The Application of Petrophysical Data to Improve Pore and Fracture Pressure Determination in North Sea Central Graben HPHT Wells

C.D. Ward, Sperry-Sun Drilling Services, K. Coghill, BP Norge U.A., and M.D. Broussard, Amoco Norway Oil Company

Pore and fracture pressure determinations are key considerations for the successful planning and drilling of North Sea Central Graben High Pressure High Temperature (HPHT) wells. Knowledge of these downhole pressure constraints can have a significant impact on drilling safety and economics. In this paper we present recent advancements to a previously presented methodology. Pore and fracture pressure is determined using wireline or WMD petrophysical data and effective stress approach. The results have increased our understanding of pressure generation mechanisms and hydrocarbon migration in this basin.

Most traditional pore pressure estimation methods use a shale disequilibrium compaction model for their calculations. We assess these methods and propose that excellent results can be obtained by deriving porosity from density or deep resistively data. This porosity, together with a lithology estimation from the gamma ray are input into an Effective Stress Loading limb (ESL) model that calculates pore and fracture pressure through all major lithologies.

Along the North Sea Central Graben axis there are two distinct pressure domains. These are separated by a low porosity (<5%) horizontal pressure seal with the Cretaceous Chalk Group. This seal occurs between ~3.4-4km, rapidly deposited Tertiary shales and Upper Cretaceous chalks are on the compactional loading limb. Here, moderate overpressures are generated by disequilibrium compaction. Below ~4km, low porosity (5-10%) Mesozoic sediments are unloaded by fluid expansion mechanisms to produce extreme overpressures.

The upper limit for the pore pressure is the fracture propagation pressure. When this is exceeded, hydraulic fracturing can occur and the fluids escape, allowing the fracture to close and pressure to build again. In this environment, primary hydrocarbon migration may be largely dependent on the rate of the hydraulic fracturing formed as a consequence of the extreme fluid pressures generated by fluid expansion.

An understanding of these pressure generating mechanisms, together with improved porosity determinations, has led to more accurate pore and fracture pressure determinations. Implementing the results into well planned and drilling can help avoid many of costly pressure related problems inherent to HPHT wells.

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