Niels Bech

The history of hydrocarbon filling of Danish chalk fields

It is well established that dynamic conditions expressed as tilted fluid contacts characterize most hydrocarbon accumulations in North Sea Chalk reservoirs. Chalk is a low-permeability, high-porosity rock and properties grade smoothly from reservoir over baffle to seal. The natural dynamic conditions prevail because pressure dissipation takes place through the rockmatrix, as fracture-supported flow often is minimal. The dynamic conditions are imposed by processes occurring on a geological time-scale and result mainly in lateral pressure differences in the water zone and even in lateral pressure differences in the oil zone. Re-equilibration of fluid contacts also occurs on a geological time-scale. These processes are of paramount importance for trap definition and impose severe restrictions on migration distances. Reservoir simulation techniques are applied, in combination with back-stripping, to the simulation of geological time-scale secondary migration and trapping. Flow simulation of the filling dynamics of a chalk reservoir shows a complex filling geometry due to the high capillary entry pressures in the low-permeability chalks. Such internal barriers will re-direct hydrocarbons and residual oil can be left on the migration route. The process of hydrocarbon charging is slow and equilibration of hydrocarbons with respect to pressure gradients, therefore, also occurs very slowly. The Kraka Field and the Dan and Halfdan fields are subjected to studies of primary oil charging and re-migration in this paper and the dynamic oil on Dan Field west flank is successfully mimicked. Results show that a time span in the order of 2 Ma is required for the hydrocarbons to reach the summit in an approximately equilibrium state from a flanking position. However, even dynamic equilibrium may not be fully obtained due to re-perturbation by tectonic movements and changing water zone pressure gradients. Results show that saturation profiles in drilled wells may appear in drainage equilibrium while under partial re-imbibition, which impairs saturation modelling.

Determination of Oil/Water Saturation Functions of Chalk Core Plugs From Two-Phase Flow Experiments

Summary A new procedure for obtaining oil/water saturation functions, i.e., capillary pressure and relative permeability, of tight core samples uses the pronounced end effect present in flooding experiments on such material. In core material with high capillary pressure, the end effect may allow determination of the saturation functions for a broad saturation interval. A complex coreflooding scheme provides the fluid distributions and production data from which the saturation functions are computed for both drainage and imbibition by a least-squares technique. A chemical shift NMR technique is used for fluid distribution determination. An undesirable interdependency of the saturation functions is avoided by their calculation from different data sets. Killough’s method is employed to account for the scanning effect in hysteresis situations for both capillary pressure and relative permeability. The procedure is demonstrated on chalk samples from the North Sea. The experimental time is intermediate between the centrifuge and porous plate methods.

Determination of Free Water Levels in Low-Permeability Chalk Reservoirs From Logged Saturations

The present paper describes a technique for determination of the free water level (FWL) in low-permeability chalk reservoirs along slanted or horizontal wells from logged saturations and porosities. The calculation is done by utilizing existing empirical correlations for drainage and imbibition capillary pressure curves. For each logged saturation and corresponding porosity the FWL is determined so that the calculated saturation equals the logged value. The method takes into account possible imbibition, which results in a FWL that is shallower than the original paleo FWL. The method has been applied to field data from the North Sea and it is shown that it can capture a tilting water level along a horizontal well. Introduction Low-permeability chalk reservoirs are characterized by high capillary entry pressures and large oil/water transition zones. Estimation of oil-in-place for a reservoir at any given time requires information on the reservoir history, a good description of the capillary pressure behavior and knowledge of the FWL. The latter is typically non-horizontal and free water level gradients of more than 100 m/km have been observed in the North Sea. In general the information about the FWL is very scarce, and consists of sometimes dubious pressure measurements. If measured at all, it is usually done in a few exploration wells only. By means of the present method the FWL may be estimated from saturations and porosities logged in production wells. It is usually assumed that the reservoir is in drainage equilibrium. However, frequently observed residual oil zones indicate that water influx have affected the saturations. (See f. ex. Albrechtsen et al.). The saturation distribution is a result of drainage as well as imbibition processes and its shape may deviate considerably from the shape of a drainage curve. The present method takes into account possible imbibition, which results in a FWL that is shallower than the original paleo FWL. Adams 2,3 presented an empirical model, the Imbibition from Drainage or IFD model, relating imbibition capillary pressure curves to the corresponding primary drainage curves. The IFD method was, however, developed for siliciclastic rocks in the Eromanga Basin in Australia and it appears to be less suited for tight chalk with high capillary entry pressures. A new technique was presented by Bech et al. 4 for the modeling of initial saturations along vertical wells in water wet oil/water reservoirs with large transition zones that are in imbibition equilibrium. The method determines the locations of the existing and the original free water levels and thus the extent of imbibition that the reservoir has undergone by matching calculated and log derived water saturations. The present paper extends this method to non-vertical wells. The capillary pressure curves including the curves representing the scanning between the drainage and imbibition curves are described by analytical expressions as shown in Skjaeveland et al.. Optionally, the scanning may be described by Killoughs method . The drainage capillary pressure shape function, the irreducible water saturation and the capillary entry pressure are modeled by the EQR method . The method is demonstrated on a synthetic example and applied to field data from the North Sea and it is shown that it can capture a tilting water level along a horizontal well. Determination of Present Free Water Level The FWL is determined for each saturation and corresponding porosity logged along the well. It is assumed that the oil accumulation was originally formed through a pure drainage process resulting in a drainage equilibrium saturation distribution and a palaeo free water level. In case that subsequent burial and/or tectonic events have lead to an influx of water, the free water level has risen and a new saturation distribution has emerged in the reservoir. This new saturation distribution is a result of both drainage and imbibition processes and can therefore not be described by a model which assumes drainage equilibrium. The changes in water saturation and FWL is illustrated in Fig. 1 for a constant-porosity vertical well. It is seen that the oil-water contact is unaltered and that a region with residual oil has been left below the rising water table. SPE 105622 Determination of Free-Water Levels in Low-Permeability Chalk Reservoirs From Logged Saturations N. Bech, SPE, P. Frykman, and O.V. Vejbæk, SPE, Geological Survey of Denmark and Greenland, GEUS