Swelling-induced Fracturing Of Organic-rich Shales In The Replacement Of Calcite/Dolomite By Calcium Sulfate

Hydraulic fracturing is the main method to improve the oil and gas permeability of tight shale reservoirs.The technology quickly injects a large amount of fracturing fluid(clear water and a small amount of chemical additives)into the shale reservoir,and breaks the shale under the injection pressure to form a complex fracture networks.The greater the number of fractures,the more oil and gas can be extracted.Shale reservoirs are rich in chemically active components such as calcite,dolomite and pyrite,which are unstable in an acidic or oxidizing environment and can induce a significant change in rock strength.However,the effect of these water-rock reactions on the formation of shale fractures is still unclear.Therefore,the traditional fracturing design did not pay too much attention to the potential impact of water-rock reactions on improving the efficiency of fracturing.We were greatly inspired by the important geological phenomena such as crystallization induced cracking(or rupture)and spontaneous fracture propagation in the transformation of minerals within the deep buried formations.In this study,the shale samples derived from the Lower Silurian Longmaxi Formation in Sichuan Basin are the main experimental objects.Sulfuric acid and ammonium persulfate solution were also used to promote the replacement reaction of calcite(4.7 wt.%)and dolomite(4.3 wt.%)within shale matrix by calcium sulfates.The mechanisms consisting of the replacement reaction and fracturing induced by crystallization stress from the calcium sulfate crystals are presented in this paper.We also evaluated the ability of microfractures initiation and propagation induced by crystallization stress.Finally,combined with the technology characteristics of hydraulic fracturing of shale reservoir,the feasibility of crystallization induced fracturing within the hydraulic fracturing shale formation is analyzed.Based on the above research,we obtained the following main research results:A quantitative characterization method for the micro structures of carbonate minerals within organic-rich shales was established.Based on the characteristics that the acid dissolution rate of carbonate minerals is much faster than that of the remaining rock-forming minerals in shale,carbonate minerals are dissolved at room temperature using 3.0 wt.%to 5.0 wt.%dilute hydrochloric acid.The changes in shale mineral and pore structure were tested by X-rays diffraction,low pressure nitrogen adsorption,in-situ scanning electron microscopy(SEM),micro-CT imaging(resolution 1 ?m),and then the pore structure parameters was extracted by image processing software(Avizo).It was confirmed by experiments in this paper that only the carbonate minerals was dissolve to form the pores within the reaction time of 48 hours,and the pore size is in the range of micrometers order,while the other rock-forming minerals maintain good stability.Based on the micro-CT imaging of the dissolution pores and the quantitative characterization using Avizo,the micro-structural parameters such as the pore-scale distribution,morphology,particle size and surface area of carbonate minerals were successful obtained.The reaction mechanism of the conversion of carbonate minerals to calcium sulfate in the confined nanoscale pore space within the shale matrix was clarified.Our experimental results found that even if the bulk solution is undersaturated with respect to the calcium sulfate(i.e.,no crystal precipitation was observed on the sample surface),a large amount of calcium sulfate crystals was still formed within the shale matrix.It was further confirmed by Micro-CT imaging that the carbonate minerals can be replaced by calcium sulfate and preserve the volume and morphology of the parent minerals.The spatial distribution and morphology characteristics of this phenomenon are completely consistent with the theory of "supersaturated thin layer of fluid at the mineral-fluid interface and interface-coupled dissolution-precipitation reaction",which is common in the mineral replacement reactions.The experiment confirmed that the crystallization stress exerted by calcium sulfate crystals can induce shale fracturing.The molar volume of calcium sulphate dihydrate(74.31 cm3/mol)is much larger than calcite(36.94 cm3/mol)and dolomite(64.35 cm3/mol).When the latter was converted into the former,the local volume increase is 101%and 15%,respectively.The matrix of organic-rich shale is too much tight,and the mineral particles are arranged in a compact manner,thus the local volume is increased to induce the expansion stress.Through industrial CT,micro-CT and SEM,fracturing was observed inside or on the surface of shale samples at various scales.It was confirmed that the crystallization stress from calcium sulfate crystals can induce the formation of micro-cracks in shale and then spontaneous propagation(no extra external force is needed).The mechanical mechanism of shale fracturing induced by the swelling stress of calcium sulfate crystal was revealed.Calcium sulfate crystal precipitates in the dissolution pores of carbonate minerals,and produces compressive stress on the pore wall surface,which eventually forms shale tensile rupture.Therefore,based on the theory of salt crystallization and Poromechanic approach,we used the shale ideal pore-throat shapes to model the formation of crystallization stress and for the calculation of crystal stress on the single-pore and macroscopic tensile stress.The calculations show that at the single-hole scale,the lower limit of the crystal stress calculated from the pore and throat morphology is 6 to 53 MPa(when the crystal-solution is in equilibrium),while the value can easily exceed 50 MPa when it was calculated by the concentration expressions of crystal stress.After being converted into macro tensile stress,it can still reach tens of MPa,which is easy to induce shale expansion.In order to obtain a wider zones that can be influenced and a bigger crystallization stress,it is preliminarily concluded that persulfate(strong oxidant)is a better choice.Calcium sulphate crystals are mainly distributed in the carbonate mineral pores(ie,in situ precipitation,confirmed by the experiments in this paper),which can reduces the clogging effect of hydraulic cracks.Therefore,this method may have a good application prospect.The concept or idea of using the crystallization stress to increase the micro fractures density during the hydraulic fracturing of shale reservoir was proposed.Based on the simplified two-dimensional plane model of natural fracture rupture,the net pressure in the crack required for natural crack shearing and tensile fracture is calculated,and the mechanical mechanism of using micro-cracks to form crystal cracks during the post-pressure "swelling" is analyzed.The favorable factors for the in-situ conversion of carbonate minerals in shale reservoirs to calcium sulfate crystals are discussed.Finally,the combination of crystallization induced cracking and hydraulic fracturing is proposed.