基于煤层气井生产数据的储层含气量校正新方法

勘探实践发现沁水盆地潘庄、潘河区块及鄂尔多斯盆地保德区块煤层气井累计产量远远大于原始计算的地质探明储量。该现象对体积法计算的煤层气资源储量提出了挑战,同时为全面“上储增效”提出了新的方向。在采用体积法计算煤层气储量时,含气面积、含气量的准确性以及煤岩密度与煤层厚度的非均质特征都会对储量参数的准确性产生影响。其中,由于取心测试过程的局限性,煤层含气量的数值常存在一定的误差。本次研究基于鄂尔多斯盆地和沁水盆地的煤层气井生产数据并结合等温吸附实验结果提出了计算储层临界最低含气量的方法(临界最低法)。将校正后的临界最低含气量与实测含气量(基于美国矿业局直接法(USBM)和史密斯-威廉姆斯法)进行对比,并剖析含气量测试损失量的地质控制机理。结果表明:在中低至中高煤阶(R_o=0.7%~2.1%)范围,临界最低法计算的含气量总体高于其它两种方法计算的含气量,临界最低法在中低煤阶至中高煤阶具有较强的适应性。在高煤阶(R_o=2.1%~2.8%)范围,临界最低法计算结果可以与取心测试结果相互验证。总体上,煤层含气量测试(USBM法)损失量受不同煤阶煤岩孔裂隙发育特征、煤体结构、含气饱和度及逸散时间的影响。含气量测试损失量与孔渗发育特征、构造煤发育程度、含气饱和度及逸散时间呈正相关。此外,针对未取心的煤层气井,可以采用钻井岩屑测试等温吸附参数进而利用临界最低法求取储层含气量,为煤层气进一步的勘探开发提供数据基础。

Coal Reservoir Gas Content Correction Based on Coalbed Methane Well Production Data

It is found that the cumulative production of coalbed methane wells in some coalbed methane (CBM) blocks (Qinshui basin: Panzhuang and Panhe CBM blocks; Ordos basin: Baode CBM block) is far greater than the original calculated geological proven reserve.This phenomenon has made a challenge on the calculated geological reserve by volume method, and put forward a new way for the overall increase of geological reserve.In the CBM reserve calculation by volume method, the accuracy of gas-bearing area and gas content, as well as the heterogeneity of coal density and thickness would all affect the accuracy of reserve parameters.Due to the limitation of coring and analysis, errors are common exist in the calculation of gas content.In this study, a minimum critical gas content calculation method (Critical Minimum Method) was proposed, based on the production data and coal isothermal adsorption experimental data from CBM wells in the Ordos and Qinshui basins.Then, the calculated critical minimum gas content was compared with the measured gas content, according to the direct measurement method proposed by the United States Bureau of Mines (USBM) and the Smith-Williams method.The geologic controlling mechanism of the D-value between the gas contents (based on USBM and Critical Minimum Method) was also analyzed.The results show that in the medium-low to medium-high coal ranks (R_o= 0.7%-2.1%), the gas content calculated by the Critical Minimum Method is generally higher than that by the USBM and Smith-Williams method.The Critical Minimum Method shows strong adaptability in this coal rank range.In the range of high coal ranks (R_o=2.1%-2.8%), the results by Critical Minimum Method can serve as mutual support with the coring sample measurement results.Generally, the D-value between these two gas contents (based on USBM and Critical Minimum Method) is affected by the pore/fracture development characteristics in different coal ranks, coal structure development, gas saturation, and escape time of the coring process.The D-value is positively correlated with the porosity and permeability, coal structure development, gas saturation and escape time.In addition, for the CBM wells with no cores collected, drilling cuttings can be used for isothermal adsorption parameters measurement.Gas content of coal reservoir can be obtained by the Critical Minimum Method, which can provide a data basis for further CBM exploration and development.