Youcef Khetib

Date of Award

December 2022

Document Type


Degree Name

Doctor of Philosophy (PhD)


Petroleum Engineering

First Advisor

Kegang Ling


Unconventional resources development was made possible by the combination of long horizontal wells and a multistage hydraulic fracture technique; this configuration enabled operators to reach economical flowrates besides the low permeabilities. For liquid rich basins like the Williston basin, the wells tend to show a high flow rate peaks during the first 24 months followed by a rapid decline and a relatively stable low flow rate and long period. Due to pressure decline liberating the dissolved gas and due to low flow velocities caused by low flowrate the wells flow intermittently, and oil, water and gas phases flow rates become highly variable with time, this instability is called slugging. As the unstable flow do have an adverse effect on the recovered reserves, it is of importance to characterize the factors influencing such flow behavior. One of the least focused on factors is the well trajectory and its influence on the flow stability. This factor is the object of this research study.The present investigation study aims to analyze the effect of the horizontal wells’ lateral section undulations on the production performance and stability, a topic requiring high-quality data and adequate experiments. The study is based on three investigation areas, first, a statistical analysis is performed on the North Dakota Industrial Commission (NDIC) Database to search for a correlation or a trend between production performance and wells trajectory, then, secondly, the numerical modeling and simulation of field and lab scale cases to explain the statistical analysis findings, and third, the flow loop experimental tests are performed to capture high quality lab data, which are used to experimentally verify the numerical simulation findings. The field data statistical analysis is performed based on ~ 40 000 wells data base from which ~ 5 400 are selected for the analysis, aiming to find correlations between the production performances and the wells trajectories parameters. The analysis showed that no quantifiable correlation could be obtained for the production performance represented by the Production Efficiency Index (PEI) and the Average Angle Change (AAC), Arithmetic Average Angle (AAA), Sinuosity Index (SI) or the Maximum Amplitude, (MA). In general, an overall negative trend was obtained with regards to the AAC, AAA and MA, as these parameters absolute value increases, the production efficiency index decreases for oil, water, and gas. Highly deviated (Larger positive or negative angles and maximum amplitudes) wells do perform poorly on average when compared to the less deviated ones with trajectories closer to flat horizontal. Important to note that the observed trend, can be influenced by the basin wide distribution of reservoir quality, with tier one wells in the center and tier 3 at the periphery, thus no conclusion can be drawn from this part. Also, no trend was obtained with regards to sinuosity which is expected to translate the effect of the undulations, a relationship which needs to be investigated using the numerical simulations and laboratory experiments. Numerical modeling and simulation were performed for both field and lab scale systems. It is shown that, high amplitude of undulations does increase the slugging severity by increasing the slug’s lengths, and reducing the slug’s frequency, while the increase of undulations number have the opposite effect as it reduces the slug’s length and increases its frequency, reducing the slugging severity. An important finding is that very low and high undulations amplitude and number, respectively, do show similar effect with an optimal configuration at intermediate values. Profile wise, it is observed that the development of slugs happens in the lateral section profile with a slug merging at the vertical section. Another numerical simulation finding is that at lower slugging severity configurations, the average oil flow rate is consistently higher reaching a 4% increase, with a possible 1% decrease at high slugging severity configurations which perform at higher pressure drop when compared to hypothetical flat lateral case. An extensive number of experimental runs was performed to explore the effect of undulation number [0:3] and amplitude [0:4 D] on the flow stability of the system, most of the previously observed trends in numerical simulation are confirmed, higher amplitude generates more severe conditions while higher undulations number reduce the slugging severity. The experimental results also confirmed the existence of an optimal configuration by the bell-shaped trends obtained for slug frequency, translational velocity and horizontal pressure drop. Although numerical simulation and experiments tends to agree on the trends, the prediction of the slugs’ severity magnitude was overestimated, slugs’ length were higher in simulation when compared to the experimental results, with slugs’ frequencies lower in simulation. Very few parameters agree when the profile results are compared. This difference is mainly due to the magnitude of liquid film fall back effect from the vertical section to the horizontal section, which is not accurately modeled in the available simulation tool. As a general conclusion, the present study demonstrate numerically and experimentally that lateral section undulation characteristics (amplitude and number) do have an effect on the slugging tendency of lateral to vertical flow systems. Additionally, higher oil recovery can be expected for optimized trajectories. The statistical analysis didn’t provide a concluding finding. (5260850 kB)