Reservoir Evaluation And Experimental Research Of Dissolution Kinetics For The Typical Carbonate Reservoir Rocks In Sichuan And Tarim Basins

Carbonate reservoirs are the most important targets in the word's oil and gas exploration, and the key issue of its geological study is their porosity formation and evolution. Dissolution experiments of typical carbonate rocks in different physical and chemical conditions can clarify the controlling factors of carbonate dissolution, and then discuss pore-hole-seam development pattern in the reservoir, which are of very important practical significance for the prediction of reservoir distribution and reservoir evaluation.Tarim basin and Sichuan basin are two important petroleum basins in China, where a large number of marine carbonates had been deposited, with Paleozoic Cambrian-Ordovician carbonates in the former and Mesozoic Pemian-Triassic carbonates in the later, which constitute the most important petroluem exploration formations. Due to the influence of various geological effects, carbonate reservoir heterogeneity is very strong, pore-seam development is complexed.Typical carbonate reservoir samples in Tarim basin and Sichuan basin, such as fine-grained dolostone of Xiaorbulak Group, granule limestone of Yijianfang Group, lamellar dolostone of Qiulitage Group, oolitic dolostone and bioclast limestone of Changxing Group, Changxing group oolitic limestone Changxing Group etc. were selected, and kinetic dissolution experiments under both normal and high temperature/pressure conditions, were applied to find the major controlling factors and distribution rule of good reservoirs.Results of Disssolution experiment of carbonate powder samples under normal temperature and pressure show that, in a supergene environment, the dissolution rates of limestone are significantly greater than those of dolostone. In Sichuan basin, the most vulnerable carbonate strata is the bioclastic limestone of Changxing Group of Permian, and then is oolitic limestone and domomitization oolitic limestone of Feixianguan Group of Triassic, the weakest is the oolitic dolostone of Changxing Group. In Tarim basin, the easiest dissolved is aggregate limestone of Yijianfang Group of Cambrian, and then is lamellar medium-grained dolomite of Qiulitage Group and fine-grained dolomite of Xiaorbulak Group, the weakest is medium-coarse-grained dolostone of Qiulitage Group of Cambrian. In addition, with creasing temperature, the dissolution rate of carbonates are increasing, and for the same lithology of carbonate, the smaller of the particle size, the faster dissolution rate.Carbonate cylinder samples under high temperature and pressure dissolve experimental results show that:In the buried environment, the dolomite dissolution rate is still weaker than the limestone's. The maximum dissolution rate of oolitic limestone in Sichuan basin appered among 100-150℃, according to the geothermal gradient in northeatern Sichuan basin, the depth at about 3000m, so this section maybe is the most favorable range to generate dissolution pores, but the dolomitization of 20%-40% oolitic limestone, erosion aggravated with temperature increasing, whicn forming good reservoir area may be deeper; In Tarim basin, dolostone dissolution rate is proportional to temperature, when the temperature exceeds 150℃, the dissolution rate of Mg ions are higher than the Ca ions in dolomite, explain that at high temperature, dolomite dissolution is much stronger than calcite, and the dissolution rate of transition temperature point may be at 200℃, according to the geothermal gradient in Tarim basin, we can speculate that the favorable depth of forming dolostone reservoir may be around 5000-8000m. In addition, the dissolution rate of carbonate is proportional to the underground fluid flow, but the pressure's effect to the carbonate dissolution is comparatively complex, generally speaking, pressure on the influence of carbonate dissolution is affected by the ions of geological fluid indirectly.Meanwhile, through SEM image analysis of typical carbonare dissolved before and after shows that carbonate original structural-fissure, seam, holes, etc. will further happen dissolution, and become favorable hydrocarbon migration and reservoir places; Non-dolomitic oolitic limestone dissolution process is relatively uniform, small pores formed by dissolution, however, dolomitization oolitic limestone is likely forming dissolved pores between grains, forming a ring solution groove around oolitic, and forming dissolved pores in grains and mould pores etc., these holes will be the main spaces of hydrocarbon migration and reservoir; In the process of lamellar dolostone dissolution, the secondary holes are most likely developed, as well oil and gas explorate targets, followed by coarse-grained siliceous dolostone, and then the medium-coarse-grained dolostone and fine-grained dolostone; Granule limestone dissolution belongs to the holistic dissolution, more difficult to form solution pores. In additon, quarz and other impurities play a supporting structrue for the rock, be helpful for secondary pores preservation.