新疆准噶尔盆地白家海凸起深部煤层气孔渗系统特征

Characteristics of the pore and seepage system of deep coalbed methane in the Baijiahai uplift,Junggar Basin,Xinjiang

【目的】新疆深部煤层气资源丰富,其中准噶尔盆地白家海凸起侏罗系煤层是深部煤层气勘探开发的有利目标区。深部煤层气孔渗系统直接决定了深部煤层气的可采性。【方法】以新疆油田2023年最新施工的彩煤2-004H评价井为基础,基于扫描电镜观察,压汞–低温液氮–二氧化碳吸附、低场核磁共振、CT扫描,变温压孔渗实验,结合数学建模,深入研究了本区侏罗系西山窑组煤的孔渗系统。【结果和结论】结果表明:(1)研究区深部煤储层孔隙类型以植物组织孔和气孔为主,孔裂隙发育,形态多为板状和狭缝状,孔裂隙连通性好,多为开放型孔。(2)全孔径表征结果显示,超微孔最为发育,其次为中孔和过渡孔,各类型孔隙发育较为均匀,总孔比表面积和总孔容较大,接近于贫煤和无烟煤,暗示了其吸附性和储集能力较强,其主要地质原因在于其煤层为特低灰分(平均灰分在5%左右)煤、煤岩显微组分中主要为丝质体和结构腐殖体,且处于构造高部位的煤储层断裂发育。(3)考虑温度和有效应力对孔隙率和渗透率的影响,建立了研究区孔隙率和渗透率与埋深的关系式。与鄂尔多斯盆地大宁-吉县区块深部煤相比,不论是常规孔渗结果,还是模拟原位储层条件下的孔渗系统,白家海凸起深部煤储层孔隙率和渗透率均较大,原位储层条件下孔隙率介于8.60%~10.21%,渗透率介于(0.04~0.21)×10^(-3)μm^(2)。即本区深部煤储层孔渗系统,不仅储气能力较强,同时其导流能力也较好。

[Objective]Xinjiang boasts abundant deep coalbed methane(CBM)resources,with Jurassic coal seams in the Baijiahai uplift of the Junggar Basin identified as a favorable target area for deep CBM exploration and exploitation.The pore and seepage system of deep CBM directly determines its recoverability.[Methods]Based on appraisal well Caimei 2-004H drilled the most recently in the Xinjiang oilfield in 2023,this study investigated in detail the pore and seepage system of coals in the Jurassic Xishanyao Formation in the Baijiahai uplift using scanning electron microscopy(SEM)observations,mercury injection capillary pressure(MICP),low-temperature liquid nitrogen adsorption,and carbon dioxide adsorption,low-field nuclear magnetic resonance(NMR),CT scanning,and porosity and permeability experiments under varying temperature and pressure conditions,as well as mathematical modeling.[Results and Conclusions]Key findings are as follows:(1)Pores in deep coal reservoirs in the study area are dominated by plant tissue pores and other pores,along with fissures,which are mostly tabular and slit-shaped.These pores and fissures feature high connectivity and are open pores mostly.(2)The characterization results of full pore size distribution show that ultramicropores are the most developed in the deep coal reservoirs in the study area,followed by mesopores and transitional pores.Various types of pores are relatively uniformly distributed,with a larger total specific surface area of pores and a high total pore volume.Therefore,the deep coal reservoirs are similar to lean coals and anthracites,suggesting high adsorption and reservoir capacities.This occurs primarily because the coal seams comprise coals with ultra-low ash content(average:around 5%),macerals in the coals are dominated by fusinite and homotelinite,and faults are found in coal reservoirs in structural high parts.(3)Given the impacts of temperature and effective stress on porosity and permeability,this study determined the relationships between both porosity and permeability of the study area and the burial depth.Compared to deep coals in the Daning-Jixian block in the Ordos Basin,the deep coal reservoirs in the Baijiahai uplift exhibit high porosity and permeability,as revealed by whether conventional porosity and permeability results or the simulation res-ults of the pore and seepage system under in-situ reservoir conditions.Specifically,the deep coal reservoirs exhibit porosity ranging from 8.60%to 10.21%and permeability from 0.04×10^(-3)μm^(2) to 0.21×10^(-3)μm^(2) under in-situ reservoir conditions.Therefore,the pore and seepage system in deep coal reservoirs in the study area boasts both a high CBM storage capacity and excellent conductivity.