Flow in fractured chalk
The aim of the project is to improve the knowledge of flow mechanisms in fractured chalk reservoirs, and to improve the simulation tools for studying such reservoirs. Water injection in fractured chalk was examined by displacement experiments on chalk samples from wells in the Danish North Sea.
The project aims at increasing the knowledge of flow in fractured chalk by conducting displacement experiments on samples of fractured chalk, and developing computer models to simulate the experimental results. In a fluid system without capillary forces, composed of two miscible water phases and a sample of fractured chalk, the distribution of flow between fractures and matrix, together with absolute permeabilities of fractures and matrix are determined. From similar experiments in an immiscible oil-water system the saturation functions for fracture and matrix are determined by history matching in a single porosity model. Comparative experiments on sample material with natural fracture and artificial fractures will attempt to identify possible difference in flow patterns between naturally fractured and artificially fractured chalk. Nuclear Magnetic Resonance (NMR) methods are used for quantitative experimental monitoring. One or more dual porosity models will be employed to investigate the extent to which the detailed results from the single porosity model can be reproduced. A two-phase method for upscaling in a matrix-fracture system will be developed
Initially the samples were analyzed in a non-fractured state, where the petrophysical characteristics of the matrix were determined. Then artificial fractueres were induced, and the samples were analyzed again. Nuclear Magnetic Resonance Imaging (NMRI) was used to obtain the spatial distribution of the fluids during the experiments. The fluid system light water - heavy water - oil, together with the sample, comprised a model of a fractured hydrocarbon reservoir where injection water displaced oil and formation water. A reservoir simulator model was used to futher investigate the flow mechanisms. Important results are: 1) Fracture permeability may vary by a factor of 10 within few centimeters. 2) During water injection the formation water forms a bank ahead of the injection front with limited mixing between injection water and formation water. 3) Counter-current imbibition is a commom flow mechanism in fractured chalk. It was investigated to which extent double-porosity models may reproduce results from detailed single-porosity models. The work was concentrated on the oil/water system. It is concluded that the single-porosity description should be used when the density of fractures is so small that they can be described individually. In other situations the double-porosity description should be used with pseudo functions for capillary pressure and relative permebility. Simulation results of small-scale models of water flooding of flow permeable fractured systems were examined. It was shown analytically how to derive the shape and transmissibility factors of matrix and fracture, in order to describe a specific fracture trajectory in the double porosity formulation. The results from these small-scale models have been used to derive a fast and easily applicable method for estimating of the magnitude of shape factors for arbitrary fracture patterns, e.g. from photos and maps
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Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| COWI A/S |
Contact
Thoravej 8
DK-2400 København NV, Denmark
Olsen, Dan (seniorforsker), 38142000, GEUS@GEUS.DK
Øvr. Partnere: COWI Rådgivende Ingeniører AS
