A new computer based cementing simulator is described that quantifies the effect of key displacement variables such a s geometry, standoff, fluid rheology and density, and flow rate. The paper shows how to optimise cement placement within the fracture and pore pressure constraints of the well and demonstrates its use through case studies from the North Sea. The technique assesses the quality of mud removal and cement placement as well a s estimating the circulating pressures throughout the job. The examples illustrate how existing rules of thumb can be confusing and how reliable cement jobs can be achieved through quantitative design procedures.
fax 01-972-952-9435. AbstractFrequently, poor cement logs are observed in low gamma ray formations (sand). Contrarily, very good bond is often noted across high gamma ray formations (shales). In some cases this has resulted in a squeeze decision where no injectivity can be obtained incurring significant costs and delayed oil. Most commonly the problem has been ascribed to an artifact of the logs, presence of mud cake, cement shrinkage and squeezing shales.However after studying many such logs the first observation is the sharp contrast between the logs
Over the course of a fo4ur-year period, during which approximately 42 wells were constructed by this operator, processes were developed to improve cement slurry design and reduce nonproductive time (NPT) during cementing operations on deepwater wells. Cementing best practices have been followed for many years; however, not all of these practices are applicable to deepwater operations, especially for the riserless sections. Results of developing (1) slurry design practices, (2) temperature prediction and mud removal modeling, and (3) the use of SSR® subsurface release plugs will be reviewed. The choice of cement blend used can have a profound impact on cost, not only in terms of failure in the event of the wrong design, but also in terms of supply logistics. As deepwater operations move farther away from shore bases, the need for a systems approach to cover all cement design needs from conductor to plugging should be considered. The challenges across this breadth of application range from dealing with shallow water flow potentials to relatively high temperature and pressure. Both conventional and foamed cement slurries can be successfully used in this endeavor when properly designed and placed. Placement design should include a thorough understanding of the static gel development behavior of the drilling fluid. Although it is generally accepted that conventional circulating temperature estimates are not appropriate for use in deep water, the use of alternatives is not always practiced. Over the course of this project, this operator realized better cement slurry performance and lower slurry costs following the correct use of wellbore thermal simulators. Attention to detail in applying the correct data to computer simulations resulted in lower slurry costs while at the same time achieved improved performance as measured by improved casing shoe integrity. Real-time monitoring and detailed pressure analysis were used to manage cementing operations and the correct application of SSR plugs. Feedback of these processes from operators and service tool personnel has resulted in improved job performance as measured through reduced NPT and well construction costs. These new processes have resulted in a 100% success rate as compared to the previous 74% success rate. Introduction In deepwater operations there is always the desire to improve logistics to reduce waste and cost associated with materials and equipment transport.[1] Distances from a shore base increase the importance of this as related to job planning. Current cement shortages in some regions of the world also require that the design take into account materials unique to a given area. Recent R&D efforts have aided in improving logistics associated with cementing the riserless casings.[2] However, this Gulf of Mexico (GOM) deepwater operator has further improved its use of bulk cement to allow flexible slurry designs during the entire drilling operation. Wells currently being drilled in deep water are being designed to put more emphasis on achieving long-term zonal isolation.3 While drilling and completion temperatures may not be as critical as they once were, the deeper wells being drilled have flowing wellhead temperatures that require strength retrogression consideration in the cement design. While this aspect adds to planning considerations, this operator was still able to maximize logistical benefits while minimizing costs.
The properties of batch mixed slurries prepared in a series of yard tests using full scale equipment are compared with laboratory tests on the same materials. Dramatic differences have been observed between conventional laboratory tests and field scale properties. By splitting the batch mixing process into distinct steps, a laboratory method is tested against the observed differences. It is shown that mixing energy is an important parameter in slurry design and that energy levels substantially above the conventional API laboratory mixing energy may degrade slurry properties such as pumping time. INTRODUCTION Batch mixing of slurries for critical jobs is a common practice when the size of the job allows, (say), less than 100 bbl. Many different types and size of batch mixer are available to cope with offshore restrictions and batch mixing offers the assurance of good quality control of the slurry prior to pumping. The density can be controlled within tight limits and the slurry is fully homogenized and can even be subjected to testing prior to committing to pumping. Although physical and chemical aspects of mixing have been investigated before1,2, there is little data on the effect of batch mixers on slurry properties and no industry accepted laboratory test procedure to design such slurries to account for the numerous parameters involved. Laboratory testing invariably follows the API procedure3 for thickening time and, although this may be adequate in some cases, it cannot universally predict field behaviour. The mixing, homogenization, pumping and displacement of the cement slurry all play a part in determining the hydration and thickening behaviour. The relative contributions from each part of the process need to be understood if failures are to be avoided. In general, it has been found that increasing the mixing energy up to the value experienced in the API laboratory test is a good thing. Many slurry properties are improved2. At levels of mixing energy greater than the API value, properties w degrade. The actual behaviour will depend on the type of slurry design, the brand of the cement as well as its API classification and, in the case of batch mixing, the volume mixed and the amount of shear energy delivered during mixing and homogenization. Again, the process needs to be viewed as a whole. Surface lines can provide extra mixing energy as, indeed, can pumping through coiled tubing. Often these contributions may not have a significant impact on slurry properties but do need to be quantified and understood. In batch mixing of relatively small volumes of slurry (1Obbl), profound changes can occur in properties compared with conventional laboratory testing. This paper describes a series of yard tests using full scale equipment coupled with considerable laboratory testing of samples of slurry taken from the yard and also mixed in the laboratory using identical material.
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