Optimizing Lateral Well Spacing by Improving Directional-Survey Accuracy
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The effect of inaccurate directional surveying on lateral wellbore spacing is commonly overlooked. The purpose of this paper is to demonstrate how inaccuracy in standard directional-surveying methods affects wellbore position and to recommend practices to improve surveying accuracy for greater confidence in lateral spacing.
Determining optimal wellbore spacing for a given field requires consideration of several factors such as hydraulic-fracture geometry and reservoir quality. These variables are simulated using advanced reservoir models to identify ideal parameters, such as lateral spacing distance, which will lead to the greatest production value. Models are then validated against field spacing tests to confirm results. However, an important consideration that is usually ignored is wellbore positional accuracy.
Most reservoir models assume accurate wellbore placement. When reservoir models do account for wellbore positional uncertainty, the approximation is only a fraction of the actual positional error expected from standard wellbore-surveying techniques. This creates a problem when positional errors occur during spacing tests, because the actual lateral spacing distance could vary significantly from the values used to create the reservoir model. If production and reservoir simulations are misinterpreted because of wellbore positional errors, then economic consequences could be significant.
Horizontal wells are directionally drilled using measurement-while-drilling (MWD) surveying to characterize the well path. Standard MWD surveying is subject to many error sources that lead to significant positional uncertainty and can result in inaccurate well placement. However, enhanced surveying methods such as use of multistation analysis (MSA) in field referencing can be an effective strategy for reducing a majority of errors. Positional uncertainty can be modeled by 3D ellipsoids of uncertainty at each survey point in the well path, which represent a statistical distribution of where the actual survey might exist.
In-Field Geomagnetic Referencing (IFR). IFR is a means of predicting the local magnetic field at a specific geographic location. It can be used to support MWD operations as a reference frame for magnetic measurements. IFR accounts for three of the four contributing factors of the geomagnetic field. These are the main field (generated by the Earth’s core), crustal field (magnetic minerals in the Earth’s crust), and steady external field (generated by charged-particle flow in the Earth’s atmosphere). The remaining contribution to the geomagnetic field is the magnetic disturbance field (generated by electric currents in near-Earth space).
An IFR model must capture a wide spectrum of wavelengths in the geomagnetic field. Satellite measurements account for the long-wavelength (266 to 2500 km) crustal field as well as the main field, secular variation, and steady external field. A local magnetic survey provides the shorter wavelengths through accurate mapping of local crustal-field anomalies. To provide a model that is continuous across the geomagnetic spectrum, the local magnetic survey is extended by merging it with a larger regional survey. The merged grid that results is then further extended to cover the longest of wavelengths by merging with the satellite measurements. The merging of these different data sets must be evaluated at the same altitude and must make a seamless transition across their boundaries. The local magnetic survey specifies only the total strength of the magnetic-field vector; however, MWD requires accurate modeling of the direction of the magnetic-field vector. It is possible to determine the direction of the magnetic field accurately by representing its vector as the gradient of a scalar potential, found by solving Laplace’s differential equation using ellipsoidal harmonic basis functions.
MSA. MSA is a method for reducing systematic errors in MWD measurements. By comparing the measured total magnetic field, magnetic dip angle, and total gravity field from multiple survey stations against the theoretical values obtained from an IFR model and a global acceleration reference model, bias and scale errors can be resolved for the MWD accelerometers and magnetometers. This enables a correction to be applied to the raw MWD measurements, which reduces uncertainty from instrument calibration, magnetic drillstring interference, and magnetic mud. Because magnetic drillstring interference is one of the largest contributers to azimuth error as expressed by the MWD tool code, correcting for it leads to a substantial reduction in positional uncertainty. If the bottomhole-assembly components are strongly magnetized while drilling, then MSA-corrected surveys may result in a significant change in wellbore position. Because MSA relies on the accuracy of the magnetic reference model for determining MWD error, it is critical to use an IFR model to achieve the most accurate survey analysis.
