Managing Drilling Losses in the Permian Using Airborne Gravity Full Tensor Gradiometry

Abstract

Shallow drilling losses are a significant problem in the Permian basin because of the presence of subsurface karst features. Karst weakens the soluble rock producing voids and caves systems that result in drilling losses. An operator drilling in Culberson County, Texas recently experienced total losses drilling four surface holes in a pair of neighboring pads located in bordering leases. Drilling into caves negatively affected operations by reducing the drilled footage per day, increasing fluid and cementing costs, and increasing the difficulty in performing satisfactory cementing jobs to cover the water table. This paper will describe the issues faced drilling with losses and explain how to manage the risk of losses by improving surface well placement with airborne gravity full tensor gradiometry (FTG) to map subsurface hazards. Airborne gravity FTG measures the directional components of the gravity field. Multiple simultaneously acquired tensor components allow identification of anomalies associated with subsurface voids. For this project, a Basler BT67 aircraft acquired data over the targeted expanse with line spacing of 328 ft. The aerial survey took place over 3 days in July 2017. Feasibility modeling using Castile formation cave systems reveals detectability of single caves larger than 10 m diameter with FTG, however networks of smaller caves are also detectable. Polygons created from analysis of negative vertical gravity tensor (Tzz) anomalies separate the cave systems into tiered risk areas. Initial analysis reveals risk at both pads where losses occurred. Extending the analysis to the entire survey, the drilling events in the drilled offset wells match with the risk interpreted for karst. FTG data and subsequent interpretation offer strong correlation to known shallow hazards and cave systems, making it a useful tool for risk assessment. It recommended to locate future drill pads in the identified moderate risk areas and that any new wells be located away from elevated risk areas. This is the first application of FTG to classify drilling risk of karst features in the Permian basin. The FTG hazard map improves operational integrity of surface location selection and is a complement to surface topography and geology considerations. The FTG data and analysis also holds promise for fault mapping and for water drilling efforts.

Paleokarst features in the Aptian carbonates of the Barra Velha Formation, Santos Basin, Brazil

“…However, there is a need for careful considerations when dealing with these karst and fracture zones, as they may pose significant hazards to well development in situations of loss of mud, risking failure of well completions which could lead to serious complications or even total loss of the assets. Asides drilling complications, karst and fracture zones typically called thief zones are difficult to map on seismic due to their sub-seismic scale in most situations and issues associated with their capture on NMR measurements (Kumar et al 2012, Mahry et al 2016, Sallee et al 2019, Maksimov et al 2021.…”

Prediction and Early Detection of Karsts—An Overview of Methods and Technologies for Safer Drilling in Carbonates

“…Based on more than eleven years of drilling experience in carbonate structures offshore Sarawak, Malaysia, total losses were encountered in more than 30% of the wells drilled [5]. In Culberson County, Texas, USA, in each case of drilling in the intervals of cave systems, losses progressed into total losses following well control incidents [6]. Drilling in naturally karstified carbonates of the Continental Shelf in Barents Sea, Norway, resulted in total losses causing weeks of non-productive time, shoot off and lost in the bottom hole assembly.…”