Coiled Tubing Drilling - A Concept for Directional Drilling in Pressure Depleted Reservoirs

Many gas reservoirs in the north-western part of Germany have been produced for decades. It has become increasingly challenging to start production from conventionally drilled holes in depleted reservoir sections. This can be due to invasion of fluid into the rock matrix causing near wellbore damage, the necessity to stop fluid losses prior to running a completion, or the reservoir pressure being insufficient for producing fluids back after drilling. During the past years a concept has been developed to re-enter carbonate reservoir wells, drill a sidetrack to the top of the reservoir, case-off the section, and run the completion. Afterwards the reservoir section is drilled on coiled tubing using nitrified fluid in order to reduce the overbalance towards the reservoir pressure and to minimize skin related inflow performance issues. This procedure has proven successful in a first step for drilling short open-hole sections without directional control and steering capability. Encouraged by the results, as a second step, a concept for drilling directional hole sections on coiled tubing has been developed using state of the art technology. The goal is to use existing wells that are suffering from severe reservoir damage issues and access virgin reservoir rock by drilling open-hole sidetracks. A portfolio of wells has been screened to find promising candidates for directional coiled tubing drilling. First choice candidates are open hole completed wells with high inclination well paths in carbonate reservoirs. Since this work must be performed through tubing, most of the candidates have an ID restriction that requires using a 2 3/4" bit. Directional coiled tubing drilling technology requires accurate downhole data for steering and tool protection. Using nitrified fluid makes it necessary to transmit data via cable instead of using mud pulse systems. Simulations for drilling open-hole sidetracks within the reservoir section have been run for various well trajectories. Results show that depending on the donor well, drilling fresh reservoir sections of several hundred meters is possible, minimizing potential damage in pressure depleted reservoirs. Success of an open-hole sidetrack can be immediately verified after drilling. A further step towards improving reservoir drainage could be multiple sidetracks resulting in multi-lateral wells.