determination of errors, specifically in areas of high magnetization. In this methodology, magnetic reference values are estimated at each station using forward and inverse modeling of surface-magnetic observations from IFR surveys. The fixed errors in magnetometer components are then calculated by minimizing the variance of the difference between the measured and unique estimated reference values at each station. A Levenberg-Marquardt algorithm (LMA) is adopted to solve the nonlinear optimization problem. Examination of this methodology using MWD data confirms more than 20% improvement in well-path-determination accuracy by comparing the results with the corrected path from the conventional MSA method and gyro surveys.

Anti-collision risk management guidelines for Alaska North Slope directional wells

The complex process of anti-collision in directional well planning has gained particular attention in the Alaska North Slope (ANS) as the economic and environmental constraints have compelled the operators to drill from already congested pads. The accuracy of anti-collision study is highly dependent on the positional accuracy of the wellbore, which in turn is associated with the ellipse of uncertainty (EoU) generated by appropriate survey tools and their assigned error models. Initially, this study compares the performance of newly developed Operator Wellbore Survey Group (OWSG) error model and the industry-standard Industry Steering Committee for Wellbore Surveying Accuracy (ISCWSA) error model. The main objective of this study was to develop potential anti-collision guidelines that can be followed when any well would be drilled in ANS by integrating the uncertainties associated with survey tool and anti-collision considerations. To achieve this, the error model results were coupled with thorough sensitivity analysis based on four parameters. First, location, to understand the effect of latitude on EoU. Second, type of well to be drilled, to understand if the anti-collision considerations are well-type specific. Third, survey tools, to understand the limits of survey tools to be used, and lastly, different targets in same sand, to understand the difference if any between the first three parameters. The results show that the positional accuracy of the wellbore degrades drastically at higher latitude areas like Alaska which implies that higher magnitude of separation factor (SF) is required for safety of drilling operations in such areas. The results also show that MWD tool with in-field referencing (IFR) and multi station analysis (MSA) provide higher positional accuracy than the generic gyroscopic tools. The results of this study provide an improved anti-collision risk management workflow for effective well planning in the Arctic and high latitude areas.

Magnetic Referencing and Real-Time Survey Processing Enable Tighter Spacing of Wells

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 175539, “Magnetic Referencing and Real-Time Survey Processing Enable Tighter Spacing of Long-Reach Wells,” by Stefan Maus, SPE, Magnetic Variation Services, and Jarod Shawn Deverse, SPE, Surcon, prepared for the 2015 SPE Liquids- Rich Basins Conference—North America, Midland, Texas, USA, 2–3 September. The paper has not been peer reviewed. Wellbore position is computed from survey measurements taken by a measurement-while-drilling (MWD) tool in the bottomhole assembly (BHA). The MWD tool uses accelerometers and magnetometers to measure the Earth’s gravitational and geomagnetic fields. Knowing the direction of these two fields and using them as a frame of reference enable drillers to calculate inclination and direction (azimuth) of the wellbore. Local high-resolution in-field referencing (IFR) models have been produced for the Permian Basin that significantly improve the accuracy of MWD azimuth measurements. Ellipse of Uncertainty (EOU) Numerous error sources are associated with MWD-survey measurements, and each error source contributes in some form to the magnitude of uncertainty that propagates along the computed wellbore trajectory. The Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) developed a framework for quantifying the magnitude of uncertainty. The Operator’s Wellbore Survey Group (OWSG), an ISCWSA subcommittee, continued development on the original error model and published a set of instrument-performance models that enables the computation of EOU for specific surveying methods. This consolidated set is referred to as the OWSG set of tool codes. Fig. 1 illustrates the difference between EOUs for standard MWD vs. advanced corrections using MWD with IFR and corrections for sag (MWD+IFR1+SAG). Generally, one can see that the large uncertainty of standard MWD can be reduced by 11 to 38% by use of IFR, while further applying multistation analysis (MSA) can reduce the uncertainty by 50 to 61%. As shown in Fig. 1, vertical uncertainty can also be reduced by advanced survey-correction methods. Bending of the BHA brings the accelerometer out of alignment with the inclination of the wellbore trajectory. This effect can be corrected for by applying corrections for sag. IFR The objective of IFR is to use local-magnetic- field measurements to produce an accurate 3D magnetic reference model for the drilled volume. The most cost-effective method is to use aeromagnetic surveys, which have been used for more than 50 years to map geology and tectonic features. Earlier IFR methods used fast Fourier transforms or equivalent-source techniques on planar 2D grids. A recent analysis showed that these grid methods can lead to significant errors in the declination and dip reference values because they assume that the crustal magnetic anomalies are entirely contained within the grid. A superior approach is to tie the grid into the global crustal field inferred from satellite measurements and represent the solution in terms of global high-degree ellipsoidal harmonic functions. The IFR method used by the authors first produces an accurate baseline magnetic field for a given reference date. This baseline for the reference date is supplied as an IFR model together with an IFR calculator to the drilling engineer. The baseline can then be extrapolated to any desired drilling date using the main field model embedded in the IFR calculator.

