TOC Identification Supports Cement Lift Pressure Calculation

Overview

While drilling wells in the relatively young formations of the deepwater Gulf of Mexico (GOM), zonal isolation of the intermediate casing strings can be a major challenge. When mud losses are observed in the weaker formations during cementing, there can be uncertainty as to the placement of cement, with top of cement (TOC) estimated using the cementing lift pressures. Acoustic logging-while-drilling (LWD) tools can be used to provide an assessment of TOC while tripping in hole to drill out cement ahead of the next hole section by analyzing the raw waveforms, which can show the contrast of free-ringing casing and casing that is bonded to the formation.

XBAT™ Service Identifies TOC in Weak Formations, Reducing Well Time

Running, cementing and displacing the plug of a 13 5⁄8-inch intermediate casing job on a deepwater GOM well, the operator observed significant mud losses at each stage of the operation. While the planned lift pressure was 700 psi, the observed final differential pressure of 650 psi put the calculated TOC at X1,430 feet. 

The XBAT azimuthal sonic and ultrasonic LWD service was included in the bottomhole assembly (BHA) and was programmed to perform real-time pore pressure analysis and seismic tie-in. While tripping in hole, waveform data inside the 13 5⁄8-inch casing were recorded and analyzed once the tool was at surface.

Waveforms and Casing Arrival Analysis Confirm TOC

The TOC analysis plot on the next page illustrates the downloaded acoustic data, while tripping in hole shows the raw waveform data during the high-frequency monopole firing for the receivers nearest to the transmitter, and the processed semblances using the waveforms from all of the receivers. A change in the waveforms is clearly seen at approximately X1,420 feet—representing the TOC. Despite the depth resolution being limited by the 12-second sample rate and the fact that regular tripping speeds were used, the TOC analysis from the XBAT service supported the calculations made from the final differential pressure. This confirmation clearly supports the potential value of using the XBAT service in future applications to identify TOC, especially if the tool is programmed specifically for the application, and trip-in or trip-out speeds are selected to optimize depth resolution.