Optimization techniques for the secondary development of old gas fields in the Sichuan Basin and their application

After nearly 60 years of development, many old gas fields in the Sichuan Basin have come to middle-late development stages with low pressure and low yield, and some are even on the verge of abandonment, but there are plenty remaining gas resources still undeveloped. Analysis shows that gas fields which have the conditions for the secondary development are faced with many difficulties. For example, it is difficult to produce low permeable reserves and to unset the hydraulic seal which is formed by active formation water. In this paper, therefore, the technical route and selection conditions of old gas fields for the secondary development were comprehensively elaborated with its definition as the beginning. Firstly, geological model forward modeling and production performance inversion characteristic curve diagnosis are performed by using the pressure normalization curve and the identification and quantitative description method for multiple sets of storage-seepage body of complex karst fracture-cavity systems is put forward, after the multiple storage-seepage body mode of fracture-cavity systems is established. Combined with the new occurrence mode of gas and water in U-shape pipes, a new calculation technology for natural gas reserves of multiple fracture-cavity systems with strong water invasion is developed. Secondly, a numerical model of pore-cavity-fracture triple media is built, and simulation and result evaluation technology for the production pattern of “drainage by horizontal wells + gas production by vertical wells” in bottom-water fracture and cavity gas reservoirs with strong water invasion is developed. Thirdly, the geological model of gas reservoirs is reconstructed with the support of the integration technologies which are formed based on fine gas reservoir description. Low permeable reserves of gas reservoirs are evaluated based on each classification. The effective producing ratio is increased further by using the technologies of well pattern optimization, horizontal-well geosteering and staged acid fracturing. And fourthly, overall simulation, optimization and prediction technology for regional pipeline net-works is developed by building a multi-node multi-link gas transmission pipeline network model. Application shows that this technology plays an important role in productivity construction, recovery factor improvement, production decline delay and production stabilization of old gas fields.