Research On Gas-water Distribution Of Low Permeability Sandstone Gas Reservoir

Abstract: The lithology, pore structure and gas-water distribution of ancient timespermian gas reservoir in Sulige Gasfield were complicated. Located in the north ofSulige Gasfield, the Su54area He8and Shang1gas reservoir was one typicallow-porosity and low-permeability sandstone lithologic gas reservoir, with poorphysical properties, small radius of pore throat and strong heterogeneity. Su54areawas in the early period of development. The He8member in Shihezi formation andShang1member were the principal producing formations in Su54area. UntilNovember2012, there were82drilled wells containing39exploratory wells and43development wells. With continuous exploratory development, there was water in theprocess of gas testing in nearly30wells and meanwhile the water yield of single wellcould be up to72m3/d. Judging from the water production interval, there was waterboth in He8and Shang1members; from the distribution of water wells, the water wellpositions were scattered, which indicated that there was always water in gas reservoirs;at present, the research degree of the main production layer was low, and thedistribution of sand body, the cause of effluent of gas well, the connectivity offormation, distribution of water body and the controlling factors in Su54area had notbeen clearly known. Additionally, the recognition of gas and water layers was difficult.In order to avoid water body and achieve high-efficient production in the process ofdrilling, and reasonably develop Su54He8and Shang1member gas reservoir, the gasreservoir in this area was selected as research object in this paper.Based on previous research achievements and data on rock core, well log,logging, water chemical analysis, slice observation and gas testing, the modernreservoir description technology and methods, such as reservoir assessment technology and log analysis techniques, were adopted to comprehensively study thechemical characteristics of formation water, recognition of gas-water layer, physicalproperties of sand body and its distribution, distribution of water body and itscontrolling factors in accordance with the development status of upper Paleozoic inOrdos Basin to ensure the high efficient development of gasfield; at the same, thisresearch was guided by discipline theories, such as sedimentoloty, petroleum geology,oilfield chemistry, reservoir geology and reservoir geology. Through the research, thefollowing achievements and understandings were obtained:1. Based on sedimentary theory, the high-resolution sequence stratigraphy, themethod of the key bed and equal height were applied to the fine strata classificationand correlation of Su54area He8and Shang1members in Sulige Gasfield, with fullyconsidering the sedimentary background and sedimentary characteristics in study area,and following the principles of “big then small, step by step”. Finally the isochronousstratigraphic framework in Su54area was established; the standards which weresuitable for the stratigrahic division and comparison of this area were set up; and thelayering of all wells in this area was unified.2. According to comprehensive log interpretation, water chemical analysis dataand gas testing data, the water production types of gas wells in Sulige gasfield weredivided into three sorts: normal formation water, desalinated formation water andraffinate; on this basis, the chemical characteristics of formation water in He8andShang1members of Sulige gasfield was analyzed. It could come to the followingconclusions: the differences between the chemical characteristics of formation waterin this two members was not significant; the Na++K+and Ca2+were the main cationsin formation water, followed by Mg2+; while the Cl-was the main anion; and theformation water produced in the two members could not be distinguished by totalsalinity and hydrochemistry characteristics coefficient. Through the comprehensiveanalysis on hydrochemistry characteristics of formation water, the results showed thatgenerally the sealing of He8and Shang1gas reservoir in Sulige gasfield was better,which was in favor of conversation of natural gas.3. By using the data of cores and thin slices of He8and Shang1members andcombining the characteristics of logging, the facies markers such as the color of rocks,sedimentary structure, petrological characteristics and grain-size characteristics ofreservoir were analyzed. Through analysis, observation and assay, it was thought thatthe upper segment of Shang1and He8members was the distributary channel depositof meandering river, and the water system of He8member was mainly the distributary channel deposit of braided river. Consequently, the micro-phases, including channelretention sediment, sideband deposition, channel bar sedimentary, flood plain andnatural levee, were divided.4. In terms of the casting slices, scanning electron microscope and physicalproperty measurement, the reservoir characteristics of He8and Shang1members wereevaluated. There were chiefly four pores developed in the reservoirs of He8andShang1members----primary intergranular pore, secondary solution pores, kaoliniteintercrstallline pore and microfissure. According to reservoir physical properties, thereservoir of He8and Shang1members belonged to typical low-porosity andlow-permeability reservoir; the sorting of pore structure was poor, with the coarse-fineskewness type as the principle; on the whole, the reservoir physical properties of thelower segment of He8were better.5. Combining the logging curve and core observation, the electricalcharacteristics of reservoir sand of He8and Shang1members were analyzed, eachsmall layer of Shang1and He8segments were furtherly subdivided, and thedistribution of sand body in study area was also analyzed. The sand body extendedalong south and north direction as a whole; the continuity of sand body in thedirection parallel with sources was better, extending widely; while the continuity waspoor in the direction perpendicular to the sources, taking on shapes of isolated points;the inhomogeneity of distribution, the continuity of sand body and changes ofreservoir physical properties directly caused the strong heterogeneity of sand body inthis area.6. Through synthesizing the perforation gas testing data and data on logging,lithology and hydrochemistry, the cause of low-resistivity gas reservoir in study areawas analyzed, with the results that the low-resistivity gas reservoir was caused by thephysical properties of reservoir, invasion of mud filtrate and matrixes of shaly; byapplying two-level recognition principle, the permeability formation andnon-permeability formation were recognized firstly, and then the gas and water inpermeability formation were recognized by many methods. Through comparison, therecognition methods of gas and water which were suitable for this area were screened.The porosity-resistivity method was mainly used, assisted by apparent formationwater resistivity method and multiple discriminate analysis method.7. In terms of multi-factors containing the geology in study area, it was thoughtthrough analysis that the geological structure was the basic factor for the distributionof gas and water, gas-water identification was guarantee factor and accumulation process was the key factor. Through combing the results of gas-water identification,the sedimentary micro-facies and the distribution of sand body were taken asbackground to predict the distribution law of water body in vertical and lateraldirections. In general, there was effluent in each interval in vertical direction whilewater body was scattered in this study area in lateral direction. By tracing water body,the cause and type of formation water were analyzed.