Technical Challenges Encountered During Coiled Tubing Underbalanced Drilling In Oman
Abstract: This paper describes two of the major technical achievements during the first year of operation of a new, purpose built, underbalanced (UB)
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CTU has been gaining importance in the oil industry. It is a multipurpose unit with one application being drilling. The major limitation of CTU was eliminated recently by inventing rotating coiled tubing which is yet to hit the commercial market. In this concept a conventional derrick based rig is modified to a Hybrid Drilling Rig. A coiled tubing unit is incorporated inside derrick structure itself at a height above the derrick floor which depends on type of derrick. The coiled tubing unit added is not the conventional one instead it is a unit that actually rotates the coiled tubing. This coiled tubing will be placed horizontally above the floor. It has a support structure passing through its center which is detachable. The use of this center structure is that it has straightening injector fixed in it and it supplies the torque to the CTU. Coiled tubing from the CTU will take a helical path around the support structure, pass through the injectors and become straight. The purpose of the paper is to discuss the advantages and limitations of this hybrid drilling rig technically and economically. Also some calculations are included which are used in the design of the rig used. This paper also outlines fatigue analysis and advantages and limitations of using rotating coiled tubing. Introduction CTU, after it was invented many decades ago has been experiencing increase in demand in the oil industry. Apart from its basic uses, it has recently gained importance for drilling. Hence with the advancement in coiled tubing drilling we can consider the CTU to be one of the most versatile equipment available to the industry that can be used for a wide range of jobs or tasks performed on the rig. The only problem with the coiled tubing was its inability to rotate which reduced its efficiency. Recently rotating coiled tubing was invented by John Van Way which was used on a workover rig as a model in Louisiana. This is the basis on which this concept, of using a CTU on a rig efficiently, may fetch us good results. This paper will give describe how a rotating coiled tubing can work on a rig. Now days whole vertical wells are drilled with coiled tubing especially in many parts of Canada but still it poses some problems when it comes to hard formations. Coiled tubing drilling is not a replacement of conventional drilling equipment for drilling conventional, directional, and horizontal wells. The real application will be on drilling projects requiring special consideration such as:Environmentally sensitive areas where noise abatement and well control must be assured.Underbalanced drilling in areas where special permission is required and may not be obtainable for conventional equipment. These projects may require oil based mud systems, or aerated or gasified mud systems.Offshore re-entry drilling where the platform cannot support a workover rig or workover rig does not exist and would be too costly to refit platform with a rig.Through tubing drilling where mobilizing a workover rig to pull the production tubing would increase project cost to the point that would be uneconomical.High pressure underbalanced drilling where conditions exceed normal equipment capabilities. So drilling with this hybrid rig requires use of the conventional rotary steerable drilling technique and reduce the costs by using coiled tubing simultaneously wherever possible, making it an overall economical drilling project. Besides coiled tubing will provide its conventional services like well unloading, cleanouts, acidizing/stimulation, velocity strings, fishing, tool conveyance, well logging (real time & memory), setting/retrieving plugs and fracturing onboard.
CTU has been gaining importance in the oil industry. It is a multipurpose unit with one application being drilling. The major limitation of CTU was eliminated recently by inventing rotating coiled tubing which is yet to hit the commercial market. In this concept a conventional derrick based rig is modified to a Hybrid Drilling Rig. A coiled tubing unit is incorporated inside derrick structure itself at a height above the derrick floor which depends on type of derrick. The coiled tubing unit added is not the conventional one instead it is a unit that actually rotates the coiled tubing. This coiled tubing will be placed horizontally above the floor. It has a support structure passing through its center which is detachable. The use of this center structure is that it has straightening injector fixed in it and it supplies the torque to the CTU. Coiled tubing from the CTU will take a helical path around the support structure, pass through the injectors and become straight. The purpose of the paper is to discuss the advantages and limitations of this hybrid drilling rig technically and economically. Also some calculations are included which are used in the design of the rig used. This paper also outlines fatigue analysis and advantages and limitations of using rotating coiled tubing. Introduction CTU, after it was invented many decades ago has been experiencing increase in demand in the oil industry. Apart from its basic uses, it has recently gained importance for drilling. Hence with the advancement in coiled tubing drilling we can consider the CTU to be one of the most versatile equipment available to the industry that can be used for a wide range of jobs or tasks performed on the rig. The only problem with the coiled tubing was its inability to rotate which reduced its efficiency. Recently rotating coiled tubing was invented by John Van Way which was used on a workover rig as a model in Louisiana. This is the basis on which this concept, of using a CTU on a rig efficiently, may fetch us good results. This paper will give describe how a rotating coiled tubing can work on a rig. Now days whole vertical wells are drilled with coiled tubing especially in many parts of Canada but still it poses some problems when it comes to hard formations. Coiled tubing drilling is not a replacement of conventional drilling equipment for drilling conventional, directional, and horizontal wells. The real application will be on drilling projects requiring special consideration such as:Environmentally sensitive areas where noise abatement and well control must be assured.Underbalanced drilling in areas where special permission is required and may not be obtainable for conventional equipment. These projects may require oil based mud systems, or aerated or gasified mud systems.Offshore re-entry drilling where the platform cannot support a workover rig or workover rig does not exist and would be too costly to refit platform with a rig.Through tubing drilling where mobilizing a workover rig to pull the production tubing would increase project cost to the point that would be uneconomical.High pressure underbalanced drilling where conditions exceed normal equipment capabilities. So drilling with this hybrid rig requires use of the conventional rotary steerable drilling technique and reduce the costs by using coiled tubing simultaneously wherever possible, making it an overall economical drilling project. Besides coiled tubing will provide its conventional services like well unloading, cleanouts, acidizing/stimulation, velocity strings, fishing, tool conveyance, well logging (real time & memory), setting/retrieving plugs and fracturing onboard.
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.
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