Experimental Study Of Porosity And Electrical Properties Of Gas Shale

Laboratory core-analyze is essential for reservoir evaluation of gas shale.The using of conventional techniques in these low permeability rocks with complex mineralogy has proven to be challenging.Studies are conducted to develop suitable techniques for gas shale measurements.The paper presents the experimental works on cleaning hydrocarbons,total porosity measurement,water saturation establishment and electrical property analyses.Due to the co-existence of fluid and solid hydrocarbons in gas shale,the cleaning target of hydrocarbons in it is quite different for that in the conventional rocks with no organic matter.It is more than a work to clean all hydrocarbons simply.The Rock-Eval II pyrogram can give visual information of the cleaning effect in gas shale,however,it has limitations in differentiating hydrocarbon components and would provide an insufficient cleaning result.Because the heating mode could sort a partial of fluid-like hydrocarbon residues（FHR,which naturally belongs to the free hydrocarbon）into the solid organic matter matrix inappropriately.In this study,the ESH（extended slow heating）pyrolysis evaluation method is proposed to evaluate the cleaning effect in gas shale by comparing the pyrograms measured on the uncleaned and cleaned aliquots.The differences between the ESH and Rock-Eval II pyrograms on samples with different kerogen richness are experimentally researched.The ESH pyrolysis technique solves those defects by dividing the pyrogram into three finer fractions,including light free hydrocarbon（S1_E）,FHR（S2a_E）and solid organic matter（S2b_E）.The evaluation criteria for the complete cleaning of free hydrocarbons in the kerogen-rich shale sample are expressed as the disappearance of S1_E and S2a_E fractions and the invariant of S2b_E fraction on the cleaned sample’s ESH pyrogram.For the kerogen-poor sample,the criterion is simplified as the disappearance of S1_E and S2a_E fractions.The results prove that the ESH pyrogram is more reasonable than other techniques,especially for evaluating the cleaning level of free hydrocarbon and the integrity of solid organic matter in gas shales.The TGA（thermal gravimetric analysis）results on clay rocks show that heating at 200℃can remove almost all the clay bound water and ensure that only the integrated matrix is left.Porosity is considered one of the most important rock physics parameter when evaluating a gas shale reservoir’s production potential.The measurement of total porosity by the Gas Injection Porosimetry（GIP）method in shale has usually been doubted.It is not rare for results to vary from different laboratories but the reasons behind these inconsistencies are not fully understood.These differences are commonly attributed to apparatus（different performances of porosimeter）,core cleaning（lacking of cleaning effect evaluation criteria and arbitrary drying）and measurement（insufficient gas injection pressure and inconsistent crushing levels）.In this study,systematic experimental investigations are conducted to study the impact of these factors in determining the GIP method.A modified porosimeter which was designed to meet the particular needs of gas shales is made.Helium is used to minimize the adsorption resulted by the molecular sieve effect in gas shale porosity measurements.Besides,the effects of gas injection pressure and particle size on porosities measured with plug samples and crushed samples are studied.Results show that the gas injection pressures play a key role in the measurement of crushed and uncrushed samples’porosity.Both the intact plug and crushed cuttings can effectively give the total porosity of gas shales.The plug sample requires a higher minimum-injection-pressure and more total diffusion time in the total porosity estimation with respect to that in the crushed samples,because these two parameters are greatly decreased by crushing.Note that the lower crushing level should be controlled not to alter the integrity of grain size composition and the pore network,where 60mesh is recommended.The porosity resulted by GIP method is essentially the total gas accessible porosity,since the helium is unable to penetrate into the throats with size smaller than the helium dynamical diameter.Experiments are designed to research water saturation establishment methods and electrical properties of gas shale.Higher field nuclear magnetic resonance with short echo space（TE）is used to identify the water saturation and the distribution of saturated pores which contribute to the conductivity.The statistical data of Archie saturation exponents from different water saturation establishment techniques are derived.Using semi-permeable plate drainage can obtain a uniform fluid distribution and a lower water saturation.A wide range（approximately 10%¹00%）of water saturations can be established by the combination of modified spontaneous imbibition and semi-permeable plate drainage techniques.Spontaneous imbibition gains water saturation from 0 to nearly the irreducible water saturation,and,semi-permeable plate drainage desaturates from 100%to the irreducible water saturation.The experimental results indicate that,the fluid distribution of imbibition occurs on the left of T_2C,which means that the exponential behavior of RI-Sw is dominated by the gaining of clay bound water.On the contrary,the fluid distribution of drainage shows on the right of T_2C,where the power behavior of RI-Sw is dominated by the decreasing of free water.The free water and some bound water are consumed in the hydrocarbon generation explains why the irreducible water saturation derived by drainage is larger than the in-situ water saturation.The water distribution established by imbibition is better to be used in evaluating the in-situ water saturation.