In 1920 Fridtjof Nansen postulated an uplift and erosion in the order of 500 m in the Barents Sea. Hydrocarbon exploration in the 1980's has since confirmed a significant uplift and erosion in this area. In 1985 Statoil initiated a multidisciplinary study to evaluate and quantify the amount of uplift and erosion. The methods used were: Shale compaction curves, Dc-exponent from drilling, vitrinite reflectance trends, diagenesis of clay minerals, apatite fission track studies, volumetric mass balance calculations of erosion and deposition, and PVT-modelling of known reservoirs. Studies of porosity and density trends in Tertiary shales from wells showed an uplift and erosion of the order of 500–1000 m in the Hammerfest Basin, increasing to about 1700–1800 m towards the Fingerdjupet Subbasin area. Vitrinite trends from exploration wells and IKU shallow drillings confirmed these results also showing increasing erosion towards the north (3–4 km erosion at Bjornoya). Depending on the paleo-drainage area chosen, volumetric calculation of the Late Tertiary deposition along the western shelf margin gives a magnitude of erosion of about 1–1.2 km in the southern part of the Barents Sea, increasing to approximately 3 km on Svalbard. Used together, the methods above give a fairly accurate estimate on the magnitude of the erosion but not on timing. Fission track studies indicate one or more phases of uplift from Late Oligocene to Pliocene/Pleistocene, while biostratigraphic redatings of sediments from the wells on the Senja Ridge suggest that a major part of the erosion is related to glacial processes during the last 2–3 Myr. Extensive residual oil staining found below the present hydrocarbon/water contacts indicates that reservoirs, such as the Snohvit field in the Hammerfest Basin, once contained larger quantities of oil. PVT-modelling has shown that this oil was lost probably as a consequence of gas expansion during uplift, and can explain in part the lack of commercial oil finds so far in the Barents Sea.

Tertiary Uplift and Erosion in the Barents Sea: Magnitude, Timing and Consequences

A drilling control system provides, in one aspect, advisory actions for optimal drilling. Such a system or model utilizes downhole dynamics data and surface drilling parameters, to produce drilling models used to provide to a human operator with recommended drilling parameters for optimized performance. In another aspect, the output of the drilling control system is directly linked with rig instrumentation systems so as to provide a closed-loop automated drilling control system that optimizes drilling while taking into account the downhole dynamic behavior and surface parameters. The drilling models can be either static or dynamic. In one embodiment, the simulation of the drilling process uses neural networks to estimate some nonlinear function using the examples of input-output relations produced by the drilling process.

Real-time drilling optimization based on MWD dynamic measurements

Comparison of different failure criteria in prediction of safe mud weigh window in drilling practice

Pore and fracture pressure modeling using direct and indirect methods in Badri Field, Gulf of Suez, Egypt

The Sichuan Basin is a prime gas-producing basin in China. Besides the giant Neoproterozoic Weiyuan gas field discovered in the central paleo-uplift, an extra giant gas reservoir in the Cambrian in this area has been confirmed in recent years. Although the lower Paleozoic in the southwest (Weiyuan area) is normally pressured, it is overpressured in the central area (Moxi–Gaoshiti area). Combined with the seal distribution, five pressure systems, including three overpressure systems, can be divided based on drill-stem test, mud weight, and sonic transit time data. Overpressures appear at depth interval of 1500 to 4900 m (4921 to 16,076 ft), approximately. Benefiting from the good sealing capacity of the gypsum in the Upper and Middle Triassic, high overpressure (pressure coefficient [r] > 2.0) has been preserved in the Lower Triassic, and the Upper Triassic and Cambrian are moderately overpressured (1.3 < r < 1.7). Mechanisms for various overpressure systems are different. Abnormally high sonic transit time in the Permian indicates disequilibrium compaction overpressure. The analyses of sonic transit time–effective stress suggest that disequilibrium compaction is the primary mechanism for the overpressure in the Upper Triassic Xujiahe Formation, but the Cambrian overpressure system is predominantly associated with fluid expansion, which mainly resulted from gas generation. Furthermore, we consider that the late thermal cracking of oil to gas is a key factor for gas and overpressure preservation in old strata. The high overpressure in the Lower Triassic marine carbonate rocks was caused by oil cracking and gypsum dehydration mechanisms. Combining the origin analysis with the burial and hydrocarbon generation histories, we constructed the Cambrian pore pressure evolution model, which is characterized by roughly normal pressure before 200 Ma, overpressuring from 200 to 90 Ma, and overpressure releasing since 90 Ma.

O.J. Shirley

Papers

Application of Drilling Performance Data to Overpressure Detection