| Member 3 of Liushagang in WS17-2 oilfield is a loose sandy conglomerate faulty reservoir with low permeability and strong water sensibility. At present, preliminary research and evaluation are being undertaken before put into production. The main problems for development we are facing include that high-quality reserves are small, the natural energy is not adequate, the tested productivity is low, reservoir heterogeneity and water sensitivity are strong. Therefore, energy feeding measures such as water injection or gas injection must be taken for effective development of this reservoir. In order to study the formation sensitivity mechanism and evaluate the effectiveness of water injection and gas injection thus determining energy feeding way and providing guidance suggestion for the development of Member 3 of Liushagang in WS17-2 oilfield, in this work, water sensitivity evaluation experiments and water injection/gas injection feasibility researches for sandy conglomerate reservoirs have been first investigated and analyzed, after that, phase compatibility during gas flooding, reservoir sensitivity evaluation and damage mechanism, two-phase flow characters have been further studied to understand the mechanism for enhancing oil recovery during injection. Based on previous study, water injection and gas injection for EOR experiments have been carried out under the real reservoir conditions. These experiments were conducted by combined long cores, debris filled and compacted long cores, combined full diameter cores to synthetically analyze the displacement efficiency and optimizing the injection scenario. The following results and suggestions can be concluded.(1) Evaluation for phase compatibility during gas flooding. The injected gas (associated gas, CO2, CH4) can swell the oil and reduce the viscosity. Besides, the gas can also extract components of the oil which makes the volume shrink and the system become light. Also, the IFT can be reduced until miscible state is reached. A better phase compatibility between associated gas, CO2 and the oil than the dry gas can be concluded because near-miscible flooding can be achieved under reservoir conditions which contributes to gas injection development.(2) The rocks are mainly with mediate porosity and low permeability. Velocity sensitivity and stress sensitivity are both small which can be neglected. However, the water sensitivity damage is big. The main mechanisms of water sensitivity include static permeability damage caused by clay lattice expansion of Illite Smectite (I S) and dynamic damage caused by osmotic hydration of illite particle surface. The latter can be given the priority. Salinity sensitivity is caused by Lattice contraction, rupture, fall off and blocking during dehydration. Both damages are irreversible.(3) Under reservoir condition, the flow ability of oil is weak, two phase flow interval is narrow, displacement efficiency is not high, the oil permeability reduces sharply by water saturation and the water permeability remains low and can not "raise the head". Liquid increasing for carrying out oil is not effective and two phase flow resistance is big. The same is to gas injection besides the low two phase flow resistance which is a unique advantage for gas flooding.(4) The formation water damage and micro heterogeneity hamper the development efficiency significantly which may cause injection problems, fast breakthrough and poor liquid enhancing effect. Displacement efficiency and recovery of oil are low when simply injecting sea water and gas, while a good development response can be achieved by sea water /gas (associated gas, CH4) alternating with a final recovery of 60.05% and 52.12%, respectively.(5) It is suggested that injection-production well pattern should be first optimized and dynamic adjustment strengthened. Meanwhile, the reservoir pressure should be maintained and production rate should be controlled to reach a injection-production balance. Moreover, it is also suggested that field trials of several technologies including associated gas injection should be carried out during the late development stage. |
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