Successful Use of the Hydraulic Workover Unit Method for Underbalanced Drilling

Abstract This paper describes two very successful underbalanced drilling operations that were performed in Belgium and in the coastal area of The Netherlands. The underbalanced drilling operations used a hydraulic workover (HWO) unit to effectively control the well pressure during drilling. This method is unique because it reduces damage to the formation and causes very little environmental risk. For the Belgian operation, drilling was preceded by a series of tests and simulations. A hydraulic workover data acquisition system was installed to provide the highest possible control of the drilling process. A 9 5/8" shoe was drilled, and a cement bond log was run, after which injectivity tests were performed at the shoe. A bottom hole assembly (BHA) with an 8 ½" bit was then used to drill into the reservoir at which point the well was predicted to go underbalance with fresh water as the drilling fluids. Although the well did not flow, the underbalance drilling systems ran flawlessly and the BHA was retrieved without any problems. The well was then cased with 7" liner. The second case history represents one of the most complex and technically challenging underbalance drilling projects to date due to the offshore environmentally sensitive location in the North Sea and the high BHP of 250 bar. The same steps of preparation as the prior well were gone through but this time the client required a zero discharge of gasses or fluids. Some of the precautions taken consisted of console enclosures, hose covers, drip pans and the entire hydraulic workover unit had to be encapsulated. The target fault was crossed successfully while meeting all of the strict environmental restrictions and client objectives. The success of the method must also be attributed to the operational planning and close attention paid to experience gained from previous drilling operations from the multidisciplined crews. The execution of the treatment utilized all of the components considered to be state-of-the-art in drilling including the use of a data acquisition system, real-time treatment pressure monitoring and HWO drilling procedures applied.