Annular Ledge Formation Method Allows Sidetracking in Multilateral Wells
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Multilateral drilling technology offers a highly effective method of enhanced oil recovery in fields characterized by complicated geological structures. This paper describes the analysis of sidetracks drilled in an open hole by an annular ledge formation method with a downhole motor in multilateral wells.
Currently, industry growth requires development of fields with complicated geology. Drilling single-bore horizontal holes for these fields is insufficient, to say nothing of drilling vertical or directional wells. Multilateral wells with multiple horizontal sidetracks are drilled with increasing frequency. The competition between downhole motors and rotary steerable systems (RSS) in multilateral-well drilling is fierce.
In the complete paper, the authors describe deployment of a method previously only available with RSS that provides advantages to downhole-motor use: annular ledge formation, which involves sidetracking in an open hole. In the case study described, a conventional motor is used without additional electronics.
Conventional sidetracking methods in an open hole with the use of motors involve sliding. During sidetracking, the sliding is aggravated additionally by a low rate of drillstring movement. If sidetracking with the conventional method is impossible, then usually a special sidetracking bit is installed and the motor-bend angle is increased. Such measures result in additional time and financial expenditure.
The annular ledge formation method allows for sidetracking in an open hole with the use of a standard bottomhole assembly (BHA) with a motor, which also will be used for drilling a lateral. The key feature of this method lies in the fact that, during sidetracking, the drillstring is constantly rotating, which allows avoidance of the problems related to sliding. When sidetracking with the use of this method, the ledge is formed over the entire circumference of the borehole, not only on the bottom wall. But, when sidetracking deeper, a trough is formed on the low side of the hole and the upper side remains unaffected. Because of this, the annular ledge formation method is described by the authors as sidetrack with rotation. The authors discuss the deployment of this method in the Vostochno-Messoyakhskoye field when drilling multilateral fishbone wells. The complete paper presents a detailed discussion of the method and different situational approaches to its use.
The field is the northernmost developed onshore oil field in Russia in the Tazovsky region 250 km from the Arctic Circle in the Arctic climatic zone. The field is developed with horizontal production and injection wells along a horizontal section length of approximately 1000 m. The geology of the reservoirs is complicated with a large gas cap. In terms of geological conditions, the PK 1-3 pay zone is irregular within the field and is represented by loose sandstones both uniform and horizontally layered. The oil-bearing formations are isolated.
The geological conditions of the Vostochno-Messoyakhskoye field limit the efficiency of fracturing for the following reasons:
- Risks of the fracturing in water-saturated and gas-saturated intervals
- Low stress/strain properties of the unconsolidated sand of the PK 1-3 formation, conditioned by widespread development of destruction zones
The operator chose the application of fishbone technology. Fishbone well design implies a main hole in the target formation and several laterals that intersect the beddings and diverge in order to increase horizontal and vertical sweep. Such a design enables a significant increase of pay-zone coverage.
The main type of fishbone well completion in the field corresponds to the Technology Advancement Multilateral 1 complexity level, according to the international classification. A screened (slotted) liner is run in the main horizontal hole. In this case, the laterals remain open.
The sidetracks in fishbone wells in Vostochno-Messoyakhskoye field have the same diameter as the main horizontal hole. In fact, each successive hole is a sidetrack for the previous hole. The main hole is a horizontal sidetrack drilled last.
Completing six kickoff operations when constructing the well with seven laterals is a necessity. With this number of holes, the time reduction for each sidetrack contributes to a sizable acceleration when drilling the entire well. An extra challenge is to perform all operations in the producing formation over one run, which effects a significant time savings.
First Results of Sidetracking With Rotation of the BHA With Downhole Motor
The technology of sidetracking in open hole with annular ledge formation was applied with the use of the BHA with a downhole motor in Vostochno-Messoyakhskoye field in May 2018. Six sidetracks were drilled in the producing formation. The horizontal projection of Well 1 is presented in Fig. 1. Well 1 is of interest because different combinations of methods were applied for sidetracks. The first sidetrack was drilled with a standard method. The trough was made under sliding without BHA rotation; then, time drilling was performed without rotation as well. The trough for the second and the third sidetrack was made by sliding, and time drilling was performed with drillstring rotation. The three remaining sidetracks were drilled with BHA rotation. The result of the approach was that the time for operations in the first well was reduced significantly. A detailed description of each sidetrack is provided in the complete paper.
Eight holes were drilled from one wellhead: one pilot hole and seven horizontal fishbone holes. The well-trajectory length totaled 6752 m, of which 3970 m fell on horizontal sections. The deepest point of the well—2796 m—is the final bottomhole of the FB-6 lateral. All horizontal sections were drilled with one downhole motor. The total runtime for the downhole motor reached 249 hours, with a guaranteed mean time between failures of 150 hours.
During the 6 months after the first sidetracks were drilled in May 2018, sufficient experience was gained to assess the results of the new method, work out the decision-making circuit, and prepare a list of recommendations for extrapolating the method to other fields.
Over 6 months, 39 multilateral wells were drilled with 130 sidetracks. Sixty-six sidetracks were drilled with trough making with rotation, eight sidetracks were drilled with trough making by sliding, and 56 sidetracks were drilled under time-drilling mode only.
Transfer to the sidetracks with rotation allowed a reduction in average time from 7–9 hours to only 3–4 hours in soft rock and up to 5–7 hours in harder formations. The record sidetracking time totaled 2 hours and 4 minutes.
Eighty percent of the wells were drilled without trips for replacement of a downhole motor. The runtime for the motor reached 200 hours and more in these wells. In some complex-trajectory wells, in which RSS was planned for drilling the last and the farthest fishbone holes, drilling the well to a final bottomhole location with only the use of a downhole motor was possible. Because of a decrease in tripping operations, well-construction time is reduced.
From the time this technology was introduced, no cases with the BHA entering the old hole were observed during tripping operations. This confirms that more-favorable conditions are created for BHA passage in an interval of hole diversion and, subsequently, for running in a liner.
Drilling fishbone wellbores by the use of a motor requires consideration of the following recommendations:
- Reduce the friction factor by adding lubricants to the drilling mud.
- Clean the borehole with tandem pills before sliding.
- Design the well trajectories without sharp azimuth turns.
- At an equal well-hole length, the wells with a direction change in the production-casing interval pose greater drilling difficulty than wells with a constant azimuth. The boundary wells of the pad in the field under review are characterized frequently by a high turn in order to hit the geological target.
- Sidetracks with rotations are drilled with more ease and speed than sidetracks with sliding.
Annular Ledge Formation Method Allows Sidetracking in Multilateral Wells
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