Research On Mechanism Of Condensate Damage And Reasonable Development Method In Tight Sand Gas-Condensate Reservoirs

Tight sand gas-condensate reservoirs, as an important part of unconventional oil and gas reservoirs, are playing a great role in oil and gas resources as well as production. Compared with general gas and oil reservoirs, the development of gas-condensate reservoirs is much complicated due to its special phase behavior of gas condensate fluid. During the production of gas-condensate reservoirs, the heavy components of gas-condensate fluid will drop out when the formation pressure reduce to below the dew point pressure, which is known as retrograde condensation phenomenon. The retrograde liquid will be left in formation in the form of liquid film and liquid drop, which will result in shrinkage of the pore space available to gas flow and reducing of relative permeability to gas phase. It is well known that the tight sand reservoir stratum has the characteristics of strong anisotropy, low permeability and porosity, large surface and high capillary pressure, whose development effect will be remarkably affected by retrograde condensation damage phenomenon. At present only few focus have been played on research of retrograde condensation damage mechanism and rational exploitation pattern, therefore this dissertation accomplished related studies in terms of physical simulation experiment and theoretical research. Relying on tight sandstone full diameter cores we firstly tested the gas-water relative permeability and starting pressure gradient at ambient temperature and low pressure conditions with different initial water saturation and discussed the damage extent and rules of water locking effect. Then we carried out 7 sets of experiments at HTHP (high temperature and high pressure) conditions to evaluate retrograde condensation damage, in which the factors of content of condensate oil, retrograde condensation zone and reservoirs' stratum are taken into account. In order to evaluate the optimum development method for matrix type and fractured type tight sand gas-condensate reservoirs with medium to high content of condensate oil, we then, in total, performed 18 sets of dynamic physical simulation experiments. Natural depletion, dry gas cycling, huff and puff with dry gas and methanol slug, pulse injection with dry gas are taken into account. Finally a new material balance equation, with consideration of stress sensitive, for fractured-matrix type gas-condensate reservoirs was derived. According to these comprehensive studies noted above, we obtained some achievements and understandings as follows:(1) Initial water saturation has a great influence on gas-water relative permeability curve and starting pressure gradient in tight sandstone reservoirs. As the water saturation increases, a sharp drop occurs in gas relative permeability while a great enlargement occurs in start-up pressure gradient, and the worse the reservoir property is the greater the influence extent will be.(2) Condensate oil content is one of the factors that influence the difference extent of retrograde condensation damage in near wellbore area and deep reservoir region. For the same area, the higher the condensate oil content is, the higher the pressure when retrograde condensation damage occurs will be, the extent of damage is more serious. The extent of damage to effective permeability in near wellbore area, when condensate content is low, is more serious than deep reservoir region. As the condensate content increases, the difference extent of damage in these two regions decreases. When condensate content rises to a critical extent, the damage extent in deep reservoir region is as serious as near wellbore area.(3) Compared with the matrix dominant reservoirs, the critical flowing saturation of condensate oil is lower and fluid (gas and condensate oil) is more easily to flow in the fissures of fractured dominant reservoirs. Gas-liquid two-phase flow will cause substantial decrease in the gas phase permeability. In a word, the fractured dominant reservoirs are more sensitive to retrograde condensation damage compared with the matrix dominant reservoirs. The permeability of fracture, however, is greater than matrix, which shows that fracturing can improve seepage environment near wellbore area as well as gas phase percolation ability, but can't reduce the damage degree of the retrograde condensation.(4) For the matrix dominant reservoirs, the condensate oil recovery factor of natural depletion, compared with other exploiting methods, is lowest. Gas producing rate and initial water saturation has little impact on the development effect of natural depletion. The condensate oil recovery is little higher, however, when the content of condensate oil is high enough to reach its critical flowing saturation. Compared with natural depletion, the development mode of dry gas cycling, as well as dry gas huff and puff, can significantly improve condensate oil recovery.(5) For the fracture dominant reservoirs, the permeability of fracture is much greater than matrix, and the phenomenon of fluid supplying shortage is significant, which result in low recovery of condensate oil when natural depletion as well as dry gas cycling and huff and puff, for the injected gas is easy to channel in fissures.(6) Considering the actual operability and process requirement, the development mode of dry gas huff and puff after natural depletion is recommended for matrix dominant tight sandstone gas-condensate reservoirs with high content of condensate oil, while pulse dry gas injection after natural depletion is recommended for fracture dominant reservoirs.