The accurate placement of directionally controlled wellbores is essential in order to maximize oil production while avoiding collisions with existing wells. Error models provide estimates of the positional uncertainty that can be expected when using a specific survey tool. For example, a Measurement While Drilling (MWD) tool will have a different ellipsoid of uncertainty when compared to a Gyro While Drilling (GWD) or a continuous gyro tool. Whichever survey tool is chosen, the uncertainty in the calculated wellbore position is dependent on the trajectory and latitude of the planned wellbore. Many drilling projects have been designed assuming specific survey instruments and favorable drilling directions. However, these assumptions are often not fulfilled due to existing wellbores, environmental safety concerns, land management issues, drilling costs and survey tools available. Different azimuthal corrections and methods of surveying have been deployed to increase confidence in the wellbore placement when drilling wells in east or west directions at high inclinations. MWD surveys with magnetic interference corrections and rate-gyro surveying on wireline are two examples of these methods. Previous GWD technology was limited to 70 degrees of inclination and was not an option for high angle and horizontal wells. With the development of an all-attitude GWD tool, it is now possible to provide accurate gyroscopic surveys at all inclinations and all directions during the drilling process in real time.This paper describes a case study in which the all-attitude GWD tool was utilized to achieve tighter control of well trajectories drilled predominantly in an easterly direction in close proximity to a number of existing wellbores. This approach allowed a significant reduction in costs since it became possible to drill all the wells from an existing pad and so avoid land management and environmental concerns initially associated with the project. Further, the accuracy of an all-attitude GWD tool allowed the operator to drill safely through an additional 1,000 feet of geological pay zone, a result that could not have been achieved safely using MWD surveys alone.

Wellbore collisions and accurate well placement are major concerns in Chevron U.S.A. Inc.'s multi-well pad drilling operations in the Piceance Basin in Colorado. Up to 24 wells are drilled from a single pad with two rows of well slots. As a result of the high well slot density, gyroscopic survey systems must be run during kickoff operations in most wells due to magnetic interference.Several methods of obtaining high accuracy rate gyroscopic (gyro) survey data have been employed, including:Incorporating rate gyro systems in the MWD string while drilling has proven economical in the more costly offshore environment for a number of years (Eaton 2005). Applying the technology successfully to less expensive onshore projects can increase efficiency and bring additional benefits in reduced logistical requirements and safer operations.

Cenovus Energy initiated an aggressive infill drilling program early in 2011 in its Pelican Lake polymer/water flood project in northeast Alberta. By mid-year, the company had drilled 52 injection and producing wells, and drilling is scheduled to continue at the same pace through year-end. Pelican Lake employs a two-phase drilling program from 18-well pads: batch-drilling the intermediate hole sections to approximately 425m MD at 70° of inclination; using follow-up rigs to drill the horizontal sections of over 2,000m in the Wabiskaw formation. Wellbore collisions in the intermediate phase and landing the wells accurately prior to horizontal drilling are critical concerns. Highly accurate gyroscopic surveying techniques are required throughout the project considering the close well spacing and congestion of existing wells. Early in 2011, the company tested a new gyro-while-drilling (GWD) system, capable of accurate performance up to 70° of inclination versus the 20° to 40° of previous models. If successful, the higher operational range would facilitate collision avoidance and accurate well placement in one operation with real-time data, potentially enhancing both the economics and the feasibility of the drilling program. Since accurate well placement was so critical—some well passes would be as close as 4 meters—the company developed a plan to evaluate the new tool: first, by surveying previously surveyed wells and comparing the results; and, second, by running back-up gyro surveys on wireline during drilling. GWD performance exceeded expectations, both in the initial tests and in the drilling operations—to the extent that the back-up surveys were eliminated, with the GWD data accepted as the definitive wellbore survey. This paper will provide the results of the initial tests and analyze GWD performance during drilling with statistics compiled from 36 wells. The savings from eliminating the back-up surveys will be presented along with other benefits and observations.

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The uncertainties associated with current wellbore surveying techniques can generate time consuming challenges when sidetracking around a fish at high inclinations. If the sidetrack is not planned with a significant safety margin and/or the bottom hole assembly does not include the proper amount of non-magnetic spacing then the MWD surveys of the sidetrack wellbore can be compromised significantly by external magnetic interference leaving the well to be drilled blind. One means of monitoring the approach between a drilling well and a secondary wellbore has been to use the MWD sensors to monitor for external magnetic interference. When monitoring the convergence of two wellbores in a close approach situation, the relative positions between the wells must be calculated. However, calculating the relative position between wells is a challenge when the inclination of the two wellbores exceeds 80 degrees. This method increases safety while drilling. Using gyro while drilling eliminates the need to run wire line conveyed gyros, saving considerable expense. Gyro while drilling (GWD) allows data to be collected frequently while the drilling progresses, minimizing the risk of intercepting the fish without slowing the drilling process. Equally it is possible to be flexible in sidetrack trajectories as any attitude is available. There is no longer a need to steer by inclination only, improving efficiency in drilling operations. This paper looks at a case study where a passive MWD ranging method uses both the output of the magnetic sensors from the MWD and the directional information calculated from an all attitude gyro while drilling tool. This allows the calculation of the spatial relationship (distance and direction), between the drilling well and the fish, at 90 degrees of inclination. As the drilling sidetrack and the fish are at 90 degrees inclination and the two wellbores are not parallel, a high inclination gyro must be used to calculate the azimuth required for passive ranging calculations. Since an electromagnetic target cannot be placed in the original hole, only a single entry ranging technique can be used. This tool combination allows the positional relationship between the drilling well and the fish to be continuously monitored until the collision risk has passed.