| Gas hydrate occurs mainly in Marine sediments and permafrost in continental regions.Most of the exploration and evaluation of gas hydrate resources are based on well logging data.As an important method to evaluate hydrate saturation,the porosity index(m)and saturation index(n)of the rock electrical parameters of Archie formula are the key to determine whether the saturation can be accurately calculated,and their values are easily affected by various factors such as reservoir lithology.The values of m and n used in the past have a wide range,and usually only one fixed value is taken from one sample or a class of samples,which will reduce the accuracy of hydrate reserve evaluation calculation by using Archie formula.In order to expand the applicability of Archie formula in different reservoirs,it is necessary to define the values of m and n more precisely.In this paper,the m and n values of hydrate-bearing sediments under different formation conditions(temperature gradient 0.02 ℃/cm,0.07 ℃/cm,0.11 ℃/cm and 0.18 ℃/cm,cooling rate of 2 ℃/h,4 ℃/h and 8 ℃/h,silt content of 0%～30%)were investigated.The obtained m and n values are used to calculate and evaluate the hydrate saturation and distribution,and the reliability of the m and n values is analyzed by comparing with the experimental data.Finally,the accurate value range of m and n and the hydrate saturation distribution of the muddy sandstone in the range of silt content in the test are given.The analysis results of this paper are as follows:1.The calculation results of hydrate formation data under different temperature gradients show that the m and n values range from 0.77 to 2.30 and 2.48 to 3.73,respectively.In this range,the relative error of Archie formula model calculation results is within 3%,which is less than the previous calculation error.In particular,the Archie formula has the best overall application effect in sand with a temperature gradient of 0.07 °C/cm,and the error is0.8%.2.Under different cooling rate conditions,the applicable range of m and n is 1.32 ～ 2.78 and 2.33～3.47 respectively.In this range,the relative error of Archie formula traditional model calculation results is within 6%.When the cooling rate is 4 °C/h,the Archie formula has the best overall application effect,and the error is 0.6%.3.The test was carried out in unsaturated sand bodies with different silt contents,and the applicable values of m and n were 1.97～2.5 and 1.04～3.04,respectively.The relative error of the optimized Archie model is less than 7 %,and the minimum relative error in the high silt content system is 3.7%.4.In the above m,n value range,the Archie formula is modified.The optimized Archie formula is used to evaluate the hydrate saturation and distribution of hydrate-bearing sediments under different conditions.It is found that with the increase of temperature gradient,the hydrate saturation in pure coarse sand gradually accumulates to both ends,while in mixed sand it mainly accumulates in the middle.With the increase of cooling rate,the hydrate saturation is higher and higher and gradually tends to be distributed at both ends.As the silt content increases,the hydrate slowly migrates to the upper part,and then mainly accumulates in the middle.When the temperature gradient is 0.02 °C/cm,the cooling rate is 4 °C/h and the silt content is 20%,the distribution of hydrate saturation is the most uniform.5.Compared with the actual formation process,it is found that the m and n values fitted by hydrate formation experiments under different conditions(temperature gradient,cooling rate and silt content)will greatly improve the calculation accuracy of hydrate reserves in hydrate sedimentary layers,which can improve the applicability of Archie formula model in evaluating hydrate reserves and distribution in sedimentary layers. |
PDF Full Text Request