Research On Development Adjustment Plan For The Sha Xi Miao Tight Gas Reservoir With Multi-layers In Xin Chang Gas Field

Tight sandstone gas reservoir is of low richness, low porosity, low permeabilityand low individual well producing rate, however, it produced natural gas in a largeproportion in our country. At present, how to exploit efficiently the large multi-layerssuperimposed tight sandstone gas reservoir is still in the preliminary stage in theworld, and don't even mention the research on the three-dimensional adjusteddevelopment plan. The Xin Chang gas field, which located at the middle section ofthe western Sichuan depression in the west part of Sichuan basin, is a large tightclastic gas field imposed by many gas reservoirs vertically. Its production lies in thetop list in our country. JS₂ reservoir, the main producing reservoir and consisting fourgas formations, produced in 2000, achieved the designed production in 2002 and ithad been producing over the designed for almost six years. Now it is difficulty toproduce steadily, so, now, the proper & timely adjustment for the gas reservoir is ourfirst target.Based on the various geologic formation, experimental, geophysical and dynamicdata, adopted by the methods of static prediction with dynamic analysis, used by themeasures of three-dimension geological modeling and numerical simulation, JS₂ gasreservoir has been quantitatively key researched on the unproduced gas distributionwith the goal of quantitative evaluation, which aims to improve the recovery andeconomic efficiency and to provide the direct & efficient guide for the furtherdevelopment.During the process of development, more and more data & knowledge achievedand more advanced fracturing technology results in the even less steering function tothe reservoir development for the original plan. That is why this article focuses on theissues as following:(1) Basic features description of gas reservoirThe fine description of structural characteristics, sedimentary microfacies andsand body spread based on the 200 wells log and the seismic data, indicates that JS₂ isa flat nose structure, the main sedimentary microfacies are distributary river course ofdelta plain with four thick & stable sand bodies. A large amount of chlorite and calcite presented and their uneven distribution lead to worse physical properties and strongmicro non-homogeneous property by the description of reservoir lithology, physicalproperty, diagenesis and non-homogeneous property. The comprehensive analysis onthe water produced wells shows that formation water exits in the formation edge andwithout unified boundary between gas and water. There are obvious characteristicsreflected on the log curve and seismic data, such as low natural gamma ray, lowneutron, low density, high sonic and medium resistivity, which can distinguish thereserves, low frequency, strong amplitude and low resistance, whose gas -bearingresponse module can give the plane prediction to the reserve. From the above researchwork, it is concluded that JS₂ is a kind of porous tight gas reservoir with abnormalhigh pressure, driven by gas and controlled by structure and lithology.(2) Three-dimension modeling of gas reservoirBased on the comprehensive description above mentioned, according to thegeological statistics theory, adopted by modeling software Petrel and under the controlof function, we simulated 100 actualizations and selected the final module whichhas the best conformity with geological knowledge. Besides, used by the method ofrandom modeling and under the control of petrofacies module, we simulated 100realizations individually for five parameters including porosity, permeability watersaturation, sonic and resistivity, and also selected the final actualization as the finalmodules which have the best conformity with facts. In accordance with the obtainedparameter module, combined with the characteristics of various gas formation, weset down the classification criteria of reserve and at the same time, we made anevaluation as listed: on the range of research area,â… andâ…¡reserves dominate in JS₂²and JS₂⁴ reservoirs, JS₂³ withâ…¢reserve andâ… andâ…¡reserves local distributed andJS₂¹ withâ… andâ…¡reserves of worse quality.(3) Reserves recalculation and evaluationReserves recalculated by using the volumetric method and the stochasticmodeling method in this paper. Based on the original proved reserves and accordingto the abundant dynamic and static datum, each parameter in the volumetric method isrecalculated and confirmed by using the new interpretation model. It is obtained thatthe reserves of Sha Xi Miao reservoir (JS₂) is 463.75×10⁸m³ through the volumetricmethod, and the reserves of each layer (JS₂¹, JS₂², JS₂³, JS₂⁴) is respectively104.85×10⁸m³, 167.75×10⁸m³, 46.56×10⁸m³, 144.60×10⁸m³. The recalculatedreserves has decreased by 71.60×10⁸m³ compared with the original proved reserves which is 534.35×10⁸m³. The layers of decreased reserves are JS₂³, JS₂⁴, the reason isthe decreasing of area. The stochastic modeling method is based on thethree-dimensional stochastic modeling, through dividing the whole reserves layer intoa lot of grids, considering the reserves parameters in each grid as stochastic variables, and