页岩油地层原油黏度的核磁共振评价方法

Oil viscosity evaluation method of the shale oil formations with NMR logging

随着页岩油气资源的规模化勘探与开发,精细地层评价发挥着越来越重要的作用.地层原油黏度作为影响页岩油开发方案、产能、产量以及采收率等的重要参数,是流体识别及定量评价的核心内容之一.然而,页岩油流体赋存状态多样且流动机理复杂,导致原油黏度指数难以准确估算.因此,建立页岩油地层原油黏度评价方法具有十分重要的意义.本文从核磁共振评价原油黏度的基础理论出发,分析并明确原油黏度、T_(2)对数平均值、绝对温度三者之间的关系,利用核磁共振方法表征不同温度下原油T_(2)弛豫特征与绝对温度的变化规律;然后,开展页岩油原油变温核磁共振实验,结合原油的黏温关系图版,分析表征原油黏度的主要参数,验证页岩油原油黏度理论计算模型的可行性;最后,通过饱含原油的页岩油岩心变温核磁共振实验,进一步明确地层条件下原油黏度、绝对温度以及T_(2)对数平均值之间的关系,改进原油黏度理论计算模型,建立了页岩油地层原油黏度的核磁共振评价方法.实际测井资料应用表明页岩油地层可动油黏度指数与实际试油得到的原油黏度相比,相对误差在5%以内,验证了该方法的应用效果.

With the exploration and development of the shale oil and gas resources,formation evaluation is playing an important role.Specifically,the viscosity of crude oil is a critical factor influencing the development strategies,productivity,outputs,and recovery rates in the shale oil reservoirs,which is the fundamental parameter of fluid identification and quantitative assessment.However,it is difficult to accurately estimate the viscosity index of crude oil due to the diverse occurrence states and complex flow mechanisms in shale reservoirs.Therefore,it is very necessary to establish an evaluation method for the viscosity of crude oil in the shale oil reservoirs.This paper analyzes and identities the relationship with the crude oil viscosity,T_(2) logarithmic mean value,and absolute temperature based on the foundational theory with Nuclear Magnetic Resonance(NMR).Furthermore,the NMR measurement has been performed to characterize the variation the T_(2) relaxation time at different temperatures with bulk oil from shale reservoirs.A model has been built to predict the viscosity of crude oil based on the T_(2) logarithmic mean value at the different temperatures.Finally,through the NMR experiments with the shale core samples saturated with crude oil,we built a similar model to predict the crude oil viscosity with the T_(2) logarithmic mean value from NMR logging in the reservoir conditions from downhole.Application results of NMR well logging data demonstrate that the relative error between the calculated moveable oil viscosity index and measured crude oil viscosity from field well tests is lower than 5%,which meaning the proposed method is accurate and effective.