Optimization of Deviated and Horizontal Wells Trajectories and Profiles in Rumaila Oilfield

Directional and horizontal wellbore profiles and optimization of trajectory to minimize borehole problems are considered the most important part in well planning and design. This study introduces four types of directional and horizontal wells trajectory plans for Rumaila oilfield by selecting the suitable kick off point (KOP), build section, drop section and horizontal profile. In addition to the optimized inclination and orientation which was selected based on Rumaila oilfield geomechanics and wellbore stability analysis so that the optimum trajectory could be drilled with minimum wellbore instability problems. The four recommended types of deviated wellbore trajectories include: Type I (also called Build and Hold Trajectory or L shape) which target shallow to medium reservoirs with low inclination (20) and less than 500m step out, Type II (S shape) that can be used to penetrate far off reservoir vertically, Type III (also called Deep Kick off wells or J shape) these wells are similar to the L shape profile except the kickoff point is at a deeper depth, and design to reach far-off targets (>500m step out) with more than 30 inclination, and finally Type IV (horizontal) that penetrates the reservoir horizontally at 90. The study also recommended the suitable drilling mud density that can control wellbore failure for the four types of wellbore trajectory.


Introduction
Horizontal and highly deviated wells became an important strategy in modern oil and gas industry because this type of wells offer great economic profits by higher production rates and recovery factors and lower development costs.
Generally, there are two primary kinds of trajectory for the borehole [1]: 1. Straight or vertical.
2. Directional, which include horizontal and deviated borehole trajectory.
The vertical borehole also termed a straight borehole hole. Nevertheless, naturally, a slight deviancy from vertical regularly happens that is associated with the features of the formation like stiffness and dip angle due to the drilling influences, ( borehole assembly, bit type, and weight on bit) [2].
Determination of suitable drilling mud density using rock failure analysis is a necessary step to gain control on wellbore instability in both vertical and deviated wells [3].
According to the mud properties, the analysis results for successful drilling recommended to sustain the lowest possible mud density in order to reduce contamination of the producing formations. However, the pressure of drilling mud (the hydrostatic pressure) would not be possibly sufficient to maintain borehole stability if the mud density is too low [4].
Values of shear failure pressure in Rumaila oilfield that are measured in pound per square inch (psi) can be converted to shear failure equivalent mud weight (EMW) in gram per square centimeter (gm/cm 3 ) by using conversion equation (2) which is a derivation of equation (1) as follows: [5].   Ru-382) [6]. The red flagged EMW values in the table points to high shear failure EMW that exceeds 1.2 gm/cm 3 (which is the mud weight regularly used to drill the same set of formations). The observed correspondent lithology for that high EMW was shale.
Shale is a fine-grained sedimentary rock that has a low permeability. Therefore, the redistribution of stress took longer time until a new hole is being drilled, leading to a possible failure in the borehole even after a few days of drilling. This is because the pore pressure in low permeable formations is very high prior to drilling compared to high permeable formations due to pore pressure not able to dissipate freely when in contact with the drilling mud [7]. also, it may possibly cause reservoir damage as the mud solids penetrate deeply into the reservoir [8]. highest average shear failure EMW at a vertical well in Rumaila oilfield. S hmin, which is also termed Fracture pressure or Tensile Failure is the pressure that fractures formations, when the minimum compressive stress and tensile strength that exceeds the formation pore pressure [6]. The lowest values of MWW are preferred and marked in green color [9].

Directional and high angle wells optimization:
Directional drilling is a broad term that concerns all required activities for design and drill a wellbore to reach a reservoir target, or number of targets, is located at some horizontal distance from top of the hole. In other words, the purpose of directional drilling is to connect the surface location with oil or gas reservoirs . Also the directional drilling can be the solution in the event of the drill pipe becoming stuck in the hole by simply drill around it or plug back the well to drill to a replacement target [10]. It was concluded from previous Geomechanically studies that the optimized direction to drill deviated wells in Rumaila oilfield is NE or SW (parallel to the maximum horizontal stress) [6].
Four primary types of well shape are considered during planning a deviated well [11]:

Type I wells:
This This technique is typically used while planning to drill wells with a one shallow producing reservoir [11].   or water cone issues leading to increase productivity of pay zones that are not been effectively depleted by vertical wells, also connection of the productive segments from the pay zones and raise production in low permeable pay zones by escalating the extent of formation exposed to the wellbore [9]. The cost of horizontal well could range from 20% up to 300% and it can enhance the production 2 to 10 times compared to a vertical well.
Hence, a reduced wells number are needed for the field development [13]. The four directional well profiles can be proposed for application in Rumaila oilfield, taking into account the field Geomechanically characteristics [6]. Also, the limitation of the volume of alteration in deviation and/or azimuth of the wellbore (Dogleg severity) commonly dogleg severity is stated in degrees for every 30 meters of path length [2]. The directional trajectory calculations can be made using the Minimum Curvature Method in excel spreadsheet [14].

1-Type I well trajectory:
As this trajectory is suitable to target reservoirs at medium depths which is suitable to penetrate Mishrif reservoir (2200-2400m depth). The proposed trajectory is displayed in

2-Type II well trajectory:
This type (S shape profile) can be recommended when the reservoir is required to be

3-Type III well trajectory:
The well profile from this type is suitable for far-off targets (>500m horizontal displacement) targeting one or multi reservoirs directionally. In this type the well profile starts with vertical section from surface till the top of Sadi Formation, then the buildup section starts and ends within Sadi Formation which is deeper than Type I and II as presented in Figure (

4-Type IV well trajectory:
Drilling horizontal wells in Rumaila oilfield can play a vital role in raising oil production rates significantly because horizontal well exposure length to the reservoir is much higher than vertical or deviated exposure (can reach more than 1 km length); beside that the horizontal drilling cuts down the cost of drilling many vertical wells , processing time and equipment needed on the surface. Other advantage of horizontal well profile is that it is ideal to access and penetrate thin reservoir beds such as sandstone beds in the upper shale and upper sandstone Members in Zubair Formation. A suggestive trajectory plan is displayed in Figure (9) and explained in the proposed four well trajectory types are summarized in Table (6). As before, the equivalent mud weight (EMW) for shear failure is decreased when drilling deviated wells in the direction of S Hmax , significant decrease in EMW will have positive effect on wellbore stability where a lower mud weight can be sufficient to maintain wellbore in stable condition. 1-Four-deviated wellbore trajectory were suggested for implementation in Rumaila oilfield bychoosing the optimal trajectory depend on the target location and recommendation to penetrate it vertically or in certain angle.