摘要
针对渝东南构造复杂区常压页岩气压力系数低、吸附气占比高、地应力复杂及压裂形成复杂缝网难度大等地质特点，在60余口页岩气水平井钻探成果和生产特征分析基础上，总结了常压页岩气生产规律，划分了生产阶段，明确了产能主控因素，提出了开发技术政策。结果表明：常压页岩气生产具有初期以排液为主，产气量较低，随着返排率增大，产气量不断增大，后期产量逐渐稳定，产量递减慢，单位压降产气量较高，单井可采储量较小的特点。可划分为纯液、过渡、稳定生产和低压排采等4个阶段，不同阶段生产特征受地层压力系数影响明显：压力系数越高，纯液生产时间越短，见气返排率降低；在过渡阶段，返排率越低，气液平衡时间越长；在稳定生产阶段，产气能力越强，单位压降产量和单井可采储量越高。产能主要受地层压力系数和有效改造体积控制，其中有效改造体积主要受控于最优靶窗钻遇率、水平段长、水平段方位以及压裂改造规模等，压力系数越高，最优靶窗钻遇率越高，水平段长越长，压裂改造规模越大，越利于提高单井产量和最终经济可采储量。在上述认识基础上，提出了渝东南构造复杂区页岩气开发技术优化政策和配套的压裂工艺参数，以指导常压页岩气效益开发。
Abstract
The normal-pressure shale gas reservoirs in some structurally complex areas of southeastern Chongqing is characterized by low pressure coefficient, high proportion of adsorbed gas and complex in-situ stress.Difficulties in forming complex fracture networks in these reservoirs are also frequently experienced during frac operations.Based on the analysis of drilling results and production characteristics of more than 60 horizontal shale gas wells in the areas, we summarized the production law, delineated the production stages, clarified the main factors controlling production capacity, and proposed an optimized technology combination for the reservoir development.It shows that the reservoirs produce mainly fracturing fluids with only a small amount of gas during the initial stage after fracturing and start to yield more gas as the flowback rate of frac fluids increases.At the later stage, gas production gradually stabilizes (depletes slowly) but features relatively high gas production per MPa of pressure drop and limited reserve covered by single wells.The whole production process can be divided into four stages: the liquid (flowback fluid only) stage, transition stage, stable production stage and low-pressure gas drainage stage.The formation pressure coefficient plays a critical role in all of the stages.During the liquid stage, higher pressure coefficient helps to shorten the liquid producing time and to initiate gas production when the flowback rate is still low.In the transition stage, it maintains a longer gas-liquid equilibrium time at lower flowback rate.In the stable production stage, it ensures a stronger gas production capacity that yields more gas with each unit of pressure drop and recover more reserves from a single well.Apart from formation pressure coefficient, the productivity of the reservoirs after fracturing operations is also conditioned by the stimulated reservoir volume (SRV) of frac operations.SRV is in turn controlled mainly by optimal hit ratios of target window, length and orientation of laterals and stimulated areas.Higher formation pressure coefficient and optimal target window drilling rate, longer lateral length, and wider stimulated areas generally signify higher single-well production and more commercially recoverable reserves.Given the above-mentioned understandings, we put forward an optimized technology combination and related fracturing parameters for the normal-pressure shale gas development in the complex areas of southeastern Chongqing.
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