Lessons Learned in Drilling Pre-Salt Wells With Water Based Muds
Authors: Lomba Rosana Fatima Teixeira , Pessanha Rubens Ribeiro , Cardoso Jr Walter Francisco , Lomba Bruno , Folsta Mauricio , Goncalves Jose Thomaz , Teixeira Gleber T. 
Published: June 2014

Abstract
Drilling operations in salt zones have gained importance in Brazil due to the discovery of large oil and gas reserves in the Pre-Salt area. Thus, the pursuit of excellence in such operations is requiring considerable development of new operational practices and technologies.
Synthetic base mud has been the first choice to drill through evaporite sections in Pre-Salt zones. Synthetic fluids application practically eliminates salt dissolution and improves caliper quality. However, depending on the salt mobility rate, stuck bit and/or stuck pipe can happen, demanding the injection of fresh water pills for its liberation. In some wells, the frequent use of those pills resulted in enlarged sections. The situation gets worse in scenarios where loss of circulation is a major issue. On the other hand, the use of non-saturated aqueous fluids to drill soluble salts (e.g. halite, tachyhydrite and carnalite) can result in localized enlargements due to leaching process. Drilling a gauge hole is a very important issue to prevent wellbore collapse and/or problems in cementing operations. However, high operating costs associated to deep water drilling is placing additional emphasis on drilling performance in order to reduce the operational time, without losing the quality of the wells. This concern raises the issues of how to most effectively improve operational performance regarding the drilling fluid selection.
The paper presents the results of the use of water based muds in the drilling of two offshore wells in Brazilian Pre-Salt area and highlights the lessons learned from the experience. Before the field application extensive lab tests and numerical simulations were carried out to support the drilling fluid design for the wells. In the first well, differences between the planned and encountered drilling conditions led to fluid replacement during the operation. On the other hand, in the second well, 2.000 m salt extension was successfully drilled to depth with no major operational problems and good drilling performance. The field application of WBM proved to be a good option to drill the salt layers in the development of Pre-Salt fields but it demands a more detailed knowledge of both lithology and stratigraphy of the evaporite section.

Introduction
The drilling fluid design for salt zones uses to focus on drilling a gauge hole to prevent wellbore collapse. Due to solubility issues, synthetic based emulsions have been selected as the first option to drill through evaporite layers in the Pre-Salt scenario. However, high operational costs associated to deep water drilling have brought special attention to drilling rates in order to reduce operational time without loss of well quality. In offshore drilling operations, the use of synthetic based drilling fluids demands the use of centrifuges to clean the cuttings before they are discharged into the sea. This procedure results in strict control of penetration rates due to cuttings processing issues. Moreover, in scenarios where lost circulation is a major issue the use of synthetic base muds may not be the best choice. Therefore, the challenge is to find the best way of effectively improving operational performance through an adequate drilling fluid selection. And it raises the question: should water base mud (WBM) or synthetic base mud (SBM) be used?
In Brazilian Pre-Salt scenario, the most common salts encountered are halite (NaCl), carnalite (KCl. MgCl2.6H2O), tachyhydrite (CaCl2.2MgCl2.12H2O) and anhydrite (CaSO4). The deposition sequence normally is the opposite of the salt solubility in water which, for these salts, follows the same sequence of easiness of reaction with water: Tachyhydrite &gt; Carnalite &gt; Halite &gt; Anhydrite. So, they might dictate some formulation choices to avoid dissolutions. In that aspect, the synthetic fluid is preferred over water based muds, because it neither has free water nor interferes in the crossed salts solubility. The well caliper tends to be in gauge when compared with the saturated water base fluid.