Self-Sealing Mechanism And Controlling Factors Of Marine Shale

Unconventional oil and gas(shale oil and gas,tight oil and gas,and coalbed methane,etc.)generally have the characteristics of self-sealing accumulation,which is essentially different from the buoyancy driven accumulation of conventional oil and gas.With the development of unconventional oil and gas exploration and development,higher requirements are put forward for unconventional oil and gas accumulation theory.In recent years,the mechanism of shale gas self-sealing has also received extensive attention.In this paper,the gas-bearing shale in the first member of Longmaxi Formation in Sichuan Basin is taken as the research object.By comprehensively applying the multidisciplinary intersection of petroleum geology theory,water film theory and rock mechanics theory and combination of geology and engineering,and through the research methods of reservoir fine characterization,theoretical calculation and rock mechanics experiment,the shale gas self-sealing system of Longmaxi Formation in Sichuan Basin has been established,the micro-forming mechanism of shale gas self-sealing has been revealed,and the macro-control factors of shale gas self-sealing have been clarified.Through gas reservoir dissection,it is clear that there are obvious differences in the basic geological and geochemical characteristics and reservoir pore characteristics of the gas-bearing shale in the first member of Longmaxi Formation in the vertical direction.The shale in the 1—3 sub-layers is mainly organic siliceous shale deposited in the deepwater continental shelf facies.The sedimentary water environment is deep and the hydrodynamic force is weak.It is a strong reduction environment.The TOC is mostly greater than 4%.The mineral components are mainly siliceous minerals,the porosity is mostly greater than 5%,the water saturation is mostly less than 20%,and the rock density is mostly less than 2.48 g/cm~3.Under the scanning electron microscope,it can be seen that the organic matter pores are largely developed.The specific surface and pore volume are very developed,and the connectivity is the best.The upward transition of these characteristics shows a gradual decrease or increase trend.The shale in the 2nd sub-member is mainly the poor organic clayey shale deposited in the shallow continental shelf facies.The sedimentary water environment is turbulent and highly oxidizing,with TOC less than 1%.The mineral composition is mainly clay minerals,with porosity more than 3%,water saturation more than 40%,and rock density more than 2.65 g/cm~3.The pore development of organic matter is low,the specific surface and pore volume are the smallest,and the connectivity is the worst.The difference in physical properties and connectivity of shale in the vertical direction caused the direct sealing of the upper shale to the lower shale.Based on this,the source-reservoir-cap combination of shale gas reservoir is divided.At the same time,the macroscopic performance of intermolecular force in shale gas self-sealing is capillary force and adsorption force,and capillary force self-sealing is the core mechanism of shale gas self-sealing.The shale gas self-sealing system is established by comprehensive gas reservoir anatomy and mechanical analysis,which lays a good foundation for the study of shale gas self-sealing mechanism.The analysis results of the fluid dynamic field of the gas reservoir show that the gas-bearing shale in the first member of the Longmaxi Formation is a unified pressure system,the gas expansion force is the power of gas migration,and the capillary force is the resistance of gas migration.The pressure difference between the adjacent layers in the gas reservoir is 0～15 MPa,and the capillary force sealing capacity of the shale in the gas reservoir is 24～38 MPa,which is far greater than the power of gas migration.In order to study the critical water saturation of shale when forming capillary sealing capacity,a geological model of water-bearing shale is established according to the pore characteristics and water distribution characteristics of shale.Under the boundary conditions of the geological model,the critical water saturation of the formation of capillary water in nanopores is calculated by water film theory,and the critical water saturation of the formation of capillary sealing capacity of shale is calculated by combining the fine characterization method of shale pore structure,and the critical water saturation of the formation of capillary sealing capacity of different TOC shale is further predicted.Therefore,the microscopic formation mechanism of shale gas self-sealing is revealed,and the self-sealing model of shale gas reservoir is finally established.The macro-control factors of shale gas self-sealing are discussed in terms of temperature and pressure conditions,rock mechanical properties and tectonic action.Based on the statistics of 46 typical shale gas wells in Sichuan Basin,the typical temperature and pressure characteristics of shale formation under 0～5 km buried depth in Sichuan Basin are defined.The calculation results show that the shallower the shale formation is buried or the lower the critical water saturation after uplift,the easier it is to form capillary force self-sealing,which reveals the control effect of formation temperature and pressure conditions on capillary force self-sealing.For the self-sealing of adsorption capacity,combining with the methane adsorption theory,it is clear that the methane adsorption capacity of the middle,deep and ultra-deep shale gas reservoirs decreases with the increase of the burial depth of the formation,i.e.,the increase of the temperature and pressure conditions.The lateral self-sealing of shale gas will strengthen with the increase of overburden pressure,and the strengthening degree of upper shale will be more obvious.The study of brittle-ductile transformation shows that shale gas reservoirs are prone to brittle fracture in the brittle zone,which affects the integrity of shale gas reservoirs.Shale gas reservoirs in the ductile zone have strong self-healing ability,so the integrity of gas reservoirs is good.In the process of formation uplift,the non-synchronous phenomenon of the brittle-ductile transformation law of the upper and lower shale also has a phased control effect on the self-sealing of the shale gas reservoir.The most direct control of tectonism on the self-sealing of shale gas reservoirs is reflected in the destruction of fractures and uplift and denudation.The vertical self-sealing of shale gas reservoir is damaged due to large faults,and the lateral self-sealing is also affected within a certain range.Uplift and pressure relief increase the lateral permeability of shale,and the exposure of the stratum further increases the pressure gradient of shale gas lateral seepage,which has a negative impact on the lateral self-sealing of shale gas reservoirs.In addition,the shale stratum uplifted to the shallow layer will undergo brittle fracture under the action of tectonic stress,causing direct damage the self-sealing of shale gas reservoir.