Research On Reservoir Parameters And Characterization Of Continental Tight Sandstone

The T3X2 (the second member of Xujiahe Formation of Upper Triassicis) in central Sichuan area is typical terrestrial clastic deposits, with low porosity and permeability and high water saturation controlled by sedimentary diagenesis, characterized by the quite gentle structure and undeveloped fractures which lead to poor gas-water differentiation and no uniform gas/water contact. However, the study methods for conventional reservoir have been used for evaluation of similar reservoir parameters for a long time, which is the major factor that leads to lower exploration successful rate since 1960's. Based on the full participation into study of different formation reservoir parameters of Xujiahe Formation in central Sichuan, the T3X2 of Tongnan is selected with the typical features of tight, low porosity and permeability. With much deeper understanding, methods for evaluation and related means are summarized to provide reference for the similar reservoirs.Based on the previous sections, core observation, microscopic identification and sedime- ntary facies analysis, this paper studies and summarizes the tight sandstone reservoir parame- ter characteristics, variations, and determining method for petrophysical property cutoffs by using a large amount of the conventional and special core analysis data, then points out the favorable sedimentary facies and exploration areas of the Second Member of the Xujiahe Formation in Tongnan.The Xujiahe Formation in Tongnan area is a set of inland fluviolacustrine alternative terrigenous clastic deposit, and the 2nd member of Xujiahe Formation mainly is delta front sedmentary system, among which the submerged distributary channels and river mouth bars are considered as the most favorable facies belts for reservoir distribution and development. Furthermore T3X2 is the tight reservoir with low and extra-low porosity and permeability, which mainly developed residual intergranular pore, secondary dissolved pore, matrix and intercrystalline micropore of clay mineral. According to reservoir lithology, physical properti- es, pore structure, casting thin sections as well as productivity data, T3X2 can be divided into 4 classes: the first reservoir and the second are easy to produce, the third has the industrial productivity after stimulation treatments, and the fourth is non-reservoir.Using the gas reservoir cores of T3X2 tight sandstone in Tongnan area, gas permeabilities of gas/water bi-phase system at permitted limit production pressure difference are tested under whole simulation conditions firstly. According to the method of the gas permeability under permitted limit production pressure difference together with gas/water bi-phase relative permeability curve, upper limit of water saturation reaching industrial productivity is 55%, which is also applied to the reserves caculation.Using the data of gas/water capillary pressure, relative permeability, and water film thickness combined with mercury injection data to study the water saturation variation with physical properties and related mathematical model, the lower limit values of porosity and permeability reaching industrial productivity is 6% and 0.1md respectively. The productivity simulation experiment indicates that the reservoir of T3X2 is 15m thickness with the porosity over 6% and permeability over 0.07md, and the production of single well could reach or exceed the commercial production standard under those parameters mentioned above. These lower limits are also proved to be reasonable by tests. On the other hand, the sandstones mainly consist of feldspathic litharenite and litharenite, and reservoir matrix display such characteristics as low porosity and permeability, and high water saturation.Based on the well logs correction and lithologic identification, the log interpretation model is set up, and log data is explained by using it. According to the lower limits of physi- cal properties, reservoir could be classified into effective and non-effective. The results of reservoir evaluation and prediction show that T3X2 is developed with bigger changes from both the vertical and horizontal. The studied area is the second reservoir and the third mostly. Effective reservoir is mainly distributed in the upper middle of T3X21 and lower middle of T3X22, and the most favorable sedimentary facies is subaqueous distributary channel and main river channel located in south-southwest of the studied area.The Second Member of the Xujiahe Formation in Tongnan is lithologic gas reservoir characterized by low porosity, low permeability and compacted sandstone, where the struct- ure is gentle and regional low-relief, water and gas could not be easily detached, and water- -bearing reservoir is general. Drilling parameters indicate that the relatively higher part of structure is more favorable to hydrocarbon enrichment and accumulation. The research also suggests that wellblock Tong5-Tongnan113-Tongnan109 and Tong3-Tongnan110 located in high reservoir capacity and fracture-developed superimposed zones are the favorable belts for oil and gas accumulation and exploration.