The Study Of Coalbed Formation Damage Mechanism During Horizontal Drilling Process

CBM is existence in coal seam as the main component of methane, adsorbed on the particle surface of the coal matrix, partly separated from the coal pores or dissolved in the water of the coal seam. CBM belong unconventional natural gas resources. It is a clean and high-quality energy which rising in nearly two decades in the international. China is rich in CBM resources. According to the latest CBM resource evaluation shows that the CBM regional area within the depth of2000m is37.5×104km2. CBM resources is about36.81×1012m3, being about13%of the world’s total CBM resources, ranking third in the world. Thus, development of coal bed methane resources in China is of great significance.With the development of foreign CBM industry and the development field expansion, development technologies is also constantly improving and enhancing a strong impetus to the development of coal bed methane ground development, especially the U.S. successfully exploring the under-balanced drilling cave completion and multi-branch pinnate horizontal well technologies. The majority CBM reservoir in our country have low porosity (Ψ<10%)、low permeability(K<1×10-3μm2) and low pressure(buried not very deep). CBM production per well is low yield and the economic efficiency is relatively poor. CBM horizontal wells are a kind of very good technology which cost not very high but yield very good. More and more horizontal wells have used in the exploitation of coal bed methane. On the basis of actively introducing, digesting and absorbing foreign advanced technology, China began to carry out the CBM horizontal drilling in2004.However, the coal seam as coal bed methane gas layer and the reservoir, its nature are very different from conventional sandstone reservoirs. Coal reservoir has developed cleat, low porosity, permeability, large surface area, low mechanical strength, and susceptible to compression, strong heterogeneity, low reservoir pressure. These characteristics determine the impact of the coal seam much larger than conventional reservoirs in drilling process, meaning that damage is much larger than conventional reservoirs in the process of CBM drilling. Especially for CBM reservoir drilled by horizontal wells which easily got damage by drilling fluids, solid particles, drilling pressure and other factors because of complex drilling techniques and process and the long drilling cycle. These damages will have a direct impact on CBM desorption, diffusion, migration, drainage and ultimately affect the development effects of CBM horizontal wells. Therefore, coal bed methane reservoir damage mechanism analysis and reservoir protection research is particularly important.This article research on the Qinshui Basin coal petrography, the use of theoretical studies and laboratory experiments the combination of technology road, analysis reservoir damage mechanism in the process of CBM horizontal drilling process from three aspects:1) During horizontal drilling, drilling fluid invasion will damage the reservoir. Especially, the drilling fluid invasion caused the expansion of coal rock, the high polymer in drilling fluid block the coal reservoir. In addition, the capillary phenomenon is one of the reasons caused coal reservoir damage.2) In the horizontal drilling process, pulverized coal existed in the feathery branches will hurt coal reservoirs too.3) CBM horizontal drilling use of underbalanced drilling, the pressure changes in the drilling process, the pressure fluctuations caused by tripping are particularly susceptible to damage coal reservoirs. Experimental results show that the filtration rate when the wellbore pressure fluctuations under the mud fluid loss is greater than the constant pressure. To reduce and avoid the damage caused by the horizontal drilling process in the CBM reservoir, the method to protect the coal reservoir has been proposed. By analyzing of the drilling fluid properties, selecting the solid-free bubble drilling fluid system for CBM horizontal drilling, the drilling fluid dynamic evaluation experiments on the coal core confirmed that the drilling fluid system can play a role to protect the coal reservoir.