Quantitative Approach to Mitigate Drilling Risk and Uncertainties Using Look-Ahead-Of-The-Bit Technology
Authors: Elshurafa B. , Leasu I. O. , Dolan J. , Santoso G. I. 
Published: November 2024

Exploration wells come with many drilling risks that must be managed. In this paper we investigate three wells with different challenges that used a logging-while-drilling (LWD) look-ahead-of-the-bit resistivity service to mitigate drilling risks in real time. The look-ahead measurements enabled continuous mapping of the formations ahead of the bit and to successfully geostop.

Without nearby offset data, the traditional approach of using well correlation and projecting ahead of the bit was not sufficient to reduce the positional uncertainty. A new approach was needed to mitigate the risks and manage uncertainties, and the LWD look-ahead while drilling service was selected. The service uses electromagnetic (EM) propagation measurements from a multifrequency transmitter coupled with several receiver subs spaced through the bottomhole assembly (BHA) to provide a deep directional resistivity measurement ahead of the bit. The EM signals are transmitted into the formation and measurements are compared to prejob modeling to resolve the formation layers ahead of the bit in real time. This method uses a real-time stochastic inversion that provides the estimation of uncertainties in identifying geological markers and identifying hazardous zones before penetration.

Three vertical wells were executed successfully with the look-ahead service. The first well had limited offset well data that showed lateral heterogeneity at the target layer. In previous wells, drilling into the target layer resulted in total losses in a depleted, fractured zone. The look-ahead service was able to detect the target layer up to 50 ft true vertical depth (TVD) before penetration and the decision to geostop made at 20 ft TVD above the target. As this was a trial well for the service, it was decided to drill into the target and confirm the inversion results.

The challenge in the second well was to map the formation layers ahead of the bit to reduce the uncertainties and to geostop 10 ft TVD above the target layer. Again, the sparse offset well data available showed heterogeneity across the target layer. While drilling the section, the geological markers were up to 270 ft TVD shallower than expected. The real-time look-ahead inversions mapped the target layer from 100 ft TVD above and the decision to geostop was made at 10 ft TVD above.

The real-time objective in the third well was to optimize casing point selection. The environment was challenging for the stochastic inversions due to several thick conductive carbonate layers alternating with highly resistive layers. While drilling, the geological markers were up to 70 ft TVD shallower than prognosis and the look-ahead service was able to detect these markers before entry. Even with these additional challenges, the look-ahead service picked the casing point 30 ft TVD below the bit.

The LWD look-ahead service has provided the means to reduce drilling risks and mitigate uncertainty. The ability to pick the geological markers ahead of the bit helped to optimize the drilling practices, pick casing points, and detect and successfully geostop prior to penetration of hazardous zones. The service has allowed for better-informed proactive decisions in real time while drilling.