Mechanical model and optimal design method of tubular strings with connectors constrained in extended-reach and horizontal wells

Abstract

Mechanical analysis and optimal design of down-hole tubular strings with connectors are the key theoretical bases of design and operation in extended-reach and horizontal drilling and completion. However, the complex mechanical interaction between spring connectors and open wellbores are not fully considered in conventional tubular models and the conventional design methods are mostly based on experience and lack of rigorous mathematical derivations. To overcome these shortcomings, a new revised mechanical model and an optimal design method of connectors on down-hole tubular strings have been built. Firstly, the local loads on a unified connector which can depict common spring and rigid connectors constrained in open wellbores are studied. The local connector loads are further introduced into the local mechanical model of tubular strings. The integral mechanical model of tubular strings is amended by the local model. Next, the integral design model of connectors while considering friction reduction, tubular centricity and cost is built. The integral design model is divided into a series of local design models according to the properties of tubular mechanical behaviors on extended-reach and horizontal wellbores. At last, the mechanical model and design method are applied to drill strings with joints and casing strings with centralizers in horizontal drilling and completion.The results indicate that the revised tubular mechanical model and connector design model are stable and fast. The coupling effects of connector and buckling, local drags on connector and radial compression of spring connectors are all included in the mechanical model, so the new model gives a more sophisticated description of mechanical behaviors of tubular strings with connectors. The design results of anti-drag joints are closely related to the clean condition of wellbores. Proper optimal design of connectors and good clean condition of wellbores can inhibit helical buckling, decrease friction factor and reduce plough effect, leading to larger lateral extending limits. Compared with the initial design results of centralizers, friction force is decreased and casing centricity is increased through optimization design. The optimization results of centralizers are closely related to the weight values of friction reduction, casing centricity and cost. Therefore, the key problem is how to assign these three weight values under actual engineering conditions.