performing grid integral for each group of stochastic variables after reservescalculation by the volumetric method, and a series of realized values of reserves arefinally figured out. Then reserves datum of different reliability can be obtained bydrawing the density function curves and accumulative probability curves according tothe distributing of reserves. This method is not only combined with geologic modelclosely, but also reserves datum of different reliability is obtained, which makingresults more external. The probability reserves P₉₀ is 470.71×10⁸m³ through thestochastic modeling method. The value is close to that of recalculated reserves and theerror is 1.5%. Based on the reserves recalculation, reserves is classified and evaluatedaccording to layer-sorting, resistivity-sizing, and testing productivity. Reserves ofgradeâ… is 186.37×10⁸m³, which is 40% of the total, and AOF is more than 5×10⁴m³.Reserves of gradeâ…¡is 183.33×10⁸m³, which is 39% of the total, and AOF is morethan 2×10⁴m³. Reserves of gradeâ…¢is 97.23×10⁸m³, which is 21% of the total andbelongs to low- profit reserves. Except JS₂¹, reserves of gradeâ… andâ…¡of other layershave been produced which is totally 288.45×10⁸m³.(4) Synthetic adjustment plan research of gas reservoirAccording to the execution schedule and development effect of the developmentplan of Sha Xi Miao JS₂ reservoir, not only geologic cognition but also developmentlayer series, well pattern, product of single well, and production scale are accord withthe practice of reservoir. But the production scale is always higher than the designedscale, which results in that the stable production period has been shortened. The mainproblems the reservoir faced are that it is difficult to keep production stable, on theother hand, recovery percent of reserves is rather low and un-produced reserves israther great. Remained reserves research shows that the present remained reserves ofreservoir is great which is composed of two parts. One is low- profit reservesdistributing in every layer but mainly in JS₂¹, JS₂³, which can not be produced due tohave no well controlling. The other is relatively high grade reserves which keepsrather high formation pressure due to big well spacing although has been produced.Thus there are two ways to keep production stable. One is to use commingledproducing wells between low- profit and high grade reserves through multi-layer fracturing technology, the other is to dispose adjustment wells (including wellwork-over) to increase recovery percent of producible reserves. Newly-increasedproduced reserves and producible reserves will provide keeping production stablewith substantial bases. According to the distribution of different grade of reserves andremained reserves, 66 adjustment wells are disposed and three adjustment programsare designed. The first program decrease present wells production by 45×10⁴m³without adding new wells and makes daily production to the level of 240×10⁴m³. Thesecond program adds 31 new wells on the base of the first program. The thirdprogram adds 35 new wells again on the base of the second one. The dynamicdevelopment indexes of each programs have been forecasted by using ECLIPSEsoftware based on history matching. The results show that the cumulative productionand recovery percent is obviously increased and the third program is better than thesecond one. Through disposing 66 adjustment wells, the production is increased by32.7×10⁸m³ (8.91×10⁸m³ from JS₂¹ and JS₂³), the recovery percent is increased by6.5%, and the stable production period is extended by 5years. Thus the third programis recommended to be the adjustment paln of the reservoir.This paper has three innovative points:1. The reserves of multi-layer tight reservoir JS₂ is calculated by using stochasticmodeling method for the first time. This method combines integral method withstatistical simulation, which not only exhibits geologic conceptions in the volumetricmethod, but also considers the uncertainty of calculative parameters of reserves. Theresults is expressed by the way of probability, and to some degree, evades thelimitation that traditional methods can only obtain one reserves value but evaluate theveracity of the value.2. Economic reserves and hypo-economic reserves are both considered aimed atmulti-layer tight gas reservoir. The method of three-dimensional Adjustment Plan forhypo-economic reserves being driven by economic reserves by multi-layer fracturingand implementing of adjustment wells. According to this idea, the three-dimensionalAdjustment Plan has been drawn. It has important direction and reference significanceto high-efficient development of large and middle scale gas fields, only the economicreserves of which are produced in the past.3. The two technologies of modern reservoir management-reservoir layermodeling and numerical model combine to one part, which is used to researchthree-dimensional Adjustment Plan of multi-layer reservoir. A researching technology system for tight reservoir Adjustment Plan has been formed, which is based on thefine geologic model, making a target of quantitative evaluation for reservoir layersand reserves, and taking the distribution of remaining gas as the researching point.