Dynamic Freeze Plug Process Proves a Viable Barrier Technique in Sajaa Field, United Arab Emirates
Authors: Powers Brian S. , Pruitt Randal D. 
Published: December 2007

Overview

A Dynamic Freeze Plug Process (DFPP, U.S Patent 5,649,594 Flak, L.H) was applied to several packerless Sajaa Field wells to achieve a second mechanical annular pressure barrier prior to removing trees for hydraulic, live well work over operations. Four wells were successfully frozen with minimal fluid lost to the formation. Several equipment and procedural modifications made after the first job improved subsequent jobs and reduced the time required to complete the freeze and achieve barrier. The DFPP is a proven technique to achieve an annulus barrier in a gas filled environment.

Background and Technical Need

BP's Drilling and Well Operations Policy requires two independent pressure barriers before any pressure envelope is broken for well interventions. The barriers must be capable of operating independently and simultaneously and must be pressure tested from the direction of flow. Dual tubing barriers are easily achievable with tubing plugs, tubing hanger backpressure valves, or a combination of the tow. The packerless wells in Sajaa Field only have one permanent annular barrier, the metal-to- metal tubing hanger seal, and during normal well operations the annulus is pressured at the surface. Thus, achieving additional annular barriers is more challenging, Sajaa well completions consist of tubing strings landed above the perforations or horizontal lateral zones, or tubing strings set in permanent packers to separate multi-lateral zones (Figures 1a. and 1b.)

Several service company barrier proposals involving various kill fluids an/or inflatable tools deployed on coilded tubing were reviewed. Due to the depleted state of Sajaa's fractured carbonate reservoir, kill fluids of any type were undesirable due to the risk of permanent and irreversible formation damage. Inflatable tools alone were thought to be too unreliable to depend upon as a second barrier, especially in the multi-lateral wells where an inflatable would have to be set in open hole. Additionally, inflatable would have required kill fluid to be placed on top of them to balance the formation pressure. This kill fluid would be difficult to remove, and some would likely be lost to the formation upon removal of the tools. If lost in the hole, the inflatable themselves might have caused irreparable lost production. Freezing the spool was discussed but thought to be impossible because the wells would not hold static fluid column to surface1–2.

Boots and Coots International Well control proposed the DFPP as a potential solution. They developed the technique for a South American client in 1995 and patented the process, but prior to the Sajaa work the process had never been performed in the field. The process seemed ideal for this application: creation of an second independent annular barrier capable of being tested from the direction of flow, applied near the surface of the well, with minimal fluid loss to the formation.

Equipment

Figure 2 shows the required surface equipment layout. The surface equipment consisted of a fresh water tank and filter. Downstream of the filter the stream was split to enable pre-chilled water to be pumped into both sides of the "D" section spool simultaneously. Each chilled water set up consisted of an air actuated chemical pump, 55 gallon drum containing coiled stainless steel control line, a pressure and valve manifold, insulated control line, and a weco flange pack off. The 55-gallon drum was filled with drinking water ice to pre-chill the water to near freezing before pumping into the casing valves. The control line was spaced so that the end was just outside the gate of inboard 2–1/16" valve.

The control line was cut the dull tubing cutter to cause the water to spray as it entered the casing.

The freeze bucket formed a fluid fit around the spool to be frozen, in the case of Sajaa wells, the "C" section (spools are lettered from the bottom up, i.e., "A" section for 20" casing, "B" for 13–3/8", etc). The bucket held dry ice and methanol, and two sizes were constructed: a round one of the 9 5/8" × 5" annuli for horizontal wells with no "C" section valves, and a rectangular one for the 7" × 5" annuli in vertical wells with two valves on each side of the "C" spool.