Evaluation and fracture optimization scheme of deep coal reservoirs in Baijiahai bulge: An example of CaiMei−2−004H Well

The target layer of Caitian 1H well in Baijiahai Bulge, the eastern margin of Junggar Basin, is Jurassic Xishanyao Formation coal, and its production rate exceeded 10 000, indicating that there is a huge potential for deep coalbed methane (CBM) gas production in this area. Taking Caimei−2−004H well, the latest deep CBM evaluation well constructed by Xinjiang Oilfield in 2023, as an example, on the basis of analyzing the spreading characteristics of the Jurassic Xishanyao Formation coal beds in the Baijiahai Bulge and the three field characteristics of the deep part of the well, we conducted an in-depth analysis of the mechanical properties of the coal reservoir in the well, such as brittleness index and fracture development, and constructed a fracturability evaluation model on the basis of which we made a study of fracturability evaluation of the Xishanyao Formation coal reservoirs in this area. Fracturability evaluation is studied, and further combined with Meyer numerical simulation, fracture optimization suggestions are given. The results show that: Through the geological and three-field characterization, it is found that the depth of the Jurassic Xishanyao Formation coal seam in the study area is more than 1 500 m, and the coal seam is stably distributed in the middle and lower part of the bulge; the distribution of the ground temperature gradient is in the range of 2.6−3.6 ℃/hm, with an average of 3.1 ℃/hm, and the distribution of the gradient is relatively flat, and the contour lines are spreading in an east-west direction; the pressure coefficient of the target reservoir is close to or less than 1, and it is a normal-anomalous low formation pressure, and the modern ground stress is mainly positive and low. The pressure coefficient of the reservoir in the target layer is close to 1 or less than 1, which is normal-anomalous low stratigraphic pressure, and the modern geostress is mainly of positive fault-type mechanical mechanism type; the triaxial mechanical experimental tests revealed that the Poisson's ratio of the coal reservoir decreases, the peak strength increases, and the brittleness index enhances when the deep high temperature and high pressure is compared with the normal temperature and normal pressure conditions. A fracturability evaluation model coupled with the brittleness index, the degree of natural fracture development, the horizontal principal stress difference and the fracturability strength was constructed, and the fracturability grading evaluation standard was proposed. Calculation results show that the fracturability index of the upper coal is 16.15, and the fracturability index is 28.04 under the simulated original reservoir conditions; the fracturability index of the lower coal is 28.07, which is higher than that of the upper coal, and the gas content of the lower coal is higher than that of the upper coal, so it is a preferred section to be developed, and the high-temperature and high-pressure environment in the deeper part of the reservoir makes the fracturability index of the coal of the target layer increase, which is more conducive to the fracturing and reforming. Based on the Meyer numerical simulation software to simulate the fracturing modification effect and optimize the displacement, fracturing fluid and proppant dosage in combination with the economic evaluation, it was determined that the optimal modification effect can be obtained with a displacement of 15 m³/min, a fracturing fluid dosage of 11.5 m³/m, and a proppant dosage of 3 m³/m. The results are of great significance for recognizing the fracturability of deep coal reservoirs and selecting the optimal fracturing section.