基于排采初期生产特征的煤层气合采地质条件分析

Analysis of geological conditions for coalbed methane co-production based on production characteristics in early stage of drainage

煤层气高效合采的基础是深刻理解合采兼容性,其中的关键在于合采地质条件的量化分析。以滇东-黔西这一典型地区为例,主要基于11口合采试验井前150 d生产历史资料,采用地质统计及因素差异对比法,分析了合采地质条件特点,提出了研究区合采兼容性阈值及工程设计优化的地质建议。研究发现,合采产层组外在地质因素对排采初期效果的影响不一:产层组底界埋深显著影响排采初期生产行为,这是单采及合采的共性特征;产层组存在一个最佳跨度,但跨度阈值因地而异;产层组层数与初期产气量关系存在极大值,与初期产水量关系存在突变点,与压裂增渗倍数呈负相关关系,压裂增渗倍数随产层数增多而降低。产层组内在地质因素并非对排采初期效果都有影响:加权平均含气饱和度及其差异指数阈值因地而异,地层能量越低及储层可改造性越差,含气饱和度差异对初期生产效果的影响越强;原始压力状态差异对见气时间的影响主要存在于不同区块之间,在同一区块内没有实质性意义。发现产层组见气时间随平均压裂渗透率增大呈指数规律缩短,压裂改造效果与煤储层原始渗透率之间不存在必然联系,统一的压裂改造措施可能导致产层组各产层属性差异拉大,抑制了整体产气潜力的充分释放。为此,提出了合采产层组分层均衡压裂改造的设计思想,目的是通过差异性改造来弱化原始储层属性差异。先前采用的储层改造技术不适应于滇东地区800 m以深的煤储层;合理确定产层组的关键在于单井储层压力剖面精细分析;影响研究区储层可压裂改造性的关键地质因素是煤体结构;实现产层组产气潜力释放最大化的工程措施是各产层之间的均衡改造;合采井憋压与否、憋压程度因区块乃至因井而异,放气时机的把握十分重要。在合采井现行排采管控制度不变的前提下,建议控制产层组底界埋深和最大跨度,优选加权含气饱和度高、饱和度差异系数小和压力状态差异系数小的产层组,注重产层组中各产层的均衡改造。

The basis of efficient coalbed methane(CBM)co-production is to understand the co-producing compatibility deeply,in which the key is the quantitative analysis of geological conditions for co-production.Taking the typical areas of eastern Yunnan and western Guizhou as examples,mainly based on the production data during first 150 days from 11 test wells,the geological conditions for co-production are analyzed with the statistical and difference comparison methods of geological factors,and the geological suggestions for co-producing compatibility threshold and engineering design optimization in the study area are put forward.It is found that the influence of the external geological factors on early drainage is not the same.Bottom depth of the gas-producing group significantly affects the initial production,which is true for both single-and co-production.There is an optimal span of the production group,but the span threshold varies from place to place.There is a maximum in the plots of the number of gas-producing seams to the initial gas production,and an inflection point in the plots of the seam number group to the initial water production.Number of the gas producing seams is negatively related to the multiple of permeability enhancement of the fractured group,and the multiple decreases with the increase of the seam number.Not every internal geological factors of the co-producing group have an impact on the initial producing behaviors.The weighted average gas saturation and its difference index threshold of the group vary from place to place.The lower the reservoir energy and the worse the reconstructive ability,the stronger the influence of gas saturation difference on the initial production effect.The influence of in-situ pressure state difference on gas breakthrough time mainly exists between different blocks,and has no substantive significance in the same block.It is found that the gas breakthrough time decreases exponentially with the increase of average fractured permeability and there is no inevitable relationship between the fracturing and the in-situ permeability of the group.Unified fracturing measure may enlarge the attribute difference between the producing seams in the group,which inhibit the full release of the overall gas production potential.For this reason,the design idea of the balanced fracturing for the co-producing group is put forward in order to weaken the original reservoir attribute difference through differential reconstruction.It is considered that the former reservoir reconstruction technology is not suitable for the coal reservoir with the depth of more than 800 m in the eastern Yunnan.The key to determine the co-producing group reasonably is the fine analysis of the reservoir pressure profile for a single well,and the key geological factor affecting the fracturing effect of the reservoir in the study area is the coal-body structure.The engineering measure to realize the maximum release of gas production potential of the group is the balanced reconstruction between the gas producing seams.It varies from block to well that whether or not the casing pressure of the co-producing well is held and what extent is casing pressure held,so that it is very important to grasp the timing of gas release.On the premise that the current production control system of co-producing well remains unchanged,it is suggested to control the bottom depth and maximum span of the co-producing group,give preference to the groups with high weighted gas saturation,small gas saturation difference and small pressure state difference indices,and pay attention to the balanced reconstruction of each production seam in co-producing group.