Circulating out minor influxes through open riser during dual gradient drilling with a partly evacuated riser
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Today, with conventional drilling technology, the cost of drilling deep and ultra-deepwater wells is very high, thus many reservoirs are classified as non-economical. Some reservoirs may also be impossible to reach with conventional drilling technology. However, progress in technology has introduced a new drilling offensive in the offshore industry called Dual Gradient Drilling. Both time and expenditures are reduced so that un-drillable wells may become drillable. A new method for handling minor gas influxes with a dual gradient system that uses a partly evacuated riser is discussed. The method is based on the dual gradient method from Ocean Riser System, called the ‘Low Riser Return System’. It is argued that a minor influx could be circulated out through an open riser, avoiding a hard shut-in and circulation through the subsea choke. When the influx is observed, mud level in the riser is increased to stop the influx. Then the gas is circulated out through the annulus, into the riser and away from the wellbore by the wiper element in the top of the riser. Evaluation of this proposed method is carried out by simulation of two distinct cases, the Macondo wellbore in the Gulf of Mexico and a typical wellbore on the Norwegian continental shelf. Simulations are conducted by a program made in Matlab, a fourth-generation programming language. Several extensions have been developed for the simulation program. Inclusion of traffic lights to indicate kick size, pressure dependent influx using productivity index, gas compressibility, a tapered wellbore, a fill-up pump for initial mud level increase and a frictional pressure calculation based on the Moody chart are the most important extensions. Results from the simulations conducted are promising. Initial gas influx volume on the Macondo case was 1.83 m3. Although this is above the proposed influx limit of 1.0 m3 for when the well should be shut in, it appears that the method should handle the influx safely. The 1544 m long 19.5” inner diameter riser gives the ability to have a large volume free of mud, allowing gas to expand when it reaches the mud surface. A long riser section is also favorable to reduce the probability for slug flow. For the well on the Norwegian continental shelf results are also promising, but with a higher degree of uncertainty regarding gas behavior in the riser. A riser length of 362 m is used, which is in the lower end of where such a dual gradient system is favorable. Influx volumes above 0.5m3 at 1675 m for the specific case will exceed the formation integrity test for the last set casing shoe. However, influxes up to 1.0 m3 should be manageable with respect to gas behavior in the riser. Full-scale field tests conducted on deepwater drilling risers described in the literature show that gas is being dispersed throughout the entire riser, rather than rising as a slug. An air volume of 8.0 m3 released at the bottom of a 950 m long riser unloaded a maximum of 12.1 m3 of mud, whereas the surface volume of gas was 2166 m3. An air bubble of 1.6 m3 released in the same conditions did not unload any mud at all, only gas cut mud was observed at surface. Water based mud with a density of 1.58 g/cc was used for the field tests. Simulations of field test scenarios show that the simulation program has room for improvement when it comes to modeling of gas flow in risers. As a consequence, results from simulations are based on the field test for flow development in the riser. Use of strengthened surface equipment to raise the level of safety is proposed, as well as the benefits of using a fill-up pump to reduce the influx size. A fill-up pump provides faster mud level increase in the riser so that the influx is stopped at an earlier stage.