A new methodology to study naturally fracture reservoir with an induced fracture model was proposed using a representative sample of the Pre-salt reservoir. A core was cut longitudinally while the fracture was simulated using a polyoxymethylene spacer (POM). This fracture configuration was adapted based on the studies performed by Lie (2013) and improved with filling the voids with spheres with controlled grain size to represent a porous medium and increase the permeability and porosity of the fracture. To study the effect of injection of low salinity waterflooding, a forced displacement test was performed under pressure conditions of 1000 psi, temperature of 63°C, and flow rate of 0.1 ml/min. The core sample was prepared at initial water saturation (Swi). This process was carried out by forced displacement and a vacuum procedure in the coreholder using synthetic formation water and dead oil of the same field as the core. The sample was aged for 34 days to simulate the wettability reservoir conditions. During the test, the syntethic seawater (SW) injection was started, and, after eight days, it was switched to ten times diluted seawater (SW10x) for 22 days. Oil production was calculated by mass balance. The X-ray computed tomography (CT) technique was used to evaluate the heterogeneity of the porosity distribution and the saturations at different injection times during the Swi process. To validate the petrophysical properties, it was performed a systematic routine for the determination of the petrophysical properties of the induced fracture model and its components: matrices and fracture. The porosity and permeability for the matrices were 11% and 31 mD for part A and are 10% and 22 mD for part B. respectively. The porosity of the fracture was analytically calculated resulting in 1.6% while the permeability of the fracture was adjusted according to the theory of flow in parallel layers resulting in 129 D. Finally, the induced fractured rock showed a porosity and permeability of 21% and 3.6 D, respectively. The Swi reached 32% and 33% by using mass balance and computed tomography (CT), respectively. Additionally, CT scans provided the Swi profiles throughtout the sample. The results of production have shown that oil recovery with injection SW was 20.8% original oil in place (OOIP) and additional recovery from the injection of SW10X of 17.33%OOIP while the final recovery was around 38.13%OOIP.
An accurate understanding of the matrix-fracture mass transfer is fundamental to the modeling of fractured reservoirs. Nevertheless, the difficulty in an appropriate representation of this process comes from the fact that matrix and fracture interact in a particular manner depending on physical mechanisms as capillary imbibition. Capillary imbibition is considered through wettability in several mass transfer formulations (also called transfer functions) as the main mass driving force between matrix and fracture. This paper provides simulation results of waterflooding in two different scales of fractured models: Core plug models and extended models (a quarter of 5-spot), aiming to evaluate the influence of wettability and flow rate alteration on the matrix-fracture mass transfer. The methodology applied is based on sensitivity analyzes of wettability and flow rates scenarios, comparing parameters involved in matrix-fracture mass transfer: capillary continuity, fluid transfer rate, and hydraulic conductivity of the fracture system.
The methodology is divided into three main parts. Initially, single-porosity models with an induced longitudinally fracture at laboratory scale are simulated, to obtain accurate models in terms of representative responses for wettability and flow rate changes. Secondly, dual-porosity/permeability models are constructed also at laboratory scale to analyze and compare answers to mass transfer. As a third stage, extended models are created attempting to analyze the impacts of sensitivity parameters of mass transfer on a larger scale. Results show that the increase of rock preference for water leads to highest oil recovery factors at low and high-water injection rates, benefiting mainly from the water spontaneous imbibition. Notably, the spontaneous imbibition in these cases is more considerable in low-rate scenarios, due to its larger contact time with water and rock. However, the increment on production may not be economically feasible, because of the long time (high pore volumes injected) needed to get this increase. In contrast, intermediate and oil-wet scenarios exhibit low oil sweep and displacement efficiency at low and high-water injection rates. Accordingly, these scenarios reach water breakthrough quickly and exhibit a less accentuated tendency to water saturation alterations if compared with a water-wet scenario.
Results from single-porosity models show a good agreement between the water saturation distributions along the length and the effect of the induced fracture, validating its use. Results also reflect the effects of the fractured porous media formulations at both model scales as well as the effects of the shape-factors. In a numerical simulation study, this work shows the importance of close interaction between the wettability, flow rate changes, and the parameters that control matrix-fracture mass transfer. At last, the significance of these sensitive parameters is also demonstrated.
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