Abstract
The accurate detection and confirmation of formation pore pressure and fracture gradient has become almost essential to the drilling of deep wells with higher than normal pore pressure. Generally, the formation pressure is the presence of the fluids “oil, gas or salt water” in the pore spaces of the rock matrix. Therefore, the fluid confined in the pores of the formation rock occur under certain degree of pressure, generally called formation pore pressure. Formation pore pressure is defined as the pressure acting on the fluids in the pore space of the rock, which is equal to the difference between the total overburden stress and grain to grain stress. Normal formation pressure is equal to the hydrostatic head of the native formation fluid or water exerting from the top of water table to the subsurface formation depth. Hydrostatic pressure is the pressure in a column of salt water or usually defined as the pressure exerted by a column of fluid, the pressure is a function of the average fluid density and the vertical height or depth of the fluid column. In most cases, the fluid vary from fresh water with a density of (8.33 Ib/gal) (0.433 psi/ft) to salt water with a density of (9 Ib/gal) (0.465 psi/ft). Consequently the hydrostatic pressure gradient of such system will be greater in terms of (psi/ft) than that of a fresh water system and will be displayed on pressure depth plot by pressure gradient line whose slope is greater than that of fresh water hydrostatic pressure. Indeed, formation pore pressure depending on the magnitude of the pore pressure, it can be described as being either normal, abnormal or subnormal. Whereas defined as follows, we had defined normal pore pressure which is equal to the hydrostatic pressure, abnormal pore pressure is defined as any pore pressure that is greater than the hydrostatic pressure of the formation water occupying the pore space, and sometimes called overpressure or geopressure. Subnormal pore pressure is defined as any formation pressure that is less than the corresponding fluid hydrostatic pressure at a given depth. The objectives of this study are:To determine the origin, nature, causes and the location of the subnormal and overpressued formations in part of Sirte basin area. To determine a graphical correlation relating formation pore pressure and fracture gradients to depth for selected areas extending fromfurther east to North West in the basin.This study is determining the pore pressure and fracture gradient, for selected fields from eastern, central and western Sirte basin, using existing correlations which utilize log and drilling data measured for the selected wells in these areas. The casing setting depths as well as the maximum and the minimum mud weight gradients to be used for future drilling activities in these selected areas in Sirte basin have been determined. The location and the magnitude of the lost circulation zones as well as the overpressured zones have been determined and correlated with depth depending on the location of the studied area in the basin. It was found that the lost circulation zones are located at depths of approximately in the range 3000 to 6000 ft from east toward west. It is concluded that the calculated pore pressure and the fracture gradients values obtained from the log data are in good agreement with the values obtained form the drilling data. It is concluded that the results obtained in this study for the eastern part of Sirte basin was satisfactory and can be used with good confident for future drilling activities in the area, where the obtained results for either the central or the western parts of the basin are not enough to draw final conclusions for future mud design programs in these areas. It is therefore recommended that further investigation and extensive study should be conducted for these two areas by gathering enough log and drilling data from different fields in these area which was not available during this study.
احمد خليفة رمضان طنيش (2009)
