Study On Features Of Phase Behaviors And Seepage Mechanism In High Sour Gas Reservoir Reservoir In Yuanba As Study Objects

In the process of the development of sour gas reservoir, phase behavior changescause sulfur deposition, which does harm to the formation and well bore; what’sworse, it might lead to gas wells off production. Deposited sulfur in the formationwould be absorbed in the appearance of the rock pores, blocking the passage of gasesand thus have a bad effect on the gas well productivity. However, the researches onthe problem are still the in the beginning stages both at home and abroad. The reporton the study of phase behaviors in sour gas reservoir is seldom found, especially thosein deeply buried depth and with high formation temperature. As a result, there is alack of the experimental methods of direct observing the separating dot and meltingdot of sulfur in the high temperature and pressure. This restricts the research on thefeatures of phase behaviors in such gas reservoir. In the study of formation seepage inthe high sour gas reservoir, the main efforts are put usually to the research on thedamage of sulfur to reservoir, without the comprehensive consideration of theinfluence on the reservoir stress sensitivity. At present, there are not any predictedexperimental and theoretical models which are used to do the research of the damageto reservoir on the basis of both the reservoir stress sensitivity and sulfur deposition.The lack of both micro and macro deep research on seepage mechanism of high sourgas reservoir seriously restricts the development of such gas reservoir. Therefore, thispaper, taking Yuanba gas reservoir (the7500m buried depth) as the research object,conducts a study on the phase behavior and seepage mechanism in high sour gasreservoir, hopefully offering some hints for the effective and efficient development ofhigh sour gas reservoir of the same kind. The following shows what we have found inthis research:(1) Establish measuring equipment testing the separating dot and melting dot ofsulfur in the high temperature and pressure; Form a set of experimental methodstesting phase behavior changes of sour gas. In the experimental studies on fluid phasebehavior in Yuanba gas reservoir, we have measured the sulfur solidification line and sulfur deposition line and then the diagrams of gas phase behavior of Yuanba’s samplehas been measured. Analysis on the potential existing situations of sulfur in theformation has shown that sulfur would be deposited when the flowing bottom holepressure is under25MPa. And once sulfur is deposited, it is in terms of liquid form.(2) Establish measuring equipment and methods testing the amount of sulfur inhigh sulfur-containing gas and the testing experiments on solubility of gas sulfursample are carried out. The amount of sulfur contained in the gas sample in thewellhead is1.0977g/m3. The saturated solubility of sulfur is6.413g/m3in its originalformation conditions. Under the same temperature, the solubility of sulfur rises withthe pressure increasing; while under the same pressure, the solubility of sulfur innatural gas rises with the temperature increasing.(3) Establish measuring equipment and methods testing micro seepagemechanism of high sour gas reservoir and study the forms, dimension and distributionof deposited sulfur in the process of micro seepage in forms of gas-liquid sulfur and inPorous Media. The results show in the condition of coexistence of fracture and matrix,sulfur is more likely to be deposited in fracture and distributed unevenly in general.Sulfur is absorbed in the pore walls in form of crystalline solid. In the range from3μmto16μm, there is high possibility of producing crystalline solid, which appears to belayered accumulation. Single junction crystal is found without obvious boundarywater chestnut. The filamentous or regular cube is not found too. The amount of sulfurcontained in the core is in the range from0.69%to15.87%, which proves that there issulfur deposited in the core.(4) Establish experimental methods of assessing impacts on the gas-liquid sulfurby both sulfur deposition and stress sensitivity and complete the experiments on stresssensitivity under different sulfur saturations in the Yuanba gas reservoir. The resultsshow that permeability and damage rate of fracture and matrix core rise underdifferent effective pressures. The stress sensitivity is neutral. With the increasing ofsulfur saturation, the stress sensitivity of core becomes weak. We conduct a study ofstress sensitivity’s influence on gas sulfur phase permeability and find the critical flowsulfur saturation is rather high, which indicates the deposited sulfur is usually not flowin the formation in Yuanba gas reservoir. With the increasing of stress sensitivity, thepermeability of its gas and liquid phase decreases. The water-containing sulfursaturation is about70%of the water and gas phase permeability in high temperatureand pressure, and the irreducible water saturation is28.10%.(5) Establish experimental models of assessing impacts on sulfur saturation changes by sulfur deposition and stress sensitivity respectively; set up a productivitypredicting model of gas and liquid sulfur phase on the basis of interaction of liquidsulfur and stress sensitivity.(6) Establish the features of gas, liquid and solid simultaneously and a modelpredicting the balance of three phases in high sour gas reservoir, which is proved bythe experimental statistics with errors within5%. After a comparison between theadvantages of fluid deviation coefficient and gas viscosity model, we take the gasviscosity model as a priority.(7) Establish an experimental method of assessing flow of sulfur deposition inthe wellbore and complete experiments of the flow of sulfur under3different forms inthe wellbore. The findings indicate: if the sulfur is deposited from formation in formof solid, it can be completely taken out of wellbore under certain flowing speed; if thesulfur is deposited from formation in form of liquid and is not evenly absorbed in thetube well, what’s more, it is firstly absorbed in the position with Crystal Nucleus, it isdifficult to be taken out; if sulfur is deposited from gas and absorbed in the tube well,it is hard to remove. The diameters of sulfur in the tube well range from238.8nmto135.0um, which suggests the formation of sulfur is a process of becoming bigger.That is to say, its formation begins from nanometer sulfur to plane sulfur and then tothe layered sulfur.(8) Based on the mathematical model about damage to reservoir by stresssensitivity and sulfur deposition, we carry out analysis on distribution ofsulfur-containing saturation in gas wells in Yuanba gas reservoir, from which, we findgenerally sulfur-containing saturation is low, with a maximum of1%in the bottomwell and it does a little harm to reservoir.(9) Predict the phase changes in the tube well by using the established model forthe three forms of phases. In the process of production, there would appear sections ofsingle gas phase sulfur flowing, gas-liquid phase sulfur flowing and gas-solid phasesulfur flowing and their combination.