Statistical and Numerical Investigation of the Effect of Wellbore Trajectories in Williston Basin Horizontal Wells and their Effects on Production Performance
Authors: Y Khetib 
Published: February 2023

Unconventional plays development requires multistage hydraulically fractured horizontal wells; the wells’ trajectories are complexes and follow different patterns, such as toe-up, toe-down, hold-toe-up, and hold-toe- down. The trajectories patterns and undulations are caused by different factors and do have different consequences on the production performances, literature lacks field data highlighting the effect of the wells’ trajectories on production performance, thus the importance of the present paper’s statistical analysis. The objective of the study is to identify the influence of wells trajectories on their performances based on the public production database provided by the North Dakota Industrial Commission (NDIC) for Williston Basin wells. To achieve the objective, it is required to characterize the undulation and trajectories, Firstly, based on the literature review, drilling methods and technologies used during the lateral section drilling do have an impact, and their selection is crucial for final undulations status and trajectories which influences the production behavior of the wells. Secondly, using an in-house developed code, 22,867 North Dakota Williston Basin drilled horizontal wells are analyzed, and the average angle change (AAC) is calculated for each individual lateral leg, along with the angles arithmetic average (AAA) and Sinuosity Index (SI). Due to the apparent erratic production behavior, a parameter has been defined called Normalized Production Efficiency (NPE) for Oil, Water, and Gas phases, specifically defined to remove 1) the effect of spatially distributed productivity variation, i.e. center of the basin is more productive than the basin periphery. 2) different stimulation treatment sizes, and 3) Early time production peaks. And then the Normalized Production Efficiency was plotted through heat maps and through scatter plots to observe its dependence on the undulations-related parameters. First, it is found that the Williston basin is dominated by toe-down wells and based on the literature review the following statements are inferred, 1) In terms of production performances when compared to the toe-up trajectories, fewer flow instabilities are expected. 2) Higher risk of water and solids accumulation are to be considered, and 3) Larger gas-oil-ratio variation during well production is to be expected. Second, the Normalized production efficiency exhibits a weak inverse correlation to the AAC, where higher AAC tends to correspond to lower normalized production efficiency meaning lower cumulative oil produced. Interestingly the normalized production efficiency parameter forms a bell-shaped scatter plot with the AAA parameter suggesting a spatial dependency as it is expected to see fewer deviated wells in the basin center compared to its flanks. After plotting the NPE for oil, AVG, and AAC on a heat map across the basin, it is shown that no special dependency is exhibited. Similar results are observed for NPE of Water and Gas. The study concludes that flat and toe-down trajectories have the highest probability of showing higher productivity regardless of the well position in the basin or its stimulation treatment. Except the described trend No claim on a quantifiable relationship, with regards to trajectory or undulations can be made and a more detailed study is required, with better assessment of formation quality variability across the basin, and parent, child wells interactions.