A Workflow for Fully Consistent Water Saturation Initialization without Capillary Pressure Scaling
- Kassem Ghorayeb (Schlumberger) | Christophe Darous (Schlumberger Oilfield Eastern Limited) | Mihira Narayan Acharya (Kuwait Oil Company) | Ealian H.D. Al-anzi (Kuwait Oil Company)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Characterisation and Simulation Conference and Exhibition, 9-11 October, Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.5.3 Scaling Methods, 5.5 Reservoir Simulation, 5.1.5 Geologic Modeling, 3 Production and Well Operations, 5.8.7 Carbonate Reservoir, 4.1.2 Separation and Treating, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics
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Discrepancies in terms of hydrocarbon initially in place volumes (HIIP) between static and dynamic models might take place because of nonlinear dependency of the saturation height function (SHF) versus porosity and averaging of height above free water level. Upscaling tends to eliminate the high and low porosity values in favor of the average porosity, which might lead to substantial changes in the resulting water saturation. Furthermore, pressure and compositional variation with depth in the dynamic model might lead to substantial contribution to the discrepancies, independent of upscaling.
We present a procedure to address the above issues and provide full consistency between static and dynamic models in terms of HIIP, without the use of capillary pressure (Pc) scaling by the reservoir simulator. A "preprocessing?? workflow is used to slightly reassign the Pc curve of each grid block so that a very close match is obtained between the two models in terms of HIIP.
Results show that HIIP obtained from the dynamic model through equilibration using the proposed procedure are within 1% of those obtained using the static model without having to use Pc scaling in the reservoir simulator which warranties realistic and physically valid Pc curves in the reservoir dynamic model initialization process.
In this paper we present a workflow that was used to ensure full consistency in terms of HIIP between static and dynamic models in the North Kuwait Jurassic Project (Chakraborty et al. 2009; Ghorayeb et al. 2011). In a previous work (Ghorayeb et al. 2009) we presented a procedure to address this problem in which the endpoint-scaling functionality in the simulator is used to modify the discrete set of capillary pressure (Pc) curves to support the provided initial water saturation. Unlike the procedure presented in Ghorayeb et al. (2009), the proposed workflow in this paper does not use capillary pressure scaling by the reservoir simulator. Instead, the Pc curve is slightly modified for each grid block in a "preprocessing?? step so that an excellent match is obtained between the two models in terms of HIIP.
The saturation height function (SHF) is defined from drainage capillary pressure data measured on core plugs. A comprehensive set of mercury injection capillary pressures (MICP) were selected to cover the full range of porosity and permeability of the matrix. Since MICP were measured on plug trim ends, mercury porosity of each sample has been compared with the corresponding air porosity of the full plug for consistency between the plug and the trim end. Measurements that exhibit obvious anomalies such as low entry pressure in low matrix permeability samples were filtered out and considered affected by induced fractures or having major conformance issues. A minority of reservoir samples has also been separated based on their MICP behavior since they belong to separate rock types, which was confirmed by petrography. Fig. 1 (left) displays the entire MICP data set color-coded according to the validation step.
MICP measurements are performed at nonreservoir conditions and with fluids (air and mercury) having very different contact angle and interfacial tension compared to the reservoir fluids. To establish the impact of fluids and confinement pressure, key samples have been selected to measure Pc with reservoir fluids at reservoir conditions. Capillary pressures were measured using porous plate techniques with crude oil and synthetic water at net overburden pressure (NOP) and elevated temperature. The results are compared with the respective MICP in Fig. 1 (right). As it can be observed, the match is very good irrespective of the shape of the Pc curves. The remaining difference can also be attributed to the different samples used for the two experiments (whole plug versus the trim end).
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