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DENVER–Drilling the optimal number of horizontal wells to efficiently drain ultralow-permeability reservoirs is critical to achieving economic value in shale and tight oil plays. In any development project, the goal is to drill the fewest number of horizontal wellsrequired to maximize reservoir drainage. That means spacing laterals at a distance sufficient to minimize overlapping drainage areas while remaining close enough to avoid stranding any reserves.

Because drainage areas are limited to fracture propagation distances, drilling laterals too far apart will result in inadequate production performance and reserves recovery. On the other hand, developing acreage with laterals spaced too tightly will increase costs, create wellbore interference issues, and add logistical challenges without necessarily boosting production or recovery.

However, optimizing lateralspacing is challenging. Many variables must be considered, including hydraulic fracture geometry and reservoir properties. One variable that often is overlooked is wellbore positional uncertainty caused by inaccurate directional surveying. Standard measurement-while-drilling directional surveying is subject to numerous error sources that can be estimated by the Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA) error model. These error sources are quantified and modeled as three-dimensional ellipsoids of uncertainty (EOU) that provide drillers a mechanism to measure wellbore trajectory positional uncertainty.

The ISCWSA error model can be used to predict the statistical distribution of wellbore positions to gain an understanding of how far actual well paths may deviate from their surveyed positions. Furthermore, some of the greatest error sources represented in the error model can be reduced significantly using in-field geomagnetic referencing (IFR) and multistation analysis (MSA) to improve MWD surveying accuracy. Performing advanced survey management analysis on raw MWD data and correcting identified systematic errors in real time improves the accuracy of well placement as a wellbore is drilled.

To reliably determine the optimal lateral well spacing, positional uncertainty should be applied to reservoir simulations and production models. Furthermore, applying IFR/MSA survey correctionsto standard MWD surveying can improve horizontal wellbore positional accuracy by 50-60 percent, leading to better decisions for optimizing field development and maximizing the value of each lateral.

Positional uncertainty can be modeled by 3-D EOUs at each survey point in the well path, representing a statistical distribution of where the actual survey may exist. EOUs are computed from tool codes using anti collision software. ISCWSA’s Operator’s Wellbore Survey Group has published a consolidated set of tool codes to represent most surveying methodologies. In one example from a shale play in Texas, evaluating a lateral longer than 10,000 feet planned with standard MWD at various azimuths showed 259-439 feet of expected lateral deviation at the bottomhole location. Applying IFR/MSA to the MWD survey reduced deviation to 129 173 feet

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