【摘要】 储层弹性与物性参数可直接应用于储层岩性预测和流体识别,是储层综合评价和油气藏精细描述的基本要素之一.现有的储层弹性与物性参数地震同步反演方法大都基于Gassmann方程,使用地震叠前数据,通过随机优化方法反演储层弹性与物性参数;或基于Wyllie方程,使用地震叠后数据,通过确定性优化方法反演储层弹性与物性参数.本文提出一种基于Gassmann方程、通过确定性优化方法开展储层弹性和物性参数地震叠前反演的方法,该方法利用Gassmann方程建立储层物性参数与叠前地震观测数据之间的联系,在贝叶斯反演框架下以储层弹性与物性参数的联合后验概率为目标函数,通过将目标函数的梯度用泰勒公式展开得到储层弹性与物性参数联合的方程组,其中储层弹性参数对物性参数的梯度用差分形式表示,最后通过共轭梯度算法迭代求解得到储层弹性与物性参数的最优解.理论试算与实际资料反演结果证明了方法的可行性.更多还原

【Abstract】 Elastic and petrophysical parameters of reservoir can be directly applied to lithology prediction and fluid identification,they are the basic elements in both comprehensive evaluation and fine characterization of hydrocarbon reservoir.Existing seismic joint inversion methods for estimating elastic and petrophysical parameters of reservoir are mainly based on either Gassmann equation,with which these parameters are inversed from prestack seismic data by stochastic optimization methods;or Wyllie equation,with which these parameters are inverted from poststack seismic data by deterministic optimization methods.The purpose of this work is todevelop a strategy for estimating elastic and petrophysical parameters of reservoir based on Gassmann equation with deterministic seismic prestack inversion.We use Gassmann equation to build up the relationship between prestack seismic data and petrophysical parameters.We treat the joint posterior probability of elastic and petrophysical parameters as the objective function under the Bayesian architecture,by expanding the the objective function with Taylor formula,the joint equations composed of physical and petrophysical parameters can thus be obtained,and the derivatives of the elastic parameters with respect to petrophysical parameters are obtained by differentiating.The conjugate gradient method is then used to find the optimal solutions of P-wave velocity,shear velocity,density,porosity,water saturation and clay content.We apply this inversion method to prestack seismic data set in Sulige gas field,which is one of the largest gas fields in central Ordos Basin in China.The main gas-producing interval is the H8 sandstone reservoir at the bottom of Lower Shihezi Formation,Lower Permian,Upper Paleozoic.Drilling results indicate that this region is a composite sandstone gas reservoir and its heterogeneous reservoirs are characterized by low permeability,low pressure and low abundance,which bring great challenge to the exploitation of this field.From the comparison of the inversion result at the well location using our method and two-step method,we can see that there are slight but noticeable differences between these two methods,especially for the elastic parameters.Our joint inversion is significantly closer to the "true"elastic and reservoir model than the two-step inversion.Later we use 2-D field prestack seismic data to assess the performance of the proposed method,and selected a 160 ms time interval of interest for the reservoir description.Three wells that intercept the section are previously characterized in two-way travel time,and used to calibrate the rock physics model and generate low-frequency starting model.The major seismic events in the inversion result show more continuity along the reference structural horizon direction,and the estimated property have a good match with the corresponding well-log curves,and commensurate with the frequency content of the seismic data,as expected.We have developed a method for inverting prestack seismic data under well-log constraints derived from Gassmann fluid substitution relations and prior petrophysical models.In our model,we use Gassmann fluid substitution relations calibrated to the well-log data for more robust modeling,which is consistent with common petrophysical knowledge.In this specific setting,the extended application to invert prestack seismic data can estimate water saturation,porosity,clay volume,P-wave velocity,shear wave velocity and density jointly.Compared with the stochastic optimization method,our method utilize gradient information of objective function,the optimal solution can be obtained more quickly in favor of large-scale production mission.Compared with Bosch′s poststack deterministic optimization,prestack seismic amplitude have more information varying with the angle of incidence.And compared with Wyllie equation,Gassmann equation is better for establishing the relationship between the reservoir parameters and prestack seismic observation data,which helps a lot to understand the impact of reservoir parameters on the elastic properties.更多还原