Research progress and breakthrough directions of the key technical fields for large scale and efficient exploration and development of coalbed methane in Xinjiang

The Xinjiang Uygur Autonomous Region has presented the expected resource conditions and work foundations for a large-scale coalbed methane (CBM) exploration and development, which shows that its CBM resources below the depth of 2000 m are 7.5 trillion m³, 450 CBM wells have been constructed, and the annual gas production has approached to 80 million m³. Xinjiang has put forward the goal of the annual CBM production of 2.5 billion m³ in 2025. Therefore, the large-scale and efficient development of CBM in Xinjiang has become an urgent and significant demand. In this paper, the main research progresses of Xinjiang CBM made in five key technical fields have been systematically summarized, including the CBM enrichment model and area optimization technology, the prediction and detection technology for sweet spot distribution, the technological strategy of the accelerated rolling development and rapid increase of reserves and productions, the geological adaptation technology system, and the cooperative exploration and development of CBM with coal, oil and gas. Then, the potential breakthrough directions have been analyzed and proposed. Research has shown that the coal reservoirs in Xinjiang show the unique geological characteristics of CBM, including widely developed multi-thick coal seams, low rank coal development, the frequent occurrence of steep coal seams, the significant deformation and structural control of coal bodies, complex hydrologic and outcrop conditions, and the gas-bearing and physical properties with three “low” (low gas content, methane concentration, and gas saturation) and five “high” (high gas intensity, porosity, stress change, reservoir pressure change, and permeability change). The Xinjiang CBM has multiple genetic types and enrichment models, including biogenic mechanism, thermogenic mechanism, and biogenic-thermogenic composite genetic mechanism, etc., and their corresponding enrichment models. Biogenic gas reservoirs or biogenic gas contribute widely. The distribution and occurrence patterns of CBM in Xinjiang show some significant differences between foreland basin and intermountain basin. Therefore, the first breakthrough direction is to innovate and form the scientific evaluation based on the principle of “two separation” (low-rank and middle- and high-rank; shallow and deep coal) and “two combination” (geological and engineering evaluation; multivariate data) and the precise target optimization technology based on “machine learning + three-dimensional geological modeling”. The sweet spot of deep CBM/CMG (coal measure gas) in Xinjiang is mainly the uplift of the depression in the basin, the depression of the uplift in the basin, and the slope around the basin margin. The potential well location is the structurally high position, and the potential reservoir is the fissure-developed primary structure coal seam or pore and fissure-developed coal bearing sand conglomerate reservoir. Then, the second breakthrough direction is the sweet spot prediction and exploration technology for deep CBM/CMG based on “new method of geophysics, rock physics and rock mechanics stratigraphy” and “new concept of geological and engineering sweet spot”. The basic principles of the accelerated rolling development are low-risk, short-cycle, high-efficiency, and multi-batch project deployment. The technical strategies of the rapid increase of CBM reserves and productions in the middle and shallow coal seams include the new well layout and construction in the new optimized block and the old well reconstruction for the increase of CBM production in the mature block. While this strategy for the deep coal seam is to give priority to deployment and development in the sweet spot in the deep but gentle slope in the large basin and the uplift in the basin. Then, the third breakthrough direction is the engineering deployment methods and technical strategies for the scientific accelerated rolling development, and efficient and rapid increase of the gas reserves and productions. The important progresses have been made in the engineering technologies of Xinjiang CBM, such as the differential optimization and deployment of the well type and pattern, the drilling and cementing with a low reservoir damage, the high reliability logging and well testing, the efficient staged fracturing with the multi-well types, and the drainage control with the low casing pressure and controlled pressure. The fourth breakthrough direction is to develop and construct a geological adaptability technology system for a large-scale and efficient exploration and development of CBM in Xinjiang. For the middle and shallow CBM and coal cooperative exploration and development, the gas extraction followed by coal mining, the co-extraction of CBM and coal, and the co-extraction of CBM and in-situ oil-rich coal should be conducted. For the deep CBM, oil, and gas cooperative exploration and development, the development of coal measure superimposed gas reservoir, the co-exploration and co-extraction of CBM and CMG, and the exploration and development of the whole petroleum system in coal-bearing sequence should be conducted. These cooperative exploration and development of deep CBM, oil, and gas is the fifth breakthrough direction, whose developments have been considered and explored. The results of this study are expected to provide the technical support and engineering decision reference for a large-scale and efficient exploration and development of CBM in Xinjiang.