Managing Circulation Losses in a Harsh Drilling Environment
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 128225, ’Managing Circulation Losses in a Harsh Drilling Environment: Conventional Solution vs. CHCD Through a Risk Assessment,’ by S. Masi, C. Molaschi, SPE, and F. Zausa, Eni E&P, and, J. Michelez, SPE, Altran, originally prepared for the 2010 IADC/SPE Drilling Conference and Exhibition, New Orleans, 2-4 February. The paper has not been peer reviewed.
Conventional drilling techniques used in harsh drilling environments are sometimes impractical or uneconomical. This was experienced while drilling through the reservoir section being investigated. This section is characterized by high pressure and formation fluids estimated to contain 18 to 20% hydrogen sulfide (H2S) and 4 to 6% carbon dioxide (CO2). The presence of extensive karst/fractures along with severe circulation losses prevents successful drilling using conventional circulation techniques. For this scenario, the pressurized mud cap closed-hole-circulation drilling (CHCD) technique was used, and proved to be the most effective method to drill through the Lower Carboniferous carbonate.
Introduction
Drilling the reservoir section is the most challenging part of the drilling activity that is being carried out at the oil field under investigation. The reservoir is a carbonate platform, and it is well known that carbonate formations (limestone/dolomite) are characterized by the presence of fractures and/or large vugs, and consequently circulation losses frequently occur while drilling. Sometimes losses are minimal, but at other times they are sufficiently large to become total losses. This situation has been experienced often in this reservoir. In fact, in certain areas, unsustainable losses occur and conventional circulation is no longer possible. Therefore, one of the main concerns is the magnitude of losses that could be faced while drilling the reservoir zone.
It must be emphasized that when using conventional drilling techniques through lost circulation zones, the standard practice is to minimize losses by attempting to seal off the thief zone before continuing drilling. This usually is done by pumping lost-circulation-materials (LCMs) pills and setting cement plugs so that the mud weight will overbalance formation pressure at all times. However, this is a long and often unsuccessful process because multiple leakoff zones may be encountered in sequence or because massively fractured zones cannot be cured. Furthermore, these methods have several disadvantages: they offer uncertain results, they are time consuming and therefore expensive, and in hydrocarbon reservoirs, they may reduce well productivity. In this scenario, the CHCD technique allows drilling in circulation-loss zones by controlling the annulus pressure and injecting a sacrificial fluid into the thief formations, thereby minimizing the nonproductive time and costs typically required to implement the traditional methods of circulation-loss control. For this field, CHCD was determined to be the most effective method to drill through the carbonate reservoir.