Coal bed methane (CBM) development in China, especially in both Qin-shui Basin and East Ordos Basin has started at a large scale. There are more than 8500 CBM wells drilled in these two basins and most of which are stimulated with induced hydraulic fracturing and multi-lateral horizontal well technology. However, some of the CBM vertical wells are low productions due to the factors of geology, drilling or producing, while some of the horizontal wells have an appreciate production in the early stage but later drop down to a very low level dramatically due to the wall collapse. Thus, a new technology, namely U-shaped well technology, is researched and applied on site in Qinshui basin for enhancing the average gas productions of CBM wells.
An optimum scheme of drilling and completion of the U-shaped well is presented based on the comparison of the price, risk and feasibility of three different schemes in this paper and used as the pilot program. In order to decrease the comprehensive cost and period of the well, such factors as the access to fuel, water and land, the environmental regulations, i.e., water disposal, mud and drilling waste disposal, and so on, should be taken into account while choosing the well site. The reservoir characteristics and physical parameters, the upper formation lithology and the technology requirements of drilling, cementing and completion should be researched in detail before doing the well design which is the basis of the success of the U-shaped well. Besides, drilling along the coal seams and connecting to the vertical well are two key steps in yielding a successful U-shaped well, therefore, it is important to monitor and adjust the well path in time based on the logging response characteristics, measurement parameters of EMWD, and the drilling and mud logging parameters. With the slotted screen completion in the horizontal well, the U-shaped well can still provide a good flow path for the water and gas even though there is a wall collapse in the horizontal well while producing. In addition, the well bores can also be cleaned up by injecting water form the vertical well and circulating out of the horizontal well when the slotted screen is block by the coal dusts. As a result, the U-shaped well technology can solve such problems as production decreasing and cycle shortening of the multilateral horizontal wells due to the wall collapse while producing.
The daily gas production of the pilot well being in production for only 160 days and in control of the casing pressure kept at 0.7–0.8MPa to produce the gas in accordance with the producing rules of CBM wells has exceeded 2000m3/d. It is still rising gradually and would come up to about 8000 to 10000m3/d in a year's production according to the numerical simulation results which exhibits the U-shaped well a promising application prospect.