Production liner cementing operations are critical due to the small cement slurry volume, the requirement to achieve good mud removal in narrow annuli and, in most cases, the need to pull the running tool out of cement at the end of the operation. The risk is increased when the operations are conducted in deepwater environment, because there is increased complexity in predicting the actual bottomhole circulating temperature due to the long water layer acting as a heat exchanger for the wellbore fluids. In deepwater environment the cement slurry suffers different heating and cooling stages, such as being heated up when batch mixed, cooled down when passing through the riser-seawater section and heated up again when sent downhole. To avoid any problem after retrieving the running tool and landing string, a synergy between the operator and service company brought an improvement to laboratory testing procedures. Non-API laboratory tests for cementing liners were developed in Brazil using temperature simulators reproducing the planned operations and performing sensitivity analysis for additives concentration, fresh water content variation, slurry density variations, contamination effects and bottomhole circulating temperature changes. These practices allow the operator to evaluate cement slurry behavior under a number of conditions that could otherwise cause major job failure or even loss of a well. This document describes in detail the procedures that significantly decrease the risks associated with liner cement jobs and the high costs related to failures in deepwater operations. Since the implementation of these measures, the operator has not experienced any liner cement job failure related to cement slurry design.
Brazilian pre-salt reservoir is mainly composed of carbonate rocks which are naturally fractured and where severe loss of circulation has often occurred. Loss of circulation during drilling or cementing is a serious issue that usually leads to nonproductive time, increase of well construction costs, and impairment of cement sheath quality, which may lead to deficient zonal isolation in the future and compromise well integrity in long term. Data analysis from studies developed by operators around the world show that approximately 12% of all nonproductive time accumulated in a certain period is due to loss of circulation occurrence. In Brazil, an analysis performed by an operator in 2014, pointed that more than 100 days were lost in pre-salt wells in operations to cure or minimize loss of circulation. This same analysis concluded that more than 170 000 barrels of drilling fluid were consumed due to this scenario of losses. It is also important to emphasize the cases where a remedial cementing job was necessary due to failure of zonal isolation achievement through primary cementing, resulting in additional costs and nonproductive time. Said that, it was important to develop and implement a new technology capable of mitigate losses during cementing operations and minimize the risk of a remedial cementing need. Since January 2017, Fibrous Loss Circulation Material has been used in the cementing jobs for production casings in the pre-salt wells successfully. Up to now, six wells were cemented using this technology; no remedial job necessary and the cementing logging evaluation showed excellent results.
To optimize drilling efficiency, some presalt wells in the Santos basin, Brazil, had their project modified to a configuration in which the entire well is drilled, cased, and cemented in only three phases, in contrast to the usual four phases, thus saving approximately 12 to 15% on the total drilling time. The saline and production zones are cased and cemented in the same phase, requiring a cement slurry design that minimizes salt dissolution and, at the same time, has specific properties for production zone isolation. The quality of this cement job is key for the success of the project, given that poor cementation could lead to direct exposure of the wellhead to the reservoir fluids, making the three-phases project unviable due to associated risks. It is important, then, to understand and control all aspects that can influence the cement job success. The presalt reservoir is formed by carbonate rocks that present a variable ratio of gases such as CO2 and H2S. To avoid future risk of gas migration through the cement sheath, the slurry used in the cementing operation must be formulated with gas-migration-control additives to reduce cement matrix permeability. The salt section is composed of layers of different salt types, with the tachyhydrite and carnallite the most soluble. The results of research conducted with salt rock cores from this basin indicated that the best salt concentration added to the cement slurry, to avoid salt dissolution, is either 15 to 20% sodium chloride or 3 to 5% potassium chloride. It is well known that the addition of salt to cement slurry impairs the slurry properties, especially free fluid and compressive strength, two important parameters to determine a successful cement evaluation. Several laboratory tests were performed until the ideal formulation was defined. The spacers were also optimized to promote superior mud removal. Cementing techniques and best practices such as effective casing centralization, mud removal, losses mitigation, and slurry density control are imperative to ensure job objectives are achieved. To the time this paper has been written, four wells had been successfully drilled and cemented using the new configuration. Fluids design and flawless execution of the job provided an excellent cement logging evaluation, proving that it is possible to enhance the well construction to save rig time and, consequently, lower the well costs.
Drilling the Marlim field at the Campos Basin has shown quite a challenge in the last few years. The field is located 110 km from Sao Tome cape at the north coast of Rio de Janeiro State, at the Campos Basin, have started commercial exploration for in 1991 and with water depths varying between 600 to 1000 m. The reservoir is composed of sandstone formation and just recently, an exploratory campaign at the field surroundings. The project of the well in the Marlim field was always complex with eight phases predicted. For the phase 4, due to the narrow fracture and pore pressure window, and also the high number of potential flow zones to be isolated, the client has decided to run a 13 5/8" stage collar in order to isolate all sandstone formations and cement the combined 14" × 13 5/8" Intermediate Casing. The idea of using the stage collar was to isolate the water holder formation, Carapebus Lambrusco sandstone, located just at the stage collar depth and then open the stage collar and cement the remaining sandstone all the way to the last water holder formation, Carapebus Marlim sandstone. The other objective of this job was to provide enough integrity to the shoe, set at the salt formation, in order to allow drilling the subsequent 14 ¾"open hole all the way to the carbonate formation. After drilled the 16 1/2" open hole of an offshore well, the intermediate casing had to be cemented with a two stage collar with sub sea release plug set, to bring the top of cement higher and isolate upper formation. Due to operational issues, there was a failure in the first stage cement job, which was confirmed with a failed formation integrity test - necessary 13.0 lb/gal to drill ahead the subsequent 14 3/4" phase. To solve the issue, to achieve formation integrity test of 13 lb/gal, it was necessary to perform a Casing Shoe Squeeze Cement job. The problem is that historically those intervals experience very narrow gaps, which means low injectivity. After several failed attempts with conventional cement, a novel technology was used combining microcement with a strong fluid loss control that could enable the cement to be injected into narrow gaps. The use of microcement alone provides rapid compressive strength development, very low rheology and combining with a fluid loss additive enables to provide the system with very high injectivity
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