A Basic Research On Improving Development Effectiveness In Fractured-Vuggy Reservoirs

Fractured-vuggy reservoir is a typical multi-media reservoir. It is mainly distributed in the Tarim Basin with great resource potential. The experiment, theoretical calculation analysis and production analysis are carried out to understand the production behavior of some fractured-vuggy development block in the Tarim Basin, considering the characteristics of the fractured-vuggy reservoir, such as poor matrix physical properties, caves and fractures being the main hydrocarbon storage place, relevant research on caves and fractures. Results are as the following:1. Different fracture-cave models including fracture-cave type, single-cave type, multi-cave type and nearby-cavity cracks type are defined according to data from seismic, drilling, logging, well testing and producing. Based on the similarity analysis, the physical modeling and theoretical model deriving are carried out. For fracture-cave reservoir, the accumulated fluid production goes up significantly with pressure drop at the late production period. For single-cave reservoir, the cumulative fluid production goes straight line with pressure drop, the slope depends on the volume of fluid in the reservoir, and the fluid production rate and pressure decline decrease single-exponential with time. For multi-cave type unit, the liquid production rate and pressure decrease multi-exponentially over time, the accumulated fluid production and pressure drop form a concave up and increasing curve. For nearby-cavity cracks reservoirs, the water cut curve typically forms " stair " shape, and the stable water cut at the " stair" only depends on the oil and water volume in the unit. All the above findings indicate that different types of fractured-vuggy reservoir will have different development characteristics.2. Based on the study of the fracture-vuggy reservoir’s development characteristics, combining data from seismic, drilling, logging and well testing, a multi-stage fracture-cave unit identification mode is established, which can be initially determined according to production characteristics.3. The reserve calculation models are established for the various fracture-cave combinations of three types of production model, cyclic water injection model, stable water cut model and no water cut model. The following parameters, such as the reserve of the fractured-vuggy reservoir, the size of bottom water, and the total effective fracture and cave volume can be calculated by this model. They are applied into the analysis for LUNGU-7 area and shows good effect.4. Through a series of water injection and gas injection physical experiments, combined with theoretical analysis, the following conclusions are available. After gas injection, energy in the production unit and period recovery gradually increases, and therefore under the same conditions, better recovery can be achieved by gas injection comparing to water injection. Reservoir type, the size of the bottom water, reservoir connectivity, effective fracture-cave volume, fracture-cave distribution and connection with the wellbore codetermine the development effect of gas injection. A prediction theoretical model is established to predict the development effects of water injection and gas injection. Then an optimization method of fractured-cave unit for gas injection is proposed, which can provide some instruction for the gas injection operation in oilfield.