Ensuring High-Quality LWD Sonic Data to Emplace Radioactive Compaction Monitoring Markers in an UHPHT Setting

Abstract

The Culzean field is an ultra-High Pressure, High Temperature (uHPHT) gas-condensate accumulation and is one of the largest development projects of recent years in the UKCS. From experience in analogue fields, compaction is likely to occur and negatively impact the deliverability of wells. With accurate geomechanical modelling such issues can be assessed and proactively mitigated.

Measurement of changes in distance between radioactive markers is a recognised technique for compaction monitoring. The measurement is undertaken throughout the life of the well and is used in the calibration of field geomechanical models. Markers were emplaced in two wells in Culzean. To ensure safe and successful emplacement of the markers a real-time estimation of rock strength was undertaken using logging-while-drilling (LWD) sonic data. Key to success was obtaining the correct data for timely decisions on the depth of radioactive markers.

The paper describes the process used to validate sonic data from an LWD sonic tool by comparison with a wireline acquisition and its later application in two additional wells where markers will be emplaced. A unipolar LWD sensor was included in the drilling assembly to transmit semblance image in real time, which was manually picked on surface for compressional and shear slowness. Rather than automatic downhole picking, this approach allowed immediate offset wells comparison, improved reliability and quality checking which increased confidence in the final product. In the initial well, the resulting logs were compared with data acquired from a wireline dipole sonic tool showing a very good match and proving "equivalent-to-wireline" quality of the measurement.

In two subsequent wells once total depth was reached, raw memory and diagnostic data were downloaded from the tool for functional and quality checks and processed to generate semblance images. After undergoing quality check, evaluation and post processing, when needed for improvement, the acoustic data was used to calculate Unconfined Compressive Strength (UCS) estimates. This then guided the selection of depths for the radioactive markers.

The use of a LWD sonic tool, providing reliable acoustic data to generate a UCS, allows a quick turnaround for mobilisation of equipment (i.e. markers) and enabled the operator to acquire data in an uHPHT environment.