Mechanism And Technological Boundaries Of Fracturing Displacement Production Enhancement In Extra-low And Ultra-low Permeability Sandstone Reservoirs

With the development of oil and gas exploration and development technology,oil and gas development in China has changed from conventional reservoirs to low-permeability,dense,and difficult-to-utilize unconventional reservoirs.However,field trials have found that there are large production differences between fracturing displacement measured well groups,and some well groups have problems such as ineffective oil production and fast-decreasing production after measures.There is an urgent need to clarify the mechanism and production rules of fracturing displacement wells to increase production and optimize the operation parameters.Using physical simulation experiments and theoretical modeling calculation methods,based on the analysis of reservoir physical properties,the mechanism of production enhancement in positive/reverse fracturing displacement（PFD/RFD）measures was revealed in this paper,which took the extra-low permeability and ultra-low permeability（ELP/ULP）reservoirs in Tamsag A area as the research object;The bounds of PFD/RFD technology were clarified;the production laws following PFD/RFD measures were summarized.The outcomes are as follows:（1）The petrological characteristics,physical properties,microporous structure,and brittleness of ELP/ULP reservoirs were investigated using scanning electron microscopy（SEM）,X-ray diffraction（XRD）,constant velocity mercury pressure（CPM）,and rock mechanical parameters.The pore throat seriously affects fluid flow in the target reservoir;the brittle mineral content of the rock samples is high,with the quartz and feldspar fractions distributed in the range of 40%–70%,and the brittleness index evaluation results indicate that the reservoir is prone to crevassing;the measured characterization parameters such as pore and throat radius ratios,structural coefficients,and expulsion and driving pressures are strongly negatively correlated with the permeability,and the average values of throat radius and relative sorting coefficients have have an insignificant relationship with the permeability,and characterization factors have a minimal correlation with porosity.（2）The influencing factors of oil stripping efficiency of displacing agents were derived from the determination of displacing agent-crude oil interfacial properties,emulsification properties,and oil stripping efficiency.From the point of view of surfactant action effect,interfacial tension below 2 m N/m,surface tension between 25～32 m N/m,and wetting angle between 10°～22°could achieve high oil stripping efficiency;and through the fluid flow equation of porous media,combined with the microscopic visualization simulation experiments,the interfacial properties of displacing agents on the state of fluid distribution were analyzed,and the main controlling factors of oil retention within the porous media were clarified,namely surfactants with low interfacial tension(<10^-2m N/m)and high wetting angle are easy to flow from the large pore channels,resulting in the phenomenon of water channeling.（3）The fracture formed by PFD measures can effectively provide energy for the reservoir to drive crude oil to the oil well,shortening the effective action distance between oil and water wells.The larger the size of the fracture,the more obvious the pressure diffusion range,the faster the pressure rise around the oil well,and the larger the production differential pressure.When there is an angle between the fracture and the connecting line of oil and water wells,the pressure diffusion range increases,and the movable area of crude oil increases,which improves the effect of production increase.Analyzing the percolation capacity of different displacing agents in porous medium and the huff-n-puff production of RFD,it is found that the percolation rate of surfactant solution directly affects the huff-n-puff production in the production process of RFD;when the characterization parameter of the percolation capacity of the surfactant solution is in the range of 1.38～1.63 m/PV,the huff-n-puff production is the highest.（4）The permeability is greater than 10×10^-3μm²,and the fracture penetration ratio should not exceed 10%of the well spacing when implementing the PFD measure.While the permeability is 20×10^-3μm²,the production capacity of non-measured wells exceeds that of all the measured well groups within 2 years.Therefore,the PFD is not suitable for reservoirs with excessive permeability.When the permeability is lower than 0.1×10^-3μm²,the reservoir cannot be effectively utilized by the PFD alone.By summarizing the seepage law of the PFD and the reservoir characteristics of the target block,it was considered that,from the engineering point of view,the local reservoir PFD,the adjustment of water absorption profiles inside the fracture,and periodic water injection can alleviate the water channeling phenomenon of the PFD wells.（5）Based on the production law of RFD,the injection limit and oil production capacity of working fluid under actual production conditions were quantified,and the concepts of limit injection volume and optimal injection volume were proposed,and the results showed that:if it is necessary to increase the degree of reservoir mobilization in a short time,the injection intensity needs to be close to the value at the time of fracture extension.In addition,if it is necessary to increase the degree of recovery from the reservoir,lower injection intensity can better mobilize the reservoir.However,to improve the ultimate recovery of the reservoir,a lower injection intensity is better to utilize the reservoir.（6）On the basis of considering the dynamic capillary force effect,a mathematical model of RFD huff-puff production was constructed,the boundaries of the smothering time of RFD were elaborated,and the fluid diffusion characteristics and production characteristics of the smothering process were analyzed.It is concluded that the bottomhole flow pressure,crude oil viscosity,reservoir permeability,and injected fluid/crude oil interfacial tension have a great influence on the cumulative production intensity of throughput and the optimal plugging time.（7）According to the effect of PFD in the application block,the rules were summarized from multiple perspectives of the effect characteristics.The effective direction and the reasons,and the effect characteristics of water injection in production wells were categorized.The production characteristics of typical well groups of different effect types were analyzed from the perspectives of the reservoir lithology and engineering parameters.Similarly,using the RFD huff-n-puff production model,the production of the RFD trial well group was fitted and predicted,and the post-measurement effect was evaluated.The research results of this paper enrich the stimulation mechanism of fracturing and driving technology,provide a basis for the selection of oil displacement agents and injection parameters in oil field fracturing and driving measures,and have important significance for the effective development of ELP/ULP reservoirs.