Fracture-matrix Transport Research Articles

Quantification of the Impact of Acidified Brine on Fracture-Matrix Transport in a Naturally Fractured Shale Using in Situ Imaging and Modeling

Quantification of the Impact of Acidified Brine on Fracture-Matrix Transport in a Naturally Fractured Shale Using in Situ Imaging and Modeling

Stabilizing CO2 foams using APTES surface-modified nanosilica: Foamability, foaminess, foam stability, and transport in oil-wet fractured porous media

Foam stability and the improvement of its transport from fracture to matrix have been two major issues in the field of foam injection into fractured petroleum reservoirs as an enhanced oil recovery (EOR) method. In the current study, we have stabilized CO2 foam using surface-modified silica nanoparticles. The modification of nanoparticles was performed using 3-aminopropyltriethoxysilane (APTES). They were characterized by the FTIR analysis and contact angle measurements. After that, foam generation and stability were investigated using static experiments. The experiments were performed at different concentrations of NPs (0.04 to 0.20 mass%), two concentrations of SDS (0.236 and 0.472 mass%), and in the presence and absence of MgCl2 salt. Additionally, the oil recovery and fracture-matrix transport properties of various injected fluids, including CO2 gas, surfactant solution, and NPs stabilized foams were investigated and compared using flooding experiments in a natively oil-wet, fractured micromodel.The results indicate that the surface modification of Silica using the APTES makes the nanoparticles more oil-wet in oil-water system and more gas-wet in air-water system. This, in turn, amplifies the migration of the modified Silica (MS) toward the water-gas interface and enhances the CO2 foam stability. The occurrence of this mechanism could be verified by foam stability experiments, where switching the foam stabilizer from Silica to MS nanoparticles led to a 33.3% increase in the maximum foam half-life. The foamability is not affected by the type of NPs; however, the foaminess is reduced due to the surface adsorption of SDS on the nanoparticle surfaces.Moreover, the stability of foams is reduced in the presence of Mg2+ ions. The reason is investigated utilizing the Mg2+ ion concentration measurements via the atomic adsorption method. Finally, the micromodel flooding indicates that the order of recovery enhancement potential of the CO2 foam stabilizers is SDS-MS > SDS-Silica>SDS. We will discuss the pore-scale phenomena, which lead to such results. We have found that during foam flow in the fractures, the large size bubbles act as viscose fluid and push the relatively smaller bubbles into the matrixes pores. This fracture-matrix transport is followed by the stable foams, and a foam bridge is formed between the fracture and matrix. Additionally, foam front is disintegrated into gas bubbles and water droplets containing surfactants and NPs, which can penetrate the dead-end pores and displace the oil in them through the formation of W/O emulsion.

Experimental and numerical investigation of tertiary-CO2 flooding in a fractured chalk reservoir

This paper describes tertiary CO2 recovery mechanisms observed in two distinct core flooding experiments. In both experiments, water flooding followed by CO2 injection into an outcrop chalk with matrix-fracture system at reservoir conditions. The objective of this paper is to study the main mechanisms that control the rate of oil recovery during tertiary-CO2 flooding. A series of experiments as well as compositional numerical simulations have been carried out to evaluate the efficiency of the process.In the first experiment (Exp-1), the fractured core is initially saturated with North Sea Chalk Field (NSCF) stock tank oil (STO) and the connate water. Whereas in the second experiment (Exp-2), the core is saturated with NSCF live-oil and the connate water. Once the reservoir condition is established (258 bara, 110 °C), sea water is injected from the bottom of the fracture and the oil is recovered from the top. Once no more oil recovery is observed, the water flooding (WF) is stopped. A “shut-in” period follows which allows preparing the rig for the CO2 flooding (CF). Right after the “shut-in”, CO2 is injected from the top of the fracture and the oil is produced from the bottom. A compositional reservoir simulator with a tuned equation-of-state (EOS) is utilized to model the experimental work. An automated history matching algorithm is developed to fit the experimental data of WF and CF periods.Good agreement between the model and experimental data is obtained. We observe a strong impact of hysteresis on fitting the fluid production during CO2 flooding in Exp-2. The sensitivity analysis shows that tertiary CO2 recovery is affected by the water saturation in the core after the secondary WF. Moreover, the fracture-matrix transport function during tertiary-CO2 flooding is dominated by the diffusion rather than the convective flux or viscous forces.The outcome of this work is an important step towards modeling the tertiary-CO2 flooding in an actual fracture-chalk system. Proper modeling of imbibition and diffusion dominated processes in a chalk system at reservoir conditions has been accomplished.

Particle-tracking simulations of anomalous transport in hierarchically fractured rocks

Complex topology of fracture networks and interactions between transport processes in a fracture and the ambient un-fractured rock (matrix) combine to render modeling solute transport in fractured media a challenge. Classical approaches rely on both strong assumptions of either limited or full diffusion of solutes in the matrix and simplified fracture configurations. We analyze fracture-matrix transport in two-dimensional Sierpinski lattice structures, which display a wide range of matrix block sizes. The analysis is conducted in several transport regimes that are limited by either diffusion or block sizes. Our simulation results can be used to validate the simplifying assumptions that underpin classical analytical solutions and to benchmark other numerical methods. They also demonstrate that both hydraulic and structural properties of fractured rocks control the residence time distribution.

