Abstract:

Gas exploration in tight sandstones of the Xujiahe Formation in Sinopec's exploration areas, Sichuan Basin, has been fruitful.However, gas in the formation is pervasive but only locally enriched in rather tight layers, which results in low reserve abundance, water production being common and gas yields being controlled by fractures.Exploitation of the gas reservoirs in the formation has therefor been a challenge.With exploration and productive capacity building being pursued strategically in an efficient way by the group in the areas in recent years, Sinopec has followed a roadmap leading to an integrated geology and engineering during a new round of exploration and development activities in the areas of the basin.Significant progresses have been made in the basic geological theories and key technologies of exploration and development.The gas accumulation pattern, summarized as "charging prior to tightening and overpressure-driven with late adjustment", and the enrichment law, briefly described as "early structure controlling enrichment zones, differential tightening process controlling reservoir physical properties and late structural activities determining well performance", are combined to establish a trap dynamic evaluation method based on paleo and modern structures and deformation history.Sweet spot classification criteria have been established with 2 categories and 4 types of sweet-spot modes being built.For tight sandstone fracture prediction, an integrated technology of multi-attributes, including maximum likelihood, structural entropy, shape index and curvature, was used to describe fault-fold fractured reservoirs.To better identify the components of tight sandstone and fluids, we performed continuous logging evaluation of grain-size parameter and neutron acoustic envelope logging evaluation of gas-bearing properties.A nitrogen gas drilling technology has been proposed to enhance well performance and stimulation procedures have been tailored for different types of reservoirs.At present, the key exploration and development issue of the Xujiahe Formation is how to realize a large-scale and effective development.Based on the research in recent years, we suggest the following domains that require a continuous attention: a basin-wide study of differential enrichment of gas in the Xujiahe Formation; a fine geological evaluation of different types of sweet spots; fine description of the interior of fracture bodies, accurate prediction of high-quality reservoirs and development of fluid and engineering technologies for production enhancement and optimization; as well as an integration of geology and engineering combined with differential exploitation strategy according to different types of reserves.Only with development priority being given to sweet spots with well-developed fractures and study being focused on the improvement of production from sweet spots in porous matrix, will an effective exploration and production of gas from these hard-to-get reserves in the Xujiahe Formation be possible.

Key words: nitrogen drilling, differential enrichment pattern, geology-engineering integration, tight sand gas, Xujiahe Formation, Sichuan Basin