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.

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.

Microbial activity is the cause of a variety of problems in water injection systems, e.g., microbial corrosion, plugging, and biofouling. Efficient monitoring of Saudi Aramco's vast water injection system requires the development of online and automated technologies for monitoring microbial activities in the system. A previous system review and technology screening has identified five single-analyte strategies [1], which were evaluated in this study with a laboratory-scale setup to determine their applicability for automated determination of microbial activity in the injection water system. Four of the five single-analyte measuring principles tested in the laboratory setup were deemed less suitable for automation and/or reliable for use in the detection of microbial activity in the company injection water system. These four principles were: luminescence assay for adenosine-5'-triphosphate (ATP), detection and electrochemical measurements of H2S, determination of pH by electrochemical sensor, and measurement of oxidation-reduction potential (ORP). The strategy of staining cells with fluorescent DNA dyes, followed by quantification of fluorescence signals, was identified to hold, with proper optimization of DNA staining and fluorescence detection, a very promising potential for integration in automated, online sensors for microbial activity in the injection water system.

Objectives/Scope Microbial growth in topsides facilities (water injection, oil production and distribution systems) is a widely recognized phenomenon leading to a range of impacts, for instance microbiologically influenced corrosion (MIC). MIC typically occurs as localized pitting and can develop rapidly (and measured in mm per year), leading to unexpected production shutdowns, environmental impact due to leaks, major unplanned repairs and increased chemical treatment costs. In severe MIC cases, the system may even need to be replaced, sometimes with corrosion resistant alloys, with marked economic consequences to operators. The threat of MIC has traditionally been very difficult to assess due to its rapid, localized, nature and due to the challenges of getting reliable information about microbial communities from system samples. Methods Recent advances in molecular microbiology technologies, particularly with respect to next generation sequencing technologies and quantitative PCR assays targeting functional and phylogenetic marker genes, have now made it possible to reliably identify and quantify a range of oilfield MIC-related microorganisms. Results and Conclusions Through case studies, this paper demonstrates how these molecular microbiology technologies can be used for monitoring, diagnosing and managing MIC on a routine basis focusing on key MIC indicator organisms and applying new approaches to interpret the MIC threat from the derived data. This paper furthermore suggests how MIC assessments can be integrated into existing corrosion management programs to target and tailor mitigation actions, minimizing the overall risk related to MIC. Novel/Additive Information These new advanced molecular microbiology tools, if properly integrated in corrosion management programs, hold potential for improving asset protection and cost savings for oilfield operators.

Induced or natural fractures in waterflooded reservoirs can have a negative impact on oil recovery. Direct connections between injectors and producers allow otherwise recoverable oil to be bypassed by the injected water, reducing the sweep efficiency and the pressure support to the reservoir. Knowledge about the number of connections, their location and size is essential to properly design a reliable conformance treatment. The Danish Technological Institute has together with Maersk Oil developed a deuterium based tracer technology which can provide information about high conductivity fractures in tight reservoirs. The method has been proven on several studies in the North Sea and allows quick and direct analysis offshore. Immediate actions based on real time results offshore can be taken and minimum response time is needed for planning further operations. The tracer used is deuterium oxide which is safe to handle and brings no environmental issues, as it is already naturally present in water. It is completely miscible with water and does not dissolve in the oil phase. The returns are analyzed directly from the produced water stream after separation using a mass spectrometer. This portable equipment allows a quick and reliable analysis with minimal sample preparation. The concentration of tracer is analyzed to give information such has breakthrough time, concentration profile and volume of tracer returned. This data is then used to determine the number of fractures, their conductivity and their relative position in the wellbore using an injector-fracture-producer model.