Blowout Prevention and Relief Well Planning for the Wheatstone Big-Bore Gas Well Project

The offshore Wheatstone LNG Project in Western Australia utilizes subsea big-bore gas wells as the preferred method of producing the field. Wheatstone wells use a 9–5/8″ production conduit from the top of the gas pay zone to the ocean floor. Wellbores of this size are necessary to match the large productive capacity of the gas reservoirs they penetrate. This producing scenario provides the obvious benefit of yielding large volumes of gas through the use of relatively few wells. Each of those highly productive wells, however, also represents a source of gas that, if accidently allowed to flow unhindered, could present an uncommonly difficult well control challenge. It is for this reason that the Wheatstone Drilling and Completions Team evaluated a wide range of possible reservoir and well architecture scenarios to fully understand the possible scale of relief well responses that might be necessary in the event of a blowout. The conclusions from this evaluation were surprising. Our originally-planned well design concept called for penetrating the Wheatstone gas reservoirs with a casing shoe set 950m vertically above. Our analysis indicated that 3–4 relief wells would be simultaneously required to bring a blowout under control. Based on these results, both the well and the drilling execution plan were redesigned to minimize the number of required relief wells. In summary, the redesign amounted to setting casing immediately (i.e., = 3m) above the gas reservoir before actually penetrating it, with the resulting benefit of reducing the required number of relief wells to 2. Although this reduction is beneficial, it should be noted that there is only one documented subsea case where 2 or more relief wells have been drilled with the intent of simultaneously pumping into both to effect a dynamic kill. Given this fact, our well control-related preparations for executing this project were more extensive than that of preceding projects.

This paper chronicles the full extent of the engineering and operational planning performed to ensure that no uncontrolled hydrocarbon releases occurred during the execution of the Wheatstone Project’s subsea big-bore gas wells and, if a blowout were to occur, that the response to such an unprecedented event would be sufficient and robust. Covered in the paper are (1) reservoir deliverability modeling, (2) dynamic kill modeling, (3) gas plume modeling, (4) relief well trajectory and mooring planning, (5) pilot hole execution planning, (6) a newly applied LWD technology for sensing resistivity vertically below the drill bit and (7) a discussion of future research that has been identified as necessary to better define the fluid injectivity capabilities of subsea relief wells.