Chalk reservoirs

Chalk reservoirs encounter some specific problems during secondary recovery of oil by waterflooding. Displacement experiments in several formations... 

T. Bjomstad, O. B. Haugen, and I. A. Hundere. Dynamic behavior of radio-labelled water tracer candidates for chalk reservoirs. J Petrol Sci Eng, 10(3) 223-238, February 1994. 

Snow, S.W. Hough, E.V. "Field and Laboratory Experience in Stimulating Ekofisk Area North Sea Chalk Reservoirs," SPE paper 18225 1988 SPE Annual Technical Conference and Exhibition, Houston. 

It may also be beneficial to alter the wettability (see Section 3.5.3) of the reservoir rock, particularly in order to shift oil-wetting regions towards neutral wetting, which is probably about the optimum for improved oil recovery [158]. Figure 3.23 (see Section 3.5.3) shows how surfactant adsorption can alter wettability, but not always in a simple manner. Such surfactant-induced wettability alteration might be considered for predominantly oil-wetting reservoirs such as chalk reservoirs in the North Sea. 

The Chalk reservoired petroleums of J Block are predominantly undersaturated black oils. The model developed to explain the variation in petroleum type within the Chalk is heavily influenced by the understanding of fluid distribution within the Pre-Cretaceous, as discussed above. 

Geochemical data are available for seven Chalk-reservoired oil samples. Five of these are from the Joanne accumulation and two are from wells on the Judy structure. Well locations are shown in Figure 15. 

Physical properties. Most of the petroleums tested from the Chalk reservoirs in the Judy and Joanne Fields are undersaturated black oils. Gas-oil ratios are generally in the range 2000 to 2500scf/bbl and bubble points are approximately 1000 to ISOOpsi below present-day reservoir pressures (Table 2). In the Judy structure, well 30/7a-4a tested gas caps in both the Ekofisk and Tor reservoirs. Petroleum tested from the Tor in 30/7a-6 also had a high GOR (4322 scf/bbl) and PVT analysis confirmed the fluid as a gas-condensate (Pd = 7185psi). However, the dew point pressure is very close to the reservoir pressures obtained from RFT pressure measurements (7150-7200 psi). Furthermore, flow rates were very low and so the test results may not be valid. 

Geochemical characterization. The tested petroleums from the Chalk reservoirs are moderately mature and form an intermediate group between the low maturity Palaeocene condensates and high maturity Pre-Cretaceous fluids (Fig. 7). Within the Chalk group itself, there are clear differences between the Joanne oils and the oils from wells 30/7a-4a and 30/7a-6. 

Migration into the Chalk reservoirs is heavily influenced by the Pre-Cretaceous charge model in terms of explaining the variability in fluid type. 

Migration into the Chalk reservoirs of the Joanne Field could have occurred by one of three conceivable mechanisms ... 

Migration of petroleum within the Chalk reservoirs from its point of entry on the Judy structure is considered to be the most likely mechanism by which the Joanne Chalk reservoirs are charged. At present, there is no structural spill from Judy to Joanne at either Ekofisk or Tor horizons. However, the Top Ekofisk depth map, flattened on an intra-Oligocene event, shows that the Joanne structure developed considerably later than the Judy structure. It is therefore possible that, during growth of the... 

It is highly probable that petroleum has migrated into the Palaeocene reservoir by leakage from the Judy and/or Joanne Chalk reservoirs. Evidence for this is as follows. 

The Judy Chalk petroleums have many similarities with the crestal Pre-Cretaceous petroleums from 30/7a-P3 and 30/7a-P9. It is probable that the Judy Chalk reservoirs were... 

It is probable that the Palaeocene reservoir was charged by vertical leakage of petroleum from the underlying Chalk reservoirs. Migration of a saturated (dew point) gas to shallower depths, and hence lower reservoir temperatures and pressures, would have resulted in condensation of liquids. The oil encountered in 30/7a-5 may be a manifestation of this phenomenon. 

MECHANICAL BEHAVIOUR OF CHALK RESERVOIRS NUMERICAL MODELLING OF... 

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