Evaluation of Geopressure Regime in an Onshore Niger Delta Field of Nigeria

Geopressure evaluation or prediction is an important exercise that must be carefully estimated when planning a drilling campaign. Formations of high pressure regime can be experienced in several areas within the bore hole depths. Being unaware of these areas that experience high pressure regime can create many uncontrolled events such as blow outs that may lead to human and material loss within the location. Geopressure regime is the pressure that is cause by the pore fluids within the pore spaces of the subsurface. At any depth, the overburden pressure is the summation of weight grains and the pore pressure at that depth [1]. Subsurface formation that is over pressured is characterized by the pore fluid pressure that is significantly below or above the normal hydrostatic pressure at a given depth [1] (Jiao, et al 1998, Dickinson, 1983). For safety, economic and efficient way of well drilling requires an accurate evaluation of the over pressure regime. Pore pressures are easily predicted for normally pressured sediments but challenging when the sediments are abnormally pressured [1]. This accurate geopressure evaluation is necessary for an efficient drilling plan in order to choose proper casing points and also design a better casing program that will allowed for a total control of the drilling and completion program [1,2]. Geopressure are evaluated through the use of the relevant subsurface data, also having knowledge of burial, stress and temperature histories, rock types and their distributions and subsurface structure and reservoir connectivity with the formation will be an advantage [1,3]. Predicting geopressure is one of the biggest challenges that is facing the oil industry, particularly as exploration moves into the deep-water areas. It has emerged from years of research that the origin of overpressure and its effect on the petroleum system are complex and difficult to quantify [1,3]. In Niger Delta field, where the shallow region of the sedimentary base has a high sand accumulation, the onset of overpressure is deeper, but the rate of the pressure-transition zone is much more rapid which contribute to drilling failures (Swarbrick et al. 2011). There are several mechanisms responsible for over pressure regime: which includes, disequilibrium compaction, load transfer, fluid expansion and buoyancy pressure. Disequilibrium compaction is attributed to be the main cause of overpressures that are generated in most fine grained sedimentary basins (Dickson, 1953) [4].