基于保压取心的页岩含气量测试新方法

A new method for testing shale gas content based on pressure-holding coring technology

含气量是页岩气储层评价和储量计算中的关键参数,其准确测试是计算地质储量和确定开发方案的基础。目前常用的现场测试方法,由于需估算损失气量,导致其数据可信度不高。为了明确我国四川盆地南部龙马溪组页岩的真实含气能力,开展了我国首口海相页岩气保压取心井的现场含气量测试。保压取心技术可以使取出的岩心保持在原始地层压力状态,能最大限度地减少页岩中气体的散失。考虑保压取心过程的特殊性,针对性地研发了页岩含气量现场测试设备,建立完整的测试流程和方法,实现了页岩含气量的带压测试。新方法需分段测试降压阶段解吸气量和常压阶段加热解吸气量,将两部分测试结果直接相加即可得到页岩总含气量,无需再估算取心过程中的损失气量。测试结果表明,该井常规取心段页岩总含气量平均仅为1.26 m^(3)/t,保压取心段页岩总含气量平均达3.77 m^(3)/t,明显高于常规取心段页岩含气量。此外,还建立了页岩吸附气量和游离气量直接测试方法,发现龙马溪组页岩游离气量均大于吸附气量,游离气量所占比例平均为64.48%,底部游离气量所占比例可达75%左右。与邻井同层位页岩气井的总含气量对比认为,采用目前的损失气计算方法明显低估了页岩的含气量,需结合保压取心结果进一步标定和建立新的损失气计算方法。建议未来增加页岩气保压取心井的试验井次,进一步明确页岩气储层的真实含气能力和储集能力。特别是对于深层页岩气、海陆过渡相页岩气等新区新领域的勘探,可优先试验保压取心。

Gas content is a key parameter in shale gas reservoir evaluation and reserve calculation,and its accurate test is the basis for calculating geological reserves and determining the development plan.At present,the commonly used field-test method needs to estimate the lost gas content,which leads to the low reliability of the test data.In order to clarify the real gas-bearing capacity of Longmaxi Formation shale in Southern Sichuan Basin,China,for the first-time,the field gas content test of the marine shale gas in a pressure-holding coring well was carried out in China.The pressure-holding coring technology can keep the core in the original formation pressure state and minimize the loss of gas in shale.Considering the particularity of the pressure-holding process,the direct testing equipment for shale gas content was developed,and a complete testing process and method were established and the gas content of shale can be measured under pressure.The new method needs to test the desorption gas in the depressurization stage and the heating desorption gas in the normal pressure stage.The total gas content of shale can be obtained by adding the two parts of test results directly,and there is no need to estimate the gas loss in the coring process.The results show that the average total gas content of pressure-holding coring section is 3.77 m^(3)/t,which is significantly higher than that of conventional coring section.In addition,the direct test method of gas and free gas content in shale was also established.It is found that the free gas content of Longmaxi Formation shale is greater than the adsorbed gas content,the average proportion of free gas is 64.48%,and the proportion of free gas at the bottom of Longmaxi Formation is about 75%.Compared with the total gas content of shale gas wells in the same layer of an adjacent well,it is considered that the current calculation method of lost gas obviously underestimates the gas content of shale,and it is necessary to further calibrate and establish a new calculation method of lost gas combined with the results of pressure-holding coring.It is suggested to increase the number of test wells of shale gas pressure-holding coring wells in the future to further clarify the real gas-bearing capacity and reservoir capacity of shale gas reservoir.Especially for the exploration of new areas such as deep shale gas and marine-continental transitional shale gas,the pressure maintaining coring test can be used as a priority.