Due to drilling economics, operators prefer extending the lateral section over drilling a new well. It presents a challenge to run casing strings to the target depth in wells where horizontal section is longer than the vertical section. Although alteration of casing weight or installation of liner may present as solutions, they are hardly economical compare to selective buoyancy methods. This paper aims to take an engineering approach on optimizingly palceing a buoyancy barrier which traps lighter fluid in the horizontal section for thepurpose of reducing friction. As seem to not have been presented in SPE conferences before, the aim of this paper is to engineeringly identify the optimum placement of the barrier, measure the efficiency improvement and identify the cases for which running a barrier is essential to run the casing string to the target depth. Operators usually run a torque and drag to measure the improvemet made by placing a barrier in kick off point. In this paper we intended to verify this practice and offer a simple rule of thumb method that can help an operator to decide on weather it is needed to run a barrier.
Referred to in this paper as flotation collar, it is installed as part of the casing string. It employs a mechanical barrier that traps the air or lighter completion fluid in the mostly horizontal section and separates it from the heavier completion fluid in the mostly vertical section of the casing string. Identifying the best position of the barrier is important to achieve the maximum hook-load. In this paper, numerous cases are studied by gradually expanding the air section and monitoring the hook-load to identify the best position for which the hook-load is maximum.
As per its physics, there is only one position of placement which results in the highest axial tensile force, also known as hook-load, when forces of gravity and friction engage. Placing the tool too high or too low in the string misuses the available tensile axial force and will not provide the best results. By using casing flotation tools as a barrier, a spectrum of thirty to eighty percent tensile improvement in the axial load are observed and more than fifty percent reduction of buckling effects in vertical section. For some cases, running a flotation tool was the only option to reach to the target depth with the casing string.
Flotation tools in long string wells are the only economical alternative to changing casing weights or running liners when it is to completing wells with extended horizontal sections. A flow-chart is presented to help with simplifying the process of choosing a flotation tool and finding its best position of placement.