Fluid Flow Simulations on Core Samples Containing Natural Fractures and Stylolites

Abstract

This study aimed to understand the impact of natural open fractures and stylolites on flow in a porous sandstone reservoir by analysing fluid flow behaviour at small scale (core sample). This analysis was done both experimentally and with fluid flow simulations within full core samples. Two core samples (seal peels) from the Snøhvit field in the SW Barents Sea were selected. These contain natural open fractures mostly associated with stylolites. Since no test data exist from the intervals with highest fracture intensity, it remained uncertain whether these fractures will affect the flow in the reservoir. Two series of flooding experiments were conducted on these core samples, one by tracer injection and the other by injecting gas (Nitrogen) in the 100% brine saturated seal peels. Computed tomography (CT) scans were used to image the fracture/stylolites system, to monitor the flooding experiments and to build a consistent simulation model. Single and two phase flow simulations were performed to mimic the experiments. First, the fractures and stylolites effective properties were determined via tracer injection experiments by matching the tracer production profile. Then, to simulate the gas injection, relative permeability and capillary pressure (Kr-Pc) curves generated from special core analysis (SCAL) measurements were applied for the matrix medium. For the stylolites, the same Kr curve was used and modified Pc applied. For the fractures, a linear Kr curve and no Pc were used. As a result, a good quality match was obtained, giving an understanding of the influence of a fracture/stylolites system on the flow within porous sandstone. Introduction The presence of open fractures is known to have significant effects on permeability and flow anisotropy in hydrocarbon reservoirs (e.g. Nelson 1981, 2001). Natural open fractures are present in the Jurassic reservoir sandstones of the Tubåen, Nordmela and Stø Formations in the Snøhvit Gas Condensate Field (Walderhaug 1992, Wennberg et al. in press a). In order to improve the understanding of the impact of these fractures on flow in the Snøhvit Field, Computed Tomography (CT) scan imaging techniques have been used on representative core samples (e.g. Wennberg et al in press b). First, CT scanning was used to describe the 3D geometrical properties of the fracture network including orientation, fracture density and fracture connectivity. Two types of fractures were observed: F1 fractures are short and stylolite related, and F2 fractures are longer, crosscutting the core and without any obvious relationship to stylolites. Second, monitoring of single and two phase flow experiments on samples containing these two types of natural open fractures was performed under 10 and 80 bar net confining pressure while using CT-scanning. The flow experiments showed that the presence of open fractures has a significant local effect on fluid flow even in a case with relatively high matrix permeability (200–300 mD). The sample containing F1 fractures showed a complex flow pattern influenced both by open fractures and stylolites. The present study aimed to further improve the understanding of natural open fractures and stylolites on flow to define the fracture/stylolites hydraulic properties to build a consistent simulation model. First, this paper focuses on the modeling of the fracture/stylolite system within two representative core samples:–Core sample A contains one long F2 type fracture crossing the sample (Figure 1);–Core sample B contains several F1 type fractures (Figure 2), which are short stylolite related. After the conversion of the CT-scan image into a core sample simulation model, hydraulic properties were determined. Permeability measurements on the whole core samples as well as mini-permeability measurements were conducted. In addition, for the fracture/stylolite intrinsic permeability determination, single phase miscible flow experiments were used.

Effective Elastic Properties of Rocks With Transversely Isotropic Background Permeated by Aligned Penny‐Shaped Cracks

“…The characterization of fractures can be done most directly by analyzing outcrops or core samples, from which first‐hand information on fracture properties, such as their geometries and distributions, can be obtained (e.g., Basquet et al, ; Zeeb et al, ). However, this method is restricted by the limited sampling volume and only provides fracture properties at sampling spots, rather than those of the whole studied area.…”

Dynamic seismic signatures of saturated porous rocks containing two orthogonal sets of fractures: theory versus numerical simulations

“…The most direct method for fracture detection and characterization is the study of outcrops or core samples (e.g. Zeeb et al 2013;Basquet et al 2008), which provides first-hand information on the geometries and distribution of the fractures in the formation of interest. However, it can only provide such information for a very limited rock volume.…”

Computed tomography scan imaging of natural open fractures in a porous rock; geometry and fluid flow

“…Basquet et al (2008) used single and two phase flow simulations to mimic the experiments described above. Firstly, the CT scan data enabled an exact representation of the fracture network in the core scale flow models.…”