A large number of natural gas resources of medium-high rank coal reservoirs has been achieved in Qinshui basin and other areas in China, which presents broad prospect of exploitation. Nevertheless, this low-porosity (3%-5%), low-permeability (0.01mD-3mD) type of reservoirs require hydraulic fracturing to realize commercial development. As relatively long period of drainage and regional depressurization are needed before the initiation of coalbed methane (CBM) production, the fractures firstly offer channels for drainage. Meanwhile, it is easy for the soft coal and proppants to be embeded. Therefore, long fractures and high conductivity through fracturing treatments are needed to gain better production performance. Thus, the key factors to get optimal effectiveness of well completion are to accurately acquire understanding of the hydraulic fractures expanding and supporting conditions.
This paper introduces a joint fracture-diagnostic technology of microseismic mapping and microdeformation measurements to monitor nine wells of two cluster well platforms of CBM reservoir. The pattern, orientation and size of fractures are obtained from microdeformation measurements, and the scope of stimulation is obtained from microseismic mapping.
Some conclusions from the studies are as follows: (1) it is hard for the fractures in coal seam to propagate, and their lengths usually range from 44 m to 119 m. The fracture length estimated from microdeformation measurements is smaller than the results from microseismic mapping. (2) A new method to estimate stimulated reservoir volume based on microdeformation measurements through calculating volume envelope is set up. The SRV estimated from microdeformation measurements is much less than the one from microseismic mapping. As the CBM reservoir has a high leak-off coefficient, the SRV estimated from microdeformation measurements is closer to the reservoir volume controlled by the effective propping fractures. (3) On the same platform, the orientation of fractures in the latter constructed well changes due to the stress interference from the former constructed well, and the orientation variation of fractures in the last constructed well is the biggest.
These understandings are of important influences on CBM fracturing design and construction, which can be used to determine reasonable stimulation volume, reasonable construction sequence and interval of platform wells based on the real fracture propagation characteristics, and finally to obtain the optimal well performance.