Investigation Into Stimulation Mechanism Of CO₂ Pre-Pad Energized Fracturing In Jimusar Shale Oil Reservoir

Depletion mining is primarily used to develop the Jimsar shale reservoir,which causes a rapid fall in formation energy and a low recovery rate.At the same time,the permeability of Jimsar shale reservoir is too low for regular injection and extraction techniques to restore formation energy.As a highly compressible fluid,CO2 can interact with rocks and crude oil under stratigraphic conditions,so it is frequently employed as an energy-replenishing medium in reservoir development.The Jimsar shale oil reservoir has been tested with CO2 pre-pad energized fracturing operation,and the stimulation effects is obvious.However,no systematic research has been conducted on CO2 pre-pad energized fracturing operation,its stimulation mechanism is still unclear,and the design of construction parameters lacks basis.Therefore,it is significant to conduct CO2pre-pad energized fracturing operation research to improve the development and recovery of Jimsar shale oil.This paper adopts a combination of experimental tests,mathematical modeling,numerical simulations,and theoretical analysis to carry out the study.The experimental tests are divided into two major aspects: the test on the action law between CO2 and Jimsar crude oil,and the test on the action law between CO2 and Jimsar reservoir.The tests on the action law between CO2 and Jimsar crude oil include the viscosity reduction law,the miscibility law,and the replacement law of CO2 on shale oil.The NMR technology was used to quantify the replacement of crude oil and analyze the replacement mechanism of CO2 on crude oil.The replacement law of CO2 on crude oil was compared with the imbibition,and the adaptability of the two stimulation mechanisms in the Jimsar shale reservoir was evaluated.The tests on the action law between CO2 and Jimsar reservoir include the dissolution of minerals by CO2 aqueous solution,the change of microscopic pore structure,the enhancement of porosity and permeability,and the physical simulation experiment of triaxial CO2 fracturing.Based on experimental laws,Darcy’s law,and the principle of mass conservation,a mathematical model of CO2 pre-pad energized fracturing operation was established.The model couples the action law between CO2 and crude oil,and the complex fracture pattern formed by horizontal well fracturing is taken into account.The mathematical model was discretely solved by the finite difference method and programmed to form a numerical simulation model,which realized the simulation of the CO2 injection-shut in-production process.Extensive numerical simulations based on the model were conducted to analyze the factors and laws affecting the effect of CO2 pre-pad energized fracturing operation,optimize the relevant parameters,predict the production capacity,and clarify the major factors.Finally,combining the experimental test results,numerical simulation results,and theoretical analysis,the contribution rate of each major factor to the stimulation effects were determined.The study shows that CO2 can significantly reduce the viscosity of Jimsar Shale oil by more than 80%.The minimum miscibility pressure between CO2 and Jimsar shale oil is 25.5 MPa,and the miscibility can be achieved under stratigraphic conditions.CO2 can replace crude oil in the pore space by extraction,and the crude oil replacement rate reaches 60%.Compared with imbibition,CO2 can replace crude oil in smaller pores whose size can be low to 0.08μm.CO2 aqueous solution dissolves carbonate rocks obviously,and the permeability increases more than 2 times after dissolution.Due to its low viscosity and strong diffusivity,CO2 facilitates the construction of complex fracture patterns during fracturing.The stimulation mechanism of CO2 pre-pad energized fracturing operation in Jimsar shale oil includes viscosity reduction,miscibility,replacement,energy enhancement,dissolution,and complex fracture,among which the main controlling factors are energy enhancement,viscosity reduction,and replacement,and the respective contributions of three factors are quantified.Under the conditions used in the simulation,CO2 pre-pad energized fracturing operation can increase production up to 30-40%.Based on the simulation,the construction parameters were optimized,including CO2 dosage,shut-in time,injection method,and fracture parameters.This study provides a theoretical basis and an optimized design method for CO2 pre-pad energized fracturing operation in Jimsar shale oil,which can help improve the development effect of Jimsar shale oil.