Managing Circulation Losses in a Harsh Drilling Environment: Conventional Solution vs. CHCD Through a Risk Assessment

Abstract

Abstract 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 18-20% of H2S and 4-6% of CO2. The presence of extensive karst/fractures along with severe circulation losses prevents successful drilling using the conventional circulation technique. For this scenario, the Pressurized Mud Cap Closed Hole Circulation Drilling (CHCD) technique was applied, and proved to be the most effective method to drill through the Lower Carboniferous Carbonate. In conventional drilling through such loss circulation zones, the typical method is to isolate the thief formation (LCM's pills, gunks and cement plugs) in order to maintain an overbalanced condition. These methods have their disadvantages: uncertain results, time and cost increase, damages on well productivity. Furthermore, it may be a long, ineffective process due to multiple leak-off zones or massively fractured zones. Where unsustainable losses occur and conventional circulation is no longer possible, the "CHCD" technique may be used to allow the continuation of drilling. This technique is not under balanced drilling. The annulus is closed and no returns are circulated to the surface. The CHCD is not a simple or an inexpensive system. It is used only when other systems for controlling losses have proven to be ineffective. Large quantities of water and mud are required as well as additional equipment and specialized personnel. An accurate evaluation of several factors has to be performed. Drilling hazard is the major issue as the primary barrier to well influx is jeopardised, but also rig time, material availability and consumption have to be considered. The aforementioned uncertainties can be addressed through risk evaluations by comparing two drilling scenarios of a typical development well where one uses CHCD technology and the other uses conventional drilling techniques. This paper will discuss the operational details of conducting CHCD as well as the risk management approach which involves the identification, evaluation and mapping of all risks involved in each scenario (qualitative risk assessment). A probabilistic model is therefore developed in order to combine all the risks identified and to address their consequences within operational time (quantitative assessment).