A New Wellbore Position Calculation Method
- C.R. Chia (Schlumberger) | W.J. Phillips (Schlumberger) | D.L. Aklestad (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2003
- Document Type
- Journal Paper
- 209 - 213
- 2003. Society of Petroleum Engineers
- 6.1 HSSE & Social Responsibility Management, 1.10 Drilling Equipment, 1.9 Wellbore positioning, 1.6 Drilling Operations, 1.14 Casing and Cementing, 1.9.4 Survey Tools, 1.1 Well Planning, 1.5 Drill Bits, 1.6.1 Drilling Operation Management, 1.6.6 Directional Drilling, 1.12.1 Measurement While Drilling, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale
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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.
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 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.
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