High Performance Wellbore Departure and Drilling System for Accessing New Target
Authors: Dewey Charles , Swadi Shantanu , Alsup Shelton , Desai Praful 
Published: March 2011

Abstract
Conventional wellbore departure and drilling systems generally require the operator to make multiple downhole trips to achieve a specific objective. For example, a window milling bottom hole assembly (BHA) is run in hole to create an exit path in the existing casing and drill sufficient rathole for the next drilling assembly. In the subsequent trip, a directional drilling BHA is run to extend the rathole and drill laterally to the target.
The industry requires an economical solution to accomplish the above objective in a single trip with good downhole dynamics control and overall BHA drillability. The re-entry system should be able to mill a window in the existing casing sufficiently large enough for obstruction free entry of the BHA and new liners. Once the BHA has exited the casing, the new system would be required to drill a full gauge lateral wellbore to the target with good directional control and minimal vibration. To solve the challenge engineers analyzed several key technological/operational issues including: Dynamic simulation of the BHA to study the nature and magnitude of the vibrationsControlling vibrations and feed rate to reduce premature cutter damage during window milling operationsUtilizing the latest force balance software for selection of shapes, sizes, number and location of cutters for maximizing on-bottom timeExplored hybrid cutting structures on bit/mill to maintain gaugeCombined knowledge and experience of subject matter experts (SME) from different engineering and operational groups within the organization including directional/geosteering personnel, fishing/remedial group and drill bit design team
The evolution of the system and the associated technology occurred in three phases over several years. In all phases a window was milled in the existing casing and a lateral was drilled to various depths. During the first phase the system was run on a conventional rotary BHA in soft and medium formations. The second phase included testing on a positive displacement motor with bent sub or bent housing. The third phase, which is currently under extensive full-scale yard testing, includes a push-the-bit type rotary steerable system.
The results obtained from all three phases indicate that a functional single-trip system for milling a window and drilling a lateral borehole is commercially feasible. The versatile system will contribute substantially to the technology required to efficiently and economically mill a window in the existing casing and then drill an extended length lateral wellbore to the target formation without tripping for equipment/bit change out. The wellbore departure and drilling system, which will be optimized with a sophisticated dynamic analysis software program, will incorporate the rate of penetration and footage benefits of polycrystalline diamond compacts (PDC) bit technology combined with the latest directional drilling tools.
The paper will include field run details of phase one/two as wells as results of ongoing full-scale yard tests along with key observations and conclusions. The authors will also include an overview of the software used for analysis and plans/capabilities of the final version of the system.