Drilling Operations. Advanced survey analysis was performed on 138 wells drilled in the Eagle Ford Shale and the Permian Basin of Texas. For the majority of these wells, survey analysis was performed in real time while drilling in order to steer the well path accurately according to the well plan. The process for real-time survey analysis occurs with each survey station. When an MWD survey is shot, the raw measurements are uploaded to a Web application where the survey undergoes a validation process to ensure that the survey is free from gross errors. Once the survey passes the initial validation, it is stored in a cloud-hosted database and accessed by analysts in a remote operating center where it is analyzed further for systematic errors such as magnetic drillstring interference or instrument bias and scale errors, by use of IFR and MSA. After the systematic errors are identified and corrected, the corrected surveys are delivered to the rigsite through the Web interface and used for directional steering.
A comparison of magnetic declination values computed from the Permian IFR model against the International Geomagnetic Reference Field (IGRF) model showed a root-mean-square (RMS) difference of 0.26° and a maximum difference of 1.32°. A similar comparison with the Eagle Ford IFR model and the IGRF model showed an RMS difference of 0.14° and a maximum difference of 0.50°. Because declination is directly applied to magnetic azimuth when calculating directional surveys, the RMS positional error for an 11,000-ft lateral wellbore resulting from use of the IGRF model would be 50 ft in the Permian and 27 ft in the Eagle Ford.
Bottomhole locations (BHLs) for all 138 well paths corrected with IFR and MSA were compared with the BHLs of the original well paths computed from each wellbore’s orginally reported MWD surveys. The RMS of the BHL horizontal positional change was 78 ft, and the maximum horizontal positional change was 269 ft. Fig. 1 summarizes these results. For wellbores that were analyzed after drilling, the difference in position represents how far the BHL was actually placed compared with what was originally measured. For wellbores that were evaluated and corrected in real time (the case for the majority of wells), the difference in position represents how distant from the plan the BHL would have been if advanced survey management had not been applied.
An important consideration is that the change in wellbore position does not always occur in the same direction. Depending on the polarity of drillstring magnetization, the wellbore direction, and the direction of the magnetic declination error, the standard MWD-measured wellbore could be to the left or right of the actual position. This is especially concerning because it means that horizontal wellbores are very likely to converge and diverge, which decreases and increases lateral spacing at the BHL.
If one considers an entire field of horizontal wells drilled with standard MWD surveying, it is easy to realize that the resulting lateral well spacing will be less than ideal. Furthermore, one should consider how inaccurate well placement could affect infill drilling. If enhanced-recovery applications or new well-completion techniques required future infill drilling, it could be quite challenging to place wells in a field of inaccurately drilled wellbores.
Reservoir permeability and fracture area are often considered the most important factors regarding lateral well-spacing optimization in shale reservoirs. How-ever, reservoir simulations and production models can be completely invalidated if underlying assumptions are incorrect, such as measured lateral spacing distance. As demonstrated by the study in the complete paper, standard MWD surveying methods are subject to large errors, which can lead to inaccurate well placement. Because most reservoir-simulation methods do not fully account for wellbore positional uncertainty, it is recommended to reduce standard MWD error by applying advanced survey corrections such as IFR and MSA in real time to ensure that the wellbores are placed accurately. Not only does accurate well placement help reduce misinterpretation of well-spacing-test results, but well-placement accuracy is also a critical requirement for maximizing shale reservoir value. Horizontal shale wells should be drilled as close to planned spacing as possible to effectively drain the entire field while avoiding overcapitalization from excessive drilling.
The most cost-effective method for achieving a substantial improvement in wellbore accuracy is to apply IFR and MSA corrections to standard MWD surveying. IFR will greatly improve the accuracy of geomagnetic reference declination, which can reduce positional uncertainty by greater than 30%. MSA is one of the most powerful forms of survey quality control and is highly effective at identifying gross errors and reducing systematic errors. This can achieve a further reduction in uncertainty for a total reduction of up to 60% compared with standard MWD surveying. A major advantage of IFR and MSA is that they can be applied in real time while drilling, which enables the wellbore to be steered with the most accurate surveys available. Placing wellbores accurately to begin with will have a positive effect on field-development and will increase the feasibility of future infill-drilling programs.
Optimizing Lateral Well Spacing by Improving Directional-Survey Accuracy
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