Confidence Limits Associated With Values of the Earth's Magnetic Field Used for Directional Drilling

Summary This paper describes updated uncertainties for use with predicted geomagnetic parameters within magnetic measurement-whiledrilling (MWD) survey-tool-error models. These models are used to define positional-error ellipsoids along the wellbore, which assist in hitting geological targets and avoiding collisions with existing wellbores. The declination, dip angle, and total field strength of the Earth's magnetic field are used with magnetic-survey tools for surveying the wellbore. These values are often obtained from mathematical models such as the British Geological Survey (BGS) global-geomagnetic model (BGGM). As the Earth's magnetic field is continually varying with time, the BGGM is updated annually to maintain accuracy. However, a global predictive model cannot capture all sources of the Earth's magnetic field, which results in uncertainties of the predicted parameters. The Industry Steering Committee on Wellbore Surveying Accuracy (ISCWSA) published an MWD-error model in 2000 (Williamson 2000). The geomagnetic-field uncertainties that are part of this model were derived from work conducted by the BGS in the early 1990s. Since then, more-accurate data from magnetic-survey satellites have been introduced into the BGGM, and the uncertainty of the predicted geomagnetic-field parameters has been reduced. The original approach to deriving the uncertainties involved separating the various error sources in the magnetic field and assessing them individually. This paper uses a simpler approach where clean orientated magnetic downhole data are simulated using geomagnetic-observatory data. Spot absolute measurements of the magnetic field made at observatories around the world are adjusted for the crustal magnetic field to make them more representative of hydrocarbon geology. The adjusted observatory data are then compared with the predicted values from the BGGM to assess the uncertainty. The uncertainties do not fit a normal distribution, so they are expressed as limits for various confidence levels. They vary with location and, in their derivation, do not assume any underlying empirical error distribution. While they also vary with time, we provide time-averaged look-up tables that should be valid for as long as there are good-quality satellite data on which to base global magnetic-field models. Options to reduce the uncertainties further using data from local magnetic surveys [in-field referencing (IFR)] and observatories (interpolation IFR) are also described. The use of the revised geomagnetic uncertainty values in the MWD-error model will reduce wellbore-position uncertainty to reflect the increased accuracy from recent improvements in geomagnetic modeling. This is demonstrated using results for the ellipsoids of uncertainty output by an MWD error model for three standard ISCWSA well profiles.

Application of Interpolation In-Field Referencing to Remote Offshore Locations

This article is a synopsis of paper SPE 49061, "Application of Interpolation In- Field Referencing to Remote Offshore Locations," by H.S. Williamson, SPE, BP Exploration; P.A. Gurden, SPE, Baker Hughes Inteq; D.J. Kerridge, British Geological Survey; and G. Shiells, SPE, Baker Hughes Inteq, originally prepared for the 1998 SPE Annual Technical Conference and Exhibition, New Orleans, 27-30 September.