Simulation Of Drilling Mud Invasion And Evaluation Of Permeability Damage In Sandstone Reservoir

To meet the growing demand for oil&gas resource and ensure crude output,recently,the drilling mud invasion and drilling induced permeability damage in the sandstone reservoirs which are widely distributed in China have attracted more and more attention.Therefore,conducting simulation of drilling mud invasion and evaluation of permeability damage in sandstone reservoir are of great significance for revealing invasion mechanisms,establishing invasion correction model for logging response,accurate reservoir logging evaluation,optimizing drilling mud performance,protecting reservoir productivity,and improving exploration and development efficiency of sandstone reservoir.This work has broken through the traditional concept of conducting drilling mud invasion research on core plug.By using the self-developed physical simulation platform in formation module,the long-term invasion experiments under different reservoir physical properties(high porosity and high permeability,low porosity and low permeability),different drilling mud systems(water-based,oil-based),different overbalance pressures are conducted.Based on the results of the physical simulation and the finite difference numerical simulation,the characteristics and the main control factors of drilling mud invasion are summarized.And the simulation results are applied to an oilfield.Combining nuclear magnetic resonance(NMR)and permeability measurement,the microscopic essence and macroeconomic performance of the drilling induced permeability damage are evaluated.The effects of the reservoir physical property and overbalance pressure on the damage are quantified.Finally,the time-lapse logging considering the permeability damage effect is realized.Compared with the conventional core plug,as a new type of experimental aliquot,the sectorial sandstone formation module made by outcrop is larger in size,better in saturation capacity,and much more similar to the in-situ formation.Its structure can ensure the seepage form of mud filtrate(plane radial flow)within it is the same as that in the actual downhole environment.Its large size is able to provide enough space and radial distance to follow the entire invasion process from beginning to dynamic equilibrium.The corresponding physical simulation platform,namely,the multifunctional physical simulation system of drilling mud invasion is excellent in overall performance,and has the advantages of high simulation degree,high utilization ratio and good maneuverability.Under the experimental conditions of this work:during the fresh water-based and oil-based mud invasion,the radial resistivity of the formation module increases in turn,and with the increase of radial depth,its increase rate slows down,the effect of the invasion on the formation module becomes weaker,the duration that takes from the beginning to the dynamic equilibrium of the invasion becomes longer.At the beginning of the invasion,the filtration rate decreases rapidly,the increase rate of the cumulative filtrate volume slows down,and the invasion depth is shallow.After a period of time,the invasion reaches a dynamic equilibrium state,the filtration rate tends to be stable in a very low level,the cumulative filtrate volume increases linearly with time,and the increase rate of the invasion depth tends to be very slow.Compared with the water-based drilling mud,the oil-based drilling mud has a higher solid content and a higher filtrate viscosity,the mud cake forms faster on the top face of the formation module,and the mud cake permeability is lower.The delay effect of the oil-based drilling mud on the continuance of the invasion is greater.The invasion depth of the water-based drilling mud is 2.8 to 3.8 times that of the oil-based drilling mud.After a long-term invasion,the invasion depth of the water-based drilling mud in the formation module with low porosity and low permeability is 1.6 times that of the one with high porosity and high permeability.The invasion depth of the oil-based drilling mud in the formation module with low porosity and low permeability is 2.0 to 2.1 times that of the one with high porosity and high permeability.The experimental results demonstrate that:reservoir physical property,drilling mud system and overbalance pressure are the main control factors of drilling mud invasion in sandstone reservoir.The numerical simulation results of drilling mud invasion under formation conditions show that:compared with oil-based drilling mud,the effect of water-based drilling mud invasion on sandstone reservoir is much greater.The water-based drilling mud has the deepest invasion depth in the low porosity and low permeability sandstone reservoir.The oil-based drilling mud has the greatest protection effect on the high porosity and high permeability sandstone reservoir.The application of the invasion simulation research in an oilfield in Western China suggests that:physical property has the greatest effect on the drilling mud invasion in sandstone reservoir.A worse physical property results in more obvious invasion characteristics and a deeper invasion depth.For a gas or water layer with a relative better physical property,the invasion characteristics are negligible.For the same sandstone reservoir,the invasion characteristics are controlled under the combined effects of fluid saturation,fluid seepage property,formation resistivity,difference in salinity between mud filtrate and formation water,etc.The experimental results of permeability damage evaluation show that:during the fresh water-based drilling mud invasion,under the combined effect of the two main damage mechanisms,the deterioration of pore structure and the permeability damage degree become more severe with the decrease of radial depth.Compared with the high porosity and high permeability sandstone reservoir,the low porosity and low permeability sandstone reservoir has a more severe damage degree and a deeper damage depth.For the same sandstone reservoir,a higher overbalance pressure result in a deeper damage depth,and a more severe damage degree at the same radial depth.The decrease ratio of movable water based on NMR T₂ distributions along the radial depth is able to establish the relationship between the macroscopic performance and the microcosmic essence of the permeability damage.An obvious positive correlation between the permeability damage rate and the decrease ratio of movable water is observed.With the variation of decrease ratio of movable water,the permeability damage rate changes faster in the low porosity and low permeability sandstone reservoir.Combining the experimental results and numerical simulation,it is clearly seen that:when the permeability of sandstone reservoir is less than 10×10^-15m²,the invasion depth considering permeability damage effect is shallower,and the difference between the invasion characteristics considering or not considering permeability damage effect becomes greater with the invasion time.When the permeability of sandstone reservoir is greater than10×10^-15m²,the effect of permeability damage on the invasion is not obvious.