Natural Gas Industry B (Aug 2021)
Volume fracturing and drainage technologies for low-pressure marine shale gas reservoirs in the Ordos Basin
Abstract
There are abundant natural gas resources in the marine shale gas reservoir of Middle Ordovician Wulalike Formation in the Ordos Basin, which is an important resource base for PetroChina Changqing Oilfield Company to increase the reserves and production of oil and gas. Compared with the other shale gas reservoirs at home and aboard, however, the marine shale gas reservoir of Middle Ordovician Wulalike Formation in the Ordos Basin has a lower formation pressure coefficient and poorer reservoir physical properties and gas-bearing property, so its production increase difficulty is higher. In this paper, horizontal-well volume fracturing was studied and tested based on the earlier vertical well tests. According to the technical idea of the staged multi-cluster massive fracturing of long horizontal section, the propagation mechanisms and morphological characteristics of fractures were studied and analyzed based on the fracturing geological characteristics of the shale gas reservoir in the Ordos Basin. On this basis, a full three-dimensional fracture model was optimally established for parameter optimization. The fracturing of the test well ZP1 was carried out with 15 stages and 103 clusters. After the fracturing, a more complex fracture network was formed with a fracture complexity index of 0.4–0.6. The microseismic monitoring zone is 579 m long and 266 m wide and the fracture is 146 m high. To address the drainage difficulty after large-volume fracturing of low-pressure shale gas in the Ordos Basin, this paper carries out a gas energized fracturing test. Considering the characteristics of reservoir physical properties, gas-bearing property and segmented fractures, 805 m3 liquid nitrogen was injected in stages during the fracturing of the test horizontal well. The formation pressure coefficient measured from pressure buildup data is increased from 0.7 to 0.8 to 1.88. The wellbore gas–liquid flow model was established and the parameters of long-period control-pressure drainage were optimized. The critical surface equipment was upgraded to achieve accurate measurement, safety and environmental protection. And the following research and practice results were obtained. First, based on the technological innovation and optimization, continuous gas–liquid two-phase flow is realized in the test well ZP1 and its production rate and pressure during the test are stable with the tested daily shale gas production at the wellhead of 6.42 × 104 m3. Second, after fracturing, the absolute open flow of the test well reaches 26.4 × 104 m3/d, which is more than 10 times higher than the production rate of the vertical well in the same block during the test. Thus, a significant breakthrough is realized in the exploration of marine shale gas in North China.
