A method is described for combining multiple wellbore surveys to obtain a single composite, more accurate, well position. Established methods for defining the wellbore position, and its associated uncertainty, rely upon accepting the position obtained from the most accurate survey instrument used in each section of the wellbore. This position is then assigned an uncertainty based on the information from this single survey instrument run. Today, when a modern wellbore is constructed, each section may be surveyed for position many times using one or more magnetic, gyroscopic or inertial survey instruments. By statistically combining the wellbore positions obtained from all of the survey instruments run in a given section of the wellbore a new position, designated the ‘Most Accurate Position’ (MAP), is calculated. The main advantage of the MAP is that its uncertainty is smaller than the uncertainty of any of the constituent surveys. The major benefits of this technique will be to facilitate the drilling of smaller targets at greater distances, allow the drilling of new wellbores in closer proximity to existing wellbores while maintaining accepted safety clearance rules, and improved reservoir delineation. Describing Well Position The wellbore trajectory is defined as a series of surveyed points in three-dimensional space typically described in a North, East and Down reference system. These points are joined together to form a continuous trajectory using a geometrical calculation method1. Most magnetic or gyroscopic survey instruments in use today provide a survey point that is referenced to measured [or along hole] depth that is obtained from the driller's pipe tally, or from a wireline spooling measurement. The survey instrument provides inclination (hole angle) and azimuth (direction) measurements. When these parameters are used to calculate trajectory with an assigned survey depth, the horizontal displacement [or North and East coordinates] from the origin, and the vertical depth [or Down coordinate] from the elevation reference can be derived. Alternatively, some inertial survey instruments measure displacement in three-dimensional space from a known initialization point, and from which all of the above parameters [including depth] can be obtained to achieve the same purpose. Wellbore Position Uncertainty Wellbore survey requirements are typically driven by the need to guide the well to a geological target, to avoid other wells, to ensure that property boundaries are respected, and to record the position of the wellbore for future reference. In order to visualize and quantify our ability to hit a target or avoid colliding with another well, position uncertainty is assigned to wellbore trajectories. This position uncertainty represents our modeled knowledge of the collective errors arising from both the intrinsic performance limitations of the survey sensors and those induced by the operating environment2. This uncertainty is defined as a statistical confidence region with an associated confidence level. In three dimensions the confidence region is most often depicted as an ellipsoid3 because ellipsoids are the constant value contours of the three-dimensional Guassian probability density function. Such a confidence region is commonly referred to as an ‘Ellipsoid of Uncertainty’ (EOU). The EOU is used in target analysis, for example, by reducing the size of the geological target by the size of the EOU to define a drilling target. In this fashion, the geological target will be achieved if the wellbore penetrates the drilling target. Likewise EOUs are used to assess collision risk by considering the proximity of the EOUs from adjacent wells.

Accurate directional borehole guidance requires Bottom Hole Assembly (BHA) non-magnetic spacing of Measurement While Drilling (MWD) systems from surrounding steel components. This is required to limit the magnetic interference and associated azimuth measurement error to acceptable levels.The current industry method of calculating the Estimated Drillstring Interference (EDI) on MWD magnetometers and the resulting azimuth error produced, are largely based on published material from the late 1970's to early 1980's. The current method models components as magnetic dipoles requiring the knowledge or "guess" of the pole strength of the BHA component and combination of components.Technological advances in drillstring design and complexity have rendered these methods inadequate. This inadequacy is due to the lack of knowledge of the pole strength information for modern components and/or the unlimited configurations of components, and the limited and simplified BHA configurations normally available in supporting software tools where the EDI calculation is performed. This paper presents the theory and method of computing the EDI using demagnetizing factors of a prolate spheroid. A worked example of this method is shown to illustrate its implementation with a comparison of the results to published dipole method examples as a reference.The operational advantages of the demagnetizing factor method are presented showing more consistency in calculation, elimination of BHA complexity limits, ease of computer implementation and system integration, and elimination of subjective and inconsistent selection of component pole strengths used in the dipole method, this providing a means of EDI calculation standardization across the industry.

