柴达木盆地东坪基岩气藏的特征及合理开发指标

柴达木盆地东坪基岩气藏为变质岩气藏,是我国发现的首个基岩气藏,2015年该气藏天然气年产能规模达到12×108m3,后续随着水侵影响的加剧,产气量大幅度下降。国内外基岩气藏开发实例均较少,对该类气藏的静、动态特征尚缺乏系统的认识,为了给今后同类型气藏的高效开发提供技术借鉴,在深化储层特征评价和气井生产历史跟踪分析的基础上,结合数值模拟技术与气藏工程分析方法,确定了该气藏适宜的井型、井网和合理的开发指标。研究结果表明:(1)东坪基岩气藏储层致密、裂缝和局部溶蚀孔洞发育,具有低孔隙度、高渗透率、大渗透率级差的特点,垂向上划分为风化破碎带、裂缝发育带和致密带,该气藏属于典型的整装构造底水气藏,底水活跃;(2)气藏初期单井产量高、压降速率小,后期受水侵影响,气井产量大幅度下降,甚至水淹停产;(3)以"整体评价、少井高产、均衡开采"为部署原则,宜采取高部位部署水平井与边部部署直井的混合井网进行开发;(4)直井合理产气量应介于3.2×10~4~7.5×10~4 m~3/d,水平井合理产气量应介于12.4×10~4~25.6×10~4 m~3/d,气井合理产量应为其无阻流量的1/4~1/3,气藏的采气速度宜控制在3%以内。

Characteristics and rational development indexes of the basement gas reservoir in the Dongping Block,Qaidam Basin

The basement gas reservoir in the Dongping Block of the Qaidam Basin is a metamorphic gas reservoir and it is the first basement gas reservoir discovered in China. Its natural gas productivity was up to 12×108 m3/a in 2015, and then its gas production rate declined greatly as the effect of water invasion got stronger. There are few development examples available at home and abroad and the static and dynamic characteristics of basement gas reservoirs have not been investigated systematically. For providing technical references for the efficient development of basement gas reservoirs, the reservoir characteristics were evaluated further and the production histories of gas wells were traced and analyzed deeply in this paper. Then, combined with the numerical simulation technology and gas reservoir engineering analysis, the well type, well pattern and rational development indexes suitable for this basement gas reservoir were determined. And the following research results were obtained. First, the basement gas reservoir in the Dongping Block is tight and fractures and local dissolved pores are developed. It is characterized by low porosity, high permeability and large permeability range. It is vertically divided into weathering fracture zone, fracture development zone and tight zone. It is a typical integrated structural gas reservoir with active bottom water. Second, the single-well production rate is high and the pressure drop rate is low in the early stage. In the later stage, however, the production rate declines greatly under the effect of water invasion and even shut down due to water flooding. Third, the arrangement shall be made according to the principle of overall evaluation, high yield with few wells and balanced production. It is necessary to adopt the combined well pattern with horizontal wells in the high position and vertical wells in the edge. Fourth, the rational gas production rate of vertical wells shall be between 3.2×104 m3/d and 7.5×104 m3/d, and that of horizontal wells shall be between 12.4×104 m3/d and 25.6×104 m3/d. The rational production rate of gas wells shall be 1/3–1/4 of absolute open flow and the gas recovery rate of this gas reservoir shall be controlled below 3%.