Advanced Technology for Predicting the Fluid-Flow Attributes of Naturally Fractured Reservoirs from Quantitative Geologic Data and Modeling

Jon E. Olson, Larry W. Lake (Department of Petroleum and Geosystems Engineering, The University of Texas at Austin), and Stephen E. Laubach, principal investigators

 

Microstructural analysis of fractures has established key processes that lead to fracture sealing. These results are being compared with a mathematical model that has been developed to simulate hydrodynamics and fluid-mineral reactions in permeable media. Fluid convection, diffusion, and precipitation/dissolution (PD) reaction inside a finite space are solved in the model? as a simplified representation of natural fracture mineralization. The problem involves mass transfer within the fluid, accompanied by chemical reaction at the fracture surface. Mass-conservation equations for components in fluid are solved in this problem, and these are coupled with chemical reaction at the fracture surface. The model shows time evolution of fracture-aperture shrinkage patterns from PD reactions. Partly cemented fractures are

created if cementation fails to fill the fracture completely or if subsequent dissolution leaches out some of the mineral. Certain sets of boundary conditions show how the fractures are completely filled by precipitation.

Successful extraction of hydrocarbons from many remaining domestic exploration and development targets depends on the creation of new approaches to predicting natural fracture attributes. This research, supported by the U.S. Department of Energy, is to develop new understanding and new technology for prediction of fracture-pattern attributes related to subsurface fluid flow. In recent years interest has increased considerably on flow and transport in low-permeability fractured rock. Groundwater flow frequently induces dissolution and cementation processes, and it is the latter with which we are concerned because the mechanisms for fracture closure are not well defined. The crux of the problem is that fractures are closed by fluid flow even when there are no flow paths apparent in the surrounding medium. In many reservoir engineering applications and field performance studies, characterization of fractures is an important issue and a useful parameter for the studies of well productivity and breakthrough behavior. The focus of the study is predicting connectivity, clustering, and aperture, fracture pattern attributes that are exceedingly difficult to measure but that can be controlling fractures for fluid movement. The diagenetic process of dissolution and partial cementation is a key control on the creation and distribution of natural fractures in hydrocarbon reservoirs. Even with extensive data collection, fracture permeability still creates uncertainty in reservoir description and the prediction of well performance. Data on the timing and stages of diagenetic events can provide an explanation as to why, when, and where natural fractures will be open and permeable.

 

References:
Laubach, S. E., 2003, Practical approaches to identifying sealed and open fractures, AAPG Bulletin, v. 87, No. 4, (April 2003) p. 561–579.

Rijken, P., Holder, J., Olson, J., Laubach, S., 2002, Predicting fracture attributes in the Travis Peak Formation using quantitative modeling and structural diagenesis, Gulf Coast Association of Geological Societies Proceedings volume (CD), v. 52, p. 837–847.

 

For more information, please contact Steve Laubach, principal investigator. Telephone 512-471-6303; e-mail stephen.laubach@beg.utexas.edu.

February 2003