Study On Correlation Between Interfacial-Transition Zone And Cementation Property At Cement–Aquiclude Interface In Coalbed Methane Well

Coalbed methane(CBM)is a much important unconventional gas resources and also a new type of clean energy.The efficient development and utilization of CBM energy is of great economic and social significance to reduce the supply of conventional oil and gas in China and build an environment-friendly society.However,it no doubt that fluid channeling occurs at aquiclude interface severely restrictes the exploration and development benefit of CBM resource.The success of cementing operation is closely related to the sealing quality of cement-mudcake-formation systerm.As to CBM wells,the intergrity of cement-mudcake-formation sealing systerm woud affect the bonding strength at aquiclude interface.The performance of the interfical-transition zone(ITZ)between cement and mudcake will seriously affect the cementation quality at aquiclude interface.Unfortunatly,there are still few reports about the ITZ in the world.In the present thesis,comparative study method was used to investigate the properties of the ITZ at the cement-mudcake interface(CMI)in CBM wells,and puts an insight into the microstructure and chemical composition of ITZ in order to reveal the essence of its weakness and come up ways to its improvement for zonal isolation.The forming process of ITZ was explored and a spacer fluid,mud cake solidification agents(MCSAs),was used to improve its weakness.Moreover,the cementation quality of aquiclude interface was assessed by shear bond strength and hydraulic bond strength;the former reflects the abilities of resisting cement sheath movement from formation,while the later reflects the abilities to prevent the communication of formation fluids.Two parameters,?(fluid channeling coefficient)and K*(equivalent permeability)were brought in to assess the channeling prevention ability of the aquiclude interface.And the influence of fluid channeling on the bonding strength of aquiclude interface was discussed.In the end,the improvement mechanism of ITZ by MCSAs was studied.And layered model of ITZ and mathematical model were set up to explore the correlation between ITZ and the cementation of aquiclude interface.This thesis was thought to provide a guidline to the optimal formulation design of effective prepad fluid.Several conclusions were obtained as following:(1)The ITZ does exist between mudcake and cement,and can be remarkably improved by the MCSAs.The reaction production of cement leachate and MCSAs can significantly improve the microstructure,chemical composition and mechanical strength of ITZ.ITZ is the phase between cement and mudcake,which was composed of calcium carbonate,calcium silicate hydration(C-S-H),and crystalline portlandite which has a growth orientation in(001)crystal plane.The porosity of ITZ decreases with the increase of the distance from cement-mudcake interface to the cement stone center.Microcracks and crystalline portlandite in ITZ were observed from the ESEM images and EDAX spectra provided the significant information about the Ca/Si ratio of the region of interest.The analysis results indicated that Ca/Si ratio of the blank samples decreases with the increase of the distance from cement-mudcake interface to the cement stone center,while for the modified sample Ca/Si ratio of the blank samples increases with the increase of the distance from cement-mudcake interface to the cement stone center.XRD analysis showed that portlandite and calcium carbonate were the mainly crystalline composition of ITZ besides the amorphous cement hydration.FTIR analysis results suggested that bands in montmorillonite structure were altered by MCSAs and became more receivable to cement slurry.The high Ca/Si ratio due to the uptake of calcium in the C-S-H interlayer and the depolymerisation of the silicate chains,which could significantly enhance the shear bond strength at simulated cement-formation interface and reduce the fluid channeling risk.The C-S-H generated at the cement-formation interface is thought to be the essential reason why the cementation quality of modified samples was bigger than the blank samples.The comparative experimental results demonstrated that MCSAs could significantly enhance the shear bond strength at simulated cement-formation interface and reduce the fluid channeling risk.(2)The methods to assess bond quality of aquiclude interface were set up,including shear bond strength and hydraulic bond strength,which were characterized by the permeates ?(fluid channeling coefficient)and K*(equivalent permeability),respectively.The fluid channeling experiments results showed that K* increased with the hydraulic strength and decreased with the increase of curing time,and K* of modified samples were smaller than blank samples under the same curing condition.Combining with the results of previous studies,the calcium silicate hydration helped enhance the cementation of the solid particles,compact the ITZ,and reduce the interconnectedness of micropores,microcracks and holes.In view of this,it was the MCSAs that improved the ITZ to own high structural strength which can resist the invasion of formation fluids.Furthermore,fluid channeling damaged seriously to the bonding quality of ITZ.(3)The equivalent critical crack length of ITZ decreased with the increase of curing time when the plane failure occurred during shear strength test,indicating that young modulus and surface energy density were improved.With respect to the mechanical strength,ITZ of the modified samples owned a better performance than the blank samples under the same curing condition.This demonstrated that the generation of C-S-H enhanced the cementation and integration of the CMI.The above laboratory experiments results verified that MCS technology is an effective method to improve the bond strength of ITZ at cement-mudcake interface,and can be used in the cementing operation to improve sealing quality at aquiclude interface of CBM wells.In summary,the findings in this thesis give the guidline to design the formulation of the prepad fluid.Therefore,when designing the prepard fluid,especially for the chemical washes,engineers should try to improve the cchemical composition of the ITZ,decrease the porosity of ITZ,eliminate the crystalline of the portlandite,even translate portlandite to C-S-H.