This paper addresses challenges faced in Coiled Tubing (CT) intervention in slimhole horizontal wells in one of the most mature fields in Oman. An intensive production optimization program has been set by the reservoir management team. The optimization activity consisted of clean-out, saturation logging, perforation and stimulation. CT units utilization played the main role in this program. Large number of these wells were completed with 2 7/8" cemented horizontal liner and 4 ½" tubing equipped with gas lift system. These wells were sidetracked using CT drilling. The length of the horizontal 2 7/8" section ranges between 1000 m and 1600 m, having the kick-off depth at about 1300 m.
CT intervention was very challenging in these well, as CT reach was limited in most of the cases. This is mainly due to the following factors:Complex well trajectory path or "snake-like" wells.Limited tools and CT pipe size due to small cross sectional area.Scale build up on completion.High production rate resulting in resisting CT passage.Corroded liner and poor cement bond resulting in preventing scale cleanout with reactive fluids.Bad liner and/or tubing conditions resulting in mechanically obstructing CT passage.
Therefore some unsuccessful cleanouts or dummy runs were held. Hence, no logging and or optimization activities could be done.
A total of 22 wells were intervened in this project. At project midway success rate was 40% and at the end success rate was increased to 60%.This paper addresses the lesson learnt throughout this project and techniques used to enhance CT reach. The technique involved the use of CT force models and reach simulator to sensitize different factors. As more jobs were performed, models were compared with actual data and some learning could be gained. The main learning was that well flow rate was the highest contributor preventing a deep reach. Hence, the improvement techniques involved manipulating gas lifting flow rate to reduce well flow rate while CT runs in hole.
Introduction
This project took place in one of the most mature and complex carbonate fields in the north of Oman operated by the Petroleum Development Oman (PDO). The field produces mainly from the lower cretaceous Shuaiba formation which is heavily faulted. Porosity ranges between 30 to 35% while permeability ranges from 1 mD at the base of the Shuaiba to 200 mD at thief zones (high permeability streaks). Most of the production comes from horizontal wells with 4 ½" cemented and perforated liners. However, this project is addressed toward a less common wells type. These wells are ultra-slim horizontal wells with 2 7/8" cemented liners. Figure-1 shows the evolution of the wells till reached the ultra-slim well type.
As part of MRC (maximizing Reservoir Contact) and production optimization plan for the field, further field development consists of high-density infill drilling by horizontally sidetracking existing, watered out wells. In order to be able to access smaller remaining oil targets in a cost effective way, a slim-hole drilling campaign was embarked on in 1998, using a standard rig and a BHI Galileo CTU drilling rig. Old 4 ½" liners are sidetracked with 3 ¾" hole and completed with 2 7/8" liner. With the remaining reserves interspersed within flushed areas the CTU unit is required for the well path flexibility it offers in accessing these reserves. Perforations were done selectively in areas with high oil saturation. Currently the field has got 40 wells completed in this manner.
