Abstract

Hydrocarbon influxes into the wellbore while drilling are typically caused by over-pressured formations, where mud weight is insufficient to control the pore pressure of the formation drilled. Formation permeability and over-pressure determine the intensity of the kick. Well kill procedures in such situations are standard industry wide. Shut in drillpipe pressure is used to determine the pore pressure of the formation, and the driller's or weight and wait method is used to remove the hydrocarbon influx from the well. Hydrocarbon influxes associated with fracture breathing are much less common, and methods to deal with such influxes are not taught within the drilling industry. This paper describes two case studies: the first, Bard-1, an exploration well in the Timor Sea, offshore Australia, describes a well control situation that was initially interpreted as over-pressure. The well could only be killed by pumping cement. Investigation showed that the influx mechanism was a fracture breathing and hydrocarbon swap out mechanism. The high mud weights used in the attempt to kill the well actually exacerbated the situation. The second case study, Jura-1 which was located nearby Bard-1, describes design and procedures used to manage a similar influx mechanism. These were implemented successfully enabling the well to achieve its objectives.

Introduction

Bard-1 was drilled in October 1998. The well is located in the Timor Sea, offshore northwest Australia. The 9 5/8" casing was set at 1460m and a formation integrity test (FIT) performed, limited to 1.41sg. While drilling ahead at 2164m mud flow increased at surface followed by significant gas readings. The well was shut in and conventional (wait and weight) kill operations commenced. After raising the mudweight to 1.31sg and then 1.36sg the well appeared to be killed, was opened and found to be static. However, after circulating bottoms up, gas peaks were registered and the well return flow increased. Despite several further increases in mud weight to a maximum of 1.48sg, the ingress of hydrocarbons into the wellbore could not be prevented. At various stages of the attempted kill the shut in drillpipe pressure (SIDPP) was reduced to zero, suggesting the well was overbalanced by the current mud weight. Approximately 70bbl of oil with associated gas were recovered to surface during well kill operations. Efforts continued for several days before the well was abandoned with a series of barite pills and cement plugs.

The subsequent investigation concluded that the ingress method was a fracture breathing/fluid exchange mechanism instead of over-pressure. The variations in bottom hole pressure due to circulation were causing charging of the fracture system near TD, which then returned hydrocarbons and mud when circulation ceased. This swap out mechanism appears to have been exacerbated by the high mud weights used to attempt to kill the well.

Jura-1 was drilled on the same structure approx. 45 km from Bard-1 in July 1999. The well was designed to enable management of a "Bard event". Casing design and wellsite procedures are described in this paper. Casing was set close to the potentially fractured formations in order to permit a low mud weight to be used and thereby minimise overbalance. Special procedures to shut down and start up the mud pumps were implemented to reduce bottom hole pressure fluctuations. Jura-1 also encountered a fracture breathing formation, with return mudflow of up to 15bbl when circulation ceased during drillpipe connections. During these flow back periods, the "flow signature" was plotted i.e. volume vs. time. Comparison of "flow signatures" helped to identify that the well remained over balanced despite flow being excessive by normal well control standards. Jura-1 was drilled to TD successfully.

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