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

The basis of efficient coalbed methane (CBM) co-production is to understand the co-producing compatibili- ty deeply,in which the key is the quantitative analysis of geological conditions for co-production. Taking the typical ar- eas 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 compari- son methods of geological factors,and the geological suggestions for co-producing compatibility threshold and engineer- ing 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 thresh- old 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 frac- tured 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 differen- tial reconstruction. It is considered that the former reservoir reconstruction technology is not suitable for the coal reser- voir 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 fractu- ring 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 va- ries from block to well that whether or not the casing pressure of the co-producing well is held and what extent is cas- ing pressure held,so that it is very important to grasp the timing of gas release. On the premise that the current produc- tion 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 produc- tion seam in co-producing group.