Abstract Ultra-low permeability, inertinite-rich, poorly cleated, dehydrated, overpressured coal seams deeper than 9000 ft (2740 m) in the Cooper Basin of central Australia represent a fundamentally different play type to shallow and “deep” coal seam gas reservoirs. Proof-of-concept gas flow was achieved in 2007. Four Patchawarra Formation coal seams were subjected to low-proppant concentration slick-water hydraulic fracture stimulation within a dedicated vertical wellbore at a depth of 9500 ft (2900 m). Gas was produced for 81/2 years at a slowly increasing base flow rate, averaging 0.1 MMscfd (2.8 Mscmd). Experimental data were gathered for characterising dynamic reservoir behaviour. This included multiple extended pressure build-up tests. The authors have previously investigated the three largest of these using time-lapse pressure transient analysis. The published results reveal a dominant bilinear flow regime, associated with an isolated domain of increasing coal fabric permeability surrounding the hydraulic fracture. This paper builds upon the time-lapse pressure transient analysis study by specifically investigating conductivity of the hydraulic fracture, which the authors postulate to have also increased. The hypothesis is tested by applying time-lapse rate transient analysis to the two specially designed pressure drawdown tests to atmospheric pressure that immediately follow the first two pressure build-up tests of the time-lapse pressure transient analysis. Between the two pressure drawdown tests, spaced 586 days apart, the wellbore flowed gas continuously for 327 days, at an average flowing bottom-hole pressure of 580 psig (4.0 MPag). Hence, there was ample opportunity for hydraulic fracture conductivity to change between the tests. Both pressure drawdown tests were monitored for 24 h using high-resolution surface pressure gauges. Each was initiated from the same surface shut-in pressure of 2500 psig (17.2 MPag). The flow pressure data are initially used to construct “diagnostic plots” that clearly identify the hydraulic fracture linear flow regime, early in each test, immediately after the dissipation of wellbore storage. Hydraulic fracture properties are then extracted using “specialty plots” that display rate-normalised pseudo-pressure difference versus linear superposition time. Comparing the slopes of the two speciality plot trends indicates that the hydraulic fracture flow property b f k f ∅ f increased during the 327-day gas flow period by a factor of 4. This is supported by a 60% increase in the initial gas flow rate from “hydraulic fracture storage”, from 7.5 to 12.0 MMscfd (212.4 to 340.0 Mscmd). Additionally, despite a significantly larger volume of “hydraulic fracture storage” gas being produced to surface during the second test, the duration of the hydraulic fracture linear flow regime is less than for the first. These observations are consistent with an increasingly more conductive hydraulic fracture over flowback time that allows compressed gas within it to discharge more rapidly.

中文翻译：

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使用延时率瞬态分析在生产超深煤层内增加水力压裂导流能力的研究：澳大利亚库珀盆地的长期试点试验

摘要 澳大利亚中部库珀盆地深度超过 9000 英尺（2740 米）的超低渗透率、富含惰性、裂理不良、脱水、超压的煤层代表了与浅层和“深部”煤层气藏根本不同的成藏类型。2007 年实现了概念验证气流。四个 Patchawarra 组煤层在 9500 英尺（2900 米）深度的专用垂直井筒内进行了低支撑剂浓度滑溜水水力压裂增产。气体以缓慢增加的基础流速生产 81/2 年，平均为 0.1 MMscfd (2.8 Mscmd)。收集实验数据以表征动态储层行为。这包括多个扩展的压力建立测试。作者之前已经使用延时压力瞬态分析研究了其中三个最大的问题。已发表的结果揭示了占主导地位的双线性流动状态，与围绕水力压裂的煤织物渗透性增加的孤立域相关。本文以延时压力瞬态分析研究为基础，专门研究了水力压裂的传导率，作者假设该传导率也增加了。通过将延时速率瞬态分析应用于两个专门设计的大气压压降测试，紧接着延时压力瞬态分析的前两个压力建立测试来检验该假设。在相隔586天的两次压降测试之间，井筒连续供气327天，在 580 psig (4.0 MPag) 的平均流动井底压力下。因此，水力压裂传导率在两次测试之间有很大的变化机会。使用高分辨率表面压力计对两种压降测试进行了 24 小时的监测。每个都从相同的 2500 psig (17.2 MPag) 地面关井压力开始。流动压力数据最初用于构建“诊断图”，在每次测试的早期，即在井筒储存消散后立即清楚地识别水力压裂线性流动状态。然后使用显示速率归一化伪压力差与线性叠加时间的“专业图”提取水力压裂特性。比较两个专业绘图趋势的斜率表明，在 327 天的气流期间，水力压裂流动特性 bfkf ∅ f 增加了 4 倍。这是由初始气体流速增加 60% 所支持的。水力压裂存储”，从 7.5 到 12.0 MMscfd（212.4 到 340.0 Mscmd）。此外，尽管在第二次测试期间产生了大量的“水力压裂储存”气体到地表，但水力压裂线性流动状态的持续时间比第一次短。这些观察结果与随着回流时间导电性越来越强的水力压裂一致，这使得其中的压缩气体能够更快地排出。“水力压裂储存”的初始气体流速从 7.5 增加到 12.0 MMscfd（212.4 到 340.0 Mscmd）支持了这一点。此外，尽管在第二次测试期间产生了大量的“水力压裂储存”气体到地表，但水力压裂线性流动状态的持续时间比第一次短。这些观察结果与随着回流时间导电性越来越强的水力压裂一致，这使得其中的压缩气体能够更快地排出。“水力压裂储存”的初始气体流速从 7.5 增加到 12.0 MMscfd（212.4 到 340.0 Mscmd）支持了这一点。此外，尽管在第二次测试期间产生了大量的“水力压裂储存”气体到地表，但水力压裂线性流动状态的持续时间比第一次短。这些观察结果与随着回流时间导电性越来越强的水力压裂一致，这使得其中的压缩气体能够更快地排出。