A survey program is designed for every well drilled to meet the well objective of penetrating the target reservoir and avoiding colliding with other 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 technology individually is not sufficient. This highest level of wellbore positioning accuracy to date is achieved by combing two 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 has been enhanced by correcting potential error sources such as misalignment of the survey package from the borehole, drill-string magnetic interference and limited global geomagnetic reference and accelerometer sensor accuracy. Further, the MWD DDS inclination accuracy is improved using an independent inclination measurement from the Rotary Steerable System (RSS). Hence the first position is derived from magnetic MWD DDS after applying In-Field Referencing (IFR), Multi-Station Analysis (MSA), Bottom Hole Assembly (BHA) sag correction (SAG), and Dual-Inclination (DI) corrections. A Second wellbore position is calculated using the latest technology in Gyro-measurement-While-Drilling (GWD). The results and comparisons of multiple runs are presented. The highest accuracy of wellbore positioning had been proven in successful case studies by penetrating a very small reservoir target on an extended reach well that was unfeasible using either the most accurate enhanced MWD DDS or the latest GWD technology. The presented case study shows how the wellbore objectives of penetrating the tight target reservoir had been confirmed by Logging-While-Drilling (LWD) images and interpretation of the subsurface team. This gave the highest accuracy of the wellbore position accuracy to date while drilling assured placing the well with higher confidence to maximize reservoir production without colliding with nearby offset wells. In reservoir sections, the wellbore survey accuracy limits boreholes' lateral and true vertical depth spacing, constraining reservoir production. In the top and intermediate sections, wellbore survey accuracy limits how close the well can be drilled in the proximity of other offset wells. This directly impacts the complexity of the directional work and the cost per drilled foot. This technique unlocks the potential to improve the wellbore positioning accuracy significantly. It demonstrates the highest wellbore positioning accuracy achieved to date when compared to the latest magnetic MWD surveys after correcting all known errors compared to the GWD.

A survey program is designed for every well drilled to meet the well objective of penetrating the target reservoir and to avoid colliding with other offset wells. The selection of the wellbore survey tools within the survey program are limited to the current accuracy available to the industry. A newly developed wellbore survey technique has proven to have superior accuracy compared to the current standard measurement-while-drilling (MWD) surveys with in-field referencing and multi-station analysis (MSA). In almost every drilling bottom hole assembly (BHA), there is an MWD survey tool to survey the wellbore while drilling. Accuracy of the MWD surveys has been improved over the years by correcting potential error sources such as misalignment of the survey package from the borehole, drillstring magnetic interference, limited global geomagnetic reference, and gravity model accuracy. This new positioning technique takes the accuracy of MWD surveys to the next level by combining surveys from two independent survey packages. The second survey package is installed inside the rotary steerable system (RSS). Surveys from both packages are retrieved while drilling. Results have been obtained from multiple runs worldwide, enabling comparisons between the new technique and standard MWD surveys from both an enhanced accuracy and true wellbore placement point of view. A proposed error model is based on both the theoretical improvements in accuracy and the empirical proof from the data analyzed. The improved accuracy while drilling assures higher confidence that the well placement will maximize reservoir production and avoid collision with nearby offset wells. In reservoir sections, the wellbore survey accuracy limits the lateral spacing, and this constrains the reservoir production. In top and intermediate sections, wellbore survey accuracy limits the well plan, and this affects how close the well can be drilled in proximity to other offset wells. This directly impacts the complexity of the directional work and the cost per drilled foot. The new technique unlocks the potential to significantly improve the wellbore positioning accuracy.

