Torsten Clemens

Simulation of polymer injection under fracturing conditions - An injectivity pilot in the matzen field, Austria

Polymer flooding leads to enhanced oil recovery by accelerating oil production and improving sweep efficiency. However, because of the higher viscosity, the injectivity of polymer solutions is of some concern and is important to understand to predict incremental oil recoveries. Achieving high polymer-injection rates is required to increase oil-production rates. In the field test performed in the Matzen field (Austria), polyacrylamide polymers were injected for the past 2 years. Coreflood experiments with these polymers showed a significant increase in apparent viscosity because of the viscoelastic properties of the polymer solutions. Also, severe degradation of the polymer solution at high flow velocities was detected. In addition to coreflood experiments, flow experiments through fractures were performed. In these experiments, shear thinning and limited degradation of the polymer solution were observed and quantified. Detailed polymer-injection simulations were conducted that included complex polymer rheology in the fractures and the matrix. The reservoir stress changes and their effects on the fractures were also taken into account as a result of cold-polymer injection. The results of the simulations matched the field data both for waterfloods and polymer-test floods. The simulations revealed two distinct phases during the injection of the polyacrylamide-polymer solution: 1. Injection under matrix conditions in an early phase resulting in severe degradation of the polymers 2. Injection under fracturing conditions after the formation parting pressure is reached, leading to limited degradation of the polymers The calibrated model was used to investigate the impact of polymer rheology and particle plugging on injectivity and fracture growth. The results of the field test and the simulations indicate that screening of fields for polyacrylamide-polymer projects needs to include geomechanical properties of the reservoir sand and cap/ base rock in addition to the conventional parameters used in screening such as oil viscosity, water salinity, reservoir temperature, and reservoir permeability.

