Anti-collision Analysis of Pad Drilling and Optimization of Wellbore Trajectory: A Field Case Study

Errors can occur in the wellbore position according to the fact the survey tools are not completely accurate; therefore, prediction of well path position is imperative for safe and cost-effective drilling operation. The aim of this paper is an analysis of collision avoidance as well as assessment and optimization of wellbore trajectory for minimizing the risk of collision by applying different anti-collision and planning techniques. Thus, anti-collision analysis of pad drilling in the Iraqi oil field has been investigated using Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) error model to estimate the wellbores position and assess their separation using different techniques available in the industry. Three actual offset wells X1, X2, X3, and one proposed principal X4 well in a drilling pad have been used in the collision avoidance model. Separation factor, ladder, and travel cylinder plots revealed a high possibility of X4 proposed well colliding with X3 actual offset well. The separation factor of 0.75 and 7.5 m center to center prove that the current design of X4 principal design doesn’t meet the anti-collision standards, accordingly, a design revision must be highly considered. The field operator hasn’t revised the well design due to the lack in the assessment of anti-collision risks, thus, the survey service company has advised the operator to modify the predetermined well trajectory due to major risk of collision with X3 offset well and the well has been sidetracked. After reviewing and optimizing the well trajectory by using slant and optimum align (curve hold curve) planning methods, the anti-collision results have been greatly improved. The results showed that, through adopting an adequate anti-collision risk assessment and the modified well design, problems associated with the execution of the improper well design could be totally eliminated.


Introduction
The development of drilling methods and techniques cause a significant improvement of well productivity. One of these effective drilling techniques is pad drilling system. Well pads play an important role in the oil industry, making the development of oil and gas fields with complicated surface conditions feasible [1].
In pad drilling, it is crucial to conduct anti-collision study for the planned wells within the pad in particular as well as between offset pads [2]. Because of low space between wells surface locations within the pad, an accurate well trajectory plan and actual offset surveys are mandatory to avoid wells collision which if not designed properly could lead to potential drilling problems and catastrophic consequence to human beings and environment.
However, an anti-collision management is usually initiated by accurate survey program of reference well, offset wells as well as the future proposed wells [3]. Hence, to minimize the wellbore position uncertainty, two tasks should be performed: i) utilizing reliable survey tools with high confidence level, ii) fitting the wellbore trajectory to the measured parameters using the most accurate trajectory planning methods. fact here are some sources of error in the measured data (measured depth, inclination & azimuth) which originates from different elements such as tool calibration performance, tool misalignment, BHA Sag, magnetic declination, magnetic interference, pipe stretch and thermal expansion [4]. Hence, the survey errors must be translated into well position errors whereas in extreme cases, these errors, if not considered can result to collision of neighboring wells and completely missing the predetermined underground target [5].
In 1969, Walstrom et al. [6] presented a method that produced too small ellipses of uncertainty.
Later, the initial thought of random survey errors was proven to be wrong. Hence, their model was rejected by the oil and gas industry.
In 1981, an alternative approach for determining the wellbore position uncertainty was introduced by Wolff and de Wardt in which the systematic survey errors sources are adopted [7].
At that time, there was similar works which led to little standardization in the method of Ellipses of Uncertainty (EOU) computation which cause confusion to the oil and gas industry. Therefore, the Industry Steering Committee for Wellbore Survey Accuracy (ISCWSA) was founded to introduce a standardized uncertainty model [8]. Built on a rigorous mathematical The key objectives and contribution of this paper are analysis of collision avoidance in addition to assessment and optimization of wellbore trajectory in complex cases such pad system drilling, that involving a low space between wells surface locations within the pad, by applying different anti-collision and planning modern techniques.

Field Background
"N" oil field is one of the Iraq's major oilfields. It was discovered more than 30 years ago and it is located in Iraq southeast. This oil field is a gentle elongated anticlinal structure. Long axis of the field is extended in a NW -SE direction. The structure is approximately 32 km long by 8.

Methodology
The Anti-collision model is based on three parameters of survey error model: Sigma confidence level of output error, scan method, and error surface. In this paper, Compass 5000.1.9.0 Landmark software from Halliburton Company was used to build the anti-collision model for four wells in pad system of N field.
1-To reduce the uncertainty associated with measurements, thereby ensuring the proper wellbore positioning, the anti-collision model was built in accordance with the well accepted and modern industry standards. In this method, ISCWSA error survey tool model was adopted and the following options were set: Output error: 2 Sigma which means (the confidence level for the survey errors, number 2 refer to 95% confidence that survey lies in the quoted error, which mean 5% chance of being outside the EOU. 2-Scanning method: closed approach 3D method, which shows a hypothetical 3D traveling spherical shape with designed reference well's trajectory. This 3D spherical would measure the distance between reference well and all offset wells in specified area, COMPASS will compute the distance and shows the values of separation factors for each comparison and gives the warning reports accordingly [10].

Journal of Petroleum Research and Studies
3-Error surface: to define the shape of the uncertainty envelope about the wellbore, the elliptical conic method was chosen which refers to an ellipse being formed perpendicular to the wellbore direction. Furthermore, to evaluate the risk of X4 well collision with offset wells, the EOU was calculated for all wells along their whole path. In these calculations, Magnetic MWD tool type was used for X1, X2 and X4 actual and planned survey data program, while EMS tool type was used for X3 actual survey program as shown in Table (1).

Results and Discussion
The separation factor computation summery which is based on the following simple algorithm: (1) Where, SF= separation factor, C-C = center to center distance in (m), R1: radius of major axis of ellipse of uncertainty of reference well in (m), R2: radius of major axis of ellipse of uncertainty of offset well in (m).
The Separation distances between reference X4 well and offset wells are introduced in Table   (2).
From this table, it appears that in these range of measured depths, X4 proposed well become close to the offset wells as indicted by center to center and EOU distances. The separation factor is less than one between X4 and X3 wells due to overlapping of the ellipses of error (negative distance value) with small Center to Center (C-C) distance, which in turn, leads to high probability of wellbores collision. Consequently, X4 planned design can't be adopted and the current design must be reviewed and rectified to meet the anti-collision standards.        The results of C-C and EOU distances after modifying the design plan is shown in Table (3). The optimization of the well trajectory design has greatly improved the separation distances and no more warning level become crossed.

Conclusions
 Wellbore trajectory planning stage is crucial for safe and cost effect drilling operations, hence, great deal of attention must be paid in designing the final principal wellbore among different prototypes planned trajectories.
 Conducting anti-collision model during wellbore planning stage is very important for assessment of proposed wellbore separation distances and positioning uncertainty with respect to offset wells especially in pad drilling systems.
 Real-time of wellbore survey must be carefully monitored and collision avoidance assessment must be reviewed in case of wellbore trajectory deviation from the planned trajectory.
 Operator, directional driller, and survey companies must work together to ensure that anti-collision policies are in place and survey programs are valid and followed during drilling operations.

Nomenclature
 ISCWSA: The Industry Steering Committee for Wellbore Survey Accuracy.
 EMS: Electronic Multi Shot Systems.
 KOP: kick off point.
 MDRT: measure depth referenced to rotary table.
 C-C: center to center distance.
 R: radius of major axis of ellipse of uncertainty.
 R1: radius of major axis of ellipse of uncertainty of reference well.
 R2: radius of major axis of ellipse of uncertainty of offset well.