| Popis: |
Drilling wells in pinnacle carbonate formations, which are gas bearing and have karstified zones, is a complex operation due to severe mud losses encountered in these formations. Currently, the safest way to drill such formations is to deploy Pressurized Mud Cap Drilling technique whereby a sacrificial drilling fluid, usually sea water, is used to control the formation fluids from flowing into the well. At the same time, a light annular mud (LAM) is injected into the annulus to prevent any gas from migrating upward in the annulus. Drill cuttings are injected into the loss zone higher up in the well and as a result there are no returns to the surface. In wells drilled using this technique, wireline logging is usually discouraged due to safety reasons. Logging While Drilling is the only method of acquiring well log data. Due to the dynamics involved in PMCD mode of drilling, these wells tend to washout quite often, resulting in LWD logs which are of dubious quality. This is due to the fact that LWD log data is acquired while the drill string is being rotated and cannot be pushed against the formation like wireline logging tools such as the density and neutron porosity tools. This usually results in log data which are heavily affected by the drilling fluid, rendering them questionable for formation evaluation. Therefore, wireline logging has been considered as an option to acquire data which is more reliable than the LWD logs as the wireline logging tools can be pushed against the wellbore wall, providing better contact with the formation. Additionally, there are other logging tools such as formation fluid sampling, formation resistivity imaging and rotary sidewall coring tools which are usually run on wireline to provide complete evaluation of the formation. The main deterrence against wireline logging in PMCD wells is safety. In order to acquire data using wireline logging safely, LAM must be continuously pumped into the well annulus to prevent fluid migration. This means some form of injection method must implemented while wireline logging tools are deployed in the well. Furthermore, as the drill string is no longer present during the wireline logging operations and the well is underbalanced, wellhead pressure control equipment must be deployed to prevent the well from blowing out. This paper discusses the planning, execution of wireline logging operations under PMCD condition and the results obtained in a well drilled in pinnacle carbonate gas field. |
