Borehole Effects from LWD Logs in Large Boreholes - A Case Study in a Well of Assam Oil Field
Prabhakara Rao Jyothula, Sairam Prasad Pasupuleti, and Ramachandra Rao Doddapaneni
WLS, ONGC, Nazira, India
Usually logs are recorded in wells after completing drilling. Thes logs are affected by borehole, invasion, temperature, type of mud and pressure. Even logs recorded using Logging While Drilling is not an exception. The main objective of LWD is to get the real time data almost without borehole effects. LWD logs are recorded while the well is being drilled. Hence they are expected to be less affected by invasion and borehole effects and to some extent by pressure. But when drilling takes inadvertently log time these logs get affected by borehole and invasion.
The main reasons for bore hole effects are 1. Type of formation (eg., shales, unconsolidated sands ), 2. Type of fluid in the borehole and its weight. 3. BHA. Even these affects can be minimized but cannot be altogether avoided if drilling takes long time.
A well drilled in Assam oil field using LWD has shown the effects of borehole and invasion on this type of logs. In the intervals logged while drilling. the logs got effected due to rugoseness created by BHA and type of formation. Since the measuring points are around three stands away from BIT, the measurements took some time after drilling. Time lapse logs recorded in different runs has shown the affects of invasion and rugosity in more pronounced manner. These effects are clearly seen in this case and are discussed in this paper.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery