FORMATION FRACTURE PRESSURE DETERMINATION WHEN DRILLING OFFSHORE HOLES

Abstract

The purpose of the article is to focus on the breakdown pressure of the well bore when drilling offshore for oil and gas. It is much lower than the one when drilling on shore and can vary significantly. Domestic articles on this topic are little to choose. One of the reasons of this situation is that drilling on the already explored offshore deposits is not performed now as the oil prices are low, and drilling from the floating drilling rigs is too expensive. Moreover, severe ice conditions and imperfection of drilling equipment for offshore drilling hinder the development of drilling works in the Northern and Eastern seas. We should also note the lack of experience of Russian drillers in the development of offshore oil and gas fields. Methods. The study uses a refined analytical calculation of the breakdown pressure of the well bore and its gradient. Results. To determine the breakdown pressure it is proposed to allow for the shelf depth and rotary table elevation above the average level of the shelf surface. The depth of the shelf is often not taken into account allegedly because of its small depths. But the described factor is not the only one that causes a short-term increase in the hydrostatic pressure. It could also be caused by daily tides, swabbing under RIH/POOH operations and etc. All together they can cause the loss of circulation. Besides, the article presents the average overburden and formation pressure gradients in tabular variants instead of graphical ones in order to facilitate the calculations. Conclusions. Analytical determination of the fracture pressure when drilling offshore wells (water holes) is an important task as it gives the real values of the fracture pressure. This can help to prevent the loss of circulation.