Directional surveys are too far apart at the usual 90 feet in unconventional laterals. While it is considered uneconomic to survey at a closer spacing, an analysis of the positive displacement motor (PDM) slide/rotate sequences or rotary steerable system (RSS) set points along with standard MWD surveying can yield much higher fidelity in survey data. This paper covers the results of applying such a technique with the objective of delivering a less tortuous and more accurately placed lateral.
Increasing the fidelity of directional surveys entails utilizing a statistical estimation-based forward tendency model. A PDM curve rate along with rotary build/drop and walk rates are balanced with the stationary surveys to capture the actual curve rates between the longer spaced surveys. A similar process is used when drilling with a RSS. This data allows for a better picture of the tortuosity within what will become hydraulic fracture stages which impacts frac development and performance. The data is also part of survey accuracy computations that now can be used to determine a more accurate placement or location of perf clusters or frac points within those stages. Examples of stage placement with and without high fidelity are shown.
The accuracy of directional inclination, as used with 90 ft survey course lengths, is such that the true vertical depth error accumulated subsequent to the curve kick-off point is usually in the range of 30 ft or less in lateral sections. However, high fidelity analyses show that left/right spacing errors are larger than 100 ft between 20% and 40% of the time, depending on wellbore direction. This was not as significant in conventional directional wells. When unconventional laterals surround a new lateral at a 500ft spacing, this left/right error becomes as significant as TVD error. This problem becomes more acute as spacings become closer.
The use of high fidelity techniques during drilling can lead to a balanced approach to PDM and rotary directional tendencies. This ability is not available without these real-time high fidelity computations. While this balancing is most pronounced with PDM’s, it is also possible with RSS’s. It will result in lower tortuosities, primarily in the laterals. Comparisons with hydraulic fracture data will detail the significance of this.
Reducing wellbore tortuosity and improving lateral placement has an impact on the “drillability” of a well. This improvement along with the better knowledge of lateral curvature has a larger impact on completion design and the ability to centralize casing for cement isolation. Hydraulic fracture design can be modified for the different curve rates now visible in laterals. This higher precision in frac placement can be used in analyzing and preventing frac hits in nearby wells where high fidelity analysis has been performed.