World Drilling Limit Envelope: Why it Shows an Irregular Triangle?

Abstract

Many scholars have carried on the related researches of world drilling limit envelope and provided the TVD (true vertical depth)-HD (horizontal displacement) charts. Those charts show that both the ultra-deep well and the extended-reach well (ERW) with extremely high HD-TVD ratio are relatively rare, while those wells have a certain TVD as well as a certain HD formed the majority part of these charts. Why the envelope shows an irregular triangle as a whole? There exist theoretical reasons except for the reasons of economic benefits and drilling needs. This paper mainly analyze the following four kinds of well: 1) the ultra-deep well; 2) the vertical well and the directional well with identical TVD; 3) the horizontal ERW; 4) two ERWs with identical measured depth (MD). These analyses can be used to explain why the world drilling limit envelope shows an irregular triangle. The research shows that drilling limits are restricted by borehole space, well deviation, bearing capacity of drilled formation, mechanical factors and hydraulic factors simultaneously. These factors working together make the world drilling limit envelope as an irregular triangle. The factors that limit the well’s MD can be divided into two categories: subjective factors and objective factors. The subjective factors can be broken by upgrading the drilling equipment and improving the drilling technology. However, the bearing capacity of drilled formation is an objective factor which cannot be changed. It is the ultimate factor that restrict the drilling limit. In addition, some optimization control measures are proposed, which are beneficial to break the world drilling envelope.