Summary A method for combining multiple wellbore surveys to obtain a single, composite, more accurate well position is described. Established methods for defining the wellbore position and its associated uncertainty rely on accepting the position obtained from the most accurate survey instrument used in each section of the wellbore. This position is then assigned an uncertainty based on the information from this single survey-instrument run. When a modern wellbore is constructed today, each section may be surveyed for position many times with one or more magnetic, gyroscopic, or inertial survey instruments. By statistically combining the wellbore positions obtained from all the survey instruments run in a given section of the wellbore, a new position, designated the "most accurate position" (MAP), is calculated. The main advantage of the MAP is that its uncertainty is smaller than that of any of the constituent surveys. The major benefits of this technique is facilitating drilling smaller targets at greater distances, allowing new wellbores to be drilled in closer proximity to existing wellbores while maintaining accepted safety clearance rules, and improving reservoir delineation. Describing Well Position The wellbore trajectory is defined as a series of surveyed points in 3D space, typically described in a north, east, and down reference system. These points are joined together to form a continuous trajectory with a geometric calculation method.1 Most magnetic or gyroscopic survey instruments in use today provide a survey point that is referenced to measured (or along hole) depth obtained from the driller's pipe tally or a wireline spooling measurement. The survey instrument provides inclination (hole angle) and azimuth (direction) measurements. When these parameters are used to calculate trajectory with an assigned survey depth, the horizontal displacement (or north and east coordinates) and the vertical depth (or down coordinate) can be derived from the origin and the elevation reference, respectively. Alternatively, some inertial survey instruments measure displacement in 3D space from a known initialization point, from which all the previous parameters, including depth, can be obtained to achieve the same purpose. Wellbore Position Uncertainty Wellbore survey requirements are typically driven by the need to guide the well to a geological target, to avoid other wells, to ensure that property boundaries are respected, and to record the position of the wellbore for future reference. To visualize and quantify our ability to hit a target or avoid colliding with another well, position uncertainty is assigned to wellbore trajectories. This position uncertainty represents our modeled knowledge of the collective errors arising from both the intrinsic performance limitations of the survey sensors and those induced by the operating environment.2 This uncertainty is defined as a statistical confidence region with an associated confidence level. In 3D, the confidence region is most often depicted as an ellipsoid3 because ellipsoids are the constant value contours of the 3D Guassian probability density function. Such a confidence region is commonly referred to as an "ellipsoid of uncertainty" (EOU). The EOU is used in target analysis by, for example, reducing the size of the geological target by the size of the EOU to define a drilling target. In this fashion, the geological target will be achieved if the wellbore penetrates the drilling target. Likewise, EOUs are used to assess collision risk by considering to assess collision risk by considering their proximity to adjacent wells. Survey Program A detailed survey program may be prepared to verify that a well's position requirements will be achieved. The survey program is a planned sequence of survey instruments to be used at different phases of the well construction. It will normally be presented as a listing indicating the survey depths for each survey tool to be used, required survey frequency, running conditions (run in cased or open hole or in drillpipe), and any special corrections or contingencies to validate the tool error model to be used for each surveyed interval. The survey program takes into consideration the available drilling room based on the proximity of existing nearby (object) wells and their respective EOUs, the expected EOU of the (subject) well to be drilled based on the performance of the specified surveying tools, any spacing requirements for adjacent future wells, a collision-avoidance safety clearance rule, the size and location of the geological target, and any relief-well-planning positional-accuracy criteria. Well Construction Well construction is conducted in a number of drilling stages or hole sections by drilling with decreasing drill-bit sizes, subsequently cementing a steel casing or liner into place in each hole section. During this process, various survey instruments will be run in different hole sections (through drillpipe) and casings (on wireline) in accordance with the survey program to achieve well-positioning objectives. It is not uncommon to have multiple surveys in one or more hole sections. For the top section, for example, we may have a measurement-while-drilling (MWD) survey obtained during the drilling phase, a gyro survey obtained after reaching the first casing point, and, perhaps, additional gyro surveys as deeper hole sections are completed. The current practice is to establish the final or "definitive" well position by using the most accurate survey in each hole section and disregarding the rest of the survey data.