No abstract

Unconventional shale gas exploitation presents complex problems in terms of radioactive waste disposal. Large volumes of saline produced water resulting from hydraulic fracturing are typically enriched in radium isotopes, up to several hundred Bq/dm3, orders of magnitude above national discharge limits. There is a need, therefore, to decontaminate the fluid prior to discharge, preferably by creating a less problematic radium-containing, solid waste form. Barite (barium sulphate) co-precipitation is a cost-effective method for achieving these objectives, provided the process can be controlled. In this work, radium recovery of ~90% has been achieved for simulant produced waters containing 100 Bq/dm3, using a single, optimised co-precipitation step. However, salinity has a significant effect on the efficiency of the process; higher salinity solutions requiring substantially more reagent to achieve the same recovery. If >90% radium removal is sought, multiple co-precipitation steps provide a much faster alternative than post-precipitation recrystallization of the barite solid phase, albeit at higher cost. The resulting solid waste has a relatively high specific radium activity but a much smaller volume, which presents a less intractable disposal problem for site operators than large volumes of radium-contaminated fluid.

The yet to be developed Liberty field was discovered in the 1980s, and then confirmed with the Liberty No. 1 well in 1997 by BP Exploration Alaska, Inc. (BPXA). The Liberty reservoir lies within the Outer Continental Shelf (OCS) in approximately 20′ of water, in Foggy Island Bay, about 20 miles ESE of Prudhoe Bay field. The reservoir is estimated to contain between 80 and 140 MMBO of reserves. In 2014, Hilcorp Alaska, LLC (Hilcorp) purchased 50% ownership and assumed operatorship from BPXA, and submitted the Development and Production Plan (DPP) to the Bureau of Ocean Energy Management (BOEM). Since then, BOEM as the lead agency, has been conducting a National Environmental Protection Act (NEPA) review which is expected to deliver the Final Environmental Impact Statement (EIS) in mid-2018. Concurrent with the NEPA review, Hilcorp submitted the Oil Spill Response Plan (OSRP) to BSEE in March 2017. A unique feature of this development in OCS waters is that wells will be drilled from an artificial gravel island using a land-based rig. One of the key elements of the OSRP is the utilization of well ignition as an early step in a well capping operation which subsequently minimizes the volume of oil that hits the ocean or ice if a well were to blowout. As prescribed in regulations, the operator must demonstrate it has a plan and resources to remediate a spill of the worst case discharge (WCD) rate. Because the Kekiktuk formation at Liberty has very high (>1 darcy) permeability, the initial WCD rate of a blowout from the full exposure to the reservoir and up 9-5/8″ casing is approximately 91,000 bopd and 84 MMscfpd. To prove the WCD rate would remain ignited, and to calculate the percent of liquid that would combust, first BPXA then Hilcorp, hired two companies, Boots & Coots Inc. with Intuitive Machines, LLC. to: research the robustness of combustion models; develop models that would determine if the blowout would remain ignited; and, to calculate the liquid carryover that would have to be recovered with spill response equipment. The companies created several models that resolved these issues, including: 1) burn efficiency, defined as the ratio of oil combusted to total oil exiting wellbore, 2) blowout flame radiant heat flux, 3) mass flux and heat content of unburned residual, and, 4) spatial distribution of unburned residual. The work was based on four broad areas: 1) research of governing physics, 2) evaluation of academic and laboratory scale measurements, 3) evaluation of a blowout database, and, 4) numerical modeling utilizing both computational fluid dynamics and first principles engineering modeling. The numerical model validated the presence of complex supersonic shock structures and calibration data for the development of the engineering burn efficiency methodology. Research results were integrated with extensive field-based blowout experience and validated numerical modeling results. Parametric analysis of key driving variables provide measures of burn efficiency margin and robustness required for approval of the OSRP. This paper presents the methodology and engineering-based solution that integrates oil blowout fire experience with aerospace engineering expertise to adequately and accurately predict burn efficiency of high rate blowouts. This methodology allows the operator to justify the use of well ignition as a step in well capping and as a means of source control, on a new field development from an artificial gravel island in the Beaufort Sea, in Arctic OCS waters.