Henan Ocean Chemical Technology Co.,Ltd.learned at the end of November 2020 that drilling fluid is the first foreign fluid in contact with the oil layer,and the fine particles that invade the near wellbore oil and gas layer come from drilling fluid and drilling fluid for opening the oil layer.Solid particles invading the formation can seal pores and pore throats.The damage caused by drilling fluid to oil and gas reservoirs is functionally related to the erosivity of filter cake.
The physical transport of solid particles in the formation,chemical reactions between solid particles and filtrate and the formation,or the combination of both can cause formation damage.If the filter cake on the wellbore is partially or completely eroded,the fine particles in the drilling fluid may cause serious formation damage.The key factors in determining the interaction between filter cake and drilling fluid flow are the key to stabilizing filter cake and its response to erosion.A reasonable design of drilling fluid and oil layer opening drilling fluid is crucial for controlling direct invasion of drilling fluid and minimizing formation damage.Forming non erodible filter cakes on the wellbore wall can reduce formation damage caused by drilling fluid and drilling fluid opening the reservoir.Indoor research results on the erosion state of filter cakes formed by different drilling fluid systems can be used to design,screen,and select the optimal drilling fluid for drilling oil reservoir sections and opening oil reservoir drilling fluids.
Because the purpose of this experiment is to evaluate the erosion characteristics of filter cake and the potential damage of drilling fluid to the formation,conventional additives and a small amount of chemical agents are added to the drilling fluid.Use an API filter press to filter for 30 minutes to prepare filter cake.We prepared filter cakes containing different salts and filter loss reducing agents,as well as those containing or not containing barite.Anionic,polyanionic,and non anionic fluid loss reducers were used.Adjust the pH value of the drilling fluid to 9.8-10 to minimize the impact of pH on the properties of the filter cake.
The erosion solution should be compatible with the fluid inside the filter cake to avoid chemical reactions between the erosion solution and the fluid inside the filter cake.Freshwater based bentonite filter cake uses fresh water as the erosion solution,while saltwater based filter cake uses saltwater as the erosion solution.
In the early stage of erosion,after removing the weakly consolidated top layer of the filter cake,the well consolidated inner layer of the skeleton is exposed.The inner skeleton layer has consistent shear strength,therefore,after the initial erosion time,the weight of the filter cake is linearly related to the erosion time.Determine the erosion coefficient of filter cake using linear regression method.
The erosion state of freshwater based bentonite filter cake and saline based bentonite filter cake containing different filter loss reducers is significantly different from that of saline based bentonite filter cake without filter loss reducers.The saltwater based filter cake without filter loss reducer is quickly eroded,and after 10 minutes of erosion,there is no filter cake material left.The erosion mechanism of various types of filter cakes is also different.Salt water based filter cakes without filter loss reducers exhibit blocky erosion,while freshwater based bentonite filter cakes and filter cakes with filter loss reducers exhibit smooth surface erosion.The filter cake containing modified starch has both surface erosion and block erosion.
The erosion coefficient of freshwater based filter cake is lower than that of barite/bentonite and saline/bentonite/starch filter cake,while the erosion resistance is much higher than that of saline based filter cake.Bentonite drilling fluid directly invades and drilling fluid filtrate invades,so the potential damage of bentonite drilling fluid to the formation is relatively small.Bentonite particles have a negative surface charge and a positive edge charge,therefore they have a complex electric field.In alkaline drilling fluid environments,the interaction force is repulsive.The net repulsive force generated during the solid-phase particle deposition process gives the formed filter cake high pressure density,strong solidity,and low permeability.The strong single molecular bonds and overlapping diffusion double layers make the fiber structure of the filter cake have strong erosion resistance.Due to the high erosion resistance of the filter cake,the integrity of the wellbore filter cake is maintained during the drilling process.After adding barite to freshwater based bentonite drilling fluid,the filter cake is highly susceptible to erosion.Due to the shielding effect of barite particles,the consolidation strength formed by charged bentonite particles and other molecular forces is weakened.
The erosion state of saltwater based filter cakes formed by sodium chloride/bentonite,potassium chloride/bentonite,and calcium chloride/bentonite indicates that drilling fluids containing electrolytes or those contaminated with salts or formation saltwater have the potential to increase formation damage.The saline based filter cake is quickly eroded by the erosion solution,and after 10 minutes of accumulated erosion time,there is no longer any filter cake material.Due to continuous direct invasion and high filtration invasion,the salt water based filter cake formed in the oil well section will cause serious formation damage.
The sodium chloride/bentonite drilling fluid containing CMC and polyanionic cellulose derivatives(PAC)significantly improved the erosion resistance of the filter cake.Due to the high erodibility of sodium chloride/bentonite filter cake containing starch,it may cause very serious formation damage.Compared with CMC,filter cakes containing PAC in the filter cake skeleton have strong erosion resistance.
The following conclusions can be drawn from the above experiment:
(1)The hydraulic action of drilling fluid flow causes erosion of the filter cake,leading to continuous direct invasion of drilling fluid and high filtration loss invasion,which can cause formation damage;
(2)When the drilling fluid contains barite and salts,it will reduce the erosion resistance of the filter cake;
(3)The characteristics of filter cake,its physical state,and the possibility of causing formation damage depend on the physical,chemical,and electrical properties of drilling fluid additives.