The Role of Diffusion for Non-Miscible Gas Injection in a Fractured Reservoir

To improve oil recovery from pervasively fractured reservoirs, gas injection can be considered. In such reservoirs, the fractures typic ally provide the flowpaths but contain a limited amount of the oil whereas the matrix often has orders of magnitude lower permeabilities but contains the oil. To improve oil recovery from such reservoirs, gas/o il gravity drainage can be applied. In this process, a gravity stable displacement can be achieved. If gas is injected which is not in equilibrium with the oil, the non-equilibrium gas components diffuse from the fracture system into th e matrix and the components of the oil diffuse towards the fracture system. This results i n a modification of the properties of the oil affected by diffusion and gravity drainage rate s accordingly. The effects of non-equilibrium gas injection into a pervasively fractured reservoir were studied at the example of an Austrian reservoir. Th is type of gas injection results in a zone of decreased oil viscosity for gases such as CO 2 and CH4 at the interface of the gas and the oil in the matrix. This zone of lower oil viscosity increases the gas/oil gravity drainage rates. The results show that the effect of diffusion can i ncrease cumulative oil production up to 25 % compared with a case neglecting the effect of dif fusion. The effect of diffusion could be determined for various parameters such as permeabil ity, porosity, fracture spacing and matrix block height. While for some of the paramete rs, the effect of diffusion scales with the square root of time (e.g. permeability), for ot hers an exponential relationship has been determined (fracture spacing). The results derived for the example reservoir can b e used more generally to screen whether the effect of diffusion should be incorporated into reservoir studies concerning nonequilibrium gas injection and how large the error c uld be in case that diffusion is neglected. 32 Annual Symposium & Workshop IEA Collaborative Project on Enhanced Oil Recovery 2 Introduction Oil recovery from pervasively fractured reservoirs is challenging due to the often large contrast in permeability of the fractures and the m atrix. Frequently, the matrix of such reservoirs contains the oil while the fractures pro vide the flowpaths. In such reservoirs, oil production can be improved by making use of gas/oil gravity drainage (e.g. O’Neill 1988, Novinpour et al. 1994, Saidi 1996, Eikmans and Hitc hings 1999). Different zones can be distinguished in reservoirs which are produced by using gas/oil gravity drainage (Saidi 1975): (1) Gas invaded zone: in this zone, the matrix is o il filled but partially gas invaded. The fractures are gas filled. The main processes occurr ing in this zone are gas/oil gravity drainage and diffusion. The oil is flowing in the m atrix down towards the oil rim. (2) Oil rim: this zone is located between the curre nt gas/oil and water/oil contact in the fractures. The fractures and matrix are oil filled. Oil is entering the fractures and flowing toward the wells. (3) Water invaded zone: This zone is characterised by the fractures being water filled and the matrix oil bearing but water invaded. The main processes are water/oil gravity drainage and water imbibition. The different zones and processes are illustrated i n Figure 1. Figure 1-Schematic diagram of a pervasively fractur ed reservoir with a large permeability contrast between fractures and matrix. The fractures are conected in the third dimension. Fractures are gas filled above the current gas/oil contact (GOC) whil e the matrix is still oil filled but gas invaded. The oil is flowing vertically downwards through the matrix in the gas invaded zone and then through the fracture system to the wells. Process Zones Gas/oil gravity drainage Diffusion Vertical flow of oil predominantly through the matrix Lateral flow of oil to the producers, predominantly through fractures Water/oil gravity drainage Water imbibition Original gas cap gas present in fractures gas present in matrix Gas invaded zone gas present in fractures matrix oil filled, partially gas invaded Oil rim oil present in fractures oil present in matrix Water invaded zone water present in the fractures matrix oil filled, partially invaded by water Aquifer water present in the fractures water present in matrix Original GOC Current Fracture GOC Current Fracture OWC Original OWC Fractures Matrix 32 Annual Symposium & Workshop IEA Collaborative Project on Enhanced Oil Recovery 3 For oil recovery of such reservoirs by gas injectio n, the performance of the gas invaded zone is essential. In this zone, the fractures are g s filled and the matrix is gas invaded. Experiments and theoretical concepts showed that in this zone, the oil is flowing down in the matrix (e.g. Saidi et al. 1979, Hagoort 1980, H ori et al. 1990, Labastie 1990, Firoozabadi and Ishimoto 1994). The change in oil saturation and oil flow in the ga s invaded zone neglecting capillary forces is shown in Figure 2. Initially, the matrix is completely oil filled. D ue to production and/or gas injection, a gas/oil contact in the fractures i s generated. Gas invades into the matrix. Two periods can be distinguished for such reservoir s, plateau gravity drainage prior to gas breakthrough and the continuously reducing oil prod uction after gas breakthrough at the bottom of the matrix (“after drainage period”) (Sai di et al. 1979, Hagoort 1980). The plateau production is the “free-fall gravity draina ge rate” governed by the density difference of oil and gas, matrix permeability and oil viscosity. Note that in this paper, gas breakthrough refers to gas reaching the bottom of t he initially oil filled matrix. Figure 2-Development of saturation profiles in a ma trix block for gas/oil gravity drainage conditions. Initially, the block is oil filled. Gas invades the block. Until the gas reaches the bottom of the blo ck (t4=tBT), the maximum gravity drainage rate can be achieve d. Then, the oil rate declines (“after drainage”). The diagram shows the oil saturation (So) versus depth (z) for different times (t 0, t1, t2, ...). tBT refers to the time of gas breakthrough at the bott om of the block. If an impermeable layer is hampering the flow of oi l in the matrix, the ultimate recovery is reduced due to the capillary hold-up. The maximum o il production rate is reduced if the block height is small compared with the fracture sp acing (Clemens and Wit 2001). Oil recovery from fractured reservoirs can be impro ved by injection of non-equilibrium gases. In such cases, the gas fills the fractures a nd components are exchanged by diffusion So

Reservoir Management of a Low Permeability Off-shore Reservoir Utilizing Water Injection into a Watered-out Horizontal Well Under Fracturing Conditions

Field X is located off-shore beneath ~100m of water. Initially, long horizontal production wells and sub-vertical water injection wells were used for the field development. When one of the oil production wells located at the edge of the field watered out, it was decided to convert it for water injection (Figure 1). Since reservoir permeability is low (tens of mD), generation of injection induced hydraulic fractures is expected.