Estimation of Fracture–Matrix Transport Properties from Saturation Profiles Using a Multivariate Automatic History Matching Method

This study focuses on the development and implementation of a numerical model of multiphase flow in a fractured core sample by describing the capillary pressure–relative permeability characteristics. An automated history matching approach is proposed to determine relative permeability and capillary pressure curves simultaneously for both matrix and fracture consistent with a core flood reservoir model performance, which relies on a commercial reservoir simulator coupled with an optimization protocol. The results indicate that the proposed approach successfully predicts relative permeability and capillary pressure curves of a fractured core sample and provides a foundation for field-scale history matching with simultaneous estimation of transport properties.

Establishment of Uncertainty Ranges and Probability Distributions of Actinide Solubilities for Performance Assessment in the Waste Isolation Pilot Plant

AbstractThe Fracture-Matrix Transport (FMT) code developed at Sandia National Laboratories solves chemical equilibrium problems using the Pitzer activity coefficient model with a database containing actinide species. The code is capable of predicting actinide solubilities at 25 °C in various ionic-strength solutions from dilute groundwaters to high-ionic-strength brines. The code uses oxidation state analogies, i.e., Am(III) is used to predict solubilities of actinides in the +III oxidation state; Th(IV) is used to predict solubilities of actinides in the +IV state; Np(V) is utilized to predict solubilities of actinides in the +V state. This code has been qualified for predicting actinide solubilities for the Waste Isolation Pilot Plant (WIPP) Compliance Certification Application in 1996, and Compliance Re-Certification Applications in 2004 and 2009.We have established revised actinide-solubility uncertainty ranges and probability distributions for Performance Assessment (PA) by comparing actinide solubilities predicted by the FMT code with solubility data in various solutions from the open literature. The literature data used in this study include solubilities in simple solutions (NaCl, NaHCO3, Na2CO3, NaClO4, KCl, K2CO3, etc.), binary mixing solutions (NaCl+NaHCO3, NaCl+Na2CO3, KCl+K2CO3, etc.), ternary mixing solutions (NaCl+Na2CO3+KCl, NaHCO3+Na2CO3+NaClO4, etc.), and multi-component synthetic brines relevant to the WIPP.

Development of the FMT chemical transport simulator: Coupling aqueous density and mineral volume fraction to phase compositions

The Fracture-Matrix Transport (FMT) code couples saturated porous medium advection and diffusion with mechanistic chemical models for speciation and interphase reactions. Previous versions of FMT simulated double-porosity transport in two dimensions on the continuum from advection- to diffusion-dominated, with a user-specified velocity field to allow double-porosity transport. However, aqueous density was assumed constant, and reactive minerals were assumed to occupy negligible volume. Both of these assumptions can be considered poor for evaporite systems, where large changes in porosity and aqueous density can result from high mineral solubilities. Further development of FMT has relaxed these restrictions, allowing aqueous density to vary with phase composition, and allowing void volume to change as minerals dissolve and precipitate. This paper describes the additional mathematical complexity and code modifications required to simulate such systems. The sensitivity of advection-dominated transport to these variables is explored briefly in a one-dimensional example.

Transport Modeling in a Finite Fractured Rock Domain

ABSTRACTThe Waste Isolation Pilot Plant (WIPP) is a U.S. Department of Energy facility intended to demonstrate the safe disposal of transuranic nuclear waste. The WIPP is located in the thick halite beds of the Salado Formation in the Delaware Basin of southeastern New Mexico. Overlying the repository is the Culebra Dolomite Member of the Rustler Formation, a dolomitic unit containing accessory minerals such as calcite, gypsum, and clays. The Culebra is the predominant continuous water-bearing unit in the area. Part of the scientific demonstration of the safety of WIPP involves understanding and estimating what might happen should the WIPP inadvertently be intruded by exploration boreholes long after the WIPP has been decommissioned. Should such a breach occur, it is possible that dissolved transuranic waste could be transported up a borehole and into the Culebra, through which it could eventually reach the accessible environment. The rates and mechanisms for actinide transport in Culebra Dolomite are being investigated as a component of repository performance assessment.The two primary mechanisms for delaying solute release from a double porosity system are called physical and chemical retardation, both of which retard solute migration relative to bulk water flow. Physical retardation occurs when solutes diffuse out of the advective transport regions (fractures) and into diffusive transport regions (matrix). Results of hydrologic field tests suggest that the Culebra behaves as a double porosity medium. Chemical retardation occurs when chemical interactions between dissolved species and mineral surfaces remove solutes from the aqueous phase. Some fracture surfaces in the Culebra are clay-lined, and, because clays can have a large potential for chemical interactions with solutes (e.g., ion exchange or adsorption), it is possible that fracture linings alone may cause significant retardation of any actinides that could reach the Culebra. This report examines these two features in Culebra transport: delimiting the parameter space where a fully two-dimensional model is needed, and examining the importance of clay linings during transport.Numerical simulations with the FMT (Fracture Matrix Transport) finite difference model in a finite, idealized double porosity system were performed to: 1) delineate parameter values for which a set of two coupled one-dimensional equations is insufficient to describe behavior and a fully two-dimensional representation is needed, 2) compare chemical retardation by ion exchange for fractures with and without a clay lining, and 3) compare physical and chemical retardation by ion exchange as a function of fracture velocity. The simulations differ from others in that the matrix is finite, and thus can become saturated with tracers introduced into the domain. The modeling suggests that the chemical retardation caused by clay linings may not be important in a double porosity system. Over a wide range, a single parameter group essentially determines the complexity of the model.