The Evolution of UDAR Technologies for Risk Mitigation in Geostopping Applications
Authors: Sinha Supriya , Antonsen Frank , Clegg Nigel , Walmsley Arthur , Vicuña Brígido , Danielsen Berit Ensted , Constable Monica Vik , Prymak-Moyle Marta 
Published: September 2023

Complications during drilling and completion operations caused by subsurface geology have a significant impact on rig time, cost, assets, and even human life, if risk and incident severity is not well understood. Risk and tolerance evaluation processes are essential for completing successful drilling programs and final casing designs. While log-based correlation methodologies can be used, they are limited to scenarios where appropriate offset well data control exists, and they only provide information after the hole has been drilled. The development of technologies that provide warning of a hazardous zone before it is penetrated are therefore desirable. Ultradeep Azimuthal Resistivity (UDAR) tools are deployed for such scenarios and provide high value when used in integrated interpretations to identify hazards ahead of drill bit.

Seismic data is used as a first step to predict and map subsurface characteristics such as pressure regimes, faults, and fluid contacts. Offset and pilot hole data further complements assessment of these features enabling more precise risk assessment. Commonly, near-bit measurements such as resistivity and gamma ray have been used for these correlations in conjunction with sonic and density measurements. The mapping of horizons from seismic data can have 10s to 100s of meters of vertical uncertainty, while offset data in exploration campaigns is typically sparse and near-bit measurements require drilling into the zone of risk. Pilot holes therefore become a costly necessity, however, if sufficient resistivity contrast exists UDAR can be used for remote boundary mapping, without drilling into the geohazards, thus reducing cost and de-risking the operation.

This paper presents several case studies where UDAR technology was deployed in near vertical to horizontal wells to map geohazards before they were penetrated using different techniques, allowing optimization of the stopping point in diverse scenarios. This includes a case where the technology was used to geostop in a horizontal section prior to penetration of a major structural sealing fault plane that bounded the productive reservoir interval. UDAR has been successfully used to manage seismic uncertainty, support the decision-making process for core point selection, reduce exposure of unstable overburden shales and geostop above abnormal and subnormal pressure zones.

Mapping a geohazard and proactively stopping at a particular depth is a complex operation and evaluation of the rock properties with respect to the sensitivity of the measurements and uncertainty in the models is important. Limitations in measurement sensitivity can lead to potential masking of top reservoir picks and increased uncertainty in both boundary positions and the inverted resistivity. Improvements such as new UDAR transmitter designs being embedded into Rotary Steerable Systems allow near-bit placement of this technology, demonstrating the continual evolution of this technology and how it assists risk mitigation in geostopping applications.