深部煤层(岩)气压后焖井及控压排采机理与实践

Mechanism and practice of post-fracturing soaking and pressure-controlled drainage in deep coalbed methane reservoirs

  • 摘要: 深部煤层气(又称煤岩气)已成为我国天然气增产的重要增长极。实践证实深部煤层(岩)气必须通过大规模水力压裂获得高产,其排采规律和开发特征与中浅层煤层气存在显著差异,但背后产气关键机理和定量化排采指标界限尚不明确。与中浅层煤层气相比,研究提出了深部煤层(岩)气井因其地质特殊性决定了二者在压裂改造强度及资源动用范围、渗吸置换效应、排采理念及方式等开发领域的三大差异,基于物理模拟试验、数值模拟、机理剖析、现场试验等多种方法,首次揭示了深层煤层气压后焖井及控压排采的内在机理,明确了科学焖井时间、返排及投产后的控制制度关键定量指标与优化策略,建立了深部煤层(岩)气井全生命周期“五步六阶段”制度体系与开发模式,现场验证效果显著。结果表明:① 深部煤层(岩)气科学开发理念是通过“合理焖井、控压生产、保持气液两相流、延长自喷期”,充分发挥压裂改造“缝控范围”(SRV)内压裂液的“压驱−渗吸−置换”效应,最大化泄气体积(DRV)内气液两相协同高效产出,从而使单井最终可采储量(EUR)最高;② 深部煤层(岩)压裂焖井过程中存在显著的气水置换效应,基于渗吸置换核磁共振试验、长岩心压力传导试验及矿场尺度数值模拟,估算压裂液焖井时间下限为2.5 d、上限为15 d,结合大吉区块深部煤层(岩)气现场开发实践,综合确定深部煤层(岩)气井合理焖井时间为3~11 d;③ 微流控试验和在线核磁共振试验均表明,若生产过程中排采过快,气相极易沿优势大通道“窜流”突进,导致大量水相滞留于次级裂隙,且有效应力越大,水相滞留量越大、产出效率越低,进而堵塞与次级裂缝相连的基质孔隙中气体产出通道;核磁试验表明,相比“快速降压”方式,采用“逐级降压”控制方式的深部煤层(岩)气采收率提升了39.1%(从41.6%至80.7%),表明“逐级降压”控制方式既能避免气相突进、保障气水稳定协同产出,也能通过“吸附态解吸补充−游离态持续供采”的良性循环,大幅提升采收率;④ 速敏和应敏试验表明,自支撑裂缝和支撑裂缝导流能力在有效应力5~20 MPa区间内呈指数型急剧下降(降幅超94%),且流速超过某临界值后会因煤粉运移或支撑剂嵌入、返吐等导致导流能力严重损伤,表明需通过控压排采方式防控渗流通道导流能力损伤,综合多种方法确定返排阶段直井单层合理返排液量为20~23 m3/d,推广至水平井(以压裂10段为例)最大日产液量不超过230 m3/d;⑤ 综合数值模拟与动态监测,明确了大吉区块不同排采阶段的合理排采制度,定量确定了千米日产气量(日配产量)为4×104 ~5×104 m3/d,建议返排阶段、上产阶段、稳产阶段、递减阶段的压降速度分别控制在0.60、0.70、0.04、0.07 MPa/d阈值以内;⑥ 基于深部煤层(岩)气焖井与控压排采试验和模拟结果,构建了以“焖井扩散促解吸、控压稳流防闭合、调控促排提产量、精细管控延自喷、举升解堵保稳产、补能解吸控递减”为核心的“五步六阶段”新型全生命周期排采制度体系。理论指导应用大吉区块31口井,实践证实新试验井的产液、产气、压力等生产指标均显著优于同生产阶段“几乎未主动焖井+快速返排放产的生产井”,实际焖井时长7~10 d,返排阶段的返排率由20.0%提升至30.1%,投产时的初始井口压力由6.45 MPa提升至11.64 MPa,相比增幅80.5%,预测自喷稳产周期延长至400~500 d,单井EUR预计提升30% 以上。研究成果可为深部煤层(岩)气井排采制度优化、提升单井产量和提高采收率等提供科学理论依据。

     

