Evolution of a Damaged Zone Caused by Water-Based Polymeric Drill-In Fluid

Abstract

Open hole completion is, nowadays, the most used completion technique in horizontal wells. Many times, for operational reasons, such completions can take many days, sometimes months, to be executed. This problem leads to a questioning about what happens to the invaded area during such exposition to the drilling fluid. To answer such question, an experimental study of invaded area aging was performed. Such procedure consisted in real-time CT-scan measurement of the invaded area by the water based polymeric drill-in fluid filtrate. The formulation used is very common at the Campos Basin, Brazil. The tested samples were submitted to in situ conditions (pressure and temperature), during all the experiment time, which took about one year. It was verified that the invaded filtrate area for this drill-in formulation increased about 40% in the first 34 hours, maintaining this level during the rest of the experiment. Besides, analysis done with a Scanning Electronic Microscope (SEM) in one of the tested samples, showed that the solids and salts from the drilling fluid were only present in the filtrate invaded area. Introduction Since the last decade, a considerably increase in horizontal well demand has happened. It is due to the advantages brought by this kind of well, for reservoir drainage and productivity. Also, the completion technique most used for such wells is the open-hole one, for operational and financial reasons. The increase of open-hole completions leaded to the development of new specific fluid systems for drilling horizontal wells named drill-in fluids (DIF). These fluid systems present behaviour of both drilling and completion fluids. Some characteristics of such fluids are1:they are specially designed for drilling through reservoir zones;Its additives lead to formation damage minimization, increasing well productivity;the mud cake provides an efficient control of the invaded area during drilling;they are biodegradable and present low toxicity. Nowadays, the water based polymeric drill-in systems are the most used. Their formulation is based on xantan gum (viscosifier), polymer derived form starch (filtrate controller), magnesium oxide (alkalinizer), KCl or NaCl (clay inhibitor) and the bridging agent, that can be NaCl or CaCO3. Many times, after drilling the well, it takes a long time to make its completion. This leads to a questioning about the real conditions of the invaded area by the DIF filtrate: what happens when it takes a long time from the end of drilling and the beginning of the open-hole completion operation? To answer such question, the Petrobras' Research Center (CENPES) performed a CT scan measurement in Berea sandstone samples damaged by a water-based polymeric fluid. The damage was created by one-year exposition of the samples to the DIF used. This work presents and discusses the main results obtained by the experiments. Fluids and core A water-based polymeric formulation, often used by Petrobras E and P during horizontal drilling in Campos Basin, was used in the aging experiments. Tables 1 and 2 show the formulation of the fluid and its properties. Two Berea sandstone samples were used in the tests (Table 3). The samples were initially saturated with synthetic water (30,000 ppm NaCl brine). After that, the samples were saturated by oil, to simulate reservoir conditions (Table 4).

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