The Comparative Study of Thermal and Chemical EOR in Unconsolidated Siliciclastic Reservoir Containing Medium Heavy Oil

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Pru Krathiam (PKM) is a small onshore, unconsolidated sandstone reservoir in Thailand containing medium heavy oil with viscosity of approximately 50 cp. Fluvial channels supplied sediments to form mouth bar sands in lake with sand thickness of 1 to 3 meters. In its 25 years of natural depletion, the field has achieved merely 1.7% recovery factor. The difficulty in production has been attributed to aquifer support combined with unfavorable mobility, and sand production. Secondary and tertiary recovery methods have been investigated, with the assumption that sufficient sand-control could be implemented. Basic EOR screening reveals that thermal and chemical methods could be appropriate for this challenging field, in addition to infill drilling. Further investigation by means of a history-matched full-field reservoir simulation model indicates that chemical flooding has the advantage over cyclic steam stimulation (CSS) in this type of reservoir and reservoir fluids. Polymer flooding using high molecular weight polyacrylamide gives significant recovery improvement. Its implementation will give an extra benefit to the field which has high initial water cut as polymer solution contacts the unswept regions of the reservoir. The oil recovery appears relatively insensitive to rock-polymer properties, i.e. adsorption, inaccessible pore volume, and residual resistant factor. Further study shows that adding alkaline and surfactant can increase oil recovery beyond polymer flooding. Generic properties of oil/water/ASP system e.g. interfacial tension and surfactant adsorption were used. ASP flooding performance seems sensitive to these properties, so extra care must be taken when designing the process. The fundamental constraint of polymer flooding and ASP flooding operation is the cost of implementation. CSS, on the other hand, still faces up severe problems with reservoir heterogeneities and high initial water saturation. Reservoir heterogeneities cause steam to disperse unevenly, leading to poor heat distribution. High water saturation results in much of the heat being absorbed by water. Mobility improvement by viscosity reduction is small for medium heavy oil and is slightly overcome by the effect of steam condensation. Introduction Pru Krathiam (PKM) is one of the fault-bounded dip closures located on the eastern flank of Phitsanulok Basin. The discovery well, PKM-A01, encountered viscous oil with 17–19 oAPI in Lan Krabu formation. Lan Krabu formation was deposited in the fluvio-lacustrine environment: fluvial sediments were transported from the east, and were deposited as mouth bar sands in the lake to the west. Evidences from grain size distribution and fossil indication match the notable characteristics of fluvio-lacustrine sediments, which are low energy aqueous deposition and the absence of marine fauna. In some areas, features such as levee, back swamp, coal and rootlets can be found. These are indications of shallow lacustrine deposits with frequent variations in the water level. Cyclicity of the deposition results in alternating lamina of clay and organic matter. Sand body size is in the range of 700 to 1100 meters in width and length, and 1 to 3 meters in thickness. The net-to-gross is in the range of 15 to 20%.

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... Characteristics of heavy oil from the north of Thailand can be concluded as shown in Table 1 (Sirisawadwattana et al., 2012). This formation is unconsolidated sandstone formation with permeabilities averaging 500 md containing medium heavy oil with viscosity around 54 cP. ...
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A small oil field in the north of Thailand has medium viscous and low gas-content heavy oil. Since conventional production methods are ineffective, thermal recovery is potentially suitable to enhance oil recovery for this reservoir. In -situ combustion is a complex EOR process used for medium to heavy crude oils. The process involves the multi-phase fluid flow through porous media with chemical and physical transition of the crude oil components under high temperature and pressure conditions. The simulation results with STARS were investigated by conducting a number of sensitivity studies with varying the parameters like gridblock sizes, air-injection rates, oxygen concentrations, and injected air temperature. The 0.5m-block size was chosen due to the optimum running time with acceptable accuracy. From the results, it can be concluded that changing injection rate from 100 Mscf/d to 400 Mscf/d does not significantly affect cumulative oil production – less than 6% incremental recovery. Increase oxygen concentration from 29% to 100% shows an increase in 40.67% oil production. Moreover, if the injected fluid temperature is increased from 80˚F to 500˚F, total oil production increases 97.14%. Furthermore, optimal operating conditions to enhance recovery of oil were also studied.
In Iran, there are a number of heavy oil reservoirs whose importance is growing as the conventional resources deplete. This study concerns the numerical simulation of cyclic steam stimulation of one of the heavy oil reservoirs. Results are encouraging and should be tested by field pilots. Heavy oil is characterized by its high viscosity. Thermal methods reduce viscosity and residual oil saturation to improve mobility and achieve an economical recovery. Cyclic Steam Stimulation (CSS) which has faster production, lower capital costs and lower pressure operations than steam-flooding is of great interest in thermal methods. Oil recovery with steam injection has been enhanced with horizontal wells by increasing sweep efficiency, the contact area opened to flow, producible reserves, steam injectivity and also by decreasing the number of wells required so that higher oil production is reached. K-Field is one of the Iranian fractured heavy oil fields with low API of 7.24 and high viscosity of 2700 cp. Although steam injection in naturally fractured heavy oil reservoirs provides extremely challenging issues, it can be considered as a potentially effective and efficient improved recovery method. In this study, using STARS, a thermal dual-porosity model was constructed based on the available measured data to study CSS. Comprehensive and comparative studies and a sensitivity analysis of various operational parameters were conducted in order to find the optimum conditions for a high RF. This work shows that oil recovery could be improved from 0.66% by cold production to more than 10% by CSS during a 10 year period.
A study was carried out to determine the geomechanical effects of polymer flooding in an unconsolidated-sand reservoir. The work involved laboratory-scale polymer injections in unconsolidated- sand blocks to identify the injectivity mechanisms, numerical analyses for fracture prediction, and geomechanical modeling of the formation to examine the potential of shear failure and containment loss during flooding. Laboratory tests under polyaxial conditions indicate that nearwellbore fracturing and permeability increase in unconsolidated sands occur at net injection pressures limited to 2.0 MPa. These findings were applied to fracture modeling. Geomechanical modeling suggests large-scale shear failure in the sand and in the bounding shale during polymer flooding. These are expected to affect both the fracture containment and the vertical-hole integrity. Finally, fracture predictions underscore the importance of the geomechanical considerations on determining the fracture dimensions and containment. Sensitivity analyses also point to the significance of binding several key parameters for fracture prediction. These include sand shale stress contrast, fluid quality and total-suspended-solids (TSS) content, fluid rheology and effective viscosity in the formation, and the filter-cake properties in the presence of polymer. This paper provides a geomechanical perspective on the generally complex problem of polymer flooding in unconsolidated formations containing viscous oil. The work also offers some insights into the critical issues that must be examined in such situations to avoid catastrophic failures. It highlights the existing technological gaps in the current predictive capabilities.