Summary Natural or induced fractures in a chalk reservoir can reduce the recovery of an oil field significantly. Therefore, the plugging of fractures with a wide range of materials has been investigated over the years. Calcium carbonate is an obvious candidate, being the main constituent of the reservoir itself. However, to apply calcium carbonate as a plugging fluid, a mechanism is required for delaying the precipitation until the chemical reaches the fracture. An enzymatically induced plugging mechanism has been suggested, in which the urease enzyme converts urea into ammonia and carbonate. This carbonate will then precipitate with calcium as calcium carbonate. However, the amount of calcium carbonate produced was relatively low and the cost of the stabilizer and high-purity-enzyme source was prohibitively high for practical use. Furthermore, the calcium carbonate precipitated as a slurry of small particles, which is deemed less efficient for fracture plugging when compared to larger crystals or aggregates. In this paper, work is presented on design of an improved plugging fluid based on enzymatic calcium-carbonate precipitation and optimization toward a field-applicable solution. The relatively expensive stabilizer and enzyme source are replaced with low-cost ingredients, and the rate of precipitation is improved. By optimizing the concentrations of the reactants, we have improved the yield of calcium carbonate from 20 to more than 200 g/L. Furthermore, the crystallization can be controlled to obtain much larger calcium carbonate crystals. Laboratory plugging experiments have shown that larger crystal sizes improve the durability of the formed plugs significantly. Introduction Different authors have proposed the use of active, urease-producing bacteria for precipitation of calcium carbonate (Ferris et al. 1996; Stocks-Fischer et al. 1999). The concept of using the urease enzyme directly without in-situ microbiological production has been proposed by Nemati and Voordouw (2003), who demonstrated delayed precipitation and plugging of packed limestone columns. The results presented in this paper illustrate how the reaction rate and reaction yield depend on the reactants. Furthermore, we present improvements to the calcium carbonate crystal size and plugging performance by stoichiometric variations and addition of various chemicals. The perspectives for field trial are discussed in the penultimate section of the paper.

Culture-based methods of traditional microbiology applied to the microbiological processes involved in souring of oil fields and microbiologically influenced corrosion (MIC) pose a threat of yielding inadequate and contradictory results. Any cultivation step will almost certainly alter the population characteristics and thus also the results on which any evaluation will be based. The need for in situ cultivation-independent methods has over the past ten years facilitated the development of several analytical methods for determination of bacterial identity, quantity, and to some extent function, applied directly to samples of the native population. This development has so far been fairly limited regarding practical application and it has only recently been transferred to the offshore industry. In this paper, we demonstrate the features and benefits of applying these novel techniques to two situations often encountered in offshore oil production in the North Sea. The new microbiology tools are based on the detection of genetic material in bacteria. The methods include direct count of specific groups of bacteria with microscopy (e.g. FISH). Additional methods (e.g. qPCR and DGGE) are based on direct extraction of cell genetic material (DNA/RNA). The paper will briefly describe these novel molecular techniques. By documenting bacterial population shifts related to water breakthrough in a nitrate treated reservoir, we showed that key bacterial populations can be identified and thereby lead to the creation of new and strengthened surveillance strategies on souring bacteria in these systems. Also, we have shown that by applying these novel techniques to aggressive corrosion attacks, especially under deposit corrosion, molecular techniques are a powerful tool in identifying the most probable corrosion process. These examples will be described and related to the offshore operation. Special focus will be given to the use of the new and improved microbiological data in relation to designing and testing remedial actions towards oil field souring and MIC. Introduction Traditional vs. New Methods While great advances have been made in media and cultivation/enumeration techniques, it is now generally accepted that only 1–10% of the viable bacteria are culturable by these classical microbiology methods (ref.1). Industries that have previously used cultivation-based methods for microbiological surveillance are therefore looking for methods that include a larger fraction, or better yet, the entire population of troublesome bacteria. The solution is within reach due to the decades of research within molecular biology and microbial ecology (ref. 2). Several methods enable the measurement of entire populations of bacteria without the limitations of cultivation. In addition, cultivation-independent techniques can be applied much faster (within a few hours to a few days) compared to traditional cultivation based techniques (with up to 30 days cultivation), resulting in a potentially faster response time to e.g. maltreatments (ref. 3). Cultivation-Independent Techniques Cultivation-independent techniques are used for bacterial identification and quantification and for determination of bacterial functionality. Fig. 1 outlines the various techniques used for obtaining this information.

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fax 01-972-952-9435. AbstractNatural or induced fractures in a chalk reservoir can significantly reduce the recovery from a field. Therefore, plugging of fractures with a wide range of materials has been investigated over the years. Calcium carbonate, being the main constituent of the reservoir itself, is an obvious candidate. In order to apply calcium carbonate as a plugging fluid, a mechanism for delaying the precipitation has previously been suggested in the literature. This mechanism involves enzymatically induced plugging based on the mechanism of the enzyme urease converting urea into ammonia and carbonate. When carbonate is formed, it will precipitate with calcium in solution forming calcium carbonate. However, the amount of produced calcium carbonate was relatively low and the costs of the used stabilizer and high purity enzyme source was prohibitively high for bulk use. Furthermore, the calcium carbonate was produced as a slurry of small particles, which is deemed less efficient for fracture plugging compared to larger crystals or aggregates.In this paper work on design of an improved plugging fluid based on enzymatic calcium carbonate precipitation and optimization towards a field applicable solution is presented. The relatively expensive stabilizer and enzyme source is replaced with a low cost ingredient, which also improves the rate of precipitation. By optimizing the concentrations of the reactants, we have improved the yield of calcium carbonate from the initial 20 g/L to more than 200 g/L. Furthermore, we have been able to control the crystallization and obtain much larger calcium carbonate crystals. Laboratory plugging experiments have shown that larger crystal sizes significantly improves the durability of the formed plugs.

In the 1990s, the wells drilled in the Dan field, located in the Danish sector of the North Sea, were primarily completed with sand fracture treatments separated with packers and pipe, which include sliding sleeves [Ref 1]. The zones were stimulated with normal 20/40 mesh sand with a tail of resin coated sand to keep the sand in the fracture.As water injection was introduced in the Dan field, some water induced fracturing of the chalk reservoir occurred. In the production well DFE-05, a fracture created by a water injector hit the sand fractured zone in the well. Most likely causing damage to the resin coated seal in the zone and loose sand formed erosion holes in the tubing. Maersk Oil investigated the possible options to isolate the damaged zone without restricting the access to the well below the damaged zone. The investigation revealed that one option was to pump an environmental friendly colloidal silica gel into the zone to lock the sand in place.Several onshore tests with frac sand and colloidal silica gel showed that it was possible to displace the uncured gel into a sand/water mixture and get the gel to set up. The resulting leak rate through a gel/sand plug at high differential pressure was low, and the sand was successfully held in place by the gel.During the course of the intervention job the primary target zone of the gel was treated with good results. The neighbouring zone above the treated zone was also found to be producing sand. This zone was therefore treated as well. After the gel had cured, the inflatable bridge plugs used to place the gel correctly were removed and the well cleaned up and production restored from the remaining zones.Six months after the treatment, the well is still producing at a stable rate with only minor traces of sand. The successful job has created valuable learning about the colloidal gel technology and the way it can work in sand fractures.