ANALYTICAL STUDIES OF WELLBORE INSTABILITY DURING WELL CONSTRUCTION IN THE MARIB UPLIFT ZONE (REPUBLIC OF YEMEN)

The development system of the Al-Sama field (oil and gas bearing basin Sab'atayn, Republic of Yemen) is implemented using vertical and directional wells. The project of additional development of the field involves drilling new directional wells with horizontal completions and the same type of sidetracks from already operated wells. The completions are planned to be located in oil-producing terrigenous deposits of the Madbi suite (Late Jurassic) in zones with high residual oil reserves. The experience gained in drilling vertical and directional wells does not allow for trouble-free drilling of horizontal completions of wells, which is confirmed by the occurrence of complications in 30 % of cases of construction of mine workings. A quantitative assessment of complications during drilling of new sixty wells shows that most of them are stuck bottom hole assembly and collapse of well walls. Collapses most often occur in sidetracks drilled from old well stock, and stuck drill pipes, in turn, occur during process downtime.A preliminary study of rocks from the intervals where complications occur showed the absence of physical and chemical activity of the reservoir systems. Petrographic analysis confirmed the fractured-brittle nature of argillite with the presence of open, partially healed microcracks in the layers along the bedding.Modeling the mechanical properties of rocks made it possible to determine the optimal range of the specific gravity of the drilling fluid for drilling horizontal well endings depending on the azimuth of the direction. Construction of horizontal well endings using the same range of drilling mud weight made it possible to create a mathematical model for predicting wellbore instability and identifying various types of complications. In the future, these models will be used as a basis for designing the drilling of new horizontal wellbores in the conditions of the Marib uplift, which will help in selecting and justifying the optimal values of drilling mud density to minimize the risk of accidents during well construction.