    Abstract: Deep coalbed methane (also known as coalrock gas) has become a crucial growth driver for increasing natural gas production in China. Practical experience has proven that high-yield production of deep coalbed methane requires large-scale hydraulic fracturing, and its extraction dynamics and development characteristics differ significantly from those of shallow coalbed methane. However, the key gas generation mechanisms and quantitative extraction threshold indicators remain unclear. Compared to shallow and intermediate coalbed methane, the research proposed that deep coalbed methane wells highlights three key differences in development geology—fracturing stimulation intensity and resource utilization scope, imbibition and displacement effects, and production methods—due to the geological uniqueness of deep coalbed methane wells. Through physical modeling experiments, numerical simulations, mechanistic analysis, and field trials, it is the first to reveal the intrinsic mechanisms of post-fracturing soaking and controlled pressure production in deep coalbed methane. The research establishes critical quantitative indicators and optimization strategies for scientifically determining soaking duration, flowback, and post-commissioning control measures. A “five steps-six stages” regulatory system and development model for deep coalbed methane wells were developed and validated with significant field results.The research results indicate: ① The scientific development concept of deep coalbed methane is to fully utilize the “permeation displacement displacement” effect of fracturing fluid through “reasonable well soaking, pressure control production, maintaining gas-liquid two-phase flow, and extending the self injection period”, ensuring efficient use of resources within the SRV transformation range, maximizing the synergistic and efficient production of gas-liquid two-phase within the drainage volume (DRV), and ultimately achieving the highest estimated ultimate recovery (EUR). ② During the fracturing and soaking process of deep coal seams, a significant gas-water displacement effect occurs. Based on imbibition-displacement NMR experiments, long-core pressure transmission experiments, and field-scale numerical simulations, the lower and upper limits of fracturing fluid soaking time were estimated to be 2.5 days and 15 days, respectively. Combining Daji field practice, the reasonable soaking time for deep coalbed methane wells was comprehensively determined to be 3 to 11 days. ③ Both microfluidic experiments and online NMR experiments demonstrate that during the flowback phase, the gas phase readily channels through dominant large pathways (gas channeling), leading to significant water phase retention in secondary fractures. Higher effective stress increases water retention and reduces drainage efficiency, subsequently blocking gas flow pathways from matrix pores connected to these secondary fractures. Compared to rapid depressurization, stepwise pressure reduction establishes a virtuous cycle of “desorption replenishment from the adsorbed state−continuous supply and production from the free state”, nearly doubling the CBM recovery factor (from 41.6% to 80.7%). ④ The conductivity of both self-propped and propped fractures decreases exponentially (by over 94%) within the effective stress range of 5−20 MPa. Furthermore, once flow velocity exceeds a critical threshold, the conductivity of propped fractures suffers severe damage due to coal fines migration, proppant embedment, or flowback. This underscores the necessity of controlled-pressure drainage to prevent damage to seepage pathway conductivity. Through a comprehensive analysis of multiple methods, it was determined that the single-layer backflow liquid production for vertical wells during the backflow phase ranges from 20 to 23 m3/d. Extending this to horizontal wells (taking 10 fractured sections as an example), the maximum daily liquid production should not exceed 230 m3/d. ⑤ Through comprehensive numerical simulation and dynamic monitoring, the reasonable production system for different production stages has been clarified, and the daily gas production per kilometer (daily allocation production) in Daji field has been quantitatively determined to be 40 000 to 50 000 m3/d. It is recommended to control the pressure drop rate during the backflow stage, production stage, stable production stage, and decline stage within the threshold values of 0.6, 0.7, 0.04, and 0.07 MPa/d, respectively. ⑥ Based on the experimental and simulation results of deep coal rock gas soaking and pressure control extraction, a new “five steps-six stages” full life cycle extraction system with “soaking diffusion to promote desorption, pressure control to stabilize flow and prevent closure, regulation to promote extraction and production, fine control to extend self jetting, lifting and unblocking to ensure stable production, and energy replenishment to control decline” as the core was constructed. Application of this system to 31 wells in the Daning Block confirmed that key production indicators—such as liquid production, gas production, and wellhead pressure—in the test wells were significantly superior to those in wells with almost no active soaking and produced under conventional aggressive methods at similar stages. After soaking time in the well 7−10 days and control method, the wellhead pressure increased from 6.45 MPa to 11.64 MPa, flowback efficiency improved from 20% to 30.1%, the predicted stable self-flow production period extended to 400−500 days, and the EUR per well increased by over 30%. The research results can provide important scientific theoretical basis for optimizing the extraction system of deep coalbed methane wells, improving single well production, and enhancing recovery efficiency.

     

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