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

Abstract

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 MWD petrophysical data and an effective stress approach. The results have increased our understanding of overpressure 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 resistivity 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 pressures 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 within the Cretaceous Chalk Group. This seal occurs between 3.5-4km and is independent of stratigraphic level within the chalk. Above 3.5km, 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 planning and drilling can help avoid many of the costly pressure related problems inherent to HPHT wells. Introduction Operators increasingly target deep HPHT prospects in many areas of the world. Accurate pore and fracture pressure determinations in these wells are key considerations. Optimurn mud weights and casing point selections are crucial to safe, efficient, and cost effective planning and drilling. Pressure related problems include well control incidents, lost circulation, differential sticking, reduced rates of penetration, and reservoir damage. These often lead to costly sidetracks, well abandonments, lost production, and even underground blowouts. The latter could lead to the expense of drilling a relief well and in extreme cases loss of rig and lives. Pressure related problems therefore represent a significant proportion of the high drilling costs associated with HPHT wells. In the Norwegian sector, average HPHT wells are four times as likely to have a well control problem, and incur an average of eight times the downtime of other exploration wells (Table 1). P. 53^

A compaction front in North Sea chalk

“…[32] The existence of a chalk compaction front may furthermore explain the presence of a low-porosity pressure seal within the chalk at depths below ∼3.5 km [Ward et al, 1994]. This low porosity seal develops at a depth where effective pressures approach about 17 MPa, which according to our results may cause pore collapse.…”

“…gesting that the observed compaction front is connected to lithological or other heterogeneities within the chalk stratigraphy. Fluid pressure reaching fracture pressure levels has been observed in the chalk below this depth [e.g., Hunt, 1990;Ward et al, 1994], whereas overpressure above this level is clearly caused by disequilibrium compaction [e.g., Osborne and Swarbrick, 1997;Japsen, 1998]. …”

Permeability and fluid flow in natural mudstones

“…There is relatively little published work on this problem but some researchers (e.g. Hunt 1990;Ward et al 1994) have suggested that carbonate-cemented horizons may form laterally extensive pressure seals in the North Sea.…”

Estimating Pore Pressure in HPHT Wells in Yinggehai Basin, China Based on Hierarchical Eaton's Method

“…Previous researches about causes of overpressure are not consistent, which increased the uncertainty and difficulty of accurately estimating pore pressure. Ward [2] posed 11 kinds of mechanisms (undercompaction, lateral tectonic loading, secondary cementation aquathermal pressuring, hydrocarbon maturation clay diagenesis, and charging from other zones etc) which possibly generate overpressure. Fan [3] classified all the mechanisms into three categories: effective stress abiding in loading or unloading and porosity approximate constant.…”