Imitation And Characteristic Analysis Of Mud Filtrate Invasion In Low Porous And Low Permeable Sandstone Reservoir

The mud filtrate invasion during drilling has caused lots of problems in oil field exploration for a long time.The drilling mud and small particles invade the original reservoirs and change the initial status of formation around wellbore,which not only cause the difficult in logging interpretation,but also affects the oil and gas productivity.With the development of low porous and low permeable sandstone reservoirs,the invasion characteristics of these formations attract more and more attention.The low porous and low permeable sandstone is characterized by poor petrophysical property,small pore throat radius and strong heterogeneity,which leads to the much more complicated effects on mud filtrate invasion.In this paper,both physical and numerical simulation of mud invasion in low porous and low permeable sandstone are studied to figure out the invasion mechanism which is of great importance to optimize the mud property and protect the formation.We first simulate the mud filtrate invasion of low porous and low permeable formation by using finite difference method(FDM),and the impacts of porosity,permeability,pressure,mud cake permeability as well as the invasion are calculated,which has deepen the understanding of different effects on mud invasion and provides a theoretical support on the design and parameters optimizing of physical experiment of mud invasion.The key parameters of physical experimental apparatus are optimized with the help of numerical simulation,the thickness of formation module is set to 0.1 meter and the radius is great than 0.5 meter.Several key technologies such as the saturating and pressure implication of large samples,the design of invasion room,the seal and pressure maintaining of the apparatus,and the real-time and dynamic detection of the invasion process have been proposed.Then,a multi physical functional mud invasion system of formation module is designed and built to realize the experimental analysis of mud invasion under complex drilling conditions.During the experiment,the real time invasion data which concludes the formation pressure,resistance and the filtration rate provides sufficient information for analyzing the invasion mechanism and characteristics.We propose a spread scope method and divide the formation into before invasion,invasion and after-invasion status according to the electrode position and mud filtrate.Under different status,the spread scope is acquired by the upper and lower electrode,and then the radial resistivity distribution profile can be calculated.The resistivity distribution is utilized to confirm the invasion time when the mud filtrate arrives each electrode.Finally,the invasion process is presented by the cross plot between invasion depth and invasion time.Although the formation pressure could not reflect the invasion depth,it indicates the filtration rate of mud filtrate.On the benefit of the micro-compressibility of the liquid and pore,the filtration rate is available to reflect the invasion speed of mud filtrate of the borehole wall,which give a direct help to check the filtration property of mud prescription and the protection of borehole wall.Mud filtrate invasion physical simulation under different conditions were carried out by formation module mud filtrate invasion multifunctional physical simulation system,combined with the mud filtrate invasion numerical simulation results,main results are summarized as followed: the internal influence factors of mud filtrate invasion including reservoir porosity,permeability,heterogeneity,etc.,and external influence factors including mud pressure differential,mud performance(viscosity,filtration performance,solid-liquid ratio,etc.),mud hydrodynamic and environment invasion time,etc.The formation of mud cake is a dynamic process during the invasive process.At the beginning of the invasion,the mud cake is gradually formed with increases in its thickness and decreases in its porosity and permeability because that the precipitation rate of the solid particles on mud cake surface is greater than erosion rate.While in the middle and later periods,the precipitation rate is approximately equal to the erosion rate,which results in dynamic equilibrium and stable mud cake properties.Calculations show that low permeable mud cake effectively delayed the continued invasion of mud filtrate.At the beginning of the mud filtrate invasion,the mud filtrate invasion velocity in low porous and low permeable sandstone reservoir is slower than that in medium-high porosity and permeability one,while in the middle and later periods of mud filtrate invasion,the former invasion velocity is faster than the latter.The difference in the mud cake permeability caused by the different mud cake formation rate is the direct cause of this phenomenon.In the same sandstone reservoir,the bigger mud pressure differential result in faster mud filtrate invasion.At the beginning of the mud filtrate invasion,since the invasion of the polymer and fine clay particles in mud,the longer invasion time and greater mud pressure differential is,the larger sandstone reservoir permeability damage rate is.The sandstone reservoir permeability damage rate will keep increasing until the mud cake is steadily formed.Although with smaller pore radius,shallower polymer and fine clay particle invasion depth and smaller permeability damage rate,the low porosity and permeability sandstone reservoir's original permeability is 2-3 orders of magnitude smaller than that of medium-high porosity and permeability one,thus the production capacity of low permeability sandstone reservoir is more seriously affected by mud filtrate invasion.Thereby optimizing the drilling mud filtration property,reducing the pressure differential within the scope of the drilling safety,improving drilling efficiency and shortening the drilling time are effective measures to reduce the mud filtrate invasion affects.