Numerical Simulation Research On The Impact Of Water-rock Interaction On The Reservoir Permeability

Ordos Basin is one of the most important energy bases in China. The UpperPaleozoic formation in North-East Ordos Basin is one of the most important oil and gasproducing areas. The exploration and development process of oil and gas are often basedon field observation and indoor experiment in the past. These two kinds of means have acertain guiding significance to understand the formation and distribution of oil and gasreservoir. But due to the complicated geological conditions in the field and the large timescale of the geological phenomena, it makes the two traditional methods have somelimitations to make a prediction in the process of oil and gas resources exploration anddevelopment. The introduction of numerical simulation method makes up for the lack offield observation and indoor experiment methods. Because the powerful solubilityproperties of groundwater, many complicated water rock reactions take place continuallyduring the contact process between groundwater and the ambient rock and soil whichcaused the dissolution and precipitation of minerals and changed the properties of thereservoir. Therefore, it is very necessary to study1) the factors affecting the diagenesisprogress and2) the reaction situations between the fluid consist of different compositionand the reservoir rock and3) the influence of various water rock reactions to thereservoir permeability.This paper takes the clastic rock reservoir of Lower Shihezi-Shanxi group and themarine carbonate rock reservoir of Majiagou group of Upper Paleozoic in the NortheastErdos Basin as the research object and takes the water-rock reaction as the main researchcontent to study the influence of water-rock interaction on the reservoir permeability byusing TOUGHREACT simulation software. In the research of marine reservoir, thispaper mainly studies the influence factors of dolomitization and the space distribution ofdolomitization; For the continental reservoir, the main research is about the comprehensive influence of a variety of water rock reaction to the reservoir permeabilityincluding CO2filling. By using the method of numerical simulation, this paper got thefollowing conclusions:1. The diagenetic fluid of seepage reflux dolomitization drove by gravity is capableof forming a certain scale dolomite reservoir under the different density and salinityconditions.2. The temperature and velocity have a significant effect on dolomitization: theeffect of dolomitization is obviously enhanced when the temperature increases and theinfluence range of dolomitization expands when the flow rate increases. Dolomitizationcan significantly improve the permeability of the reservoir.3. The dissolution and precipitation of minerals are different due to the differentconditions of water chemical composition in different diagenetic stages: In the earlydiagenetic stage A, the minerals dissolved obviously include carbonate minerals, clayminerals, feldspar and sulfate minerals. Quartz is the main precipitation mineral. Due tothe volume changes of the minerals resulted from dissolution are greater than those ofprecipitation, the porosity increased from the initial value0.3to about0.36after50Maand the permeability of the reservoir have increased significantly. It increased from theinitial5mD to8.7mD. In the early diagenetic stage B, CO2intrude into the reservoir. Inthis stage, the H+derived from the dissociation of carbonic acid caused the fluid pH ofthe reservoir decreasing. And the reservoir pH remained at around5.34. The dissolvedminerals are mainly calcite, K-feldspar, kaolinite, illite and so on. According to thesephenomena, due to the dissolution of kaolinite and illite, the excess silicon precipitated inthe form of quartz which acted as the predominant authigenic mineral decreasing theporosity and permeability of the reservoir. In this stage, the volume of the solid phase ofthe reservoir increased which resulted in a decline in reservoir porosity and permeability.The late diagenetic stage A, the atmospheric water infiltrated in the reservoir. BothCa-smectite and Na-smectite dissolved in the state. Kaolinite, illite and pyrite continueddissolution; quartz, dawsonite and ankerite were the main minerals which continuedprecipitation and reduced the permeability of the reservoir. In this stage, the changes of volume fraction caused by the mineral precipitation were still greater than the changes ofvolume fraction caused by mineral dissolution. Therefore, the porosity and permeabilityof the reservoir continued to decrease. In the late diagenesis period B, part of the initialminerals had disappeared which included calcite, feldspar and smectite after three stagesof diagenetic period; kaolinite and illite continued to be dissolved, quartz and ironminerals continued to precipitated; The porosity and permeability of the reservoir isreduced to about0.1, and6.310-2D respectively because of the dissolution andprecipitation of minerals, through the mineral transformation of the4stage of diagenesis.