Formation sampling with extreme overbalance in the Gulf of Mexico

The Gulf of Mexico is known for some of the toughest drilling and wireline logging conditions in the world. An operator drilled a challenging 8.5” deviated well to 33,000 ft in a field where virgin pressures in adjacent zones can differ by over 5000psi. The first wireline run in hole was an oil based mud imaging tool (without Petromac Taxis) which became stuck across one of the problematic low pressure sands and had to be fished. As a consequence the operator insisted on using Petromac for all remaining runs. When the imaging tool was re-run it recorded outstanding data with no sticking, overpulls or spiraling – the smooth tool motion resulted in clear, crisp images. On the same job, Wireline Express successfully conveyed a 7500lbs reservoir sampling string with the probe oriented to the high side of the wellbore – enabling fast build ups and sample clean ups, with one sampling station setting an industry record of 12,000 psi ΔP (flowing). After the final reservoir sample was taken from this problematic low pressure sand, the sampling string became temporarily stuck. However because the tool was oriented probe-up the toolstring weight ‘peeled’ the probe from the high side of the wellbore and the tool was immediately able to move up and down freely. The final run was a sidewall coring tool. An 85% recovery rate was achieved despite the high overbalance (1000-5000 psi) – another industry record. Tool Taxis prevent differential sticking for imaging, sampling and coring tools in a 5000psi overbalanced well Fig1. Formation Testing Toolstring on Taxis Further details of this operation can be found in SPE-201218-MS