This paper discusses the most important drilling hazards related to borehole instability and pore pressure in deep water wells. A case study from a deep water well, which has experienced wellbore stability and loss of circulation problems, especially over intervals with higher angles of hole deviation, is presented. The answers provided by the study included properties, such as, recommended mud-weights (minimum & maximum) as a function of angle of deviation, rock strength and computed pore pressure. The determination of rock strength along with the optimum mud-weight windows improved overall the drilling performance in the next deep water well by minimizing washouts, loss of circulation and optimizing casing design by elimination of unnecessary casing strings. Moreover, improvement of bit performance was achieved by using the predicted rock-strength values. As a result, drilling time and well construction costs were significantly reduced.
Wellbore stability and pore pressure related issues, especially in deep water blocks, cause frequently serious problems during drilling, logging and production operations. The narrow pore pressure and fracture gradient (PPFG) windows necessitate multiple casing strings to reach the target reservoir formations. Errors in predicting PPFG windows could result in significant loss of rig time and even failure of wells.
The removal of rock during the drilling operation disturbs the natural rock formation that is being subjected to a state of compressive in-situ stresses at depth. The redistribution of the stresses around the hole thus produces stress concentrations at or near the borehole wall. Such a high stress buildup around the wellbore may lead to a number of hole problems that include stuck pipe, borehole collapse, sloughing shale and excessive fill. Several billions of dollars are lost each year worldwide directly or indirectly caused by wellbore stability and pore pressure related problems. Therefore, in order to fully obtain the benefits of the directional drilling technology, wellbore stability analysis and pore pressure prediction has been of increasing demand in the planning stage of the wells, especially in deep water where exploration and field development costs are very high.