Carbonate Gas Reservoir Damage Description And Applied Research

Carbonate is a kind of important reservoir rock. Although the carbonate is 20 percent of sedimentary rock, the reserve of carbonate is 50 percent of total reserve in the world. The research on formation damage in carbonate reservoir is far from enough, which is not suitable with its importance, comparing with the depth of understanding on damage mechanism in clastic rock. The research on formation damage in carbonate reservoir is focus on preventing and resisting fluid loss in early period. For a long time, the fluid sensitivity of carbonate has been regarded as weakness, or non-existing, because of the relatively simple mineral and the little proportion of clay mineral. It is the limited comprehension on damage mechanism that restricts the further research on carbonate formation damage.The characteristics of carbonate reservoir in Northeast of Sichuan include abundant types of formation, considerable difference of vertical physical property, numerous natural fractures and strong heterogeneity. The area is the ideal site to study carbonate gas reservoir, because of amount of clay mineral in some horizon, a few salt limes, high content of hydrogen sulphide in gas reservoir, some asphalt and sulphur deposition in formation. The damage mechanism of carbonate gas reservoir is systematically described, based on Leikoupo, Jialingjiang, Feixianguan and Changxing groups. The results of the latest research on the damage mechanism in carbonate gas reservoir are concerned by collecting large numbers of literatures. The object of research is carbonate gas reservoir in Northeast of Sichuan, Sichuan Basin. The damage of carbonate gas reservoir is systemically described by lots of laboratory experiments and using core analysis and test measure in advance, from these aspects of fluid sensitivity, stress sensitivity, aqueous phase trapping, capillary imbibition and the interface of reserve mineral.Meanwhile, the peculiar characteristic of damage is revealed, the new understands and theories are presented, the damage description and damage control technique are more consummated and the fetters of thought on damage mechanism are liberated. The reservoir exists a curtain sensitivity such as strong alkali and water sensitivity .The sensitivity of fluid is strengthened for the salt lime existing. Meanwhile, the new mechanism of alkaline damage is presented that alkaline reacts with dolomite to form the brucite.For complicated interstitial space and fracture system in carbonate formation, the stress sensitivity behavior of pore spaces is investigated, the degree of sensitivity is evaluated by the new method of stress sensitivity coefficient and damage mechanism is discussed. Also the stress sensitivities are studied by contrasted carbonate rock with clastic rock, based on thecharacter of rock mechanics. The result of investigation indicates the stress sensitivity of carbonate matrix core is weak, and the deformation belongs to elastic deformation, which is caused by early cementation and later dolomitization that cumbers the carbonate rock to be compacted further. The stress sensitivity of fracture is severe, which deformation belongs to plastic deformation, the stress sensitivity of man-made fracture is more severe than that of natural fracture. The stress sensitivity of tight sandstone is more severe than that of tight carbonate rock, because that the cementation degree of tight sandstone is weak than that of tight carbonate and there are plastic materials easy to distortion in tight sandstone, such as debris, clay mineral, and so on. The other important cause is that quite many micro-fractures grow in tight sandstone.The water saturation of sealed cores shows that the initial water saturation is far from the irreducible water saturation ,which is abnormal low water saturation in carbonate gas reservoirs, During the operations of drilling, completion and simulation, once the water-base working fluid contacts the gas reservoirs ,the fluid can spontaneous imbibition to gas formation, under the extra capillary pressure, reduce the permeability of gas phase severely, that is aqueous phase trapping. Through the experiments of capillary spontaneous imbibition, the law of capillary spontaneous imbibition is discussed, how physical property, pore geometry and initial water saturation affect spontaneous imbibition is analyzed. The result shows that the higher physical property, the faster the rate of spontaneous imbibiton.For exploiting the gas reservoir with high content of sulphur, the techniques of drilling, completion and exploitation face on austere challenge and the special demands on drilling fluids performance and formation damage control technique is brought forward. In the process of exploitation, with the formation temperature and pressure decreasing, sulphur element that dissolved in hydrogen sulphide is deposited to solid state, then blocks pore and throat, thus causes formation damage. It is proved that the effect factors of sulphur deposition include velocity of gas well, sulphur content and formation physical property, and so on. The interface characteristic of formation mineral in the gas reservoir with sulphur is investigated. That calcium sulfide covered on the mineral with film shape is revealed by XRD and spectral analysis. The film of calcium sulfide can affect the wettability and strengthen the fluid sensitivity, such as acid sensitivity, salinity sensitivity, and so on. The complex problems in the process of drilling, completion and exploitation in gas reservoir with high content are analyzed, the effects that damage of sulphur deposition affected by sulphur saturation, velocity of gas production and physical property are discussed. Meanwhile the countermeasure of formation damage control technique in gas reservoir with high content ispresented.The law of pressure changing in the process of removal of temporary filter cake isdiscussed. The result of experiment indicates that the break pressure existing during the removal of filter cake, only when the pressure of removal is bigger than the break pressure, the filter cake can be removed. The temporary plugging technique is improved further. Finally, the necessary of temporary plugging technique applied on this area is discussed. The degree of temporary plugging is used to confirm the buildup time of temporary filter cake. Meanwhile, based on large numbers of experiment results, the drill-in fluid system using temporary plugging technique adapt to the area and the anticipated effect is attained in the field application.